PT06

Michael L. White, Met-L-Chek, Santa Monica,California

Jeffrey F. Cook, JFC NDE Engineering, Idaho Falls,Idaho

Robert J. Lord, Boeing Company, St. Louis, Missouri

6Surface Preparation and

Cleaning

C H A P T E R

Other than removing loose dirt withnonabrasive techniques and cleaning witha simple solvent wipe, a nondestructivetesting inspector is unqualified to performmost cleaning operations. However,the inspector needs to know (1) whatcontaminants and surface conditionsadversely affect the liquid penetrant test,(2) the cleaning techniques or otherprocesses required to eliminate theadverse conditions and (3) theconsequences of using those techniquesand processes. Knowledge of theseconditions and techniques enables theinspector to request the proper operationsand be assured that the surface is properlyprepared for liquid penetrant testing.

The mesh numbers used in this chapterto express the coarseness of grit used inmechanical processing correspond toparticle diameters, as explained in thisvolume’s discussion of measurementunits.

Test Object Preparation forLiquid Penetrant TestingThe first step of the liquid penetrant testprocess is the preparation and cleaning ofthe test surface and any surface connecteddiscontinuities. Surface preparation aidswetting and flow of the liquid penetrant.Surface conditions, e.g., soilcontamination or surface irregularities,can reduce the effectiveness of the testprocess by interfering with (1) wetting ofthe test surface by the liquid penetrant,(2) entry of the liquid penetrant intodiscontinuities and (3) subsequentbleedout of liquid penetrant to formdiscontinuity indications.

Such interfering surface conditionsmust be eliminated by surface preparationbefore application of liquid penetrant.The surface preparation techniqueselected must effectively remove thepotential sources of interference withoutdamaging the parts being processed.Reliable liquid penetrant testing cannotbe expected unless the parts to be testedare free from contamination. Foreignmaterial adhering to the surface orcontained within the discontinuities orleak passageways, as well as the surfaceeffects cited above, can produce erroneoustest indications or prevent indicationsfrom forming.

After surface preparation and cleaning,any residues of cleaning agents, includingwater, must be removed. The part anddiscontinuity surfaces must be not onlyclean but also dry.

Surface ConditionsInterfering with LiquidPenetrant TestingTest material surface conditions that mayinterfere with the application of liquidpenetrants or their entry into surfaceconnected discontinuities may beclassified into two groups:(1) contaminants on the surface or withindiscontinuities that prevent surfacewetting and capillary flow of the liquidpenetrants; (2) contaminants or surfaceconditions that physically block theentrances to discontinuities so that liquidpenetrants cannot enter.

Similarly, surface conditions orcontaminants that trap liquid penetrantor tracer liquids to produce falseindications or inhibit or preventextraction of liquid penetrant retained inleaks or discontinuities duringdevelopment, can be classified into twogroups: (1) porous, adherent coatings orcontaminants providing interstices thatretain liquid penetrants on test objectsurfaces or impede their entry intounderlying discontinuities or leaks; and(2) coatings on interior surfaces ofdiscontinuities or leaks that have highsurface energies and resist extraction ofthe liquid penetrant from thediscontinuities into the developer coating.

Types of SurfaceContamination Found onTest ObjectsSurface contamination and contaminationwithin leaks or surface connecteddiscontinuities can be of many types,including (1) preservative, forming,machining or lubricating oils and otherliquids containing organic constituents;(2) carbon, varnish and other tightly heldsoil; (3) scale, rust, oxides, corrosionproducts and weld metal and weld fluxresidues; (4) paint and organic protectivecoatings; (5) water, hydrates or other

162 Liquid Penetrant Testing

PART 1. Effects of Test Object SurfaceContamination or Irregularities

residues left after evaporation of water;(6) strong acids, alkalis or otherchemically active residues, includinghalogens; (7) polishing, forming, drawingor buffing lubricants; (8) residues fromprevious liquid penetrant testing or liquidleak testing; and (9) surface treatmentssuch as phosphate, chromate conversioncoatings, black oxide or temper colors.

In some cases, sources ofcontamination can be very subtle anddifficult to identify. For example, oneaerospace manufacturer encounteredproblems resulting from contaminationby the adhesive backing on sanding disksthat came off and adhered to weldsurfaces during mechanical cleaning. Theeffects of each of these types ofcontaminant will be discussed below.Tables 1 and 2 summarize examples ofconditions interfering with penetrantaction and suggest treatments for removalor correction of interfering conditions.

Effects of Contamination by Oils,Grease, Organic Fluids or ResiduesThe performance of liquid penetrants canbe affected by contamination of testsurfaces and surfaces withindiscontinuities by various processingsubstances. Typical examples of suchcontaminants include preservative,forming, drawing, cutting, machining orlubricating oils; wax or crayon marks;dyes; greases; and ultrasonic testcouplants such as glycerine. Manyanalogous contaminants can result fromshop environments or industrialpollution.

Oily or greasy surfaces tend to impairthe action of liquid penetrants (1) bypreventing formation of a continuouslayer of liquid penetrant over the surfacearea being tested or (2) by filling orblocking leaks and surface connecteddiscontinuities so that the liquidpenetrant cannot enter and fill theircavities. Contamination by addition of a

163Surface Preparation and Cleaning

TABLE 1. Contaminants that are on the test object surface or contained in voids or discontinuities and that interferewith liquid penetrant action during processing, with removal procedures or corrective treatments.

1. Preservative, forming,machining or lubricatingoils

Most oils fluoresce under ultraviolet radiation.This fluorescence can obscurefluorescent liquid penetrant indications orproduce false indications.

Oily surfaces also tend to impair the action of theliquid penetrant.

1. vapor degreasing2. hot tank alkaline cleaning3. steam cleaning4. solvent emulsion cleaning5. water emulsion cleaning6. solvent washing

Types of Contaminants Interference Effects Removal Procedure or Treatment

2. Carbon, varnish or othertightly held soil

Surface soils tend to adsorb or absorb penetrant,resulting in background color or fluorescence.

These contaminants may also obstructpenetration into defects, impede wettingaction or bridge discontinuities.

1. solvent type carbon removera

2. alkaline type removersa

3. wire brushing (caution)a,b

4. vapor or sand blastinga,b

3. Scale, rust, oxides andcorrosion

Surface soils tend to adsorb or absorb penetrant,resulting in background color or fluorescence.

These contaminants may also obstructpenetration into defects, impede wettingaction or bridge discontinuities.

1. alkaline or acid type removal procedures2. wire brushingb

3. vapor or sand blastingb

4. electrocleaning

4. Paint coatings Paint coatings impede wetting.Paint coatings may also obscure or bridge surface

discontinuity openings.

1. solvent type paint removers2. alkaline type paint removers3. abrasive removal proceduresb

4. burning

5. Water Water impedes wetting and penetration. 1. air dry2. force dry with dry air3. oven dry at elevated temperature

6. Strong acids or alkalines Strong acids or alkalines impede wetting andpenetration.

These contaminants may also also may react withpenetrant to decompose or degrade dyes orother constituents.

1. rinse with fresh water2. use neutralizing rinse, fresh water rinse and

dry

a. Agitation such as used in ultrasonic cleaning may be beneficial with this surface treatment.b. Mechanical processes that peen or smear surface material may act to close openings into discontinuities so that liquid penetrant indications cannot form.

Such abrasive cleaning techniques are often prohibited or require a subsequent acid etching treatment to reopen the discontinuity to the part surface so thatpenetrant can enter.

second liquid to a liquid penetrant willmodify the surface tension and changethe wetting characteristics of the liquidpenetrant, typically with undesiredresults. When a crack is coated with amaterial that has a surface energydiffering from that of the liquidpenetrant, the penetrating action can bechanged.

Many oils can fluoresce underultraviolet radiation. This fluorescence ofoil may obscure a true liquid penetrant orleak indication or result in a falseindication. Removal of all contaminationby oils or greasy fluids from the surfacesof the test object and any surfaceconnected discontinuities is essentialbefore liquid penetrant testing. A checkcan be made with ultraviolet radiation toensure proper removal of fluorescing oils.

Effects of Contamination byCarbon, Engine Varnish or TightlyHeld SoilTightly bonded contaminants such aslayers of carbon, engine varnish and othersoils are difficult to remove from testsurfaces. They can interfere seriously withliquid penetrant testing. They typicallyimpede surface wetting by liquidpenetrant and may bridge over or obstructentry of liquid penetrant intodiscontinuities. Such contaminant layerson the test part surface tend to absorb orcollect liquid on their surface; they mayalso absorb or assimilate liquid into theinterior of the contaminant layer. Thisleads to background fluorescence orvisible dye staining of the surfaces of testobjects, reducing indication contrast andvisibility. For these reasons suchcontaminants should be removed fromtest parts before application of liquidpenetrant.

Effects of Scale, Rust, Oxides,Corrosion Products and WeldResiduesScale, rust, oxides and corrosion productsmust also be removed from test surfacesbefore application of liquid penetrants.These surface layers tend to coverdiscontinuities so as to block entry ofliquid penetrant. They may also causeconfusing test indications by trapping andholding liquid penetrant on the surface ofthe parts. The removal of scale, rust andoxides should be accomplished bytechniques that do not close entrances tothe discontinuities that may be presentand that will not impair the ultimateserviceability for which the part isintended. In particular, it is desirable toavoid techniques that will cover leak ordiscontinuity openings by peening,smearing or cold working the surfaces.Where the surface is suspected of havingbeen adversely affected, a treatment toreopen the discontinuities is requiredbefore liquid penetrant testing (seeelsewhere for metal removal techniques).Care is also required to ensure thatresidues left from cleaning processes arenot retained to affect liquid penetrantprocessing or evaluation.

Effects of Paint, Organic Coatings,Carbon and VarnishContaminationPaint, varnish, organic coatings, enamelsor carbon contamination can completelyprevent detection of discontinuities byliquid penetrant testing. Although someconventional paints are readily removedwith standard procedures, recent advancesin paint technology have resulted infinish systems with unique adherence anddurability. Total removal of such adherentcoatings can be very difficult, even withspecial products and processes developedfor this purpose. Even after removal of themajor part of such coatings, residues maybe left that obscure or bridgediscontinuities in the underlying metal or


TABLE 2. Test object surface irregularities and conditions interfering with liquid penetrant action during processing, withremoval procedures or corrective treatments.

1. Surface roughness reduces ease of rinsing; increases difficulty ofhand wiping

1. abrasive polishinga

2. electropolishing

Types of Contaminants Interference Effects Removal Procedure or Treatment

2. Smeared metal resultingfrom forming operationsof surface abrasion

may cover flaws and prevent liquid penetration 1. acid etching2. electropolishing

a. Mechanical processes that peen or smear surface material may act to close openings into discontinuities so that liquid penetrant indications cannot form. Suchabrasive cleaning techniques are often prohibited or require a subsequent acid etching treatment to reopen the discontinuity to the part surface so thatpenetrant can enter.

test material. Particular problems can arisewhen paint, other coating material orresidues from paint removal are firmlylodged within surface connecteddiscontinuities. In these cases, no cavity isleft for entrapment of liquid penetrant. Toensure reliable liquid penetrant testing,surface layers including materialsimbedded within the discontinuities mustbe removed completely before applicationof the liquid penetrant.

Effects of Water Contamination ofTest Surfaces, Leaks and SurfaceDiscontinuitiesSpecial precautions are needed when testparts may have been in contact withwater, such as during water rinsing afterchemical cleaning, before liquid penetranttesting and leak testing. It is importantthat all traces of water be completelyremoved before application of liquidpenetrants. Water must be removed notonly from the surface of the parts but alsofrom any surface connecteddiscontinuities that may be present;otherwise, water within discontinuitieswill minimize or prevent the penetrant’sentry into surface discontinuities.Generally, oil based liquid penetrants tendto be immiscible with water.

An excellent means for drying smallerparts before application of liquidpenetrants is with the dryer normallyused to dry test parts. However, in manycases, drying at room temperature untilthe surfaces appear dry will be adequatefor liquid penetrant testing if time is not afactor. It is desirable to perform tests todetermine the effects of drying for eachspecific liquid penetrant testing system.However, much greater difficulty isinvolved in removal of water from leakpassageways before leak testing. Vacuumdrying before leak testing is helpful whereit is feasible. A preferred procedure is toprevent water from contacting test objectsbefore leak testing.

Effects of Contamination byStrong Acids or AlkalisStrong acids or alkalis used to clean testparts before liquid penetrant or leaktesting can impede surface wetting andpenetration of test media into surfacediscontinuities if proper water rinsing anddrying are not performed. These acids andalkalis may react to decompose or degradedyes or other active constituents of liquidpenetrants. In particular, acid andchromate residues may adversely affectthe dyes by decomposing them, resultingin weak or faint test indications.Chromate residues also absorb ultravioletradiation, which leaves less radiation toexcite fluorescence in the liquid

penetrant. For this reason, it is desirableto rinse thoroughly any test objectsurfaces that have been exposed to acidsor chromates before liquid penetranttesting. All residues of acidic etchingsolutions such as those applied to removesmeared surface metal must be eliminatedby thorough washing.

