Practical Guidelines Inspect Repair HDG Coatings 2008

8/12/2019 Practical Guidelines Inspect Repair HDG Coatings 2008

1/24

HOT DIPGALVANIZERSASSOCIATION

SOUTHERN AFRICA

The

Association is

a technical

information

centreestablished for

the benefit of

specifiers,

consultants,

end users and

its members

PRACTICAL GUIDELINES

FOR THE INSPECTION AND REPAIR OF

HOT DIP GALVANIZED COATINGS

PRACTICAL GUIDELINES

FOR THE INSPECTION AND REPAIR OF

HOT DIP GALVANIZED COATINGS


2/24

Hot Dip Galvanizers Association Southern AfricaUnit 4, Quality House, St. Christopher Road, St. Andrews, Bedfordview

P O Box 2212, Edenvale, 1610

Telephone: (011) 456-7960 Fax: (011) 454-6304

Email: [email protected]

www.hdgasa.org.za

Design and layout by Sandra Addinall Telephone: (011) 868-3408 Fax: (011) 900-1922 Email: [email protected]

Printed by Camera Press Telephone: (011) 334-3815 Fax: (011) 334-3912 Email: [email protected]

Second Edition March 2008

The Association is a technical information centre established for

the benefit of specifiers, consultants, end users and its members

The copyright of all materia l in this catalogue and its supplements, particularly material

which is identified by the symbol , i s expressly reserved. No part of this catalogue may

be reproduced or distributed in any form or by any means, without the prior written

permission of Hot Dip Galvanizers Association South Africa.

Whilst every effort has been made to ensure the accuracy of the information contained inthis publication, no responsibility can be accepted for any errors or for any direct or

consequential loss or damage resulting from such information.


3/24


4/24

HDGASA 2008

Contents

Introduction .................................................................................... 3

Test Sample Selection .................................................................... 3

Coating Thickness and Uniformity .................................................. 3

Thickness Testing ............................................................................ 5

Surface Conditions Acceptance, Rejection and Responsibility ...... 8

Reference areas .............................................................................. 15

Uniformity Testing .......................................................................... 15

Adhesion of the Coating ................................................................ 16

Testing Adhesion ............................................................................ 16

Appearance .................................................................................... 16

Removal of Wet Storage Stain (White Rust) .................................... 17

Coating Repair Procedures By the Galvanizer ............................ 17

Site Repairs...................................... 18

Applicable Standards ...................................................................... 19

Aims and Objectives of the Association.......................................... 20

Publications Available from the Association .................................... 20

2


5/24


6/24

the relationship between corro-

sion protection and coating thick-

ness is approximately linear, i.e.,

the service life is doubled if the

coating thickness is doubled.

There is however a limit to the

maximum coating thickness that

can be obtained.

The factors which influence over-

all coating thickness are a combi-

nation of several variables. The

galvanizer can alter zinc temper-

ature, time of immersion and rate

of withdrawal from the molten

zinc. These factors can be used to

marginally alter coating thick-

ness, particularly with reactive

steels. The formation of the pro-

tective iron/zinc alloy layer is a

diffusion process. As with all dif-fusion processes, the reaction

proceeds rapidly at first and

slows down as the layer becomes

thicker. Higher bath temperatures

and longer immersion times will

generally produce thicker alloy

layers.

The thickness of the outer zinc

layer of the coating is indepen-

dent of immersion time.

Thickness of this layer is deter-

mined by the rate of withdrawal

from the zinc bath and the

degree of drain-off of molten

zinc. A fast rate of withdrawal

causes an article to drag out

more zinc. This results in a heav-

ier coating, although the distribu-

tion of the zinc layer may be

uneven. Wiping of the coating

during withdrawal materially

reduces its thickness and thus its

protective value. It is discour-

aged except where necessary for

the smoothness demanded by

conditions of service such as for

small bore pipe and conduit.

Tubes hot dip galvanized in

accordance with SANS 32 / EN

10240 utilise steam blowing

internally and an air ring exter-

nally, to remove excess zinc.

Some variation in internal coat-

ing thickness is acceptable within

50mm of tube ends. With fabri-

cated articles, local differences in

the drain-off, because of the

shape of the article and the angle

at which different surfaces leave

the bath, may also result in some

variation in coating thickness.

Other factors influencing the

coating thickness may be beyond

the control of the galvanizer.

The chemical composition of the

steel plays a major role in deter-

mining the thickness and appear-

ance of the final coating. Certain

steel compositions tend to accel-

erate the growth of the iron/zinc

alloy layer so that the hot dip gal-

vanized coating may have a

matte finish with little or no outerzinc layer. This coating also tends

to be thicker than the typical

bright hot dip galvanized coating.

The galvanizers control over this

condition is limited. Steels con-

taining the elements, phosphorus

in excess of 0.02%, or silicon from

0.03% to 0.15% and above

0.25% or combinations of both

elements, are particularly prone

to heavier coatings consisting

mainly of iron/zinc alloys. Due tothe iron/zinc alloy growth in

these coatings, extending to the

outer surface, the final appear-

ance of the coating can be dark

grey in colour distributed evenly

or unevenly over the surface.

The surface condition of the steel

before hot dip galvanizing also

affects the thickness and smooth-

ness of the final coating. Steels

which have been abrasive blast

cleaned, or left to weather and

rust for some time prior to hot

dip galvanizing, can produce

substantially thicker coatings

than those normally produced on

chemically cleaned steel.

The mass, shape and degree of

cold working of components to be

hot dip galvanized also influences

coating thickness and uniformity.

When a fabricated article consists

of both heavy and light sections, a

variation in coating thickness

between the sections may result.

4 HDGASA 2008

TABLE 2

MINIMUM COATING THICKNESS ON ARTICLES THAT ARE NOT CENTRIFUGED

SANS 121 / ISO 1461 SABS 763

Category and Local Mean Article Thickness,thickness coating coating Type m.(t) mm thickness thickness General

m* m* Application

t 6 70 85 A1, B1, D1, 85E1, F1

3 t < 6 55 70 A2, B2, F2 65

1.5 t < 3 45 55

t < 1.5 35 45 A3 45

B3 55F3 45

t 6 70 80 D1 85

t < 6 60 70

* Local coating thickness is defined as the mean of the measurements taken withina specified reference area. Mean coating thickness is the control sample numberaverage of the local coating thickness values from each reference area.

Thickness legend - 3 t < 6 = thickness less than 6mm but greater and equal to3mm.

Where only one reference area is required according to size of the article, themean coating thickness within that reference area shall be equal to the meancoating thickness given in the above table.

PROFILES

CASTINGS


7/24

Immersion time will vary accord-

ing to the relationship of the sur-

face area of an item to its mass.

The galvanizer has little control

over this situation.

Combining heavy and light sec-

tions in a single component may

also result in unacceptable distor-

tion (refer to HDGASA wall chart,

Design for Hot Dip Galvanizing

and Facts About Hot Dip

Galvanizing Practical Guidelines).

Since the time to the appearance

of first rusting of the base steel

is usually determined by the

thinnest portion of the coating,

an evaluation of galvanizing

quality must take into account

both the minimum thickness ofthe coating and its distribution.

Specifications for hot dip galva-

nizing recognise that variations

in coating thickness are inherent

in the process. The minimum

thickness is generally defined as

an average or mean thickness of

the coating on specimens tested

and/or a minimum thickness for

any individual specimen.

