galvanizing inspection of structure
TRANSCRIPT
HOT DIPGALVANIZERSASSOCIATION
SOUTHERN AFRICA
TheAssociation is
a technicalinformation
centreestablished for the benefit of
specifiers,consultants,
end users andits 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
HOT DIP GALVANIZERS ASSOCIATION
SOUTHERN AFRICA
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
The Association is a technical information centre established for the benefit of specifiers, consultants, end users and its members
The copyright of all material in this catalogue and its supplements, particularly materialwhich is identified by the symbol ©, is expressly reserved. No part of this catalogue may
be reproduced or distributed in any form or by any means, without the prior writtenpermission 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.
HDGASA © 2005
Foreword“Practical Guidelines for the Inspection and Repair of Hot Dip
Galvanized Coatings”, has been compiled with reference to the new
international specifications. These include:
SANS 121 / ISO 1461 Hot dip galvanized coatings on fabricated ironand steel articles – Specifications and test methods.
SANS 32 / EN 10240 Internal and/or external protective coatings forsteel tubes– Specification for hot dip galvanized coatings applied inautomatic plants.
SANS 14713 / ISO 14713 Protection against corrosion of iron and steelin structures – Zinc and aluminium coatings – Guidelines.
Although specifications SABS 763 and SABS 0214 have been replaced,
for practical reasons reference has also been made to:
SABS 763 Hot-dip (galvanized) zinc coatings (other than oncontinuously zinc coated sheet and wire).
The guidelines take into account recognised engineering principles,
inspection practices and practical experience accumulated over the
years by the Hot Dip Galvanizers Association of Southern Africa
established in 1965.
The guidelines have been formulated for general information only and
although practically enforceable, they are not intended as a substitute
for competent professional examination and verification as to accuracy,
suitability and/or applicability, by a trained HDGASA inspector.
The publication of the material contained in this booklet is not intended
as a representation or warranty on the part of the Hot Dip Galvanizers
Association of Southern Africa.
Anyone making use of this information assumes all liability arising from
such use.
Due to the frequent use of misleading terms such as “cold
galvanizing” to describe zinc rich paint and electrogalvanizing for
zinc electroplating, the specifier who requires hot dip galvanizing
for corrosion control, should specify that all components shall be
hot dip galvanized in accordance with the requirements of
SANS 121 / ISO 1461 or SANS 32 / EN 10240 as applicable.
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HDGASA © 2005
ContentsIntroduction .................................................................................... 3
Test Sampling.................................................................................. 3
Thickness and Uniformity ................................................................ 3
Thickness Testing ............................................................................ 5
Surface Conditions – Acceptance, Rejection and Responsibility ...... 8
Reference areas .............................................................................. 15
Uniformity Testing .......................................................................... 15
Adherence of the Coating .............................................................. 16
Testing Adherence .......................................................................... 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
IntroductionHot dip galvanizing is one of themost widely used methods ofprotecting steel from corrosion.
As a final step in the process, thehot dip galvanized coating isinspected for compliance withspecifications. Interpretation ofinspection results should bemade with a clear understandingof the causes of the various con-ditions which may be encoun-tered and their effects on the ulti-mate objective of providing cor-rosion protection.
This manual has been designed toassist in testing, inspection, andthe interpretation of test results. Itdeals with numerous surface con-ditions, their origins, and theireffects on protection from corro-sion. This manual also considersundesirable design and fabricatingfeatures as well as unacceptablehot dip galvanizing practice.Although it is difficult to coverevery condition, the manual cov-ers many of the conditions fre-quently encountered in practice.
To effectively use this manual,inspectors should remember thatthe main purpose of hot dip gal-vanizing is to protect steel fromcorrosion. The length of timethis protection can reasonably beexpected to last is called its “ser-vice life or time to first mainte-nance”. This is defined as thetime taken for the appearance onan article of 5% surface rust. Theservice life of a hot dip galva-nized coating is directly pro-
portional to the thickness ofthe coating. Corrosion protec-tion is greater when the coat-ing is thicker. Thus coatingthickness is the single mostimportant inspection check todetermine the quality of a hotdip galvanized coating.