Effects of Surface Contaminationby FingerprintsThe effects of fingerprints from manualhandling of tests parts are not alwaysrecognized as sources of contamination.Some individuals’ fingerprints are acidicto the extent that they can etch orcorrode a highly polished surface. Evenwhen etching does not occur, the oilsassociated with fingerprints can inhibitwetting by a liquid penetrant. Cotton orrubber gloves are recommended for partshandling where fingerprints are a knownproblem or where it is suspected that theycould cause trouble during liquidpenetrant testing. This can be particularlyimportant on critical surfaces of machineparts, aircraft components or nuclearcomponents.

Effects of Residues fromPrior Visible Dye LiquidPenetrant Testing or LeakTestingResidues retained from previous liquidpenetrant or leak tests must be classifiedas contaminants that may affectsubsequent tests adversely. If furtherliquid penetrant testing will be performedon the part (either as further processingtakes place or when it is in service),thorough and prompt postcleaningshould be practiced (see elsewhere aboutcleaning techniques after testing).

Visible dye liquid penetrant residuescontaining red dyestuffs act as ultravioletradiation filters. When mixed withfluorescent liquid penetrants, theseresidues can appreciably diminish ordestroy fluorescent indication brightness.Consequently, visible dye liquid penetranttesting should be prohibited whenfluorescent liquid penetrant tests willfollow. When reinspection of a test objectis essential and preceding tests have usedvisible dye penetrants, the reinspectionshould also be made with visible dyeliquid penetrant, if possible. If suchreinspection is not possible, tests usingcracked reference specimens and theliquid penetrant systems in questionshould be performed to verify theeffectiveness of any planned subsequentfluorescent liquid penetrant testing.


Effects of Lack ofCleanliness in LiquidPenetrant Processing AreasLack of cleanliness in liquid penetranttesting areas is a potential source of testobject contamination. If test objects areplaced on surfaces or in containers thathave previously been used for parts inprocess, they may be contaminated byliquid penetrants, emulsifiers, solvents,water or developers. If, before the testprocess, the test operator handles testparts with hands contaminated withmaterials used in the liquid penetranttesting, these contaminants may bedeposited on the surfaces of thepreviously cleaned test parts. Any suchprior contamination of the test objectscan interfere with the proper functioningof the liquid penetrants and either canlead to false indications or can obscurevalid indications. When doubt exists, itmay be preferable to subject test objectssuspected of being contaminated tothorough recleaning and retesting.


Techniques of PrecleaningTest Objects for LiquidPenetrant TestingBoth precleaning (before testing) andpostcleaning (following liquid penetranttesting) are vital steps in the test process(Fig. 1). Methods for precleaning of testobjects include (1) detergent cleaning,(2) vapor degreasing, (3) steam cleaning,(4) solvent cleaning, (5) acid or alkalinecleaning, (6) abrasive cleaning, (7) paint,varnish and carbon removal,(8) electrocleaning, (9) agitation or

ultrasonic cleaning, (10) salt bathdescaling, (11) etching and(12) combinations of these techniques.

The cleaning process selected must beeffective in removing soil or contaminantsbut must not damage the test objects.Different cleaning processes may berequired for different metals, alloys ornonmetallic objects and for differenttypes of contaminants and surfaceconditions.

Surface preparation by nondestructivetest personnel is usually limited toremoval of water or loose dirt andremoval of oils and greases with solvents.However, test personnel should be alert to


PART 2. Procedures for Cleaning Surfaces beforeLiquid Penetrant Testing

FIGURE 1. Flowsheet for cleaning processes used with liquid penetrant testing.

Alkaline

Incoming parts

Steam Vapor degrease Solvent wash Chemical

Mechanical Paint stripper Ultrasonic

Dry

Etch(optional)

Inspect

Water rinse Mechanical wash

Dry

Vapor degrease Solvent soak Ultrasonic clean

Preclean

Inspect

Postclean

Outgoing parts

all surface contamination and request thatappropriately skilled personnel performmore thorough cleaning as required.

Selection of CleaningProcessesSelection of cleaning processes forremoval of soils and other contaminantsfrom metallic test objects is influenced bythe following factors: (1) types ofcontaminants to be removed(2) composition or alloy of the test object,(3) degree of cleanliness required forproper operation of liquid penetrant testsand (4) cost and time factors.

In addition, the quantity of similar testobjects to be cleaned, their size and shapeand the ease with which they can behandled are factors to be considered.

Multiple small test objects can oftenbest be cleaned by immersion in liquidcleaners or by mechanized processes suchas vapor degreasing, dipping or sprayingof parts carried by conveyor lines. Largeparts that cannot be placed in cleaningtanks may be cleaned by spraying bothcleaners and rinsers onto part surfaces.

Parts to be cleaned in the field or atfixed locations in shops or testlaboratories can often be handled bywipe-on, wipe-off techniques. In wipingtechniques, cleaning or removal fluids areapplied by lintfree rags moistened orwetted and wiped over test surfaces.Excess fluids and contaminants areremoved by wiping with dry rags orlintfree paper towels. In each case, cautionis required to ensure that cleaning andrinsing fluids are free from excessivecontamination that could reduce theireffectiveness. Hazards to personnel fromtoxic or flammable materials must becontrolled or prevented. The cleaningagents must also be selected to benondamaging to test object materials andsurfaces. Wiping material that is lint freeor has lint that does not fluoresce shouldbe used.

Precautions in Cleaning ofMetallic SurfacesAlthough liquid penetrant testing isapplicable to a wide variety of testmaterials, metal surfaces are by far themost commonly tested. Various metalsmay be involved, including aluminum,magnesium, titanium, carbon and lowalloy steels, stainless steels, hightemperature nickel base alloys and copperalloys. The choice of cleaning chemicalsshould be specific to the method to becleaned. For example, a highly alkaline orcaustic solution may be effective for

cleaning magnesium or magnesium alloysbut can act as a strong etchant onaluminum alloys. Alternatively,mechanical procedures (such as gritblasting) recommended for removingscale from some steels may causeexcessive smearing of softer metals suchas aluminum, magnesium or titanium.

Cleaning by polar inorganic chemicals,as in alkaline cleaning or acid etching, isnot generally recommended forassemblies that contain exposed joints orjunctions that could entrap the cleaningmaterials. The entrapped materials cannot only contaminate liquid penetrantmaterials used later but can also causecorrosion of parts after liquid penetranttesting.

Precautions in Cleaningand Processing ofNonmetallic SurfacesNonmetallic surfaces such as plastics andelastomers or ceramics and glass can havedifferent wetting characteristics thansurfaces of metals. The nonmetallicchemistry may resist removal ofcontaminants. In addition, damagingeffects can occur from the liquidpenetrant materials. For example, liquidpenetrant testing of acrylic plastics mightresult in crazing. Preliminary trials onreject or scrap surfaces and/orconsultation with the supplier of the testobject materials or the liquid penetranttest materials is advisable in questionablecases. The same precautions for cleaningmetallic surfaces apply to cleaning ofnonmetallic surfaces because cleaningtechniques used for metallic surfaces maybe ineffective or may damage thenonmetallic material. Because liquidpenetrant testing of nonmetallics is aminor application, the followingdiscussion deals with metals.

General Sequence of Stepsfor Precleaning TestSurfacesCleaning before liquid penetrant testingof production parts generally has thefollowing sequence: (1) removal of lightoil or grease by treatments such asdegreasing, emulsion cleaning, solventwiping, alkaline cleaning or steamcleaning; (2) rinsing if water basematerials are used in the previous step;(3) removal of oxide, scale or rust bymechanical means such as grit blasting orby chemical means such as acid picklingor alkaline descaling; (4) etching if metalmight be smeared; (5) rinsing to remove


all residues left from cleaning; and(6) drying to remove all traces of waterleft from rinsing.

However, it should be noted that manyinservice parts have an accumulation ofdirt mixed with oils that is not removableby the techniques listed above.

Problems RequiringComplex CleaningProceduresComplex problems encountered inprecleaning of test objects for liquidpenetrant tests may require severalcontaminant removal techniques. Forexample, scale removal by acid treatmentmust normally be preceded by alkalinecleaning or by some other technique thatremoves the oil so that the acid can reactwith surface contamination. Acids are notgenerally good cleaners for oily soils.

The removal of scale and carbonaceousdeposits such as those on used jet engineblades usually requires a multistagecleaning process. Procedures for cleaninghighly critical products may necessitatecareful processing with pure cleaningproducts, followed by rinsing withdeionized or distilled water. Such criticalapplications are best considered on anindividual basis.

Types of SoilThe term soil refers to undesired materialon a surface that is not an integral part ofthe surface. Oil, grease, dirt and loosescale are soils. On the other hand, adecarburized skin or excess hardchromium are not considered to be soils.Soil can be classified into seven broadgroups: (1) pigmented drawingcompounds, (2) unpigmented oil andgrease, (3) chips and cutting fluids,(4) polishing and buffing compounds,(5) rust and scale, (6) carbonaceousdeposits and (7) miscellaneouscompounds such as lapping compounds,ultrasonic test couplants and residue frommagnetic particle testing.

Removal of Pigmented DrawingLubricantsAll pigmented drawing lubricants aredifficult to remove from metal parts. Theyusually contain substances such aswhiting, mica, graphite, white lead, zincoxide, bentonite, flour, molybdenumdisulfide and soaplike materials. Graphite,white lead, molybdenum disulfide andsoaps are the most difficult soils toremove from metallic test objects. Vapordegreasing or solvent cleaning can remove

common unpigmented oils and greasesfrom hot metals and alloys. Animal andvegetable oils that saponify at slow ratesand are insoluble in water can be removedby soaking or spraying with 80 °C (180 °F)alkaline solutions.

Removal of Oily Soils ContainingHard ParticlesSoils containing metal or abrasiveparticles in an oil phase offer specialremoval problems. The oil phases arereadily removed with solvent or vapordegreasing but the solid particle residuesare difficult to remove from metallicsurfaces except by agitation. When soilscontain solid particles, the baths ofcleaning compounds must usually beagitated by a mechanical device, excitedby ultrasonic vibrations (ultrasoniccleaning) or applied by pressure spraying.

Difficulties in Removing Residuesof Magnetic Particle Testing orUltrasonic TestingResidues left on test surfaces by wetmagnetic particle testing are difficult toremove and can interfere with subsequentliquid penetrant testing. It is essential todemagnetize the parts before attemptingto clean off these residues. Vapordegreasing acts to set the magneticparticles rather than to remove them. Theextremely powerful magnetizing fieldsused in magnetic particle testing result inrestricting the openings of surfacediscontinuities with magnetic particles. Itis so difficult to remove the magneticparticles that it is better not to try toreinspect the part with liquid penetrants.If both test methods are to be used, theliquid penetrant test should be used first.

Liquid penetrant testing should alsoprecede ultrasonic testing. The fluidcouplants used with ultrasonictransducers can also block or fill surfacediscontinuities and may be quite difficultto remove.

Avoidance of AbrasiveMethods That Peen, Smearor Cold Work MetalsSurface preparation by severe abrasionthat peens, smears or cold works metallicsurfaces should be avoided in precleaning.Peening or cold working of metal surfacestends to close discontinuities. Thustechniques such as grit blasting, sandblasting, emery cloth, wire brushing ormetal scraping should be used only withcaution and only when no othertechnique will suffice. Specificationsusually prohibit such abrasive cleaning


techniques, particularly on soft materialssuch as aluminum.

When abrasive treatments areunavoidable and the metal surface issuspected to having been adverselyaffected, a treatment such as etching toreopen the surface discontinuities ismandatory.

Restrictions on Grit Blasting orSoft Wire BrushingAerospace industries frequently prohibitgrit blasting or wire brushing of partsbefore liquid penetrant testing. In otherapplications, grit blasting with soft grit orgentle brushing with soft wire brushesmay be used. These soft abrasiontechniques are not so objectionable whenused on metallic materials withRockwell C hardness of 40 or greater.However, soft wire brushes may causesmearing when the brush material wearsoff and is deposited on the hard metalsurface. Wire brush residue is alsoobjectionable because it is a source ofmetal contamination. Carbon steel wirebrushes are allowed only on carbon andlow alloy steel. Austenitic stainless steelwire brushes that have not been usedpreviously on other materials may be usedmanually on austenitic stainless steel toprepare its surface for liquid penetrantapplication.

Restrictions on Halogen,Sulfur, Potassium orSodium CleaningCompoundsSome metals and alloys are sensitive tocertain elements such as halogens, sulfur,sodium and potassium. The aerospace andnuclear industries have been concernedabout effects of halogenated solvents suchas have been used in vapor degreasing,which can have damaging effects onhalide sensitive materials, particularlytitanium and its alloys. (Some companiesforbid vapor degreasing with halogenatedsolvents to eliminate any possible effectsof chlorides or fluorides from thedecomposition products.) Cleaningcompounds that contain sulfur shouldnot be used with nickel or nickel basealloys. Restrictions are placed on halogencontaining compounds on stainless steelsand on both halogen and sulfurcontaining compounds for use on nickelbase alloys for nuclear power industrialapplications. Restrictions on surfacetreatments of metals for militaryequipment are cited in applicablespecifications.

In the United States, precautions onhalogenated solvents are moot in light of

regulations prohibiting the productionand use of ozone depleting substances.