When measurements are taken

to determine the uniformity and

thickness of a hot dip galvanized

coating, 5 or more coating thick-

ness readings should be taken in

each reference area. Reference

areas should be taken approxi-

mately 100mm from the ends of

the article to avoid end effects.

Usually the end of an article

which leaves the bath last will

carry a thicker coating. This is

particularly so towards the edge,

where, at the time of drainage,

the last few drops of zinc tend to

agglomerate as a result of sur-

face tension.

The minimum coating require-

ments specified in SANS 121 /

ISO 1461 for different materialthicknesses and classes of work

are summarized in table 2 and

table 3. For comparitive purpos-

es, these tables compare thick-

ness equivalents to the old SABS

763 specification.Table 4 indi-

cates the minimum coating

requirements specified in SANS

32 / EN 10240 for different

classes of coating.

Specifications do not stipulate

maximum upper coating thickness

limits, but excessively thick coat-

ings on threaded articles are

undesirable. In order to ensure

effective tensioning, the coating

thickness on fasteners should

not exceed a maximum of 65m,

this applies particularly, to high

strength bolts and nuts.

Variance in coating thickness.

A requirement for a thicker coat-

ing (25% greater than the stan-

dard intable 2) can be request-

ed for components not cen-

trifuged, without affecting speci-

fication conformity.

NOTE: Where steel composition

does not induce moderate tohigh reactivity, thicker coatings

are not always easily achieved.

Thicker coatings are more resis-

tant to severe environmental con-

ditions, but can be more brittle

and may require special handling.

The efficacy of corrosion protec-

tion of a hot dip galvanized coat-

ing (whether light or dull grey) is

approximately proportional to

coating thickness.

Thickness TestingThere are several methods to

determine the thickness of the

zinc coating on a hot dip galva-

nized article. The size, shape and

number of pieces to be tested, will

most likely dictate the methods of

testing. The specified test meth-

ods are either destructive or non-

destructive and are detailed in

SANS 121 / ISO 1461. Identical

methods are detailed in SANS 32

/ EN 10240. The most practical

tests are the non-destructive type,

such as gauges utilising the elec-

tromagnetic principle.

1. Electromagnetic Testing

Method.

Instruments, which rely on

electromagnetic principles

are probably the most widely

used devices to determine

the thickness of hot dip

galvanized coatings. Electro-

5HDGASA 2008

TABLE 3

MINIMUM COATING THICKNESS ON ARTICLES THAT ARE CENTRIFUGED

SANS 121 / ISO 1461 SABS 763

Category and Local Mean Article Thickness,thickness/diameter coating coating Type m.

(t) or () mm thickness thickness Generalm* m* Application

20 45 55 C1 55

6 < 20 35 45 C2 45

< 6 20 25t 3 45 55 C1 Washers 55

D2 45

t < 3 35 45 C2 Washers 45

* Local coating thickness is defined as the mean of the measurements taken withina specified reference area. Mean coating thickness is the control sample numberaverage of the local coating thickness values from each reference area.

Thickness/diameter legend - 6 < 20 = diameter less than 20mm but greaterand equal to 6mm.

Where only one reference area is required according to size of the article, themean coating thickness within that reference area shall be equal to the meancoating thickness given in the above table.

FASTENERS

OTHER

ARTICLES

(INCLUDING

CASTINGS)


8/24

HDGASA 2008

magnetic instruments mea-

sure coating thickness in

specifically-identified local

areas of the article. The aver-

age thickness (and there-

fore mass) of the coating is

calculated from measure-

ments taken at a suitably

large number of points dis-

tributed over a reference

area on the surface of the

article. There are a number

of electromagnetic gauges

which can be used to give

accurate measurements of

the zinc coating thickness.

These gauges give reliable

thickness readings provided

they are properly calibrated

and the manufacturers

instructions are observed.

The most commonly used

type of gauge is a portable

electronic instrument. The

gauge uses a temperature-

compensated magnetic trans-

ducer to measure the mag-

netic flux changes that occur

when the probe (a magnet)

is separated from a ferrous

metal substrate by a non-

magnetic coating such as

zinc. The output signal from

the probe is proportional to

the distance of separation

and, therefore, to coating

thickness. The probe signal

is amplified and indicated

on a meter calibrated to

show coating thickness.

These battery powered

instruments have typical

accuracies up to 5% and

have the advantage of not

requiring recalibration with

probe orientation. Similar

electromagnetic instruments

with even greater accura-

cies are available for labora-tory use.

The specification requires that

the thickness gauges shall (in

terms of ISO 2178) be -

Capable of minimizing

errors in reading caused

6

TABLE 4

MINIMUM COATING THICKNESS ON STEEL TUBES TO SANS 32 / EN 10240COATING QUALITY A1 A2 A3

Mandatory Minimum local coating thicknesson the inside surface except atthe weld bead 55m 55m 45m

Minimum local coating thicknesson the inside surface at the weld

bead 28m 1) 1)

Options Minimum local coating thickness

on the outside surface 2) 2) 2)

COATING QUALITY B1 B2 B3Mandatory Minimum local coating thickness

on the outside surface 55m 3) 40m 25m

1) This requirement does not apply

2) This requirement applies when the purchaser specifies Option 1

3) Option 3 specified (if >55m required, purchaser to specify according to

SANS 121 / ISO 1461)

Coating qualities A and B refer to end application with quality A being for gasand water installations and B for other applications. The number following thequality letter refers to specific requirements in terms of coating thickness.

NOTE: In South Africa, SANS 32 / EN 10240 to quality A1 replaces the previousSABS 763, B4 coating.

A

When curved surfaces are dealt

with, particular care should be

taken to ensure that the thickness

gauge is capable of obtainingreadings to an acceptable level of

accuracy.

by the magnetic perme-

ability, dimensions, sur-

face finish and curvature of

the article being tested;

Capable of measuring

the thickness of the

coating being tested

within an accuracy of

10% or 1.5m which-

ever is betterA

, and

Calibrated by taking read-

ings at zero (or near zero)

thickness and at thickness-

es of at least 100m, on

suitable non-ferrous shims

placed on acceptable stan-

dard pieces of metal of

composition, thickness

and shape similar tothose of the articles

under test.

To avoid possible errors in

the use of magnetic instru-

ments, certain precautions

should be taken:

Follow the manufactur-

ers instructions. The

instrument should be

frequently recalibrated

against non-magnetic

film standards of known

thickness.

The base steel should be

backed up with similar

material if thinner than

the critical thickness for

the magnetic gauge or

the instrument should

be recalibrated on a sub-

strate of similar thick-

ness.

Readings should not be

taken near an edge,

hole, or inside corner.

Readings should not be

taken on curved surfaces


9/24

HDGASA 2008

without additional cali-

bration of the instrument.

The test surface should be

free from surface contami-

nants such as dirt,grease,

and corrosion products.

Test points should be

taken in each reference

area to avoid obvious

peaks or irregularities in

the coating.

A sufficient number of

readings should be taken

to obtain a true average.

2. Testing threaded articles

by fitting mating parts.

The zinc coating on externalthreads shall be free from

lumps and shall not have

been subjected to a cutting,

rolling or finishing operation

that could damage the zinc

coating. The zinc coating of

an external standard metric

thread that has not been

undercut shall be such as to

enable the threaded part to

fit an oversized tapped nut

in accordance with theallowances given intable 5.

On bolts greater than M24,

undercutting of bolt threads

is frequently preferred to only

oversizing of nut threads.

Threaded articles shall fit

their mating parts and, in

the case of assemblies that

contain both externally and

internally threaded articles,

it shall be possible to screw

mating parts together by

hand.

