Handbook of Galvanizing

8/10/2019 Handbook of Galvanizing

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INTRODUCTION

Unlike m any manufacturing p rocesses, the h ot dip galvanizing of fabricatedsteelwork involves m any variables t hat c an impact on the ap pearance a ndcharacteristics of the nished product.

Hot di p galvanizing is p rimarily an industrial protective coating processdesigned to p rovide a tough and durable p rotective coating for st eel that willgenerally outlast the d esign life of the el ement to w hich it is a pplied.

As the benets of hot dip gabeen recognised, there h as b een an increasing demand for architecturalstandards f or h ot dip galvanized coatings w hich can satisfy th e a estheticrequirements of this segm ent of the sp ecication.

This sp ecial CORROSION MANAGEMENT sect ion illustrates m any of thedefects an d variations t hat c an occur w ith hot dip galvanized coatings. Given anunderstanding of the cau ses and effects of these ph enomena, it is possi ble forboth speciers and operators to opti mise t he qu ality a nd a ppearan ce of a h otdip galvanizing p roject.

FACTORS AFFECTING HOT DIP GALVANIZED COATINGS

The f actors aff ecting hot dip the a ppearance an d characteristics of hot dipgalvanized coatings i nclude:

1. The s ize an d sh ape of the i tem.2. The st eel chemistry.3. The st eel surface con dition.4. The d esign of the i tem with respect to ga lvanizing.5. The m etallurgy of the ga lvanizing process.

The h ot dip galvanizing p rocess i nvolves i mmersing st eel items i n molten zincat 450 oC after pretreatm ent to rem ove orga nic materials, rust and m ill scale.This h ot dipping p rocess gi ves t he ga lvanized coating i ts u nique ch aracteristics.

SIZE AND SHAPE OF THE ITEM

Zinc freezes a t 420 oC, so t here i s very little su perheat in the m olten zinc a s t heitem is w ithdrawn from the b ath of molten zinc. Thicker sec tions w ill retain

their he at l onger an d promote b etter drai nage of the m olten zinc from theirsurfaces t han thinner sect ions.


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THE STEEL CHEMISTRY

The rat e a t which molten zinc r eacts w ith steel to form the ga lvanized coating i sdependent on the st eel chemistry. Pure i ron has a very l ow reaction rate an d forthis reason , galvanizing kettles a re m anufactured from steel of this t ype.

Structural grades of steel always con tains al loying elements, the m ost commonof which are c arbon, manganese and si licon. Sulfur and phosphorous ar eresidual elements ari sing from the r aw material used t o m ake t he st eel,although some spe cial steels h ave the se el ements ad ded deliberately.is an issue w here t he h ighest standards of surface a ppearance a re requ ired.

Of these el ements, silicon and phosphorous h ave t he m ost signicant effect onthe ga lvanized coating's c haracteristics, with silicon being th e m ost common

steel alloying additive affecting coating appearance

STEEL SURFACE CONDITION

The su rface con dition of the st eel will be re ected in the g alvanized coating intwo ways. Rougher s urfaces h ave a higher s urface area p er unit of surface areaand thus g enerate t hicker g alvanized coatings. Hot r olled sections typ icallyproduce h ot dip galvanized coatings 3 0-50% thicker t han smooth, cold rolledsurfaces.

Galvanized coatings o n steel with gross su rface d efects su ch as p itting willconform to t he d efects a nd follow the su rface p role of the st eel.

While t he coa ting will provide t he sa me d egree of protection from corrosion, theappearance m ay not be acceptable t o t he en d-user and thus st eel surface q uality

The an gle with which the work can be withdrawn from the bat h will inuencethe d rainage ch aracteristics. One-dimensional items t hat are sho rt enough todip vertically will produce sm oother an d more u niform coatings t han two- or 3-dimensional three-dimensional items w here som e su rfaces w ill be a t shallowwithdrawal angles t o t he m olten zinc s urface.

DESIGN OF THE ITEM FOR GALVANIZING

The d esign of a fabrication is cr itical in determining the su rface q uality o f the

nished product. Unless t he p re-treatment chemicals c an penetrate t o a llsurfaces of the i tem, the i naccessible su rfaces w ill not be g alvanized.