Removal of Soluble Oilsand GreasesThe removal of unpigmented oil andgrease such as common shop oils andgreases, rust preventive oils, lubricatingoils, drawing lubricants and quenchingoils can be effected by several differentcleaning processes, including (1) vapordegreasing, (2) hot tank alkaline cleaning,(3) steam cleaning, (4) solvent emulsioncleaning, (5) water emulsion cleaning,(6) solvent washing or wiping, (7) highpressure spray or air assisted spray and(8) combinations of above cleaningtechniques.

The spray cleaning techniques may beused either in an automated cleaningsystem or by using hand held hoses orwands. Agitation is beneficial in tankapplication and ultrasonic agitation isparticularly effective in certainapplications.

Protection of Cleaned TestSurfaces against CorrosionAfter cleaning to remove surface oils andgrease, test objects that are thoroughlyclean and dry should (1) have the liquidpenetrant applied immediately or (2) beplaced in clean, dry temporary storageand (3) possibly be treated with corrosionpreventive materials. Clean steel parts canrust in a few hours in a humidatmosphere. If rust preventives are usedafter precleaning, another cleaningoperation is required before liquidpenetrant testing. Magnesium is alsosusceptible to corrosion if not treated.

Vapor DegreasingOne of the most common techniques ofpreparing test parts for liquid penetranttesting is vapor degreasing. This process isparticularly suitable for removal of solubleorganic contaminants such as mineral oilsand greases. Unfortunately, vapordegreasing is not effective for removal ofsolid contaminants such as carbon,varnish, paints, scale, corrosion productsor oxides. Other means of removal arerequired. In some cases, restrictions areplaced on vapor degreasing of chloridesensitive metals and alloys withhalogenated solvents (see discussionelsewhere on restrictions on halogen,sulfur, potassium or sodium compounds).

When steel or other ferrous metal partsare vapor degreased, the metal is usually


highly susceptible to atmosphericcorrosion if the air has high humidity orcontains acid vapors, combustionproducts or other corrosives. All acids,cyanides or oxidizing agents should beprevented from contaminating thesolvent used for cleaning steels. If thesolvent should become acidic, it canattack steel, aluminum and magnesiumalloys, which can lead to solventdecomposition and formation ofcontaminants that can affect stainlesssteels adversely.

Mechanism of Soil Removal inVapor DegreasingIn vapor degreasing the hot vapors of avolatile solvent are used to remove oils,greases or waxes from metallic test objectsin preparation for liquid penetranttesting. A steel tank fitted with a heater,solvent reservoir, condensing coil andremovable cover is used to heat thesolvent to boiling, generating a vaporzone above the solvent. The vaporcondenses on the relatively cool metalsurface of parts placed in the vaporzone. The condensed solvent dissolves theorganic contaminants on the part.Contaminated solvent condensate thendrips back into the tank reservoir, carryingthe contaminates into the bath. Duringevaporation only clean solvent vapors areproduced so that test parts are givenadequate exposure to clean soilfreesolvent. The test objects come out of thevapor degreasing operation clean and dry,but warm. After cooling, the test objectsare ready for application of the liquidliquid penetrant if no other forms ofcontamination exist.

For many years, the preferred solventfor vapor degreasing was1,1,1-trichloroethane (methylchloroform), which was considered lesstoxic than trichloroethylene orperchloroethylene). All three have beenwidely used because they arenonflammable. More recent studies havefound that there are significant healthrisks with the latter two and1,1,1-trichloroethane is an environmentalhazard as an ozone depeleter. Vapordegreaser manufacturers have developedequipment that can more safely useflammable solvents as vapor degreasingfluids. Chemical manufacturers continueto try to develop safer solvents for thiscleaning method.

Personnel Hazards and Safetywith Vapor DegreasingPersonnel using vapor degreasing systemsshould be aware of health hazardsresulting from excessive inhalation ofchlorinated hydrocarbon vapors, which

adversely affect the mucous membranes ofthe respiratory system. Symptoms ofexcessive inhalation or absorption includeheadaches, fatigue, coughing, nausea, lossof appetite and loss of sense of balance.Long term exposure may result in kidneyand liver damage.

High temperatures and high intensityultraviolet radiation, such as produced byarc welding, can oxidize or decomposechlorinated hydrocarbon vapors toproduce the highly toxic and dangerousgas phosgene (a poison gas once used inwarfare). Other products ofdecomposition include hydrochloric acid,carbon dioxide and dichloroacetylchloride, which aid corrosion and arestrong irritants as well.

Like other fluids used in liquidpenetrant testing, prolonged exposure ofthe skin to vapor degreasing solvents canextract oils from the skin, resulting incracking of the skin and dermatitis. Vapordegreasing work areas should beventilated properly. Chlorinatedhydrocarbons should not be used in areasclose to welding or heat treatingoperations or other sources of hightemperatures. Smoking should beprohibited in areas used for vapordegreasing because phosgene gas may beformed inside the burning tobacco andthe smoker will be exposed to it.

Disposal of sludge residues fromcleaning operations must follow federal,state and local regulations and should notbe burned or discharged into steams orsewers (see discussion elsewhere on liquidpenetrant effluent). The sludge residue istoxic and may be flammable because itcontains oil and grease collected duringthe degreasing operations. Direct contactwith hot residue is dangerous. Sludgeresidues should be disposed of in coveredcontainers that should not be airtight.

Solvent Precleaning of TestObjectsIn some field applications an inspectormay be able to clean small areas with arag dampened with solvent. The solventcleaner usually supplied with the handwipe penetrant system for removal ofexcess penetrant may also be adequatefor precleaning oil and grease residuesfrom test objects before the liquidpenetrant is applied. The operator shouldmake certain that sufficient time haselapsed after use of solvent precleaners forthe cleaner to have evaporated completelyfrom any discontinuities that may bepresent, as well as from the exposedsurfaces, before applying liquid penetrantto test parts.


For more extensive solvent cleaning, testobjects are immersed and soaked in tanksof common organic liquid solvents. Thesolvents are normally used at or nearroom temperature. Oil, grease and loosemetal chips are removed from metalsurfaces, with or without agitation.Ultrasonic vibration is sometimes used toloosen soils such as abrasive compoundsfrom deep recesses or test objectdiscontinuities open to the part surface.

After solvent cleaning, parts are driedat room temperature or by external heat(such as that from a steam coil). Becausecontamination removed from test parts isretained in the solvent, both cleaningefficiency and the final cleanliness of testobjects can decrease with continued use.Cleanliness requirements dictate when theused solvent must be replaced with newor reclaimed solvent.

Cleaning inconsistency is a problemwith solvent tank cleaning because therate of resoiling of part surfaces increaseswith greater contamination of the solvent.Most metals can be cleaned in commonsolvents unless acid or alkalicontamination is introduced into thesolvents. Solvent cleaning is often used incombination with separate acid oralkaline cleaning procedures for removalof complex soils.

Common organic solvents includealiphatic petroleum or chlorinatedhydrocarbons (similar to those used invapor degreasing) or blends of two ormore solvents. Aliphatic petroleumsinclude such familiar fluids as kerosene,naphtha, mineral spirits and Stoddardsolvent. Other solvents include alcohols(ethanol, isopropanol or methanol),ketones, benzol, toluol and glycol ethers.

Precleaning by Spraying orWiping of Solvent CleanersTest objects that are located in the field orare too large to be immersed in tanks canbe cleaned by spraying or wiping thesurfaces with solvent.

Caution. When spraying a flammablesolvent, dispersion of low flash pointliquids creates an explosion hazard.Spraying is typically done withconventional paint spraying equipment,airless spray, small bench type sprayers orby aerosol spray cans. Where feasible, thesprays should be directed into a ventedfume collecting hood. Solvent cleaning isalso often used as a preliminary stepbefore acid or alkaline cleaning to removesoils that could interfere with the actionof these chemical means ofdecontamination.

Field Use of PressurizedSpray Cans ofCleaner/Remover SolventsPortable liquid penetrant kits for field useusually contain spray cans of liquidpenetrant, developer and cleaner/remover.The cleaner/remover serves a dualpurpose. It may be used to (1) clean thesurface before liquid penetrantapplication, although other strongersolvents are also used for precleaning inthe field or (2) remove excess surfaceliquid penetrant on completion of theliquid penetrant dwell.

When used to preclean, the spraysolvent should be applied liberally anddirectly on the surface to be cleaned.Contaminants and excess solvent canthen be removed with dry, lintfree clothor paper towels. Adequate time forevaporation of remaining surface solventmust be allowed to dry before liquidpenetrant application.

When solvent removers are used forremoval of excess liquid penetrant,caution must be excercised to preventremoval of the liquid penetrant fromshallow flaws. The solvent should beapplied sparingly to a cloth that is thenused to wipe off the excess surface liquidpenetrant. Never spray solvent directly onthe part surface to remove excess liquidpenetrant.

Personnel Hazards and Safetywith Solvent CleaningPersonnel using solvent cleaningtechniques should be aware of the hazardsof fire (with flammable solvents) andtoxicity (with chlorinated hydrocarbonsolvents and ketones). The flash pointsand permissible toxicity concentrationsare given in Table 3.1,2 Areas used forsolvent cleaning should have adequateventilation to remove fumes and preventaccumulation of vapors in explosive ortoxic concentrations. Flammable solventsshould be stored in safety cans or closedmetal containers. Open flames or heaterswith exposed coils must not be used inareas where solvent cleaners are available.Smoking should be prohibited in solventcleaning areas, including areas used forsolvent wiping and drying.

Operators should be warned not toexpose hands or skin to solvents becausethey dissolve skin oils and can lead todermatitis or cracking of the skin.Protective gloves and ointments to restoreskin oil should be used, when operatorsmust be in contact with solvents. Vaporsof chlorinated hydrocarbons and alcoholcan have potentially lethal anestheticactions when they are inhaled. Use in


closed areas, as within tanks, can bedangerous with high concentrations oftrichloroethylene or perchloroethylene,both of which have a strong narcoticeffect.

Hand Wiping in SolventCleaning of Test ObjectsSolvent application by hand wiping orspray wand can be used, in lieu of vapordegreasing, in precleaning of test objectsfor liquid penetrant testing. In manyinstances, the nature of the test objectsurface, the test object size and location(as in the case of inservice testing) orother considerations dictate solventcleaning by hand wiping. This is timeconsuming but may be unavoidable insome cases.

Precautions in Safe Use ofFlammable Solvents during HandWipingGenerally speaking, hand wipe solventsare either flammable or toxic or both. Thelevels of flammability and toxicity canvary with various types of solvents but inall cases it is essential that precautions betaken to ensure adequate ventilation and toobserve fire safety precautions. Theextremely flammable ketone solvents suchas acetone and methyl ethyl ketone andaromatic solvents such as benzene andtoluene and their vapors are potentiallyexplosive. They should be treated with thesame precautions as gasoline.

Generally speaking, petroleumsolvents, although flammable, arerelatively less dangerous. Observance offire precautions and assurance of adequateventilation are nevertheless warranted.Some solvents such as ketones are watermiscible whereas petroleum solvents,chlorinated solvents and aromaticsolvents are water immiscible. Thus, thiswater miscibility characteristic may be ofconcern for some solvent wipingapplications. Water miscible solvents canquickly remove water and many organicsubstances from surfaces, thus providing amore thorough cleaning. However, somesoils and contaminants are not soluble inwater but are soluble only in waterimmiscible solvents.

Precautions in Safe Use of ToxicSolvents during Hand WipingChlorinated solvents are generally toxic and,although they are generallynonflammable, adequate ventilation mustbe provided to avoid any physiologicaleffects from breathing their vapors. Thesesolvents are also of concern in someapplications because they containchlorine that can react to form chlorides,with detrimental effects on certain alloys.The fast evaporating solvents fall intoeither the chlorinated or highlyflammable class, so the property of fastdrying can be realized only through acompromise on safety. The petroleumsolvents, although relatively lessflammable, are also slower drying.However, this loss in drying time may bemore than compensated for by the addedsafety factor.

Fluorinated solvents are relatively lesstoxic than chlorinated solvents. However,they are halogenated and contain fluorineand chlorine and their use is undesirablefor cleaning metals sensitive to halides.The fluorinated solvents are also moreexpensive but have the advantage ofleaving no residue following evaporation.


TABLE 3. Flash points and relative toxicity values ofcommon cleaning solvents.1,2

Flash Pointa Threshold Limit Valuesb,c

Solvent °C (°F) µL·L–1 mg·m–3

Aliphatic PetroleumsKerosene 65 (150) ___ ___Mineral spirits 15 (59) ___ ___Naphtha, high flash 45 (110) ___ ___Naphtha, VM&Pd 10 (50) 300 2000Stoddard solvent 40 (105) 100 525

Chlorinated HydrocarbonsChloroform none none 10 49Methylene chloride none none 50 174Perchloroethylene none none 50 3391,1,1-Trichloroethane none none 350 1900Trichloroethylene none none 50 269Trichlorotrifluoroethane none none 1000 7600

AlcoholsEthanol, specially denatured 14 (57) 1000 1900Isopropanol 12.7 (54) 400 980Methanol 12 (54) 200 260

Other SolventsAcetone –18 (0) 750 1780Benzol (benzene) –11 (12) 1f ___Ethyl cellosolve 44 (110) 5 18Toluol (toluene) 8 (46) 50 188

a. Tag closed cup.b. Key, M.M. et al. Occupational Diseases — A Guide to Their Recognition.1

c. Permissible exposure limits given as threshold limit values (TLVs) byAmerican Council of Government and Industrial Hygienists2 in parts permillion and in milligram per cubic meter of air atmospheric pressurerefer to time-weighted average (TWA) values of concentrations pernormal 8 h work day. Warning — check against local regulations andmost recent federal and state regulations.

d. Varnish makers’ and painters’ naphtha.e. Manufacture and use both illegal in the United States.f. 1 µL·L–1 per 8 h day.