Coating thickness, however, isonly one inspection aspect.Other features include the uni-formity of the coating, coatingadhesion and appearance.Possible brittleness and de-fects, which arise from incor-rect design and fabrication,also need to be assessed bythe inspectorate.
While minimum specified stan-dards must be satisfied, their rel-ative importance varies accord-ing to the end use of the prod-uct. For example, the aestheticappearance of hot dip galvanizedstructural steel in an obscuredapplication is different from thatwhere a product is used as anarchitectural feature. An aware-ness of the end use of the prod-uct and the capability of the hotdip galvanizing process is essen-tial for good inspection.
Inspection of hot dip galvanizedproducts, as the final step in theprocess, can be most effectivelyand efficiently conducted at thegalvanizer’s plant. Here, ques-tions can be raised and answeredquickly, inspection speeded upand, the time saved is beneficial.
Occasionally, due to rough han-dling and possible site modifica-
tions, which include cutting andwelding, coating repairs becomenecessary. Repair procedures andthe recommended repair materi-als are covered in this manual.
The quality standards to which ref-erence is made in the followingsections are in accordance withInternational and South AfricanNational Standards’ specifications.
Test SampleSelectionTo effectively evaluate hot dip gal-vanized coatings, it is essentialthat randomly selected specimensbe representative of an inspectionlot. An inspection lot is a singleorder or batch awaiting despatch.For products required to complywith SANS 121 / ISO 1461 theminimum number of articles fromeach inspection lot that forms thecontrol sample shall be in accor-dance with table 1 of the specifi-cation.
Typically, the test unit for compli-ance with SANS 32 / EN 10240shall be, for each size, one tubeper first 500 tubes and subse-quently one per 1000 tubes foroutside diameters greater than21.3mm and one per 2000 tubesfor smaller diameters.
Unless otherwise specified by thepurchaser, an acceptance inspec-tion should be arranged andundertaken before the productsleave the premises of the galva-nizer.
CoatingThickness andUniformityThe thickness of the hot dip gal-vanized coating is the primaryfactor in determining its lifeunder given service conditions.The thicker the coating, the bet-ter corrosion protection it offers.For most atmospheric conditions,
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TABLE 1
Number of articles in the lot Minimum number of articles in the control sample
1 to 3 All
4 to 500 3
501 to 1200 5
1201 to 3200 8
3201 to 10 000 13
> 10 000 20
the relationship between corro-sion protection and coating thick-ness is approximately linear, i.e.,the service life is doubled if thecoating thickness is doubled.There is however a limit to themaximum coating thickness thatcan be obtained.
The factors which influence over-all coating thickness are a combi-nation of several variables. Thegalvanizer can alter zinc temper-ature, time of immersion and rateof withdrawal from the moltenzinc. These factors can be used tomarginally alter coating thick-ness, particularly with reactivesteels. The formation of the pro-tective iron/zinc alloy layer is adiffusion process. As with all dif-fusion processes, the reactionproceeds rapidly at first andslows down as the layer becomesthicker. Higher bath temperaturesand longer immersion times willgenerally produce thicker alloylayers.
The thickness of the outer zinclayer of the coating is indepen-dent of immersion time.Thickness of this layer is deter-mined by the rate of withdrawalfrom the zinc bath and thedegree of drain-off of moltenzinc. A fast rate of withdrawalcauses an article to “drag out”more zinc. This results in a heav-ier coating, although the distribu-tion of the zinc layer may beuneven. Wiping of the coatingduring withdrawal materiallyreduces its thickness and thus itsprotective value. It is discour-aged except where necessary forthe smoothness demanded byconditions of service such as forsmall bore pipe and conduit.
Tubes hot dip galvanized inaccordance with SANS 32 / EN10240 utilise steam blowinginternally 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 inthe drain-off, because of theshape of the article and the angleat which different surfaces leavethe bath, may also result in somevariation in coating thickness.