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Conversely, if the m olten zinc cannot completely drain from the w ork, zincpuddles w ill freeze in pockets an d ca use probl ems.

The t echniques of designing for ga lvanizing are w ell understood by galvanizersand consultation in the d esign stage of fabrication will always resu lt in a betterquality ou tcome.

The m ain issues t o b e a ddressed in designing for gal vanizing to m inimisedefects are a s f ollows:

1. The i tem needs t o b e su spended during g alvanizing. The p rovision of liftinglugs t o el iminate t ouch marks f rom wire or c hains i s an example of designimproving quality.2. The i tem needs t o be h ave a dequate ve nting a nd draining p rovisions t o

ensure t he smooth ow of zinc into an d ou t of the item.3. The d imensions of the i tem need t o comfortably t within the ba thdimensions t o al low adequate withdrawal angles. Double di pping of the i temwill frequently produce a better qu ality than single d ipping an item that can bebarely su bmerged in the ba th.

THE METALLURGY OF THE GALVANIZING PROCESS

Many of the su rface i mperfections t hat are c haracteristic of hot dip galvanizedcoatings a re a f unction of the m etallurgy a ssociated with the p rocess.

Prior t o g alvanizing, the st eel is ch emically pre-treated and at the nal stage, isimmersed in a zi nc ammonium chloride (ZAC) preux solution. This processputs a t hin lm of ZAC crystals on the st eel surface.

When the s teel is immersed in the m olten zinc bat h. A number of reactions t akeplace. These are:

1. The p reux is act ivated by t he m olten zinc an d performs a nal cleaningaction on the st eel surface. Oxidation products are p roduced by this r eactionwhich contain a combination of zinc chloride a nd zinc oxi de. These oxi dationproducts are l ighter t han zinc an d oat to t he su rface. This i s cal led zinc ash .2. When the st eel reaches ga lvanizing b ath temperature, the zi nc r eacts w iththe st eel to form a seri es of zinc-iron alloys. Some of these zinc-iron alloycrystals oat off the su rface of the st eel and enter t he zi nc ba th.3. The zi nc bath becomes s aturated with iron i n solution, either f rom workentering the b ath or f rom the st eel galvanizing kettle. This satu rati on level istypically a bout 250 p arts of iron per m illion at galvanizing tem peratu re.


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4. Over t ime, these zi nc-iron crystals coal esce an d sink to t he b ottom of the zi ncbath. This f orms a m ushy layer c alled dross. The d ross is p eriodically rem ovedby scoopi ng i t from the ba th with special equipment.Zinc dross i s crystalline an d has a m uch higher melting p oint than zinc (650 oCversus 4 20 oC).5. The m olten zinc s urface i s c onstantly ox idising, and this oxi de lm is al wayspresen t on the zi nc su rface t o a greater or l esser degree.

In the p rocess of galvanizing a steel fabrication, the p resen ce of ash and drossmay h ave t he following effects on the ap pearance of the h ot dip galvanizedcoating:

If the i tem comes i n contact with the d ross l ayer i n the b ottom of the b ath, thedross can get caught in or on the work and form a rough , lumpy d eposit.

If areas of the i tem are i naccessible for ski mming, ash may stick to t he su rfaceof the ga lvanizing a s i t is w ithdrawn from the ba th. If the rate of withdrawal varies or st ops d uring th e ex tract ion of thegalvanized item from the b ath, oxide l ines m ay appear on the su rface of thegalvanizing causing variations i n the sh iny appearan ce of the coati ng. On smooth-surfaced work such as t anks or large d iameter pipes, dross crystalsoating in the ba th may b e caught up on the ga lvanized coat ing. These drosspimples give t he coati ng a gritty a ppearance.

If surface p reparati on is n ot do ne correct ly, the zi nc w ill not r eact with a dirtyor ox idised steel surface a nd a miss in the coa ting will resu lt.

STEEL DEFECTS AND THEIR EFFECT ON GALVANIZED COATINGS

Hot dip galvanized coatings, as d escri bed above, are n ot coatings in theconventional sense, but alloys t hat form on the st eel to m ake t he ga lvanizedsteel item a composite m aterial.

As such, galvanized coatings, coveri ng up defects i n the st eel, and are m ore l ikely to h ighlight steel surfacequality problems.