Removal of PaintPaint can obscure or bridge surfacediscontinuities in underlying metals oftest objects and must be removed beforeliquid penetrant testing to obtain effectiveliquid penetrant test indications. Paintremovers are commercially available fromvarious manufacturers for both in-placeand dip tank application. Because paintremoval technology is somewhatcomplex, the recommended approach isto enlist assistance from a supplier ofgood paint removers. Recent advances inpaint technology have resulted in finishedsystems removable only with specialproducts. Factors that influence the easeof paint removal include (1) surfacepreparation before painting, (2) type ofpaint primer, (3) type of paint used,(4) number of paint coats, (5) age or cureof the paint finish, (6) type of paintremovers used and (7) nature of thesubstrate.

The treatments for paint removal fromtest objects before liquid penetrant testinginclude (1) solvent type paint removers,(2) alkaline or acid type paint removers,(3) abrasive removal procedures and(4) burning or ignition.

Critical structures cannot tolerate theuse of any products or paint removalprocedures that may be damaging to theirmetals or alloys. This requires carefulattention when abrasive techniques areused for paint removal. Ignition orburning off of paint layers should neverbe used on aircraft. When solventcleaning techniques are used, it isessential to remove traces or residues fromsolvents and other contamination bytechniques such as have beenrecommended for unpainted metallicparts.

Removal of Carbon,Varnish and Other TightlyHeld SoilCarbon and varnish removal is similar topaint removal and many of theprocedures and products used for removalof paint are also used for removal ofcarbon and varnish. Solvent carbonremovers are available for brush or sprayapplication or for use in dip tanks. Diptank carbon removers are available thatoperate at ambient or elevatedtemperatures. Carbon can also beremoved by alkaline products, dependingon the conditions and the nature of thecarbonaceous deposits. Mechanical meansare also used for removal of this type ofsoil.

The treatments for removal of carbon,varnish and other tightly held solidsinclude (1) solvent type carbon removers,either brush-on, spray-on or tank types,(2) alkaline or acid based removers, eitherbrush-on, spray-on or tank types, (3) wirebrushing or other abrasive cleaning,(4) blasting with vapor, sand, seed or glassbeads (see cautions cited elsewhere on gritblasting), (5) electrocleaning and(6) combinations of these techniques.

Removal of Scale, Rust,Corrosion Products andOxidesScale, rust, corrosion products and oxidescan usually be removed from test surfacesby chemical means, provided the surfacesare accessible. Acid scale removal iscommonly used, with the type of acid oracids selected with consideration offactors such as (1) the type of metal oralloy surface, (2) the severity of the scale,(3) the dimensional tolerances permittedand (4) other limiting factors.

Alkaline type rust removers ordescaling products are often effective andmay be preferable to acid cleaning,depending on the circumstances. After theuse of either alkaline or acid products, it isnecessary to rinse the test objectsthoroughly to ensure that acid or alkalineresidues are not retained on test objectsurfaces where they might later affect theperformance of the liquid penetrant.

Removal of High TemperatureAlloy OxidesUnique high temperature alloys are usedin the hot sections of aircraft jet enginesand rocket chambers and in nuclearapplications. These alloys operate underconditions that result in deposits ofcomplex oxides on the surface of metallicparts. Removal of these oxides requiresspecial procedures that usually involveseveral cleaning stages. Among thosecommonly used are (1) high temperaturealkaline cleaning processes, (2) fused saltsdescaling processes, (3) acid cleaningprocesses, (4) processing in alkalineoxidizing solutions, (5) electrocleaningand (6) combinations of above processes.Vapor or dry abrasive blasting (used withdue caution, where permissible) iscommonly required as a final step inremoval of tenacious oxides that areintegrally bonded to the alloy surfaces.High temperature alloys sensitive tocertain elements such as chloride, sulfur,sodium and potassium are subject torestrictions that must be considered.

Removal of these oxides is mandatoryif liquid penetrant testing of the high


temperature alloy parts is to be successful.It is recommended that specialists inremoval of high temperature oxides beconsulted for recommendations andassistance in selecting and controlling thecleaning processes.

Alkaline CleaningAlkaline cleaners remove oily soils frommetals by detergent action that reducessurface and interfacial tensions andpermits the cleaning compound to wetthe soils, seep under them and displacethem. This action is attributed to builders(usually sodium compounds) that providealkalinity. They loosen, disperse andemulsify soils removed from the metalsurface. In water solution, alkalinecleaners reduce the viscosity of the soiland the water transmits agitation to thework surface and flushes away the soils.Cleaning action is provided by soaps anddetergents added to the cleaningcompounds that act as surface active orwetting agents.

The cleaner may function by themechanism of high alkalinity needed forsaponification reactions and mustdissociate to provide ions. Most soaps orsynthetic detergents are more efficient atpH values from 7 to 13. This activealkalinity is one of the working agentsand is continually lost by saponificationand neutralization reactions and bydragout on parts removed. This type ofcleaner should provide alkalinity that iscontinuously available and ionized toreplace the losses in active alkalinity.

The alkaline cleaner must also dispersesoils removed from the test objects so thatfluid close to the part surface does notbecome so highly contaminated as toredeposit soils on the surfaces beingcleaned. The builders should be soluble incold water and have no affinity for theobject being cleaned. Traces of cleaner oralkali remaining on test objects afterrinsing are objectionable because theymight cause dermatitis or other healthhazards or interfere with the action ofliquid penetrants during the later testoperations.

Acid Precleaning of TestObjectsAcids are not generally good cleaners foroily soils, so oxide or scale removal byacid means must normally be preceded byalkaline cleaning or by some othertechnique that removes the oil so that theacid can react at the test object surfaces.Acid cleaning processes use solutions ofmineral acids, organic acids or acid salt

and a combination of wetting agents anddetergents, with or without heating, toremove oxides, shop soil and othercontaminants from metal surfaces.

Acid solutions with 40 to 60 percenthydrochloric acid or 6 to 8 percentsulfuric acid can be used at roomtemperature for removing soil and lightrust. Phosphoric acid mixed with ethyleneglycol monobutyl ether is widely used forremoving grease, oil, drawing compoundsand light rust from ferrous alloys. Citricacid is used to clean rust from iron andsteel without attacking the metal; it is alsoused in small concentrations incombination with sulfuric, phosphoricand other common acids to clean rustwithout attacking base metal.

Many proprietary acid compounds areavailable and consultation with thesuppliers is recommended in selection ofacid cleaners for various test objectmaterials and contamination conditions.Inhibitors may be added to cleaners usedon ferrous metals to reduce acidconsumption and attack on base metal.

Salt Bath Descaling andDeoxidizingSalt bath descaling and deoxidizing withreducing and electrolytic processes areeffective in attacking heavy, tightlyadhering scale that can form on carbonand alloy steels, nickel and cobalt basealloys and some grades of stainless steel.The test objects are typically immersed ina 370 °C (700 °F) bath of molten sodiumhydroxide containing 1.5 percent sodiumhydride. Immersion into the hot salt bathresults in unequal thermal expansion ofscale and base metal that cracks the scale.The molten salt then penetrates throughthe cracks and chemically reduces theoxides to lower oxides or metal. Followingthe salt bath treatment, the test parts arewater quenched. Thermal shock maycause the cracks to open further. Variousamounts of scale can be blasted off themetal surface during quenching.Following quenching, the parts can berinsed in water to reveal a bright, cleanmetal surface.

Advantages of salt bath descalinginclude the following.

1. It can be used for all carbon and alloysteels, tool steels, stainless steels,nickel or cobalt alloys and refractorymetals and alloys based on copper,nickel, cobalt, molybdenum and otherrefractory metals.

2. It provides very efficient removal ofscale from surface discontinuities.

3. It decreases the time required fordescaling all grades of stainless steel.


4. It is less destructive to smooth finishesthan mechanical descaling.

5. It virtually eliminates metal loss andavoids surface pitting and etching.

Disadvantages of salt bath descalinginclude the following.

1. It uses high operating temperatures of370 to 540 °C (700 to 1000 °F), whichrequire special heating equipment andmeans for parts handling.

2. It results in distortion of thin gagematerials of 0.8 mm (0.03 in.) gageand thinner.

3. It is not usable with aluminum,magnesium, zinc, cadmium, lead andtin, because of either their reactivity orlow melting points.

4. It can embrittle titanium and titaniumbase alloys and can actually ignitetitanium materials.

Emulsion PrecleaningEmulsion cleaning can remove manytypes of soils from test objects, includingpigmented drawing lubricants,unpigmented oils and greases, cuttingfluids and residues from polishing, buffingor magnetic particle testing. Emulsioncleaning provides rapid superficialcleaning and typically leaves on the worka thin film of oil that provides someprotection against rusting. This oil can beremoved by subsequent vapor degreasingor by two stage rinsing with an initialrinse (typically in agitated cold water) anda final rinse with hot water at 65 to 95 °C(150 to 200 °F) to preheat the test objectsand aid in drying.

In some cases, emulsion cleaningfollowing alkaline cleaning is used toprovide temporary protection againstrusting of ferrous parts. However, unlessthe residual oil film is completelyremoved before liquid penetrant testing,careful tests might be required to ensurethat it could not interfere with liquidpenetrant action.

The emulsion cleaner system typicallyinvolves stable emulsions of twoimmiscible liquids, such as a hydrocarbonand water, whose stability is aided byaddition of a suitable emulsifying agent.Depending on the nature of thehydrocarbon solvent, the cleaning is doneat temperatures of 60 to 80 °C (140 to180 °F). The solvent is often of petroleumcompounds of naphthenic, paraffinic oraromatic types. The low boiling solventsare usually more effective in removingsoils but increase hazards of fire orevaporation loss as boiling and flashpoints are approached. Both stable singlephase and unstable multiphase emulsioncleaners exist. The latter are used for themost difficult to remove hydrocarbon oils

such as lapping or buffing compounds oroxidized oils. Cleanliness of the emulsionsolution must be considered. The solutionshould be replaced when it leaves smut ordeposits on the test objects or its cleaningaction has diminished noticeably.

Removal of Strong Acidsor Alkalis from TestSurfacesStrong acids or alkalis might be presenton test object surfaces if rinsing wasinadequate following cleaning by acids,salt baths or alkaline solutions or ifcontamination with such materials hadoccurred during handling, storage or use.Acids or alkalis act to impede wetting andpenetration of liquid penetrants intodiscontinuities. They may also react withliquid penetrant materials to decomposeor degrade dyes or other activeconstituents. Removal of acids or alkaliscan be achieved by (1) rinsing with freshwater or (2) rinsing with a neutralizingrinse followed by a fresh water rinse. Aftereither of these treatments, the test objectsshould be thoroughly dried by thetechniques described for removal of water.

Removal of Water fromTest SurfacesIf test parts have been in contact withwater, it is important that all traces ofwater be removed before liquid penetranttesting or leak testing. Water must beremoved not only from the surface of thepart but also from surface discontinuitiesthat may be present; otherwise, liquidpenetrant will be prevented from enteringthe discontinuities. The dryer customarilyused with liquid penetrant testing (to dryparts after water washing to removesurface liquid penetrant or to dry parts towhich wet developer has been applied) isan excellent means for drying test partsbefore testing. Water can also be removedby blowing it off with clean air (oilcontaminated air should not be used).The surface could also be allowed to airdry, though air drying under staticconditions can take an excessively longtime; auxiliary drying techniques areusually used for this reason.

Alternative drying techniques includewiping of the test objects with clean,absorbent rags or rinsing test objects withfast drying, water soluble solvents, e.g.,volatile alcohols, followed by drying. Heatlamps and other radiant heat sources canalso be used. Warning: Most fast drying,water miscible solvents are flammable ortoxic and should be used with caution.


Smoking and open flames should not bepermitted in areas where such solvents areused. Good ventilation is always essential.

Removal of Fingerprintsfrom Test SurfacesMost cleaning techniques will removefingerprint contamination on test objects.However, in critical applications, it maybe necessary to use special proceduressuch as electrocleaning or specialfingerprint removal products. Themandatory use of cotton gloves forhandling test parts may be justified wherecontamination from fingerprints, duringtest handling or operations, is suspectedof causing trouble.

Special Surface TreatmentsSurface treatments such as phosphate,chromate conversion coating and blackoxide are typically very adherent and tendto become integral portions of the testobject metal surface. Some of thesecoatings can act physically to impedesurface wetting and entry of liquidpenetrants into leaks or discontinuities.Some phosphate and chromate coatingscan react with the liquid penetrant andsuppress fluorescent brightness. Ingeneral, it is recommended that the testbe performed before the process thatapplied the coatings, if practicable. Whenthe coatings are already in place, suppliersof the coatings process should beconsulted for recommendations onremoval of the surface coatings.