3. The Chemical Stripping

(Gravimetric) Test (to ISO

1460).

This method is applied

where material is inspected

after hot dip galvanizing.

Since this is a destructive test

method, it is generally not

suitable for the inspection of

large or heavy items unless

smaller or representative

specimens can be substituted

for them (see Test Sampling).

The test specimen is cleaned

with naphtha or other suit-able organic solvent, rinsed

with alcohol, dried and

weighed. It is then stripped

of the zinc coating by immer-

sion in a solution containing

3.5g of hexamethylenete-

tramine and 500ml concen-

trated hydrochloric acid in

1litre of water. The stripping

of the coating is complete

when evolution of gas ceas-

es. After washing and drying,the specimen is weighed; the

differences in mass before

and after stripping divided by

the surface area of the test

specimen gives the mass of

coating per unit area. In the

case of threaded articles,

such as bolts and screws, the

determination is made on an

unthreaded portion of the

article.

The stripping test gives an

accurate average coating

mass of the zinc coating.

However, it does not pro-

vide any information on

how evenly the coating is

distributed.

To test compliance with

SANS 32 / EN 10240, two

test pieces shall be taken

from the tube to be tested.

These should be between 30

and 600cm2 in surface area,

with length between 50 and

150mm taken at least

600mm from the tube end.

4. Metallographic

Examination.

Where the hot dip galvanized

coating composition and

thickness are of interest,

microscopic examination is a

reliable tool. This test is a

requirement for the testing of

compliance of Coating A1 to

SANS 32 / EN 10240. This

very accurate method uses a

small polished and etched

cross-section of the hot dip

galvanized component to

provide information about

the relative thicknesses of the

alloy and the free zinc layers

which comprise the hot dipgalvanized coating.

The fol lowing procedures

should be adhered to

Water should not be

used as a lubricant at any

stage during the polish-

ing procedure due to

staining or mild corro-

sion of the galvanized

layer.

The etchant should be

2% (max) Nital, i.e. (2ml

concentrated HNO3 in

100 ml of 95% ethanol

or methanol).

Important disadvantages of

this technique are that

Specimens cut from the

hot dip galvanized arti-

cle are required,

Coating thickness mea-

sured only applies to a

very limited area, it does

not indicate the variation

in coating distribution on

the article and,

It is necessary to exam-

ine a number of speci-

mens to determine the

average coating thick-

ness on the hot dip gal-

vanized article.

7

TABLE 5

OVERSIZE TAPPING ALLOWANCE FORHOT DIP GALVANIZED NUTS

Nominal size of Allowancethread (mm)

M8 to M12 0,33

M16 to M24 0,38

>M24 = M27 0,43>M27 = M30 0,47

>M30 = M36 0,57

>M36 = M48 0,76

>M48 = M64 1,0


10/24

8 HDGASA 2008

Surface Conditions (SC)LEGEND:

A - Accept R - Reject N - Negotiate C - Clean REP - Repair

RESPONSIBILITY: G - Galvanizer D - Designer B - Builder/Fabricator S - Steel Type/Surface

SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP AVAILABLE BATH

2 ASH DEPOSITS A / NAsh deposits are grey,

non-metallic deposits

consisting of zinc oxide

that have been

deposited on the hot

dip galvanized coating.

C/REPG If

necessary

4 BLACK STEEL A / RINSPECTION.Inspection prior to hotdip galvanizing isextremely important.

Although the recommended

quantity of sodium dichromate

is about 0,15 to 0,3%,

occasionally when topping up,

more is added. This often

results in a dark yellow to

brown colour on the galvanized

surface. The darker colour will

provide enhanced initial

corrosion protection.

1 APPEARANCE OF ASODIUM DICHROMATE.A small amount of

sodium dichromate is

generally added to the

quench water bath for

passivation.

G

Maintain concentration of

sodium dichromate at about

0,15 to 0,3%.

Zinc oxide deposits can take

place when the component is

dipped or when it is removed

from the bath.

The coating is normally intact

underneath the ash deposits.

Ash must be removed and the

coating thickness verified for

conformance to the

specification requirements.

Remove ash from all liquid

conveyance pipes.

There are several causes of bare spots. These include:Overdrying. If the time between fluxing and hot dip galvanizingis prolonged or the drying temperature is too high, the barrierprotection provided by the flux may be lost. This is indicatedby a rusty appearance on the ungalvanized article, which canresult in coating discontinuities after hot dip g alvanizing. G

Excess Aluminium. A condition sometimes referred to as black -spots may occur if the aluminium content of a bath becomes toohigh. No trouble should be experienced if flux concentration iscorrect and the aluminium content of the bath is maintainedbelow approximately 0,007%. GFurther causes are:Blowouts; flux deposits; stains and inclusions; mechanical damage; touchmarks; uncoated surfaces caused by - surface contaminants, scale or sand;

welds and weld spatter.See SC Nos 6, 16, 19, 30, 33, 34, 35 and 37

Components should be checkedfor distortion caused duringrolling or fabrication. Check forappropriate vent, fill and drainageholes: removal of weld slag andspatter; venting of overlappingsurfaces; unsuitable joiningmaterials; temporary identificationmarkings; clearance for movingparts and potential distortion dueto the process.

Insufficient inspection prior tohot dip galvanizing can be the causeof dispute.

G

3 BARE SPOTS. A / RAlthough excluded fromSANS 121 / ISO 1461,bare spots of about 5mm 2

(2,2 x2,2mm), due tosmall localised flaws,

are adequately protectedby the sacrificial propertiesof zinc and will have verylittle effect on the servicelife of the coating. Wherenecessary, such spots maybe repaired using one of thespecified repair methods.Gross uncoated areasare a cause for rejec- REPtion. See Coating IfRepair Procedures. necessary


11/24

9HDGASA 2008

SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP

5 BLASTING DAMAGE. RSweep blasting, donecorrectly, substantiallyincreases paintadhesion and finalcoating appearance butdone incorrectly canresult in coatingdamage.

6 BLOWOUTS. AStaining and coatingdefects aroundunsealed weld areasand vent holes. Similarto stains caused by

weeping . See SC 28.

C / REPIf

necessary

7 CLOGGED HOLES . AZinc film clogging orpartly bridging holes.

CIf

required

8 CLOGGED THREADS. R

Threaded componentsor attachments havethreads clogged withzinc

C / REP

9 DESIGN STRUCTURES AIN MODULAR LENGTHSAND OPTIMUMWIDTHS TO SUIT

AVAILABLE BATHSIZES.

C / REP

10 DISCOLOURATION AAFTER HOT DIPGALVANIZING CAUSEDBY GRINDING OROTHER RESIDUES.

CIf

possible

Incorrect nozzle pressure;nozzle angle; sweepingdistance; size of abrasive andrecycling of grit.

A hot dip galvanized coating willbe partially or fully destroyedby excessive blasting. Refer tothe HDGASA Code of Practice.

D/B

Pre-treatment chemicalspenetrating sealed overlapareas through the required ventholes and escaping duringimmersion in the molten zinc.

This effect tends to damage theflux coating, causing localiseduncoated areas.

Pre-heat item prior toimmersion in zinc bath to dryout overlap area as much aspossible.

G/D

Molten zinc has a high surfacetension and will not easily drainfrom holes under 8mm indiameter.

Make holes as large as possible.Removal of molten zinc overthe bath and utilization ofvibrators will reduce thelikelihood of clogging.