Steel like m ost manufactured p roducts, can be p rone t o a vari ety of commondefects, most of which are a ssociated with the cast ing, rolling andmanufacturing processes.

The su rface a rea of the st eel per un it of area h as a si gnicant effect on how thezinc r eacts w ith the st eel to form the ga lvanized coating. Rough surfaces w illreact wore vi gorously th at smooth surfaces to form thicker ga lvanized coatings.


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Hot rolled surfaces with mill scale m ay h ave 30 50% more surf ace are a perunit t han smooth, cold rolled surfaces. Abrasive blasting will articiallyincrease su rface r oughness an d i s a w ay to deliberatel y increase c oatingthickness f or h eavy d uty a pplications.

Galvanized coa tings are t otally u nlike p aint lms i n the way i n which theyconform to sh arp edges or corners. The su rface t ension effects w ith paint lmstend to t hin the p aint lm out over corners an d edges.

Hot dip galvanized coatings, on the ot her h and, tend to b e t hicker on edges an dcorners because t he cr ystals i n the a lloy layer c an are ou t and have m ore ro omto gr ow around t he r adius.

For t his r eason, hot dip galvanized coa tings m ay exa ggerate t he ap pearance ofdefects w here rol ling d efects c ause sh arp ri dges on the st eel surface.

Small particles o r sl ivers of steel can be rol led into th e steel s su rface, and thesemay b e al most invisible t o t he n aked eye. However, when the st eel is pi ckledand the su rrounding oxi de i s r emoved from these s hells an d scales, as t hey a recalled, the ga lvanized coating can take on a p imply a ppearance as t he zi ncreacts m ore q uickly w ith these sm all, loosely a ttached steel slivers.

The m anufacturing op erations perf ormed on some s teel sections can also s howup in the f orm of visual defects, the m ost common of these b eing striations.These are n arrow (typically a bout 1-2 m m wide) longitudinal raised l ines i n thecoating usually running the f ull length of the sect ion.

The m ost common cause of striations i s st ress l ines on the su rface of the st eelcaused by cold working of the sec tion. Heavy g auge rod w ill sometimes s how aspiral striation pattern arising from the rod or w ire bei ng passed through a

roller/straightener p rior t o nal manufacture.

Welded section will frequently h ave m uch thicker c oatings i n the w eld areas.While t his i s rarel y a problem on structural steel sections, it can cause a estheticproblems on high quality fabrications su ch as w rought iron and security d oors,where t he weld h ave been ground ush a nd t he item is t o be pa inted a ftergalvanizing.

This ph enomenon is cause by t he differing chemistry of the weld metalcompared to t he p arent metal. Most weld metals c ontain high levels of silicon,


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and the h igher si licon in the w eld metal will result in thicker ( sometimes 3 -4Xas t hick) galvanized coatings.

A reverse of this phenomenon can occur hermm) are ame-cut. The anges of welded beams are a common example. Theame cut edges of these h eavy plate s ections w ill frequently show a widevariation in coating t hickness f rom zone t o zon e i n the ame-cut area.

This i s cau se by irregular oxi dation of some of the con stituents i n the st eel onthe su rface, giving the ame cu t surface a variable m etallurgy compared to t heparent metal.The zi nc coating in these ame cut areas m ay a ppear as a ser ies o fsharply dened plateaus, with low thickness c oatings w here si gnicantoxidation has t aken place, and normal coating th ickness, typically w ell over t heminimum standard requ ired, in less affected a reas.

EMBRITTLEMENT

Embrittlement of the st eel in the ga lvanizing p rocess i s n ow a rare p roblem, butshould always be taken into con sideration as the sel ection of steel grade a ndprocessing m ethods.

There a re t hree t ypes of embrittlement encountered in galvanizing. These are:

1. Hydrogen embrittlement2. Stain-age em brittlement3. Liquid m etal embrittlement

Hydrogen embrittlement affects s usceptible s teels; namely st eel whose yi eldstrength goes beyond t he 800-1000 m Pa r ange.

Hydrogen atoms f rom the p ickling a cid penetrate t he grai n boundaries of these

types of steels a nd can cause b rittle f ractures u nder cert ain conditions. Highstrength steels can be galvanized and standard p re- or p ost-galvanizingtreatments a re ava ilable t o el iminate t he ri sk of hydrogen embrittlement.