Other factors influencing thecoating thickness may bebeyond the control of the galva-nizer.
The chemical composition of thesteel plays a major role in deter-mining the thickness andappearance of the final coating.Certain steel compositions tendto accelerate the growth of theiron/zinc alloy layer so that thehot dip galvanized coating mayhave a matte finish with little orno outer zinc layer. This coatingalso tends to be thicker than thetypical bright hot dip galvanizedcoating. The galvanizer’s controlover this condition is limited.Steels containing the elements,phosphorus in excess of 0.03%,or silicon from 0.05% to 0.12%and above 0.3% or combinationsof both elements, are particularlyprone to heavier coatings con-sisting mainly of iron/zinc alloys.Due to the iron/zinc alloygrowth in these coatings,extending to the outer surface,the final appearance of the coat-ing can be dark grey in colourdistributed evenly or unevenlyover the surface.
The surface condition of the steelbefore hot dip galvanizing alsoaffects the thickness andsmoothness of the final coating.Steels which have been abrasiveblast cleaned, or left to weatherand rust for some time prior tohot dip galvanizing, can producesubstantially thicker coatingsthan those normally producedon chemically cleaned steel.
The mass, shape and degree ofcold working of components to behot dip galvanized also influencescoating thickness and uniformity.When a fabricated article consistsof both heavy and light sections, a
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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 µm.(t) mm thickness thickness General
µm* µm* Application
t ≥ 6 70 85 A1, B1, D1, 85 E1, F1
3 ≤ t < 6 55 70 A2, B2, F2 65
1.5 ≤ t < 3 45 55
t < 1.5 35 45 A3 45 B3 55 F3 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.
PROF
ILES
CAST
INGS
variation in coating thicknessbetween the sections may result.Immersion time will vary accord-ing to the relationship of the sur-face area of an item to its mass.The galvanizer has little controlover this situation.
Combining heavy and light sec-tions in a single component mayalso result in unacceptable distor-tion (refer to HDGASA wall chart,“Design for Hot Dip Galvanizing”).
Since the time to the appearanceof first rusting of the base steel isusually determined by thethinnest portion of the coating,an evaluation of galvanizing qual-ity must take into account boththe minimum thickness of thecoating and its distribution.Specifications for hot dip galva-nizing recognise that variations incoating thickness are inherent inthe process. The minimum thick-ness is generally defined as anaverage or mean thickness of thecoating on specimens testedand/or a minimum thickness forany individual specimen.
When measurements are takento determine the uniformity andthickness of a hot dip galvanizedcoating, 5 or more coating thick-ness readings should be taken ineach reference area. Referenceareas should be taken approxi-mately 100mm from the ends ofthe article to avoid end effects.Usually the end of an articlewhich leaves the bath last willcarry a thicker coating. This isparticularly so towards the edge,where, at the time of drainage,the last few drops of zinc tend toagglomerate as a result of sur-face tension.
The minimum coating require-ments specified in SANS 121 /ISO 1461 for different materialthicknesses and classes of workare summarized in table 2 andtable 3. For comparitive purpos-es, these tables compare thick-ness equivalents to the old SABS763 specification. Table 4 indi-cates the minimum coatingrequirements specified in SANS32 / EN 10240 for differentclasses of coating.
Specifications do not stipulatemaximum upper coating thick-ness limits, but excessively thickcoatings on threaded articlesare undesirable. In order toensure effective tensioning, thecoating thickness on fastenersshould not exceed a maximumof 65µm, this applies particu-larly, to high strength bolts andnuts.
Variance in coating thickness.A requirement for a thicker coat-ing (25% greater than the stan-dard in table 2) can be request-ed for components not cen-trifuged, without affecting speci-fication conformity.
NOTE: Where steel compositiondoes not induce moderate tohigh reactivity, thicker coatingsare not always easily achieved.Thicker coatings are more resis-tant to severe environmentalconditions, but can be morebrittle and may require specialhandling. The efficacy of corro-sion protection of a hot dip gal-vanized coating (whether lightor dull grey) is approximatelyproportional to coating thick-ness.