Need for Cleaning of TestObjects after LiquidPenetrant TestingSome form of posttreatment of test partsis generally required on completion of theliquid penetrant test process. For example,rust preventive steps may be required forcarbon steel, because surfaces are leftrelatively clean and free of light oils bythe liquid penetrant process. Exactingsurface cleanliness may be required if thetest object is to be installed in a liquidoxygen system. Whether or not the posttreatment of the test object surface shouldbe the responsibility of the inspectiondepartment or of the productiondepartment would depend on factors suchas the need for swift action to preventcorrosion of test objects and the physicallocation of the available cleaningfacilities. Regardless of whereresponsibilities lies, personnel responsiblefor liquid penetrant testing should bethoroughly cognizant with posttreatmentcriteria. They should know the effects ofliquid penetrant processing on thesubsequent deterioration, processing orserviceability of test parts. Also, at theminimum, test personnel should beresponsible for removal of residues left onthe surface by the liquid penetrantprocess. This would apply especially toresidues conducive to corrosion andwould include developer coatings undermany test conditions.

Questions to be asked in determiningwhether or not posttreatment is necessaryand in selecting the cleaning proceduresand materials to be used include thefollowing.

1. Is the condition of surface onconclusion of the liquid penetrantprocess likely to interfere with asubsequent process?

2. Is it conducive to corrosion or similareffect?

3. Does the ultimate use of the testobject demand cleanliness?

After completion of the liquidpenetrant test process, includinginterpretation of test indications andsorting of test objects into acceptable,salvageable and reject categories, all tracesof liquid penetrant materials or othercontaminants should be removed from

test objects that are acceptable for service.As a general rule, the sooner the liquidpenetrant and developer residues areremoved by a cleaning process aftertesting, the easier it will be to removethem.

The purpose of postcleaning is toensure that no deterioration of or damageto the test objects can occur later as aconsequence of the liquid penetrant testprocess and to remove any residues thatmight interfere with subsequentprocessing. In the event that repairwelding is required, liquid penetrantresidues can have detrimental effects onweld areas. Special care is required if thetest objects will be used in nuclear serviceor with liquid oxygen (LOX) systems.Traces of hydrocarbons can lead to violentexplosions when the test parts come intocontact with liquid oxygen. Extremecleanliness is also essential for nuclearcomponents.

In general, care should be taken toprotect the test objects from corrosion ordamage during parts handling, storage orassembly. In some cases, by mutualagreement with the user or customer,protection against rusting or corrosionmay be applied to preserve parts untilready for use.

Procedures for CleaningTest Objects FollowingLiquid Penetrant TestingWhether or not post treatment other thandeveloper coating removal is required ordesirable following completion of theliquid penetrant process must bedetermined by conditions including(1) likelihood of corrosion, (2) texture ofthe surface, (3) intended use of the testobject and (4) subsequent processingscheduled. Cleaning following liquidpenetrant testing can generally beaccomplished by one or more of theprocedures discussed elsewhere in thischapter. Developer residues tend to besomewhat tenacious at times. Part of thistenacity may be caused by electrostaticattraction between the developer particlesand the metallic surface of the testedparts. The longer the developer coating —aqueous or nonaqueous — remains ontest parts, the more difficult it will be to


PART 3. Procedures for Postcleaning TestObjects after Liquid Penetrant Testing

MOVIE.Postcleaning.

remove. Developer coating removalshould be done as soon as practical aftercompletion of liquid penetrant testing.Combinations of one or more cleaningtechniques may be necessary during postcleaning, depending on (1) the types ofliquid penetrant test resides involved,(2) the length of time these residues havebeen left on the test objects and (3) thecleanliness required or specified.

Ultrasonic agitation of cleaning fluidsmay be especially beneficial for removingliquid penetrant process residues,particularly for tight clearance crevices,blind holes and other surface conditionsthat tend to trap and hold residues.However, the user is cautioned thatultrasonic cleaning has certain limitationsand it is advisable to explore these beforeadopting this method. Care must also beexercised to ensure removal of all maskingand plugging materials.

Cleaning Action of LiquidPenetrant TestingProcessesThe liquid penetrant process is itself acleaning operation. Liquid penetrant oilsdo a job of dissolving organic soils,especially if the contact time or liquidpenetrant dwell time is extended. Theseliquid penetrant dissolved soils will beremoved. Further, developer particles areabsorbent and soak up oils, solvents andsimilar contaminants. Thus, oncompletion of the liquid penetrantprocess and removal of the developercoat, the surface of the test object may bemore nearly free of certain contaminantsthan when the test process began.

Postcleaning to RemoveLiquid Penetrant ResiduesThe cleanliness condition followingtesting will vary with processingtechniques, liquid penetrant materials andsurface texture. Some liquid penetrantssuch as water washable liquid penetrantswith a low level of water solubility may beapt to leave a film of surface liquidpenetrant on test objects. However, somewater washable liquid penetrants, whenproperly washed off, leave a cleanersurface than lipophilic (oil base)emulsifiers. Rough surfaces such as sandcasting surfaces tend to retain liquidpenetrant in a multitude of pores. Sometest processors, fearing washout of liquidpenetrant from relevant wide opendiscontinuities, favor underwashing. Theyintentionally leave a surface film of liquidpenetrant. However, where the test

involves either self-development (usingno developer) or a dry powder developersystem, there will be little absorbentparticle action to remove this liquidpenetrant residue. So, in addition toremoval of the developer coat,postcleaning (or posttreatment) is moreoften than not required to remove liquidpenetrant residues.

Liquid PenetrantEmulsifiers in Postcleaningof Liquid PenetrantResiduesAlthough not a common practice and notas thorough as many other procedures,the emulsifier used to removepostemulsifiable liquid penetrants can beused for removing residual surface liquidpenetrant during postcleaning. The partsare subjected to a prolonged emulsifierdwell time. This procedure may berelatively inexpensive, depending on thecost of the emulsifier and the rate atwhich it becomes contaminated. In someinstances, such as in field testing of only afew parts, emulsification may be the mostpractical postcleaning technique. Whenthe volume of testing is low and therequirement for a special cleaningprocedure is absent, the emulsifierapproach may be preferable.

Postcleaning of Test Partsto Be Plated, Anodized,Painted or CoatedIf the test object is to be plated, anodized,painted or treated with similar coating,any oil film or developer particle surfacecontamination will interfere with theadhesion of the applied coating.Normally, the department responsible forapplication of such coatings will have acleaning process. This cleaning wouldprecede the coating application whetherthe part arrived from the liquid penetrantstation or from some other source. Theliquid penetrant test department willseldom be responsible for delivering thetest objects in a condition ready foranodizing, plating or even painting.

Prevention of Corrosion of CarbonSteel or Magnesium PartsVery often, on completion of the liquidpenetrant testing process, the test surfaceis cleaner than when the parts werereceived for testing. Rust on carbon steelsurfaces and corrosion on magnesiumsurfaces will appear almostinstantaneously under most climatic


conditions. Immediate rust prevention isrequired. There are several approaches tothis problem. A rust inhibitor additivemay be put into the final rinse water forpostinspection cleaning to removedeveloper. Following final cleaning, thepart may be treated with light oil, atemporary protective coating or acommercial corrosion inhibitor. Additiveslike sodium nitrite and sodium chromateare environmental hazards and their use isrestricted.

Reasons for RemovingDeveloper from Parts afterTestingThe developer is the last material appliedin the liquid penetrant process. It may beone of several types: dry powder,nonaqueous, aqueous suspension,aqueous soluble or one of the lesscommon types such as resin developer.After completion of the liquid penetranttest process, including interpretation ofthe test indications and sorting of testobjects into acceptable, salvageable andrejectable categories, the developercoating should be removed. If the coatingis not removed, the developer mayinterfere with subsequent processing suchas anodizing or act to absorb and holdcorrosion conducive moisture. Even ifthere is not subsequent processing and nocorrosion concern, the developer residuemay interfere with the proper function ofthe parts. There is generally at least acosmetic need to remove the developercoating before delivering the part.

Removal of Dry PowderDevelopersDeveloper removal techniques must fitthe developer type. For example, thefluffy dry powder developer, unlessapplied by an electrostatic spraytechnique, will only adhere to areas thatare wet with liquid penetrant (or otherliquid substances), although powder maylodge in crevices. So, dry powder removalis either unnecessary or can beaccomplished by a plain water wash, anair blast or similar uncomplicated means.

Removal of Aqueous SuspendibleDevelopersAqueous wet suspendible developers aremore difficult to remove than dry powderdevelopers are. Typically, the aqueous wetsuspendible developer is dried on the testpart surface by means of an oven.Removal is best accomplished by apressure spray water wash that containsdetergent. Hand wiping with a

nonabrasive medium, water and detergentis also effective.

Removal of Aqueous SolubleDevelopersThe aqueous soluble developer is acombination of white powders, wettingagents, corrosion inhibitors and similaringredients, all of which dissolve in water.Cleaning is not complicated, because allof the developer ingredients are watersoluble. Plain water will quickly flushmost water soluble developers from thetest object surface. The mechanical actionof a pressurized spray will facilitateremoval.

Removal of Nonaqueous, SolventSuspendible DevelopersThe nonaqueous developer, typically amixture of suspendible absorbent particlessuch as calcium carbonate in a volatilesolvent such as alcohol, is usually sprayedon the test object surface. In a highpercentage of applications, this developeris applied to relatively small areas, such asa weld, from an aerosol spray can.

Small areas lend themselves to handwiping. Most nonaqueous developers wipereadily from the surface with a dry, cleancloth or soft bristle brush sufficientlyclean to meet requirements. However, formore thorough removal, a wipe withwater moistened toweling, followed by adry wipe, may be in order.

Also, even though the area may besmall, there may be threads, crevices andsurface recesses where trappednonaqueous developer particles cannot bereached by wiping. A forceful water spray,plain or reinforced with detergent, will beeffective in removing trapped developerunder these conditions. Where the testobject is too large for manual wiping, thissame water spray technique is the logicalanswer for removal of the nonaqueousdeveloper coating. Ultrasonic cleaningwould also be very effective.


Precleaning InserviceAircraft Components forLiquid Penetrant TestingIn liquid penetrant testing, duringoverhaul of aircraft, cleaning is the mostimportant process if the test is to be valid.Where aircraft are returned from a servicetour for rework or repair, the cleaning andits effect take on a special significance. Ona returned aircraft, the airframes andengines with their related componentswill all vary in surface conditions. Thesepart surface conditions affect the test aswell as the cleaning requirements beforeliquid penetrant testing. Other factorsthat must be considered include (1) theamount of disassembly required, (2) partconfiguration, (3) material compositionand (4) surface coating previously applied.

Disassembly of Aircraftand Engine for Reworkand InspectionAircraft returning for rework or repair aredisassembled to the degree necessary toprovide an airworthy product for anotherservice tour. This disassembly includesboth the aircraft and engines. The amountof disassembly an aircraft componentreceives will govern its techniques ofprecleaning for the fluorescent liquidpenetrant test. In general, the precleaningmay be divided into two major categories:(1) precleaning of components that arecompletely disassembled and processedthrough feeder shops and theirfluorescent liquid penetrant test stations;(2) precleaning of components andstructures that are fluorescent liquidpenetrant tested only in selected areas.Each of these categories entails differentprecleaning processes.

Preparation ofComponents forInspectionBefore induction into rework facilities, theaircraft and engines(s) are cleaned bydetergents, solvents, steam or othertechniques to remove soils, greases etc.

that accumulate on inservice equipment.Following induction into rework facilities,those parts requiring liquid penetranttesting begin precleaning processes thatvary with the alloys, surface coatings andgeneral condition of the part and whetherit is to be processed through feeder shops,remain on the aircraft or be part of acomponent. Those parts to be processedthrough feeder shops will be discussednext.

With cleaning techniques where partsare processed literally from tank to tank,precleaning is tailored to the parts. Theseparts being disassembled have very fewrivets or fasteners to cause entrapmentand usually offer good smooth surfaces.Structures and components that remainon the aircraft or are not disassembledrequire a different approach.

Chemical Cleaning ofComponents Scheduled for FeederShopsComponents scheduled for feeder shopsare removed from the engine or aircraftand disassembled and their parts arecleaned and tested separately. Chemicalcleaning is a preferred technique and isused as much as practical for paintstripping, derusting and descaling ofparts. However, mechanical cleaning isrequired on some parts.

As most parts and components onaircraft are painted as protection againstthe operating environment, paintstripping is usually the first precleaningprocess used. Parts are then chemicallycleaned to remove corrosion products,grease, carbon, dirt etc. This aids inensuring that surfaces are free of anyforeign materials that would eitherprevent the liquid penetrant’s entry intodiscontinuities or would hold the liquidpenetrant on the surface, causing falseindications or masking relevantindications. Etching of turbine blades isbut one example of the variety ofprecleaning required to condition a partfor liquid penetrant testing.

Restrictions on MechanicalCleaning of Components ofInservice AircraftEven though chemical cleaning is thepreferred technique, mechanical cleaning


PART 4. Cleaning Requirements for FluorescentLiquid Penetrant Testing in Aircraft Overhaul

is required at times. Whenevermechanical precleaning is used, thefollowing restrictions are imposed.

1. Rotary disks and wheels areacceptable only when used at lowspeeds.

2. Small areas are cleaned by handsanding. The sanding motion follows aback and forth pattern in the samedirection as a suspected discontinuity.

3. Abrasive blasting may be used only tothe extent that the surface does notbecome peened to a degree that couldseal a discontinuity or contaminatethe opening with abrasive residue. Theblast dwell is kept to the absoluteminimum required to clean the metalsurface. The area blasted is restrictedto the proximity of that area to betested.