Consultation with the g alvanizer i s essen tial where f abrications i nvolving theuse of high strength steel are i nvolved.

Strain-age e mbrittlement is c aused by excessi ve col d working o f the st eel priorto g alvanizing. The h eat of the g alvanizing process a ccelerates t heembrittlement proc ess, so su sceptible st eels p resent a long-term embrittlementrisk, even if they were n ot galvanized.


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The work has t o be heat ed from around ambient temperature t o 450 oC almostinstantaneously a s i t enters the m olten zinc.

Not all of the i tem is h eated at the sam e t ime a s i t has t o be l owered i nto t hebath at a rate t hat is saf e f or t he op erators a nd consistent with the a bility o f themolten zinc t o ow over an d into t he i tem.

At the galvanizing temperature, regains i t as i t cools dow n. Much research has b een done on the eff ect of thegalvanizing process on steel strength, and this h as r evealed that there i s n omeasurable l oss, and in most cases, there i s a slight i ncrease, in yield strengthafter g alvanizing.

Thin plate sect ions are m ost prone t o d istortion, and this i s caused by the rap id

differential heating and cooling of the sect ions.

Thin steel that has b een processed into h ollow sections or col d-formedstructural shapes i s m uch more s table an d i s r outinely ga lvanized withoutproblems.

Fabrications t hat ar e asym metrical, or hav e si gnicant weldments on one si deof the sect ion are i n a highly stressed state d ue t o t he h eat effects of the w elding

processes. Hot di p galvanizing will stress rel ieve these se ctions a nd distortionmay result, typically cambering or s weeping beam sections.

Fabrications c ontaining elements of signicantly different section thickness a reanother pot ential problem area, and high stresses c an be generated during t heheating a nd cooling cycl e t hat may cause d istortion. Platforms w ith checkerplate welded to structural framing is a c ommon problem area t hat can beavoided by galvanizing the thin and thick sections as sepa rate el ements.

The ga lvanized has t he res ponsibility t o h andle t he w ork in a w ay that w illreduce th e risk of distortion.

Lifting p oints l ocated a t the op timum points t o su pport the fabrication so t hatits ow n unsupported weight does n ot contribute t o i t bending or s agging d uringgalvanizing are a galvanizers resp onsibility.

The way t he item is immersed in the m olten zinc and the post galvanizing

quenching process may all have a n inuence on the d imensional stability of thenished p roduct. Competent galvanizers s hould b e aw are of these i ssues an d be


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prepared to a dvise t he cl ient on design details t hat can minimize d istortionproblems.

Where d istortion is u navoidable, particularly with products su ch as w aste b insand trailers, clients are n ow prepared to acc ept the a ppearance of the t hinmetal plate sect ions b ecause of the d ramatic i ncrease i n service l ife p rovided bythe coa ting.

The a cceptance of unavoidable d istortion during hot dip galvanizing will thusalways be a trade-off between appearance an d performance. Only t he client candecide.

PICKLING CORROSION

Hydrochloric aci d is u sed for p ickling steel prior t o ga lvanizing. Somegalvanizers add i nhibitor t he t he aci d but most operate at acid con centrationlevels of around 10%.

At this level, the awill not react w ith the b ase st eel at a very high rate.

The excep tion is w ith high sulfur s teel. The m ain application for t hese t ypes ofsteel is f or s mall part manufacture of machined components.

The su lfur i s ad ded to t he st eel to w eaken its grai n structure so t hat duringhigh speed machining op erations, the chi ps break off in short sections an dfacilitate th e m achining operations.

These s teels are u sed f or n on-critical components such as t hreaded ttings a ndrepetition machines st eel non-stressed components.

From time t o t ime, threaded tting su ch as soc kets, spigots, nipples an d bushes

are w elded into fabricated assemblies. Attack by the a cid on these t hreadedcomponents c an quickly d issolve t he t hreads an d render t he i tem unserviceable.

The cor rosion of the t hreaded area i s of ten held to b e cau sed by zinc at tack inthe g alvanizing bath, but it is p urely a function of acid attack during thepickling process.