Thickness TestingThere are several methods todetermine the thickness of thezinc coating on a hot dip galva-nized article. The size, shape andnumber of pieces to be tested,will most likely dictate the meth-ods of testing. The specified testmethods are either destructive ornon-destructive and are detailedin SANS 121 / ISO 1461.Identical methods are detailed inSANS 32 / EN 10240. The mostpractical tests are the non-destructive type, such as gaugesutilising the electromagneticprinciple.
1. Electromagnetic TestingMethod.Instruments, which rely onelectromagnetic principlesare probably the most widely
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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 µm.
(t) or (ø) mm thickness thickness General µm* µm* Application
ø ≥ 20 45 55 C1 55
6 ≤ ø < 20 35 45 C2 45
ø < 6 20 25
t ≥ 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 ≤ ø < 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.
FAST
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SOT
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ARTI
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(INCL
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used devices to determinethe thickness of hot dip gal-vanized coatings. Electro-magnetic instruments mea-sure coating thickness inspecifically-identified localareas of the article. The aver-age thickness (and thereforemass) of the coating is calcu-lated from measurementstaken at a suitably large num-ber of points distributed overa reference area on the sur-face of the article. There area number of electromagneticgauges which can be used togive accurate measurementsof the zinc coating thickness.These gauges give reliablethickness readings providedthey are properly calibratedand the manufacturer’sinstructions are observed.The most commonly usedtype of gauge is a portableelectronic instrument. Thegauge uses a temperature-compensated magnetic trans-
ducer to measure the mag-netic flux changes thatoccur when the probe (amagnet) is separated from aferrous metal substrate by anon-magnetic coating suchas zinc. The output signalfrom the probe is propor-tional to the distance ofseparation and, therefore,to coating thickness. Theprobe signal is amplifiedand indicated on a metercalibrated to show coatingthickness. These batterypowered instruments havetypical accuracies up to 5%and have the advantage ofnot requiring recalibrationwith probe orientation.Similar electromagneticinstruments with evengreater accuracies are avail-able for laboratory use.
The specification requiresthat the thickness gaugesshall (in terms of ISO 2178)be -
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TABLE 4
MINIMUM COATING THICKNESS ON STEEL TUBES TO SANS 32 / EN 10240
COATING QUALITY A1 A2 A3
Mandatory Minimum local coating thickness on the inside surface except at the weld bead 55µm 55µm 45µm
Minimum local coating thickness on the inside surface at the weldbead 28µm 1) 1)
Options Minimum local coating thickness on the outside surface 2) 2) 2)
COATING QUALITY B1 B2 B3
Mandatory Minimum local coating thickness on the outside surface 55µm 3) 40µm 25µm
1) This requirement does not apply
2) This requirement applies when the purchaser specifies Option 1
3) Option 3 specified (if >55µm 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.
AWhen curved surfaces are dealt
with, particular care should betaken to ensure that the thicknessgauge is capable of obtainingreadings to an acceptable level ofaccuracy.
� Capable of minimizingerrors in reading causedby the magnetic perme-ability, dimensions, sur-face finish and curvatureof the article being test-ed;
� Capable of measuring thethickness of the coatingbeing tested within anaccuracy of 10% or1.5µm whichever isbetter
A, and
� Calibrated by takingreadings at zero (or nearzero) thickness and atthicknesses of at least100µm, on suitable non-ferrous shims placed onacceptable standardpieces of metal of com-position, thickness andshape similar to those ofthe articles under test.
To avoid possible errors inthe use of magnetic instru-ments, certain precautionsshould be taken:
� Follow the manufacturer’sinstructions. The instru-ment should be frequent-ly recalibrated againstnon-magnetic film stan-dards of known thick-ness.
� The base steel should bebacked up with similarmaterial if thinner thanthe critical thickness forthe magnetic gauge orthe instrument should berecalibrated on a sub-strate of similar thick-ness.
� Readings should not betaken near an edge, hole,or inside corner.