D/G

Insufficient centrifuging or poor

drainage of threaded attachmentson withdrawal from the galvanizingbath.

The correct centrifuging equipment

or post galvanizing thread cleaningby heating and wire brushing oroversize tapping of nuts, willgenerally remove clogging. Ifnecessary specify delivery of boltsand nuts in nutted up form.

G

Double dipping can sometimes beused to hot dip galvanizefabrications that are too long or

wide for single immersion. If the

fabrication exceeds the bath size,members in the fabrication mayrequire touching up.

Touch up and repair can be avoided,the cost of hot dip galvanizingreduced and the overall qualityimproved, if the design of the

component is restricted to the length,width and depth of the galvanizingbath.

D/B

Material stored in contact with rustysteel, or iron filings can causesurface rust staining.

Once the cause has been removedthe stains will gradually disappear.

B


12/24

10 HDGASA 2008

12 DRAINAGE SPIKES. A / NSpikes and teardropsof zinc often appearalong the edge of acomponent after hotdip galvanizing.

C / REPIf

possible

The hot dip galvanizing processoccurs at a molten zinc temperatureof 450 deg C. This is at the lowerend of the stress relievingtemperature for treating steel. Thus,any inherent rolling or weldingstresses in the fabrication, are likelyto be released. This may result in adimensional change, i.e. distortion.

11 DISTORTION. A / NDistortion is theunwanted warpingthat occasionallybecomes evident afterhot dip galvanizing

REPIf

D/G possible

The edge most likely to have thesespikes is the last to leave the bath on

withdrawal. This applies particularlyto complex fabrications.

Drainage spikes are easily removedat the bath while still molten but

with complex fabrications, thesolidified spikes will be removed bycareful fettling by the galvanizerprior to inspection.

G


Use symmetrical designs; Use sectionsof similar thickness; Stiffen unsupportedthin wall sections; Use preformed mem-bers with the correct minimum bendradii; Use balanced or staggered weld-ing techniques; Make use of tempo-rary braces on thin walled sections suchas troughs, cylinders and angle frames.Avoid quenching after galvanizing.Components can be straightened afterhot dip galvanizing.

14 ENTRAPMENT OF ASH. REP

Ash which has not been (if smallremoved from the surface uncoatedof the molten zinc prior to areaimmersion of steel can be Rtrapped on the steel (if large,surface as it is immersed negotiable)and result in an uncoatedsurface beneath thetrapped ash.

Inadequate skimming of ash from

the molten zinc surface prior todipping.

On removal of entrapped ash, small

uncoated surfaces shall be repairedaccording to the requirements ofSANS 121 / ISO 1461. Large defectsare a cause for rejection and requirestripping and re-galvanizing.

G

13 DULL GREY OR AMOTTLED COATINGAPPEARANCE.Dull grey or mottledcoatings can appear asa dark grey circularpatter, a localised dullpatch or, may extendover the entire surfaceof the component.

This appearance indicates thepresence of extensive iron/zinc alloyphase growth, caused by steels withhigh reactive levels of Silicon andPhosphorous in steels.

Although not as aestheticallypleasing as a coating with free zincon the surface, a dull grey coatingprovides similar or better corrosionprotection.

D/S

15 FLAKING OR R / NDELAMINATION OFCOATINGNo adhesion of zinc to

steel surface. Thick,rough coating.

High phosphorous content (greater)than 0,02% can cause the entirecoating to delaminate from the steel.

Use a steel that has a phosphorouscontent of lower than 0,02%.

D / S

16 FLUX DEPOSITS, STAINS AAND INCLUSIONSFlux deposits or stains fromthe galvanizing process mayadhere to the steel orbecome included in thecoating. Flux residues are

black, brown, grey oryellowish non-metallic C / REPdeposits consisting mainly Ifof ammonium chloride. necessary

Flux deposits or stains may occur as aresult of excessive "dusting" withammonium chloride on withdrawalfrom the molten zinc. Flux inclusionscan occur when a surface flux blanketis applied to the zinc surface (wetgalvanizing). Flux blankets are

normally only used for specialisedprocesses, e.g. galvanizing of tubesand fasteners.

Flux deposits or stains should beremoved and the underlying coatingmeasured to determine whether itconforms to the minimumrequirements of the specification.

G


13/24

11HDGASA 2008

Inadequate repair of a damagedsurface on the hot dip galvanizedcoating prior to the application of apaint coating.

Make use of the correct repairmaterials and application procedures

when touching up cut or welded hotdip galvanized components.See Coating Repair Procedures.

B

R

REP

The use of chains, wire ropes,dragging or dropping of thecomponent onto a hard surface, cancause mechanical damage. This isparticularly relevant with thickbrittle coatings.

Warning labels, highlighting a thickcoating and possible damage ifmanhandled, should be attached bythe galvanizer, before thecomponent is transported. The use ofnylon lifting slings is recommended.

G / B

No matter how the zinc coating isapplied, the coating life isproportional to its thickness in agiven environment. Oftenelectroplated fasteners withinsufficient coating thickness areincorrectly used in externalenvironments.

Specify hot dip galvanized fastenersto SANS 121 / ISO 1461, whererequired. Alternatively overcoatfastener with an approved zinc richpaint or epoxy. See Coating RepairProcedures.

D / B

Dross pimples result from agitationof the dross layer at the bottom ofthe bath or from dragging materialthrough the dross layer. They appearas small, hard lumps on anotherwise normal galvanizedsurface. Blisters may be formed by

hydrogen, which is absorbed duringpickling and diffused at galvanizingtemperatures.

Due to the shape and / or drainageconditions of some components, thehoist crane has stopped and startedupon withdrawal of the items from

the molten zinc.

No effect on corrosion resistance.The overall appearance becomesuniform in time.

G

The galvanizer should avoiddisturbing the dross layer at thebottom of the bath,by controllingimmersion depth and drossingregularly. Since dross pimplesrepresent minor disturbances incoating uniformity, they do not

affect corrosion resistance.

G

This difference in coating thickness,is brought about by a combinationof a more reactive silicon killedsteel, and/or high phosphorousresulting in a thicker coating and aless reactive aluminium killed steel,resulting in a coating thicknesssometimes below that required inthe specification. Should thegalvanizer be asked to regalvanizein accordance with the specification,the resultant coating thickness onthe reactive steel will be excessivelythick, resulting in a brittle coatingmore susceptible to damage.

Select the same steel for fabricatinga component. If need be, accept aconcession request by the galvanizer,

when the thinner coating is possiblybelow specification.

D / B

A / NAcceptCon-

cessionRequest

R

REPIf

accep-table


17 DISCOLOURATION OFTHE PAINT COATINGOVER HOT DIPGALVANIZING AFTEREXPOSURE TOTHE ENVIRONMENT

18 COATING THICKNESSPROVIDED ONFASTENERS USED TOASSEMBLE HOT DIPGALVANIZEDSTRUCTURES

19 MECHANICAL DAMAGE. AMechanical handling ortransport damage canoccur, particularly withextremely thickcoatings, which tend tobe brittle in nature.

REPIf

necessary

20 OXIDE L INES. ALight aluminium oxidefilm lines on a hot dipgalvanized surface.

21 PIMPLES OR BLISTERS. APimples or blistersformed during hot dipgalvanizing are usuallyassociated with surfaceimperfections such asdross inclusions.

CIf

necessary

22 REACTIVE AND NON-REACTIVE STEELS,WELDED TOGETHER.Variations in coatingthicknesses can arise

when reactive andnon-reactive steels are

welded together.Efforts to increasecoating thickness onthe less reactive steel

may result in anundesirably thick andbrittle coating on themost reactive steel.