4. Steel wire brushes, either flat or rotary,are not used on nonferrous metals.

Etching is at times used to assist inopening the discontinuities. However,etchants and the etching procedure areauthorized only by the materials orquality laboratory having cognizance ofthe test procedures.

Final Precleaning by VaporDegreasingVapor degreasing is a good technique offinal precleaning. The use of chlorinatedsolvents to vapor degrease titanium isprohibited. The final precleaning oftitanium parts is with a Stoddard orspecialty solvent followed by oven dryingat 50 °C (120 °F). Drying is necessary asthe less volatile solvent could becomeentrapped in cracks even though thesurface may appear dry.

Precleaning of Selective orLocalized Areas of Aircraftor ComponentsPreparation of selected, localized surfaceareas can range from cleaning a completewing spar to a small attach fitting. Somelarge helicopter transmission housing andengine accessories cases also fall into thiscategory. Because these parts cannot betaken into the cleaning tanks, theprecleaning techniques are much morerestrictive.

Again, precleaning usually starts withpaint stripping. Chemical stripping isgenerally used but mechanical stripping issubstituted for some components havingphenolic resins or baked enamels.Mechanical stripping is usually gritblasting of the approximate area to betested. Many areas will have sealantsapplied that must be removed, usually by

scraping with a nonmetallic scraperfollowed by chemical removal.

Assembled aircraft parts and structureswill contain fasteners, overlapping jointsand a myriad of other protrusions thatmake precleaning difficult. Extreme caremust be used to remove the residue fromaround abutting surfaces and protrusions.A fiber brush is used for this operation.

Following the paint stripping, the areasare washed with a safety solvent anddried. Drying is at times assisted by airblasting with clean air. Oil contaminatedair is prohibited. The final precleaning isdone by the liquid penetrant inspector.The entire area is precleaned with a liquidpenetrant precleaning solvent and dried.

Causes and Prevention ofImproper Preparation ofAircraft StructuresThe results of the fluorescent liquidpenetrant test for either of the twoprecleaning categories depend on thethoroughness of the precleaning. Laxityin any of the above processes can bedangerous. Improper or incompleteprecleaning may be the result of manyfactors; time is a major one. Sufficientdwell time is required to dissolve orremove the extraneous materials that caninterfere with the test procedures. Tryingto rush parts or areas through precleaningwill probably account for more poor teststhan any other factor.

Incomplete paint removal is anexample of hurrying the precleaning. Theitems are coated with strippingcompounds, the compounds and residueare removed and the item is processed toits next station. Many times a part willappear clean to a casual observer but theitem will still have traces of primer intact.These minute traces are often mistakenfor stains caused by anodizing or othertreatments. In some cases, residues ofsoftened paint and/or paint removers arefluorescent. Other examples includeinadequate derusting (or corrosionremoval) and descaling.

In addition to the problems previouslymentioned, there are those encounteredwhere cleaning is required aroundfasteners, attachments etc. After strippingthe areas around these, protrusionsusually contain small amounts of paintresidue softened by the stripper. They alsotypically have scratches and tooling markswhich, through a period of time, havebecome corroded. This paint residue andcorrosion serve as a blotter for the liquidpenetrants, leading to unwantedfluorescent background.


Conditions Interferingwith Liquid PenetrantTesting of InserviceAircraftConditions listed below and many othersinterfere with fluorescent liquid penetranttesting for the reasons given.

1. Remaining scale, rust and paintresidue will absorb liquid penetrantand thereby give false indications ormask relevant indications.

2. Residue remaining around fastenerspresents an additional maskingproblem and causes such excessivebleedout that cracks extending fromthese areas have to propagate from3 to 6 mm (0.125 to 0.25 in.) to beidentified.

These conditions also tend to causeovercleaning and overremoval of liquidpenetrant by inspectors with limitedexperience. This is especially true ofsolvent removable liquid penetrantprocess, referred to as Method C in SAEAMS 26443 and ASTM E 1417.4 Whenimproper cleaning conditions are detectedby the inspector, the parts or areas mustbe recycled through the precleaningprocess and all subsequent liquidpenetrant processing steps. This is costlyin both time and production delay.

Other factors affecting the test systemare the maintenance of the liquidpenetrant tanks. Liquid penetrantcontamination is one concern; anotherconcern is a production line where partsare continually fed into the liquidpenetrant tanks. Hot parts out ofdegreasers, hot alkaline cleaners or ovenscan eventually heat the liquid penetrantsufficiently to possibly change the liquidpenetrant’s solvent system, with aresultant change of sensitivity. Thispossibility, even though remote, shouldbe considered.

Quality Control duringPrecleaning for LiquidPenetrant TestingTo provide adequate precleaning, qualitycontrol must start at the first cleaningprocess and continue throughout. Withproper quality control, parts with residualpaint, corrosion etc. will be stopped andproperly precleaned before proceeding tothe next process. A savings in both timeand money is realized by this technique,as the item does not have to back trackthrough the system.

A little extra time to test eachprotrusion and fastener area for residue

will, in the end, save time. These areascan be readily cleaned with a little solventand a fiber brush. This little effort willprovide much better testing of thesecritical areas and also can avoid recyclingdue to excessive bleedout. To summarize,fluorescent liquid penetrant can provideeconomical and reliable testing after theitems to be tested are properly precleanedand processed. There are no short cuts inprecleaning for a reliable test.


Liquid penetrant testing can be aneffective test method only if thediscontinuities are open to the surface sothat liquid penetrant can enter. Whenperformed before liquid penetrant testing,some mechanical processes maskdiscontinuities by causing surface materialto deform plastically and sealdiscontinuities. The so called soft metalssuch as aluminum and titanium areespecially susceptible to deformation bymechanical processing. This problem is ofspecial concern to aircraft manufacturingand transportation industries that usethese metals.

Types of MechanicalProcessing PrecedingLiquid Penetrant TestingDuring the fabrication of componentssuch as in aircraft, a number ofmechanical processes may be applied tothe part. Titanium and aluminum partsare frequently machined to their finalconfiguration. Steel parts are often gritblasted to remove heat treat scale.Titanium and aluminum parts arecommonly tumble deburred or liquidhoned to improve the surface conditionand remove burrs resulting from previousoperations. Shot peening is used to inducea favorable stress pattern in the surface ofsome parts. Many parts are machined andthen sanded to remove machining marks.Liquid penetrant testing often is appliedat several stages in the fabricationsequence and may directly follow any ofthe above mechanical processes.

Need for ChemicalRemoval of SurfacesDamaged by MechanicalProcessingThe examples described elsewheredemonstrate the importance of chemicalremoval of surface material that has beenplastically deformed by mechanicalprocessing before liquid penetrant testingof titanium, aluminum and steel. Ifchemical removal (etching) of surfacemetal is not feasible, liquid penetranttesting should be carried out before the

mechanical process in question or adifferent test method should be used. Inaddition, for a single mechanical processsuch as grit blasting, the required amountof chemical removal can vary, dependingon the operating parameters chosen forthe mechanical processing. Consequently,further investigation is needed for thoseprocesses or operating parameters notspecified in the following examples.

Examples of MechanicalObliteration of CrackIndications in AluminumAlloysMcFaul has reported unique photographicevidence of obliteration of liquidpenetrant indications of surface cracks inaluminum alloys.5 These tests indicatedthat obliteration of cracks by mechanicalsurface processing (except for shotpeening) can be overcome by a mild etch,if a postemulsifiable liquid penetrantsystem is used. The postemulsifiableliquid penetrant, compared to a waterwashable liquid penetrant, is moreimpervious to the nullifying effects ofpossible residual acids and othercontaminants.

To conduct the tests, patterns of tightcracks with depth of 0.2 to 3 mm (0.01 to0.125 in.) were generated in 10 mm(0.375 in.) thick blocks sliced from 75 ×100 mm (3 × 4 in.) aluminum alloy barsby repeated heating to 540 °C (1000 °F)and quenching in ice water. After aboutthe fifth quench, a network of highthermal stress, low cycle fatigue cracksappeared. These crack specimens weremilled sufficiently to clean up both topand bottom surfaces of the blocks to a0.8 mm (0.03 in.) root mean square (rms)surface finish.

Liquid penetrant tests were made witha lipophilic postemulsifiable liquidpenetrant system and a nonaqueousdeveloper. Liquid penetrant dwell timewas 15 min. Excess liquid penetrant wasthen flushed off with water to reduce thesubsequent emulsification time,established at 1 min. After emulsification,the parts were again thoroughly washedwith a spray rinse and checked withultraviolet radiation. Specimens were thendried for 10 min in an 80 °C (180 °F)


PART 5. Influence of Mechanical Processing onEffectiveness of Liquid Penetrant Testing

circulating hot air oven, allowed to cooland sprayed with a very light coat ofnonaqueous developer. A minimum of 5min developing time occurred beforephotographing the fluorescent indicationsunder ultraviolet radiation. Where therewas no initial evidence of a returningcrack pattern, as much as a half hour ofdeveloping time was allowed.

Recovery of Crack Test Specimensfor Further TestsTo remove residual liquid penetrant andreuse the specimens, they were cleanedfor about 18 h, in a vapor degreaser. Toprobe the effectiveness of the cleaningtechniques, these specimens were againsprayed with nonaqueous developer,allowed a development time of 3 h andagain examined under ultravioletradiation. This technique of cleaning forremoval of residues from prior liquidpenetrant processing proved satisfactory,because no evidence of liquid penetrantremained. The cracked test blocks werethen given various mechanical surfacetreatments and again subjected to theliquid penetrant test procedures. Resultsof obliteration of liquid penetrantindications by mechanical treatments andtheir restoration by chemical etching areshown in the photographic sequences ofFig. 2 through 11.

Effect of Honing or Lapping onLiquid Penetrant Crack Indicationsin Aluminum AlloysFigure 2 simulates the effects on visibilityof fluorescent liquid penetrant crackindications of final honing or lapping ofthe inside diameter of an aluminumhydraulic cylinder. As shown in Fig. 2c,honing completely obliterated allevidence of cracks. Because an etch of thelapped cylinder surface would destroy thehydraulic seal effect of the lapped surface,it was necessary to conduct liquidpenetrant testing before honing orlapping. Note in Figs. 2d and 2e thatanodic etching or milling restored thecrack indications on the cracked testblock.

Effects of Sanding on LiquidPenetrant Indications of Cracks inAluminum AlloysFigures 3 through 5 show effects of powerand hand sanding on fluorescent liquidpenetrant indications of cracks inaluminum alloys. In Figs. 3b, 4b and 5b, itis evident that the finer the grit size usedin sanding, the more severe the maskingof crack indications becomes. Scraping,whose effects are shown in Fig. 6, can beused to avoid etching after sanding. Ifsanding and subsequent etching are used,one of the several mild aluminum alloy


FIGURE 2. Effect of honing or lapping on liquid penetrant indications in cracked aluminum test specimens: (a) original crackpattern of standard quench cracked block; (b) crack pattern after removal of 0.06 mm (0.0025 in.) of material using 30 gritwheel with coolant and controlled feed and speed; (c) crack pattern after removal of 25 µm (0.001 in.) by simulated cylinderhoning operation; (d) crack pattern after removal of 0.008 mm (0.0003 in.) per side by standard preanodic etch method;(e) crack pattern after removal of 0.06 mm (0.0025 in.) by milling.

(a) (b) (c)

(d) (e)


FIGURE 4. Effect of hand sanding using 240 grit on liquid penetrant indications in crackedaluminum alloy test block: (a) original crack pattern of standard quench cracked block;(b) crack pattern after removal of 0.05 mm (0.002 in.) material by hand sanding using 240aluminum oxide grit paper; (c) crack pattern after removal of 0.008 mm (0.0003 in.) per sideby standard preanodic etch method; (d) crack pattern after removal of 0.06 mm (0.0025 in.)material by milling. Note that crack obscuring effect of 240 grit is noticeably more than thatindicated by use of coarser 180 grit.

(a) (b)

(c) (d)

FIGURE 3. Effect of hand sanding using 180 grit on liquid penetrant indications in cracked aluminum alloy test block:(a) original crack pattern of standard quench cracked block; (b) crack pattern after removal of 0.05 mm (0.002 in.) by handsanding using 180 grit aluminum oxide paper; (c) crack pattern after 10 min preanodic etch, removing about 0.005 mm(0.0002 in.) per side; (d) crack pattern after additional 5 min etch, making total of 0.008 mm (0.0003 in.) material removedper side; (e) crack pattern after 0.06 mm (0.0025 in.) was removed by milling.

(a) (b) (c)

(d) (e)

etchants can be used. As seen in Figs. 3cand 3d, a 15 min etch in a preanodizedcleaning etch bath restored the crackpattern.

Effects of Shot Peening on LiquidPenetrant Indications of Cracks inAluminum AlloysFigures 7a and 7b show that all liquidpenetrant indications were masked bypeening. A 15 min etch partially restoredthe crack pattern, as shown in Fig. 7c.Figure 7d shows that milling 0.1 mm(0.004 in.) from the aluminum surfacefully restored the center portion of the

crack pattern (where peening was lesssevere). By milling to remove 0.1 to0.25 mm (0.004 to 0.010 in.) from thesurface, the entire crack pattern wasrestored, as shown in Fig. 7e.