HDGASA © 2005
� Readings should not betaken on curved surfaceswithout additional cali-bration of the instrument.
� The test surface shouldbe free from surface con-taminants such as dirt,grease, and corrosionproducts.
� Test points should betaken in each referencearea to avoid obviouspeaks or irregularities inthe coating.
� A sufficient number ofreadings should be takento obtain a true average.
2. Testing threaded articles byfitting mating parts.The zinc coating on externalthreads shall be free fromlumps and shall not havebeen subjected to a cutting,rolling or finishing operationthat could damage the zinccoating. The zinc coating ofan external standard metricthread that has not beenundercut shall be such as toenable the threaded part tofit an oversized tapped nut inaccordance with the allow-ances given in table 5.
On bolts greater than M24,undercutting of bolt threadsis frequently preferred tooversizing of nut threads.The allowance should beincreased to 0,4mm.
Threaded articles shall fittheir mating parts and, inthe case of assemblies thatcontain both externally andinternally threaded articles,
it shall be possible to screwmating parts together byhand.
3. The Chemical Stripping(Gravimetric) Test (to ISO1460). This method is appliedwhere material is inspectedafter hot dip galvanizing.Since this is a destructivetest method, it is generallynot suitable for the inspec-tion of large or heavy itemsunless smaller or represen-tative specimens can besubstituted for them (seeTest Sampling). The testspecimen is cleaned withnaphtha or other suitableorganic solvent, rinsed withalcohol, dried and weighed.It is then stripped of the zinccoating by immersion in asolution containing 3.5g ofhexamethylenetetramineand 500ml concentratedhydrochloric acid in 1litre ofwater. The stripping of thecoating is complete whenevolution of gas ceases.After washing and drying,the specimen is weighed;the dif ferences in massbefore and after strippingdivided by the surface areaof the test specimen givesthe mass of coating per unitarea. In the case of thread-ed articles, such as boltsand screws, the determina-tion is made on anunthreaded portion of thearticle.
The stripping test gives anaccurate average coatingmass of the zinc coating.However, it does not provideany information on howevenly the coating is distrib-uted.
To test compliance withSANS 32 / EN 10240, twotest pieces shall be takenfrom the tube to be tested.These should be between30 and 600cm2 in surfacearea, with length between
50 and 150mm taken atleast 600mm from the tubeend.
4. MetallographicExamination. Where the hot dip galvanizedcoating composition andthickness are of interest,microscopic examination is areliable tool. This test is arequirement for the testing ofcompliance of Coating A1 toSANS 32 / EN 10240. Thisvery accurate method uses asmall polished and etchedcross-section of the hot dipgalvanized component toprovide information aboutthe relative thicknesses of thealloy and the free zinc layerswhich comprise the hot dipgalvanized coating.
The following proceduresshould be adhered to –
� Water should not be usedas a lubricant at any stageduring the polishing pro-cedure due to staining ormild corrosion of the gal-vanized layer.
� The etchant should be 2%(max) Nital, i.e. (2ml con-centrated HNO3 in 100ml of 95% ethanol ormethanol).
Important disadvantages ofthis technique are that –
� Specimens cut from thehot dip galvanized articleare required,
� Coating thickness mea-sured only applies to avery limited area, it doesnot indicate the variationin coating distribution onthe article and,
� It is necessary to examinea number of specimens todetermine the averagecoating thickness on thehot dip galvanized article.
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TABLE 5
OVERSIZE TAPPING ALLOWANCE FOR HOT DIP GALVANIZED NUTS
Nominal size of Allowance thread (mm)
M8 to M12 0,33
M16 to M24 0,38
8 HDGASA © 2005
Surface Conditions (SC)LEGEND:
A - Accept R - Reject N - Negotiate C - Clean REP - RepairRESPONSIBILITY: 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 hot dip galvanizing is extremely 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 about0,15 to 0,3%.
Zinc oxide deposits can take place when the component is dipped or when it is removedfrom the bath.