14/24

HDGASA 200812

The rougher surface will produce athicker coating and result in alonger service life.

S

Rough surfaces, typical of coatingson corroded steel surfaces, can behot dip galvanized satisfactorily. Thecoating will, however, reflect thetexture of the substrate. Othercauses of rough surfaces includeuneven cold working, overpickling, ahigh galvanizing temperature and /or extended immersion in themolten zinc.

The thicker coating produced will

provide greater corrosion protection.Except when the coating tends toflake off or delaminate see SC 15.

S

Rough, heavy coatings refer to hot

dip galvanized components showingmarkedly rough surfaces. This caninclude coatings that have agenerally rough surface and, in somecases, groove type surfaceconfigurations, tree bark effectcaused by variations in surface steelanalysis.

Provided the steel / casting surfaceis reasonably smooth, correctlycentrifuged articles will provide anacceptable finish.

G

Efficient centrifuging, will generallyremove excess zinc and provide asmooth and attractive exterior.

The stains can be easily removed bymeans of bristle brushing. Shouldthe component be destined for acorrosive area, the crevice should besealed with a sealant after cleaning.

D/B

The salts from acid or flux that havepenetrated porous welding orbetween contact surfaces duringpickling can weep after hot dipgalvanizing and water quenching,producing a stainedarea.

A

A / R

R

C / REPIf

accept-able

A

C / REPIf

necessary


Steel may occasionally includelaminations, laps, folds and non-metallic impurities, which result inslivers rolled into the metal surface.Defects of this type are sometimesdetected before or after pickling,but may only become apparentafter hot dip galvanizing.

Surface flaws in the base materialmay be removed by local grindingafter hot dip galvanizing followed byrepair of the affected surface. Minorsurface defects will not adverselyinfluence coating life.

S

A

REPIf

necessary

24 ROLLING DEFECTSIN STEEL.

These defects may bebroadly classified assurface discontinuitiesin the steel that havebeen elongated duringrolling.

Excessive cleaning of the coating,particularly the edges, by mechanicalmethods,can result in uncoatedareas.

The affected areas usually onlyappear after the component isinstalled. Care should be exercisedby the galvanizer to avoid overcleaning.

G / B

23 REMOVAL OF ZINC RCOATING BYEXCESSIVE CLEANINGUnless otherwiseagreed, the galvanizer

will limit cleaning of REPthe final coating by At themechanical means to galvanizer

that required in the orspecification. alterna-

tivelyat site

25 ROUGH COATINGS,CAUSED BY STEELSURFACE CONDITIONS

26 ROUGH HEAVY

COATINGS, CAUSEDBY A ROUGH SURFACEAND / OR THECHEMICALCOMPOSITION OFTHE STEEL.TREE BARK EFFECT

27 ROUGH HEAVYCOATINGS CAUSED BYINSUFFICIENTCENTRIFUGING

28 STAINS CAUSED BYWEEPING.


15/24

HDGASA 2008 13

Ensure all paint or grease isremoved prior to hot dipgalvanizing. Make use of suitablemarking pens for temporary

identification. Correctly positionadequately sized vent holes.

B

Residues, such as oil based paint,grease, oil or labels on the steelsurface or incorrectly positioned ventholes, can result in localised

ungalvanized areas in an otherwisecontinuous galvanized coating.Defects after galvanizing can vary incolour from grey black to brown

while no galvanized coating hasbeen formed.

33 UNCOATED SURFACESCAUSED BY STEELSURFACECONTAMINANTS OR

ENTRAPPED AIR.

A / N /R

REPIf

necessary

These ungalvanized areas may occurin a linear pattern on angles,channels or other rolled products.

They can also appear on cast ironproducts.

S / G

Sand on cast iron or scale on thesteel surface is generally caused bythe process used to form or roll theproduct. A localised ungalvanizedarea in an otherwise continuouscoating can occur if scale or sandfrom the moulding or rolling is not

removed by acid pickling or abrasiveblasting.

34 UNGALVANIZEDSURFACES CAUSED BYSCALE OR SAND.

R / N

REPIf

accept-able


Minimise contact betweencomponents and jig connections.(Loosen jigging wire). Smallcomponents can be centrifuged.

G

Articles entering the galvanizingbath should not be in tight contact

with each other. Jigging wire shouldalso be loosely attached toeliminate wire marks. Where acomponent has been resting onjigging or dipping equipment, anuncoated area or touch mark couldappear.

Small additions of aluminium to themolten zinc, brightens the coating.

S / G

ASurface appearances may varyaccording to the chemicalcomposition of the steel. Coolingrate has a direct effect on thesurface brightness and spangle size.Faster cooling usually results in abrighter coating with a smallerspangle size.

A

REPIf

necessaryand

accept-able

Although not particularly attractive,

this condition does not adverselyaffect coating performance.Protuberances and lumps, whichinterfere with mating surfaces areunacceptable.

G

This condition can occur over the

entire surface or in isolated areas.Uneven drainage also includes dripson the ends of parts, runs nearholes. The cause is withdrawal speedtoo high or low ga lvanizingtemperature.

A

CIf

necessary

32 UNEVEN DRAINAGE.

Uneven drainage resultsin an uneven or lumpyarea on which zinc buildup has occurred.

31 TYPICAL SPANGLEDHOT DIP GALVANIZEDCOATING.A typical hot dip galvanizedsurface is shown in theexample. The surface issilver grey in colour andnot necessary but often hasa spangled effect (zinccrystals) in a range ofsizes.

The galvanizer should ensure all zinchas been removed from the inside ofthe pipe by longer immersion times.

G

Heavy walls and thick flanges usedin the manufacture of piping can actas a heat sink when immersed inmolten zinc. This effect considerablylengthens the immersion time.Occasionally the galvanizer willremove the pipes before all the zinchas melted from the inside of thepipe.

29 TIGHTLY ADHERENTLUMPS OF ZINC ONTHE INSIDE OF HEAVYWALLED STEEL PIPING

R

C / REPIf

accept-able

30 TOUCH MARKS.The zinc in the galvanizingbath should have freeaccess to all componentsurfaces or small uncoatedor damaged areas canresult.


16/24

HDGASA 200814

Prepare surface for repair byroughening the surface by sweepblasting or some other approvedmethod. Loosely applied zinc metal

sprayed coating at the perimeter ofthe repair should be removed by

wire brushing. If not removed, thereis no compromise in the corrosionresistance.

G / B

In order for zinc metal spraying toadhere on application, the damagedgalvanized surface must beadequately roughened by sweep

blasting or other approved methods.As it is difficult not to overspray,excess zinc metal spray looselyadheres to the surrounding coating.

39 ZINC METAL SPRAYEDREPAIR APPLIED TOINADEQUATELYBLASTED SURFACES

OR NOT WIREBRUSHED AFTERAPPLICATION.

A

C

The loosely adherent zinc splashesare easily removed. An experiencedgalvanizer can ensure the coatingoverlap on double end dippedsurface, is not visible.

G

When hot dip galvanizing anunusually deep fabrication by doubledipping, moisture on the surface ofthe steel contacts with the moltenzinc causing splashes of zinc toloosely adhere to the already hot dipgalvanized surface.

40 ZINC SPLATTER.Splashes and flakes ofloosely adherent zinc,caused by moisture onthe steel surface whenhot dip galvanizing.