Effects of Vapor Blasting on LiquidPenetrant Indications of Cracks inAluminum AlloysFigures 8a and 8b show that nearly allliquid penetrant indications were maskedby vapor blasting (liquid honing). Asshown in Fig. 8c, the crack pattern wasnearly restored by a 15 min etch.


FIGURE 5. Effect on liquid penetrant indications from combination of hand sanding of crackedaluminum alloy test block with 80 grit paper and finish grind using motor driven 240 gritquill: (a) original crack pattern of standard quench cracked block; (b) crack pattern after finishgrind; (c) crack pattern after removal of 0.008 mm (0.0003 in.) material per side by standardpreanodic etch; (d) crack pattern after removal of 0.06 mm (0.0025 in.) material by milling.

(a) (b)

(c) (d)

FIGURE 6. Effect of hand scraping using carbide tip scraper on liquid penetrant indications in cracked aluminum alloy testblock: (a) original crack pattern of quench cracked block; (b) crack pattern after removal of 0.12 mm (0.005 in.) by handscraping entire surface; (c) crack pattern after removal of 0.008 mm (0.0003 in.) per side by preanodic etch. Note that somestreaking and slight loss of crack pattern is evidenced in Fig. 5b due to variations in scraping technique.

(a) (b) (c)

Effects of Tumble Deburring onLiquid Penetrant Indications ofCracks in Aluminum AlloysFigure 9 illustrates that tumble deburring,like shot peening, masked nearly all liquidpenetrant indications (Fig. 9). A 15 minetch restored most of the largest crackindications as shown in Fig. 9c but failedto restore the multitude of cracks inevidence after a mill cut removed0.06 mm (0.0025 in.) of surface, as shownin Fig. 9d.

Effects of Grit Blasting on LiquidPenetrant Indications of Cracks inAluminum AlloysFigures 10a and 10b show that nearly allof the liquid penetrant indications weremasked by grit blasting with 150 meshalum oxide grit. In aerospace industries,grit blasting of spar caps is oftenconsidered to be a suitable technique forthe blending of surface irregularities aftermachining. However, as with sanding ofaluminum surfaces, a 15 min etch aftergrit blasting was required to restore thetotal crack pattern shown in Fig. 10c.

Comparison of Chromic AcidBleedback and Fluorescent LiquidPenetrant Crack Indications in7075-T6 Aluminum AlloyFigure 11 compares two types of crackindications on the same 7075-T6

aluminum alloy test block. Figure 11ashows the original crack pattern revealedby fluorescent liquid penetrants.Figure 11b shows the crack patternproduced by the old technique usingchromic acid bleedback for detection ofcracks and discontinuities open to partsurfaces. The chromic acid stain is lightbrown in color and low in contrastcompared to the bright fluorescent liquidpenetrant indications.

Figure 11c shows that use ofpostemulsifiable fluorescent liquidpenetrant on anodized aluminum alloysurfaces is quite satisfactory in this casewhere the cracks are relatively wide.Fluorescent liquid penetrant indicationsof tight cracks may be obscured byfluorescent background generated by theporous anodized surface.

Investigation ofMechanical ProcessesReducing Liquid PenetrantSensitivityAn investigation has been reported6

whose objectives were to (1) identifyseveral mechanical processes that reducethe effectiveness of subsequent liquidpenetrant testing of aluminum, titaniumand steel and (2) determine techniques torestore liquid penetrant sensitivity afterthose mechanical processes. Quenchcracked steel and stress corrosion cracked


FIGURE 7. Effect of standard shot peen process on liquid penetrant indications in cracked aluminum alloy test block:(a) original crack pattern of standard quench cracked block; (b) crack pattern after shot peen; (c) crack pattern after removalof 0.008 mm (0.0003 in.) material per side by standard preanodic etch process; (d) crack pattern after removal of 0.12 mm(0.005 in.) material by milling; (e) crack pattern after removal of an additional 0.12 mm (0.005 in.) by milling.

(a) (b) (c)

(d) (e)


FIGURE 8. Effect of standard vapor blastoperation on liquid penetrant indications incracked aluminum alloy test block:(a) original crack pattern of standardquench cracked block; (b) crack patternafter vapor blast; (c) crack pattern after0.008 mm (0.0003 in.) was removed perside by standard preanodic etch; (d) crackpattern after removal of 0.06 mm(0.0025 in.) material by milling.

(a)

(b)

(c)

(d)

FIGURE 9. Effect of standard tumble deburrprocess on liquid penetrant indications incracked aluminum alloy test block:(a) original crack pattern of quench crackedblock; (b) crack pattern after tumble deburroperation; (c) crack pattern after 0.008 mm(0.0003 in.) material per side was removedby standard preanodic etch; (d) crackpattern after 0.06 mm (0.0025 in.) wasremoved by milling. Note that crackspecimen was tumbled with another groupof production parts of about same weight.

(a)

(b)

(c)

(d)

titanium were subjected to grit blasting,shot peening, liquid honing, tumbledeburring and sanding and the resultanteffect on liquid penetrant effectivenesswas noted. In addition, quench crackedaluminum was machined, tumbledeburred, sanded and subsequently testedwith liquid penetrant. Following this,

chemical etching was used in incrementalsteps on each of the materials todetermine the amount of materialrequired to be removed to restore theoriginal liquid penetrant indications. Therequired amount of surface material to beremoved by etching listed in Table 4summarizes their recommendations. This


FIGURE 10. Effect of grit blasting using 150 alum oxide grit on liquid penetrant indications in crackedaluminum alloy test block: (a) original crack pattern of quench cracked block; (b) crack pattern after grit blast;(c) crack pattern after 0.008 mm (0.0003 in.) per side was removed by standard preanodic etch. Note thattextured surface of block in Fig 5c definitely affected spread of liquid penetrant during development period.Because liquid penetrant pattern was returned in its entirety, no further work was done with this example.

(a) (b) (c)

FIGURE 11. Effect of chromic anodic anodize treatment on liquid penetrant indications in cracked aluminumalloy test block: (a) original crack pattern as developed in 7075-T6 block by quench crack method;(b) chromic acid bleed back stain after anodizing; (c) crack pattern as developed by liquid penetrant processafter anodizing. Note that here, as in grit blasted specimen, surface texture promoted increased spreading ofliquid penetrant during development.

(a) (b) (c)

TABLE 4. Amount of surface material to be removed by etching to restore liquid penetrant indications.

Surface Material to Be Removed______________________________________________________________________________________________________________

Titanium 4340 Steel 300M Steel AluminumProcess µm (in. × 10–6) µm (in. × 10–6) µm (in. × 10–6) µm (in. × 10–6)

No. 120 aluminum oxide grit blasting 7.5 300 1.8 70 0.8 30 ___ ___No. 50 aluminum oxide grit blasting 100.0 4000 1.5 60 0.5 20 ___ ___Liquid honing 2.5 100 1.8 70 0.8a 30 ___ ___Shot penning 100.0 4000 4.5 180 2.5 100 ___ ___Tumble deburring 2.5 100 1.5 60 1.0a 40 5 (200)Sanding, 100 grit no effect 1.5a 60 no effect 5 (200)

Sanding, 180 grit 2.5 100 1.8a 70 no effect 5 (200)Finish sanding of O-ring grooves ___ ___ ___ ___ ___ ___ 1.3 (30)Conventional machining ___ ___ ___ ___ ___ ___ 5 (200)

a. Process reduced the strength of the indications only.

required amount was shown to vary withthe mechanical cleaning process and withthe test object materials.6

Evaluation of Effect ofMechanical Processing ofAluminum on LiquidPenetrant SensitivityQuench cracked 2024 aluminum alloyspecimens were investigated using theprocedures shown in Figs. 12 and 13. Theliquid penetrant materials used in theevaluation were (1) a water washablefluorescent liquid penetrant and (2) alipophilic fluorescent postemulsifiableliquid penetrant system used with twotypes of wet developer. The specimenswere cleaned after each liquid penetranttest by vapor degreasing to removeresidues of liquid penetrants andprocessing materials before retesting.

The effects of four mechanicalprocesses were investigated: (1) tumbledeburring, (2) sanding with 100 gritemery cloth sanding disks with an airmotor unit in the same manner asstandard milled part, (3) sanding with

180 grit butterfly type sanding air motorunits and (4) machining. Quench crackedspecimens were processed, liquidpenetrant tested, etched and again liquidpenetrant tested to determine the extentof etching necessary to restore liquidpenetrant indications lost duringmechanical processing.

Techniques for Measuring Extentof EtchingCoupons to be used for measuring theextent of metal removal by etching werecut from each specimen containingcracks. The coupon surfaces weremachined to a 0.8 µm root mean square(3.2 × 10–5 in. root mean square) finishand opposite surfaces were machinedparallel to 0.01 mm (0.0004 in.) across thelength of the coupon. Then the couponsand parent specimens were etchedaccording to the procedures indicated inFigs. 12 and 13. The thicknesses ofmaterial removed by etching weremeasured in millionths of an inch andcapable of readings accurate to fivedecimal places. The measured values havebeen converted to micrometer in Table 4and Figs. 12 and 13. Before beingmeasured, the specimens were held in atemperature controlled environmentovernight.


FIGURE 12. Etching sequence after tumble deburring crackaluminum panels.

Tumble deburr

Liquid penetrant test

Etch with proprietarydeoxidizer (3 to 10 min to

remove 1.3 µm or0.00005 in. of material)


Etch with proprietaryalkaline etch cleaner

Etch withnitric-chromic-hydrofluoricacid etch (1 L nitric acid,

0.1 L hydrofluoric acid and0.5 kg chromic acid)

30 s etch(remove

1.3 µm or0.00005

in.)

60 s etch(remove

2.5 µm or0.0001 in.)

120 s etch(remove5 µm or

0.0002 in.)

5 min etch(remove

2.5 µm or0.0001 in.)

10 minetch

(Remove5 µm or

0.0002 in.)

20 minetch

(remove7.5 µm or0.0003 in.)

Liquid penetrant test Liquid penetrant test

FIGURE 13. Etching sequence after sandingof cracked aluminum panels.

Sanded with 100grit emery cloth

Sanded with 180grit butterfly type

sanding units



remove 1.3 µm or 5 × 10–5 in.of material)


Etch for 10 min with nitric-chromic-hydrofluoricacid etch (1 L nitric acid, 0.1 L hydrofluoric acid

and 0.5 kg chromic acid to remove 5 µm or0.0002 in.) of material

Etch with proprietary alkalineetchant for 15 s to remove0.6 µm (0.000025 in.) of

material


Effects of Etching of TumbleDeburred or Sanded AluminumAlloysSome of the liquid penetrant indications,particularly those of fine cracks, wereobliterated when the 2024 aluminumalloy specimens were either tumbledeburred or sanded. After etching with aproprietary alkaline etch cleaner for 60 sor with a nitric-chromic hydrofluoric acidetch cleaner for 5 to 10 min, the waterwashable liquid penetrant indicationsobtained without developer were restoredas shown in Fig. 14. It was necessary toremove 5 µm (0.0002 in.) of material torestore the cracks that were obliterated(see Table 4). The proprietary alkaline etch

cleaner produced a better surfacecondition than the nitric-chromichydrofluoric acid etch cleaner. The acidetch cleaner pitted the surface somewhat.This increased the fluorescent backgroundon part surfaces during subsequent liquidpenetrant testing. In addition, the acidetch cleaner discolored the surface and itwas necessary to etch with the proprietaryalkaline etch cleaner for 15 s to removethe discoloration. This discolorationmight have had no effect on the liquidpenetrant process.

Effects of Etching Conventionaland High Speed MachinedAluminum

Conventional machining is used inmany industries to reduce plate and


FIGURE 14. Effect of sanding and etching on liquid penetrantindications in 2024 aluminum specimens (water washableliquid penetrant without developer): (a) after cracking;(b) after sanding; (c) after sanding, etching with deoxidizer,and nitric-chromic-hydrofluoric acid etch.

(a)

(b)

(c)

FIGURE 15. Effect of conventional machining on liquidpenetrant indications in 2024 aluminum specimens (waterwashable liquid penetrant with dry powder developer):(a) after machining, (b) after etching.

(a)

(b)

forging material to the final partconfiguration. To determine the effect ofmachining of aluminum on thesensitivity of a subsequent liquidpenetrant test, quench cracked 2024aluminum specimens were producedusing the method previously described.The specimen surfaces were machined andliquid penetrant tested with a waterwashable liquid penetrant and dry powderdeveloper. Next, the specimens wereetched and again liquid penetrant tested.As indicated in Fig. 15, it was determinedthat it was necessary to remove 5 µm(2 × 10–4 in.) to restore the liquidpenetrant indications after conventionalmachining.

High speed machining is a machiningapproach designed to minimize toolvibrations and maximize metal removalrate. For high speed machining ofaluminum, the surface speed of the cuttertypically exceeds 12.7 m·s–1

(2500 ft·min–1). Quench cracked 2024aluminum specimens were producedusing the method previously described.The specimen surfaces were then highspeed machined with a spindle speed of34 000 rotations per minute and a feedrate of 219 mm·s–1 (518 in.·min–1). Aftermachining, the specimens were liquidpenetrant tested with a water washableliquid penetrant and dry powderdeveloper. Next, the specimens wereetched to remove 5 µm (2 × 10–4 in.) ofmaterial and liquid penetrant testedagain. As indicated in Fig. 16, it wasdetermined that high speed machiningdid not degrade liquid penetrantindications in aluminum and that etchingis unnecessary.