The coating is normally intact underneath the ash deposits. Ash must be removed and thecoating thickness verified for conformance to the specification requirements.Remove ash from all liquidconveyance 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 barrier protection provided by the flux may be lost. This is indicated by a rusty appearance on the ungalvanized article, which can result in coating discontinuities after hot dip galvanizing. GExcess 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 maintained below 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 No’s 6, 16, 19, 30, 33, 34, 35 and 37
Components should be checkedfor distortion caused during rolling or fabrication. Check for appropriate vent, fill and drainageholes: removal of weld slag andspatter; venting of overlappingsurfaces; unsuitable joiningmaterials; temporary identificationmarkings; clearance for movingparts and potential distortion due to the process.
Insufficient inspection prior tohot dip galvanizing can be the causeof dispute.
G
3 BARE SPOTS. A / RAlthough excluded from SANS 121 / ISO 1461,bare spots of about 5mm2
(2,2 x2,2mm), due to small localised flaws, are adequately protected by the sacrificial properties of zinc and will have very little effect on the service life of the coating. Where necessary, such spots may be repaired using one of thespecified repair methods. Gross uncoated areas are a cause for rejec- REPtion. See Coating IfRepair Procedures. necessary
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SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP
5 BLASTING DAMAGE. RSweep blasting, done correctly, substantially increases paint adhesion and final coating appearance but done incorrectly can result in coating damage.
6 BLOWOUTS. AStaining and coating defects around unsealed weld areas and vent holes. Similarto stains caused byweeping . See SC 28.
C / REPIf
necessary
7 CLOGGED HOLES. AZinc film clogging or partly bridging holes.
CIf
required
8 CLOGGED THREADS. RThreaded components or attachments have threads clogged with zinc
C / REP
9 DESIGN STRUCTURES AIN MODULAR LENGTHSAND OPTIMUM WIDTHS TO SUIT AVAILABLE BATH SIZES.
C / REP
10 DISCOLOURATION AAFTER HOT DIP GALVANIZING CAUSEDBY GRINDING OROTHER RESIDUES.
CIf
possible
Incorrect nozzle pressure;nozzle angle; sweepingdistance; size of abrasive and recycling of grit.
A hot dip galvanized coating will be partially or fully destroyed by 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 to immersion in zinc bath to dry out overlap area as much as possible.
G/D
Molten zinc has a high surfacetension and will not easily drain from 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 poordrainage of threaded attachmentson withdrawal from the galvanizingbath.
The correct centrifuging equipment or post galvanizing thread cleaningby heating, wire brushing oroversize tapping of nuts, will generally remove clogging. If necessary specify delivery of boltsand nuts in nutted up form.
G
Double dipping can sometimes be used to hot dip galvanize fabrications that are too long orwide for single immersion. If thefabrication 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 cause surface rust staining.
Once the cause has been removed the stains will gradually disappear.
B
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12 DRAINAGE SPIKES. A / NSpikes and teardrops of zinc often appear along the edge of acomponent after hot dip 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 the unwanted warpingthat occasionally becomes evident afterhot dip galvanizing
REPIf
D/G possible
The edge most likely to have thesespikes is the last to leave the bath onwithdrawal. This applies particularlyto complex fabrications.
Drainage spikes are easily removedat the bath while still molten butwith complex fabrications, thesolidified spikes will be removed byfettling by the galvanizer prior toinspection.
G
SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP
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. REPAsh 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 uncoated surface beneath the trapped ash.
Inadequate skimming of ash from the molten zinc surface prior todipping.
On removal of ash, small uncoatedsurfaces shall be repaired accordingto the requirements of SANS 121 /ISO 1461. Large defects are a causefor rejection and require strippingand 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 aesthetically pleasing as a coating with free zincon the surface, a dull grey coatingprovides similar or better corrosionprotection.
D/S
15 FLAKING OR R / NDELAMINATION OF COATINGNo adhesion of zinc tosteel surface. Thick,rough coating.
High phosphorous content (greater)than 0,03% causes entire coating todelaminate from the steel.
Use a steel that has a phosphorouscontent of lower than 0,03%.