A

C


Weld slag deposits should beremoved by the fabricator by meansof abrasive blast cleaning. Thedeposit can also be removed byproper chipping or wire brushing.Shielded arc welding as opposed tostick welding is preferred forcomponents which are to be hot dip

galvanized.

B

A localised ungalvanized area near aweld can be caused by weld slagdeposit, weld porosity or weldundercut. Oxide deposits andresidues from welding are resistantto normal pickling acids and must beremoved before the work is pickledand hot dip galvanized.

35 UNGALVANIZED AREAIN THE VICINITY OF AWELD

Coating repair can be done by zincmetal spraying or a zinc rich paint orepoxy, providing the productconforms to the requirements of thespecification. (See Coating RepairProcedures).

D / B

Conventional drilling and boltingafter hot dip galvanizing ispreferred. Should welding or a non-conventional method of fixing beused, resulting in damage to thecoating, an approved repair methodis necessary.

36 USE CONVENTIONALFIXING METHODSSUCH AS BOLTS ANDNUTS, OR REPAIRDAMAGED COATINGSCAUSED BY WELDINGOR NON-CONVENTIONALFIXING METHODS.

R / N

REPIf

necessary

A

REP

Loosely adherent weld spatter shouldbe removed prior to hot dipgalvanizing. Although not acceptablein terms of the specification thepresence of tightly adherent weldspatter after hot dip galvanizing willnot affect the corrosion resistantproperties of the coating.

B

Weld spatter is caused by weld poolexplosions when improper weldingparameters are used, or if thematerial is dirty or contaminated.

37 WELD SPATTER.Weld spatter is oxidised,normally spherical expelled

weld metal, that is fused ornot onto the surroundingmaterial during welding.

A / N

Wet storage stain ceases when the

cause is eliminated. If the coatingthickness at the affected area is equalto, or greater than the minimumrequired in the specification, it is not acause for rejection, other than foraesthetic reasons. The latter is subjectto discussion with the end user prior togalvanizing. Customer is to exercisecaution during transport and storage.

G / B

Wet storage stain (zinc hydroxide) is

formed on freshly galvanizedsurfaces which are in close contact inpresence of moisture. Freshlygalvanized coatings react with theenvironment until such time as astable zinc carbonate film is formedon the coating surface.

38 WET STORAGE STAIN

OR WHITE RUST.Wet storage stain or

white rust as it iscommonly called, is a

white voluminousdeposit that isoccasionally found onthe surface of a freshlygalvanized coating.

A

CIf

necessary


17/24

Reference AreasThe number and position of refer-

ence areas and their sizes for the

electro-magnetic test method

shall be chosen with regard to

the shape, varying material thick-

ness and size of the article/s inorder to obtain a result as repre-

sentative as possible.

The number of reference areas,

dependant upon the size of the

individual articles in the control

sample, shall be as follows:

a) For articles with significant

surfaceB

area greater than

2m2, at least three reference

areas shall be taken on each

control sample.

b) For articles with a total signif-

icant surface area greater

than 2m2, manufactured from

material of several different

thickness, at least three ref-

erence areas shall be taken

on each specific material

thickness.

c) For articles with signifi-

cant surface area between1 000mm2 and up to and in-

cluding 2m2 inclusive, each

article in the control sample

shall have at least one refer-

ence area.

d) For articles with a total signif-

icant surface area between

1 000mm2 and up to and

including 2m2 inclusive, man-

ufactured from material of

several different thickness,each different material thick-

ness, representing the con-

trol sample shall have at least

one reference area, per mate-

rial thickness.

e) For articles with less than

1 000mm2 of significant sur-

face area, enough articles shall

be grouped together to pro-

vide at least 1 000mm2 surface

for an individual reference

area. The number of reference

areas shall be as given in the

last column of table 1.

Hence, the total number of

articles tested equals the

number of articles required to

provide one reference area

multiplied by the appropriate

number from the last column

oftable 1 related to the size

of the lot (or the total number

of articles galvanized if that is

less). Alternatively, sampling

pro-cedures selected from

SANS 2859-1 / ISO 2859-1

shall be used.

Note: 2m2 is typically

200 cm x 100 cm and1 000 mm2 is typically

10 cm x 1 cm.

For case a) and b) on each article

taken separately in the control

sample, the local coating thick-

ness within the reference area

shall be equal to or greater than

the mean coating thickness val-

ues given intable 2 or 3 or the

local coating thickness values

given intable 4.

In cases c) to e) the coating thick-

ness on each reference area shall

be equal to or greater than the

local coating thickness values

given intable 2, 3 or 4 as appro-

priate. The mean coating thick-

ness on all reference areas on

material of equal thickness in a

sample shall be equal to or

greater than the mean coating

thickness values given intable 2

or 3 as appropriate.

When more than five articles have

to be taken to make up

a reference area of at least

1 000 mm2, a single magnetic

measurement shall be taken on

each article if a suitable area of sig-

nificant surface exists: if not, the

gravimetric test shall be used.

Within each reference area,

which should be at least

1 000 mm2, a minimum of five

magnetic test readings shall be

taken. If any of the individual

readings is lower than the values

intable 2 or 3, this is irrelevant

as only the mean value over the

whole of each reference area is

required to be equal to or

greater than the local thickness

given in the table.

Thickness measurements shall not

be taken on cut surfaces or areas

less than 10mm from edges,

flame cut surfaces or corners.

UniformityTestingVariations in coating thickness

are not usually of concern withmaterials hot dip galvanized after

fabrication provided the mini-

mum coating thickness require-

ments are satisfied, (except for

fasteners - see note on maximum

coating thickness belowtable 3.)

Where the coating thickness is

not uniform, the service life of

the galvanized coating will gen-

erally be governed by the

amount of zinc available at the

place where the coating isthinnest rather than by the over-

all or average thickness of the

coating.

Standard magnetic measuring

instruments are quick and conve-

nient methods for determining

local coating thickness. By tak-

ing a number of readings on a

hot dip galvanized surface, uni-

formity as well as actual thick-

ness can be easily checked.

If small specimens are available,

it may be possible to use the

Preece test as a measure of coat-

ing uniformity. This test pro-

vides limited information as to

the coating thickness, but serves

only to locate the thinnest spot in

a zinc coating by chemically

removing the zinc.

There are many limitations on

the use of the Preece test; con-

sequently, it is no longer

referred to in specifications for

15HDGASA 2008

B

A significant surface can be

defined as a surface whichimpacts on the performance of

that article.


18/24

HDGASA 2008

standard adhesion tests must

take this into account. The

requirements for transportation,

handling and erection should be

evaluated against the additional

corrosion protection afforded by

the thicker coating.

Testing AdhesionTesting adhesion is not necessar-

ily a true measure of the adhe-

sive strength of the metallurgical

bonding of the hot dip galva-

nized coating to the base steel. It

serves, however, as an indicator

of the adhesive properties of the

coating.

Paring Test. This simple but effec-tive test is conducted by cutting or

prying the hot dip galvanized

coating with a sharp knife.

Considerable pressure is exerted

in a manner tending to remove a

portion of the coating. Adherence

is considered satisfactory when it

is possible to remove only small

particles of the coating.

It should not be possible to peel

any portion of the coating in theform of a layer so as to expose the

underlying iron or steel in advance

of the knife point. Although not

mentioned in SANS 121 / ISO

1461, this test has shown practical

significance as a test for adhesion.

For compliance with SANS 32 /

EN 10240, the most popular test

is cold flattening in accordance

with SANS 8492 / ISO 8492. Test

pieces not less than 40mm in

length are flattened between par-

allel flat platens as shown in

table 6. No cracking or flaking of

the coating shall occur on the sur-

face away from the cut surface.