BLOWOUTS AND BLEEDING

Molten zinc at galvanizing t emperature i s on ly a bout 30 d egrees ab ove i tsfreezi ng point. The v iscosity of the m olten metal limits i ts a bility to p enetrate


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small cavities an d crevices, and gaps l ess t han 1 mm in width may n ot allowpenetration by the m olten zinc.

Many fabricated items ha ve over lapping su rfaces, and during t he p re-treatmentprocess, pretreatment chemicals w ill penetrate t hese over laps an d may n otdrain out because su rface t ension holding t he l iquor i n place.

When the item enters t he gal vanizing bath, any m oisture t rapped i n thesecrevices w ill rapidly b oil and eject steam and pretreatm ent residues on to t headjacent surface of the w ork.

This con tamination interferes w ith the zi nc's a bility to rea ct with the steel anduncoated areas adjacent t o th ese ` blowouts will result.

Another defect arising from unsealwhich occurs ou t of the overl ap after ga lvanizing.

This i s com monly referred to a s ` acid staining. Hydrochloric a cid vaporizes a tbelow 200 d egrees C so t here i s n ever an y a cid residues l eft in or on the i temafter g alvanizing.

The st aining that oc curs i s c aused b y residual ux residues t hat h ave b een

desiccated by the h eat of the g alvanizing p rocess, re-hydrating from contactwith atmospheric m oisture.

The b rown liquor bl eeding f rom overlaps i s n ot particularly a ggressive to zi ncbut causes u nsightly staining of the w ork and highlight the l ack of a galvanizedcoating, and a potential for corr osion, in the u ncoated overlap area.

FLAKING

Flaking of galvanized coatings can occur on reactive st eels w hich produce t hickalloy l ayers i n the g alvanizing process.

In thick react ive steel plate, coating thickness' in the ord er of 500 microns m aybe p roduced. These al loy l ayers are ver y h ard and inexible an d differentialexpansion or con traction of the st eel can cause a reas o f the a lloy l ayers t o sh earoff the st eel surface.

Localized aking can occur w ith these t ypes of coatings i f there a re su bject toimpact or poi nt pressure f rom lifting slings or h andling e quipment.


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On some fabrications, particularly welded b eams an d h eavier sections that havebeen assembled u sing su bmerged arc welding t echniques, the w eld areas m ayshow evidence of a seri es of small pinholes al ong t he w eld bead. These pi nholesare general ly less t han 1mm in diameter.

The sm all size of the p inholes, combined with the t hickness of the ga lvanizedcoating on the weld m etal (typically 200 microns or more)means t hat they h aveno measurable effect on coating performance.

The cau se of this p henomenon has b een attributed to sm all particles of sub-arcwelding sl ag powder bei ng fused into t he w eld surface. These r efractoryparticles are n ot affected by the p re-treatment processes an d remain on thesurface t hroughout the ga lvanizing p rocess an d form small barriers to t hegalvanized coating's f ormation.

PUDDLING, DROSS AND SPLATTER

Puddling cau sed by p oor drainage, dross caught in an item and sp latter fromthe zi nc bath e nding u p in or on the su rface of the ga lvanized item areprocessing issues. Some of these, such as rem oval of splatter, are d ealt with ininspection/QA stage. design issues are t he m ain cause of puddling and can onlybe el iminated by good design.

CONCLUSION

With the exc eption of blowouts an d bleeding, and where t he ga lvanized coat ingis m issing, most galvanizing d efects h ave n o eff ect on the coa ting;s d urability. Infact, many defects , because t hey resu lt in thickening of the coa ting ordeposition of much more zi nc or z inc-rich residues, coating l ife m ay be i ncreasedsignicantly.

For example, a d ull or mottled g alvanized coat ing m ay b e 2X or 3X as t hick as astandard shiny coating t hat complies w ith the rel evant standard. The t hickergrey coati ngs w ill thus h ave a coating l ife 2 X or 3X as l ong a s t he st andardcoating.

Where a esthetic i ssues a re i nvolved, hot dip galvanized coatings ca n only d elivera level of quality within the l imitations of the p rocess.

However, an understanding of the i ssues of design, steel composition and the

hot dip galvanizing process w ill go a long w ay to en sure t hat h ot dip galvanized


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coatings on structural fabrications c an be p roduced to a st andard t hat willsatisfy a rchitect ural applications.

Documents

Handbook of Galvanizing