D / S
16 FLUX DEPOSITS, STAINS AAND INCLUSIONSFlux deposits or stains fromthe galvanizing process mayadhere to the steel orbecome included in thecoating. Flux residues areblack, 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 arenormally 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
11HDGASA © 2005
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 procedureswhen 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 thick brittle 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 byhydrogen, 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 fromthe 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 notaffect 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
SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP
17 DISCOLOURATION OFTHE PAINT COATING OVER HOT DIP GALVANIZING AFTEREXPOSURE TOTHE ENVIRONMENT
18 COATING THICKNESSPROVIDED ON FASTENERS USED TOASSEMBLE HOT DIPGALVANIZED STRUCTURES
19 MECHANICAL DAMAGE. AMechanical handling ortransport damage can occur, particularly with extremely thick coatings, which tend to be brittle in nature.
REPIf
necessary
20 OXIDE LINES. ALight aluminium oxidefilm lines on a hot dipgalvanized surface.
21 PIMPLES OR BLISTERS. APimples or blisters formed during hot dip galvanizing are usually associated with surfaceimperfections such as dross inclusions.
CIf
necessary
22 REACTIVE AND NON-REACTIVE STEELS, WELDED TOGETHER.Variations in coatingthicknesses can arisewhen reactive and non-reactive steels are welded together. Efforts to increase coating thickness on the less reactive steel may result in an undesirably thick and brittle coating on the most reactive steel.
HDGASA © 200512
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 willprovide greater corrosion protection.Except when the coating tends toflake off or delaminate see SC 15.
S
Rough, heavy coatings refer to hotdip galvanized components showingmarkedly rough surfaces. This caninclude coatings that have agenerally rough surface and, in somecases, groove type surfaceconfigurations, “tree bark effect”caused 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 stained area.
A
A / R
R
C / REPIf
accept-able
A
C / REPIf
necessary
SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP
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 be broadly classified as surface discontinuities in the steel that have been elongated during rolling.
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 BY EXCESSIVE CLEANINGUnless otherwise agreed, the galvanizer will limit cleaning of REPthe final coating by At themechanical means to galvanizerthat required in the orspecification. alterna-
tivelyat site
25 ROUGH COATINGS, CAUSED BY STEELSURFACE CONDITIONS
26 ROUGH HEAVY COATINGS, CAUSEDBY A ROUGH SURFACEAND / OR THE CHEMICAL COMPOSITION OF THE STEEL.“TREE BARK EFFECT”
27 ROUGH HEAVY COATINGS CAUSED BYINSUFFICIENTCENTRIFUGING
28 STAINS CAUSED BYWEEPING.
HDGASA © 2005 13
Ensure all paint or grease isremoved prior to hot dipgalvanizing. Make use of suitablemarking pens for temporaryidentification. 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 localisedungalvanized areas in an otherwisecontinuous galvanized coating.Defects after galvanizing can vary incolour from grey black to brownwhile no galvanized coating hasbeen formed.
33 UNCOATED SURFACES CAUSED BY STEEL SURFACE CONTAMINANTS 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 notremoved by acid pickling or abrasiveblasting.
34 UNGALVANIZED SURFACES CAUSED BY SCALE OR SAND.
R / N
REPIf
accept-able
SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP
Minimise contact betweencomponents and jig connections.(Loosen jigging wire). Smallcomponents can be centrifuged.
G
Articles entering the galvanizingbath should not be in tight contactwith 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 theentire surface or in isolated areas.Uneven drainage also includes dripson the ends of parts, runs nearholes. The cause is withdrawal speedtoo high or low galvanizingtemperature.
A
CIf
necessary
32 UNEVEN DRAINAGE.Uneven drainage resultsin an uneven or lumpy area on which zinc build up has occurred.
31 TYPICAL SPANGLED HOT DIP GALVANIZEDCOATING.A typical hot dip galvanizedsurface is shown in the example. The surface is silver grey in colour and not necessary but often hasa spangled effect (zinc crystals) in a range of sizes.