AppearanceThe ability of a hot dip galva-

nized coating to meet its pri-

mary objective of providing

corrosion protection should be

the chief criteria in evaluating

the coatings acceptability.

The basic finish requirements of

the hot dip galvanized coating

are that it be:- relatively smooth,

- continuous,

- free from gross imperfec-

tions,

- free from sharp points (that

can cause injury), and

- free from uncoated areasC

.

The above points are of particu-

lar importance when a subse-

quent organic coating (duplex

coating) is to be applied.Smoothness and lack of rough-

ness achieved by mechanically

wiped products, such as contin-

uously galvanized sheeting or

wire, are not to be used as

the criteria for accessing batch

hot dip galvanized products.

Roughness and smoothness are

relative terms where the end

use of the product must be the

determining factor in setting

tolerances.

The hot dip galvanized coating

should be continuous to provide

optimum corrosion protection.

16

hot dip galvanizing after fabrica-

tion. When this test is applied,

its scope and limitations should

be well understood and no

attempt should be made to

draw undue inference from the

test results.

Adhesionof the CoatingAdhesion is concerned with the

practical conditions of transporta-

tion, erection and service. The

hot dip galvanized coating should

be sufficiently adherent to with-

stand handling consistent with the

nature and the thickness of the

coating in normal use of the arti-cle, without peeling or flaking.

Bending or forming, other than

mild straightening after hot dip

galvanizing, is not considered to

be normal handling.

When certain grades of steel or

very heavy steel sections are hot

dip galvanized, coatings may

occur, which are thicker than

usual. The galvanizer has limited

control over the development ofthicker coatings. Thick coatings

are a function of the chemical

composition of the steel or the

longer immersion time required

for massive items. Heavy hot dip

galvanized coatings, usually

greater than 250mm thick, may

be more brittle than a typical

coating. Consequently, applica-

tion and interpretation of the

TABLE 6.DEGREE OF FLATTENING FOR TESTING COATING ADHERENCE

FOR TUBES

Tube type Distance between platens

Square 75% of side

Rectangular tube 75% of shorter side

Round 21.3mm 85% of outside diameter

Round > 21.3 48.3mm 80% of outside diameter



Round > 114.3mm 65% of outside diameter

C

To be essentially free from

uncoated areas is best described

in SABS 7634.3.2 b). The area of

an individual bare spot or thin

area shall not exceed 5mm2. The

combined area of bare spots or

thin areas shall not exceed

25mm2

per metre of length or persquare metre of surface of an arti-

cle.


19/24

Handling techniques for hot dip

galvanized articles may require

the use of chain slings, wire or

other holding devices to immerse

the material into the galvanizing

bath if suitable lifting fixtures are

not provided on the item.

Chains, wire and special jigs used

to handle the items may leave a

mark on the hot dip galvanized

item. These marks are not always

detrimental and reason for rejec-

tion. Should these marks, be

greater than 5mm2

per chain

mark and expose the bare steel,

suitable repair should be carried

out using the procedures indicat-

ed in SANS 121 / ISO 1461. See

also Coating Repair Procedures.

Differences in the lustre andcolour of hot dip galvanized

coatings do not affect corrosion

resistance and the presence or

absence of spangle has no

effect on coating performance.

The well-known spangled effect

found on some hot dip galva-

nized surfaces is simply a factor

of primary crystallisation. It is

chiefly dependant upon the zinc

bath chemistry, the rate of cool-

ing, the method of pickling, thesteel chemistry, and the thick-

ness of the coating. In fact, dull

grey or patchy matte grey

hot dip galvanized coatings

give service lives equal to or

greater than bright or spangled

coatings. Variations in coating

appearance or finish are impor-

tant only to the extent that they

wil l af fect corrosion per for-

mance or the intended use of

the article. The primary func-

tion of a hot dip galvanized

coating is corrosion protection.

Specific requirements beyond

the standard set out in SANS

121 / ISO 1461, shall be com-

municated to the galvanizer in

writing or discussed at the con-

tract review, prior to the work

being hot dip galvanized.

The information supplied may

include

a) The composition and any

properties of the metal that

may affect the quality of the

hot dip galvanized coating.

b) Identification of significant

surfaces. See page 15B

for

definition.

c) A visual standard should be

established if a special finish

is required.

d) Any particular treatments

that are required or not

required before or after the

process.

e) A variance in coating thick-

ness. See notes belowtable 3.

f) The acceptable method of

repair, if this is found to be

necessary see also Coating

Repair Procedures.

Removal of WetStorage Stain(White Rust)Although in extreme cases the

protective value of the coatingmay be impaired, wet storage

stain attack is often superficial

despite the relative bulkiness of

the corrosion product. Where

surface staining is light and

smooth without growth of the

zinc oxide layer as judged by

lightly rubbing fingertips across

the surface, the staining will

gradually disappear and blend

in with the surrounding zinc

surface as a result of normal

weathering in service.

When the affected area will not

be fully exposed in service or

when it will be subjected to a

humid environment, wet storage

staining must be removed, even

if it is superficial. This is essential

for the basic zinc carbonate film

to form. The formation of this

zinc carbonate film is necessary

to ensure long term service life.

Light deposits can be removed by

cleaning with a stiff bristle (not

wire) brush. Heavier deposits can

be removed by brushing with a

5% solution of sodium or potassi-

um dichromate with the addition

of 0.1% by volume of concentrat-

ed sulphuric acid. Alternatively, a

10% solution of acetic acid can be

used. These solutions are applied

with a stiff brush and left for about

30 seconds before thoroughly

rinsing and drying.

Unless present prior to ship-

ment from the galvanizer, the

development of wet storage

stain is not the responsibility of

the galvanizer. The customer

must exercise proper caution

during transportation and stor-

age to protect against wet stor-

age staining.

Coating RepairProcedures

BY THE GALVANIZER

In terms of SANS 121 / ISO 1461

a galvanizer may repair a coating

by either zinc metal spraying or

zinc rich epoxy or paint. The latter

method must conform to certainrequirements in the specification.

The preferred method of repair is

by zinc metal spraying. Repair will

only be necessary if bare spots are

present, usually caused by inade-

quate cleaning, air entrapment

or if mechanical damage has

occurred.

The total uncoated areas for

renovation by the galvanizer

shall not exceed 0,5% of thetotal area of the component.

For articles equal to an area of

2m2; 0,5% represents a maxi-

mum area of 100cm2 or 100mm

x 100mm.

For articles equal to an area of

10 000mm2; 0,5% represents a

maximum area of 50mm2 or

7mm x 7mm.

No individual repair area shall

exceed 10cm2 or 10mm x

100mm.

17HDGASA 2008


20/24

If uncoated areas are greater

than 0,5%, the article shall be

regalvanized, unless otherwise

agreed between the purchaser

and the galvanizer.

Zinc Metal Thermal Sprayed

Coatings

Method

The damaged area is to be lightly

blasted using preferably a pencil

blasting nozzle or the surround-

ing coating should be masked in

order to limit damage.

A zinc metal sprayed coating is

then applied to the abrasively

blasted surface to a thickness at

least 30m greater than the

minimum specified zinc coatingthickness, or equal to the sur-

rounding coating thickness,

whichever is the greater. The

coating repair should overlap

the surrounding zinc by about

5mm. The repaired area is then

wire brushed (preferably stain-

less steel) to remove loosely

adhering over sprayed zinc.