Effects of Etching of Finish SandedAluminum O-Ring GroovesTo investigate the effect of finish sandingof O-ring grooves, two aluminumspecimens were tested as indicated inFig. 17. The specimens were liquidpenetrant tested with water washableliquid penetrant and nonaqueous wetdeveloper. Sanding obliterated most of thecrack indication in the O-ring grooves.The indications were restored by etchingwith a proprietary deoxidizer to remove1.3 µm (5 × 10–5 in.) of material (Fig. 18).

Recommended MetalRemoval by Etching ofSanded Aluminum PartsThese results indicate that aluminumparts subjected to (1) a sanding operationwith 180 grit butterfly type sanding unitsor (2) a sanding operation with 100 gritemery cloth sanding disks or (3) tumble

deburring, should be etched to removeabout 5 µm (2 × 10–4 in.) of materialbefore any liquid penetrant test. Also,aluminum parts subjected to a finishsanding operation with 320 to 400 gritpaper should be etched to remove at leastabout 1.3 µm (5 × 10–5 in.) of materialbefore performing any liquid penetranttest.

Evaluation of Effect ofMechanical Processing ofSteel on Liquid PenetrantSensitivitySpecimens of AISI 4130 and AISI 1018steels were cracked by carburization and


FIGURE 16. Effect of high speed machining on liquidpenetrant indications in 2024 aluminum specimens (waterwashable liquid penetrant with dry powder developer):(a) after machining, (b) after etching.

(a)

(b)

grinding (see Fig. 19). AISI 4340 and300M steels were intentionally quenchcracked (see Fig. 20). Thereafter, the 4340steel was heat treated to the 1400 to1550 MPa (2.0 × 105 to 2.2 × 105 lbf·in.–2)strength range and the 300M steel washeat treated to the 1850 to 2050 MPa(2.7 × 105 to 3.0 × 105 lbf·in.–2) strengthrange. The specimens were then testedwith a water washable fluorescent liquidpenetrant. A 5 min liquid penetrant dwelltime was used and excess liquid penetrantwas removed using tap water with a spraynozzle operating at 280 kPa (40 lbf·in.–2).After washing, the specimens were ovendried at 60 °C (140 °F) for about 5 min. A


(a)

(b)

FIGURE 19. Typical grinding crack pattern in carburized AISI 1018 steel.

FIGURE 18. Effect of sanding and etching onliquid penetrant indications in aluminumO-ring grooves (water washable liquidpenetrant with nonaqueous wet developer):(a) after sanding; (b) after sanding andetching with proprietary deoxidizer.

FIGURE 17. Etching sequence after sanding of crackedaluminum O-ring grooves.

Quench crack two specimens

Machine O-ring grooves

Vapor degrease and liquidpenetrant test


remove 1.3 µm or0.00005 in. of material)

Sand with 320 to 400grit paper

Etch for 10 min withnitric-chromic-hydrofluoricacid etch to remove 5 µm

(0.0002 in.) of material

Etch with proprietarydeoxidizer (3 to 10 min toremove 1.3 µm or 0.00005

in. of material)

Sand with 320 to 400grit paper



Etch with proprietarydeoxidizer (3 to 10 min toremove 1.3 µm or 0.00005

in. of material)









End



End

Specimen A Specimen B

nonaqueous wet developer, applied withspray cans to the AISI 4340 and AISI300M steels, remained in contact with thespecimen for 15 min. No developer wasused on the AISI 4130 and AISI 1018steels. Following the initial liquidpenetrant testing, the specimens weresubjected to a trichloroethylene vapordegrease to remove residual liquidpenetrant.

After the initial liquid penetrant test,the test specimens were subjected tovarious mechanical surface processingoperations, details of which are given inTable 5. After the mechanical processingthe specimens were cleaned and retestedusing the initial liquid penetrantprocedure.

Technique for Measuring ofExtent of EtchingThe AISI 4340 and AISI 300M steelspecimens where chemically etched afterthe second liquid penetrant test if themechanical processing operation was

found to have affected the intensity orthe number of crack indications. Etch ratecoupons were used to monitor the etchingprocedure as described previously foraluminum. The etching was accomplishedin incremental steps using a hydrochloricacid pickle solution. After each etchingstep, the specimens were liquid penetranttested using the initial procedure to notethe change in the liquid penetrantindications.

Effects of MechanicalProcessing of Steels onLiquid PenetrantIndicationsThe effects of mechanical processing onliquid penetrant indications aresummarized in Table 6. The effect on the


FIGURE 20. Typical quench cracks in 6.3 mm (0.25 in.) thickAISI 4340 steel (water washable liquid penetrant withnonaqueous developer).

TABLE 5. Mechanical processing parameters for steel.

Operation Parameters

Tumble deburring 1.5 to 2 h with triangular aluminumoxide chipsa

Sanding (air motor units) 100 grit emery cloth sanding diska;180 grit butterfly typea

Grit blast 500 kPa (75 lbf·in.–2) 120 grit aluminumoxidea; 500 kPa (75 lbf·in.–2) 50 gritaluminum oxidea

Liquid hone 700 kPa (100 lbf·in.–2) 220 gritaluminum oxidea; 550 kPa(75 lbf·in.–2) 220 grit aluminumoxideb

Shot peen peening intensity of 0.008 Almen (steelshot)a

a. Quench cracked 4340 and 300M steel.b. Grinding cracked 4130 and 1018 steel.

TABLE 6. Summary of mechanical processing effects on liquid penetrant indications. Blank boxes indicate that tests werenot performed under those conditions.a

1018 and TitaniumOperation Aluminum 4340 Steel 300M Steel 4130 Steel (6AI-4V)

Grit blasting, 120 grit aluminum oxide not performeda masked someb masked someb not performeda masked mostc

Grit blasting, 50 grit aluminum oxide not performeda masked someb masked someb not performeda masked mostc

Liquid honing not performeda masked someb reduced strengthd masked mostc masked someb

Shot peening not performeda masked mostc masked mostc not performeda masked mostc

Tumble deburring masked someb masked someb reduced strengthd not performeda masked someb

Sanding, 100 grit masked someb reduced strengthd no effecte not performeda no effecte

Sanding, 180 grit masked someb reduced strengthd no effecte not performeda masked someb

Finish sanding O-ring grooves masked mostc not performeda not performeda not performeda not performeda

a. Test not performed under indicated condition.b. Masked some indications.c. Masked most or all indications.d. Reduced strength of indications only.e. No effect.

liquid penetrant indications varieddepending on the mechanical process andthe particular steel. For example, liquidhoning of AISI 1018 and AISI 4130 steelsresulted in a total loss of liquid penetrantindications. However, liquid honing of300M steel only reduced the strength ofthe indications (Fig. 21). The mostdetrimental mechanical processes wereshot peening (in the case of AISI 4340 and300M steels) and liquid honing (in thecase of AISI 4130 and AISI 1018 steels).These treatments resulted in a total loss ofindications (see Fig. 22). The effectivenessof the liquid penetrant testing wasreduced to a lesser degree by grit blastingand tumble deburring (in the case ofAISI 4340 steel) and by grit blasting (inthe case of AISI 300M steel). For thoseprocesses, finer indications were lost butthe larger indications remained (seeFig. 23).

Several of the processes only reducedthe strength of the liquid penetrantindications as opposed to masking all orsome of them completely. These processeswere sanding (both 100 and 180 grit) inthe case of 4340 steel and liquid honingand tumble deburring in the case of 300M

steel. Finally, both 100 grit and 180 gritsanding had no observable effect on300M steel relative to either the numberof liquid penetrant indications or theirstrength.

Recommended MetalRemoval by Etching toRestore Indications in SteelThe amount of etching required to restoremasked liquid penetrant indications alsovaried depending on the mechanicalprocess and the material (see Table 4). Forexample, the average required amount ofetching varied from 0.5 to 5 µm (7 × 10–5

to 2 × 10–4 in.) for 300M steel. Etching ofhigh strength steels is not always feasiblebecause of the possibility ofembrittlement. Consequently, when it isnot feasible to etch, these test resultsindicate that it may be necessary to use asupplementary testing such as magneticparticle testing along with liquidpenetrant testing if parts are to be testedafter grit blasting or liquid honing.


FIGURE 21. Effect of liquid honing on liquid penetrant indications in 300 M steel (waterwashable liquid penetrant without developer): (a) before liquid honing; (b) after liquidhoning.

(a)

(b)

Evaluation of Effect ofMechanical Processing ofTitanium on LiquidPenetrant TestingSensitivityStress corrosion cracked specimens ofannealed Ti-6Al-4V titanium alloy werealso investigated for effects of mechanical

processing on liquid penetrant testsensitivity. Half of the specimens werefabricated with the principal grain flowparallel to the thickness of the specimen;the remaining half were fabricated withthe principal grain flow perpendicular tothe thickness of the specimen. Eachspecimen was initially liquid penetranttested using the parameters described forsteel, except that a 10 min liquidpenetrant dwell time was used. A


FIGURE 22. Effect of shot peening on liquid penetrant indications in 300M steel: (a) ascracked; (b) after shot peening; (c) after etching.

(a)

(b)

(c)

nonaqueous wet developer was used onthe specimens.

After the initial liquid penetrant test,the test specimens were subjected to themechanical processes described in Table 7.After the processing the specimens werecleaned to remove residual liquidpenetrant, then retested using the initialliquid penetrant procedure.

The specimens were chemically etchedafter the second liquid penetrant test ifthe mechanical processing operation wasfound to have affected the intensity orthe number of crack indications. Etch ratecoupons were used to monitor theetching, as previously described foraluminum and steel. The etching wasaccomplished in incremental steps using a


FIGURE 23. Effect of grit blasting on liquid penetrant indications in 300M steel: (a) as cracked;(b) after grit blasting; (c) after etching.

(a)

(b)

(c)

TABLE 7. Mechanical processing parameters for titanium.

Operation Parameters

Tumble deburring deburred for 1.5 to 2 h using triangularaluminum oxide chips (No. 3)

Sanding (air motor units) 100 grit emery cloth sanding disk, 180grit butterfly type

Grit blast 620 kPa (90 lbf·in.–2) 120 grit aluminumoxide

620 kPa (90 lbf·in.–2) 50 grit aluminumoxide

Liquid hone 550 kPa (80 lbf·in.–2) 220 grit aluminumoxide

Shot peen peening intensity of 0.008 Almen(steel shot)

modified nitric-hydrofluoric acid picklesolution. After each etching step, thespecimens were liquid penetrant testedusing the initial liquid penetrantprocedure to note the change in theliquid penetrant indications.

Effects of MechanicalProcessing of Titanium onLiquid PenetrantIndicationsThe effect of mechanical processing ontitanium can be seen in Table 6. The effecton the liquid penetrant indications varieddepending on the mechanical process. Forexample, grit blasting and shot peeningresulted in a total loss of liquid penetrantindications (see Fig. 24) and were themost detrimental processes. Consequently,it can be seen that if liquid penetranttesting of titanium is performedimmediately after grit blasting or shotpeening without etching, the test wouldbe highly ineffective. The effectiveness ofthe liquid penetrant testing was reducedto a lesser degree by liquid honing,tumble deburring and 180 grit sanding(see Fig. 25). For those processes, finerindications were lost but the largerindications remained. Finally, 100 gritsanding had no observable effect oneither the number of liquid penetrantindications or their strength.

Recommended MetalRemoval by Etching toRestore Indications inTitaniumThe amount of etching required to restoremasked liquid penetrant indications alsovaried depending on the mechanicalprocess and the material (see Table 4). Theaverage required amount of etching forTi-6Al-4V alloy varied from 2.5 to 100 µm(0.0001 to 0.004 in.).


FIGURE 24. Effect of grit blasting on liquid penetrantindications in titanium (Ti-6AI-4V) alloy: (a) as cracked;(b) after grit blasting; (c) after etching.

(a)

(b)

(c)

FIGURE 25. Effect of liquid honing on liquid penetrantindications in titanium (Ti-6AI-4V) alloy: (a) as cracked;(b) after liquid honing; (c) after etching.

(a)

(b)

(c)

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2. Threshold Limit Values for ChemicalSubstances and Physical Agents andBiologic Exposure Indices. Cincinnati,OH: American Conference ofGovernmental Industrial Hygienists(1995).

3. SAE AMS 2644, Inspection Materials,Penetrant. Warrendale, PA: Society ofAutomotive Engineers (1996).

4. ASTM E 1417, Standard Practice forLiquid Penetrant Examination. WestConshohocken, PA: American Societyfor Testing and Materials (1995).

5. McFaul, H.J. “Effect of FinishingProcesses on Detectability of SurfaceFlaws by Penetrant Process.” MaterialsEvaluation. Vol. 23, No. 12. Columbus,OH: American Society forNondestructive Testing (December1965): p 577-582.

6. Cook, J.F., R.J. Lord and R.J. Roehrs.“Quantitative Evaluation of the Effectof Mechanical Processing on theEffectiveness of Penetrant Inspection.”Materials Evaluation. Vol. 32, No. 7.Columbus, OH: American Society forNondestructive Testing (July 1974)p 133-141.


References