Wire brushing provides the

added benefit of sealing the

pores that may be present in thesprayed coating.

Zinc Rich Epoxy or Zinc Rich

Paint

Method

The defective area shal l be blast-

ed as above or abraded with

abrasive paper (roughness 80

grit). All dust and debris must

be completely removed. In the

event of moisture being pre-

sent, all surfaces are to be prop-

erly dried.

A zinc rich paint or epoxy con-

taining not less than 80% of zinc

in the dry film (53% by volume),

should be applied to a thick-

ness, 30m greater than the

minimum specified for the gal-

vanized coating or equal to that

of the surrounding galvanized

coating, whichever is greater.

The paint coating should over-

lap the surrounding zinc by

about 5mm.

The preferred product is a two

or three component zinc rich

epoxy.

SITE REPAIRS

The preferred method of repair is

by zinc metal thermal spraying.

Due to the remoteness of mostsites, however, and the unavail-

ability of metal spraying equip-

ment, repairs by zinc rich epoxy

or zinc rich paint have to date

generally been more popular.

Site repairs should be limited to

small coating defects and areas

that have been cut or welded

on site.

Should excessive amounts of

grease or oil be present at the

affected area, it should be

removed by means of an

approved solvent. All residues

are to be thoroughly removed

by washing with clean water.

The affected area should then be

abraded with abrasive paper

(roughness 80 grit) or alterna-

tively thoroughly cleaned using,

preferably a stainless steel brush.

All dust and debris should becompletely removed.

Repair can now be carried out

using an approved product.

Single pack zinc rich paints are

good materials and can easily

be applied. They, however,

require several coats to achieve

a reasonable repair. Multiple

coats will also necessitate

longer drying times betweencoats.

Until recently, the approved

products for repair were only

available in large containers.

Due to the large quantities

involved and short pot life

when mixed, the products

proved to be expensive and

wasteful.

Products are now available in

two component form, packed

for convenience in handy, easy

to use squish packs. Two of

these products are approved

and available from the Hot Dip

Galvanizers Association of

Southern Africa and all of its

members.

18 HDGASA 2008


21/24

HDGASA 2008

APPENDIX A

Applicable Standards

SANS 121 / ISO 1461 Hot dip galvanized coatings on fabricated iron

and steel articles - Specifications and testmethods

SABS 763 Hot dip (galvanized) zinc coatings (other than

on continuously zinc coated sheet and wire).

SANS 32 / EN 10240 Internal and/or external protective coatings for

steel tubes Specification for hot dip galva-

nized coatings applied in automatic plants.

SANS 14713 / ISO 14713 Protection against corrosion of iron and steel in

structures Zinc and aluminium coatings

Guidelines.

SANS 2178 / ISO 2178 Non-magnetic coatings on magnetic substrates -

Measurement of coating thickness Magnetic

method.

ISO 1460 Metallic coatings Hot dip galvanized coatings

on ferrous materials Gravimetric determina-

tion of the mass per unit area.

SANS 8492 / ISO 8492 Metallic materials Tube Flattening test.

The Hot Dip Galvanizers Association of Southern Africa is indebted to the American Galvanizers Association

and Industrial Galvanizers Corporation (Pty) Ltd of Australia, for permission to include some of the contents

of their publication on inspection after hot dip galvanizing.

19


22/24

HDGASA 2008

The Hot Dip Galvanizers Association ofSouthern Africa

Introduction:

The Hot Dip Galvanizers Association of Southern Africa was founded in 1965 and

its membership represents over 80% of the available hot dip galvanizing

capacity of Southern Africa.

Charter Statement:

The primary role of the Hot Dip Galvanizers Association Southern Africa

(HDGASA) is to promote a higher level of acceptance of, and confidence in, hot

dip galvanized products and of ferings on a national basis in order to increase the

demand from potential customers.

The HDGASA is simultaneously an advisory body and independent authority,

representing the end-users, consumers and specifiers of hot dip galvanized steel

products in the corrosion control industry, ensuring quality standards, customer

satisfaction with the corrosion prevention system supplied by hot dip galvanizers

of Southern Africa. This includes duplex systems.

Strategic Goals:

The primary strategic marketing goal of the HDGASA is to grow market share,

to develop new applications and promote profitability of Association members

by creating a greater acceptability and demand for its products (all lines and

variations) from fabricators, specifiers and large corporate end-users.

The secondary strategic marketing goal is to increase awareness, acceptability,

demand and sustainability of hot dip galvanizing as a corrosion prevention

system thus creating a greater demand from fabricators, dealers, specifiers and

merchants aimed at growing the overall size of the market for all hot dip

galvanizers, as well as the steel and zinc industries of Southern Africa.

The tertiary strategic marketing goal of the Association is the establishment and

enhancement of liaison and networks with stakeholders and influencers of the

marketplace in order to ensure the sustainability of this system.

Objectives:

Promoting the use of hot dip galvanizing for cost effective corrosion

protection in applications where its use is appropriate.

Providing technical assistance and advice for specifiers, fabricators and end

users while also recommending alternative protective methods where

appropriate.

Identifying and investigating potential new applications for hot dip

galvanizing.

Participating in development projects on behalf of industry by providing

assistance in the form of technical consulting, practical recommendations

and assistance with the preparation of design specifications.

Providing assistance with quality control during fabrication and hot dip

galvanizing.

Disseminating technical knowledge by providing a consulting and training

service as well as the publication of technical literature.

Providing training and education for member companies to ensure a high

standard of quality and service throughout the hot dip galvanizing industry.

20

Publications

Available fromthe Association

Steel Protection by Hot Dip

Galvanizing and Duplex

Systems.

Practical Guidelines for the

Inspection and Repair of Hot

Dip Galvanized Coatings.

Specification for the

Performance Requirements

of Duplex Coating Systems.

Code of Practice for Surface

Preparation and Application

of Organic Coatings Applied

to New Unweathered Hot

Dip Galvanized Steel.

Wall Chart - Design for Hot

Dip Galvanizing.

Information CD.

Facts About Hot DipGalvanizing Practical

Guidelines.

HOT DIP GALVANIZING HAS BEEN USED TO PROTECT STEEL FROM CORROSIONFOR MORE THAN 170 YEARS. APPLICATIONS FOR WHICH HOT DIP

GALVANIZING IS SUITABLE ARE NUMEROUS AND VARIED AND THE DEMAND

CONTINUES TO INCREASE.


23/24

RELIABILITYThe hot dip galvanized coating is for med by a metallurgical reaction

between suitably cleaned steel and molten zinc. This results in the

formation of a series of iron/zinc alloys which are overcoated with

relatively pure zinc. The process entails total immersion of

components in both pretreatment chemicals and molten zinc. This

ensures uniform protection and coating reliability even on surfaceswhich would be inaccessible for coating by other methods.

DEPENDABILITYEase of inspection and dependability in service are beneficial featuresof a hot dip galvanized coating. The cathodic protection of steel by

zinc ensures that corrosion of the underlying steel cannot occur as

long as zinc is present. Even at discontinuities on the coating,

corrosion creep under the surrounding zinc is not possible.

PREDICTABILITYThe durabil ity of a hot dip galvanized coating is determined by the

degree of corrosion of zinc in a specific environment and the

thickness of the coating. Corrosion of zinc is normally uniform, thusdurability of a hot dip galvanized coating is predictable in most

applications.

ISSUE DATE: MARCH 2008


24/24

Documents

Practical Guidelines Inspect Repair HDG Coatings 2008