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 ADHERENT LUMPS OF ZINC ON THE INSIDE OF HEAVY WALLED STEEL PIPING
R
C / REPIf
accept-able
30 TOUCH MARKS.The zinc in the galvanizing bath should have freeaccess to all component surfaces or small uncoated or damaged areas can result.
HDGASA © 200514
Prepare surface for repair byadequate blasting. Loosely appliedzinc metal sprayed coating at theperimeter of the repair should beremoved by wire brushing. If notremoved, there is no compromise inthe corrosion resistance.
G / B
In order for zinc metal spraying toadhere on application, the damagedgalvanized surface must beadequately blasted. As it is difficultnot to overspray, excess zinc metalspray loosely adheres to thesurrounding coating.
39 ZINC METAL SPRAYED REPAIR APPLIED TO INADEQUATELY BLASTED SURFACES OR NOT WIRE BRUSHED AFTER APPLICATION.
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 of loosely adherent zinc, caused by moisture on the steel surface when hot dip galvanizing.
A
C
SC DESCRIPTION CAUSE EFFECT / REMEDY A/R/N EXAMPLERESPONSIBILITY C/REP
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 dipgalvanized.
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 AREA IN THE VICINITY OF A WELD
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 CONVENTIONAL FIXING METHODS SUCH AS BOLTS AND NUTS, OR REPAIR DAMAGED COATINGS CAUSED BY WELDING OR NON-CONVENTIONAL FIXING 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 expelledweld metal, that is fused ornot onto the surrounding material during welding.
A / N
Wet storage stain ceases when thecause is eliminated. If the coatingthickness at the affected area is equalto, or greater than the minimumrequired in the specification, it is nota cause for rejection, other than foraesthetic reasons. The latter is subjectto discussion with the end user.Customer is to exercise caution duringtransport and storage.
G / B
Wet storage stain (zinc hydroxide) isformed 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 is commonly called, is a white voluminous deposit that is occasionally found on the surface of a freshlygalvanized coating.
A
CIf
necessary
Reference AreasThe number and position of refer-ence areas and their sizes for theelectro-magnetic test methodshall be chosen with regard tothe 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 theindividual articles in the controlsample, shall be as follows:
a) For articles with significantsurface
Barea greater than
2m2, at least three referenceareas shall be taken on eachcontrol sample.
b) For articles with a total signif-icant surface area greaterthan 2m2, manufactured frommaterial of several differentthickness’, at least three ref-erence areas shall be takenon each specific materialthickness.
c) For articles with signifi-cant surface area between 1 000mm2 and up to and in-cluding 2m2 inclusive, eacharticle in the control sampleshall have at least one refer-ence area.
d) For articles with a total signif-icant surface area between 1 000mm2 and up to andincluding 2m2 inclusive, man-ufactured from material ofseveral different thickness’,each different material thick-ness, representing the con-trol sample shall have at leastone reference area, per mate-rial thickness.
e) For articles with less than 1 000mm2 of significant sur-face area, enough articles shallbe grouped together to pro-
vide at least 1 000mm2 surfacefor an individual referencearea. The number of referenceareas shall be as given in thelast column of table 1. Hence,the total number of articlestested equals the number ofarticles required to provideone reference area multipliedby the appropriate numberfrom the last column of table1 related to the size of the lot(or the total number of articlesgalvanized if that is less).Alternatively, sampling pro-cedures selected from SANS2859-1 / ISO 2859-1shall beused.
Note: 2m2 is typically 200 cm x 100 cm and 1 000 mm2 is typically 10 cm x 1 cm.
For case a) and b) on each articletaken separately in the controlsample, the local coating thick-ness within the reference areashall be equal to or greater thanthe mean coating thickness val-ues given in table 2 or 3 or thelocal coating thickness valuesgiven in table 4.
In cases c) to e) the coating thick-ness on each reference area shallbe equal to or greater than thelocal coating thickness valuesgiven in table 2, 3 or 4 as appro-priate. The mean coating thick-ness on all reference areas onmaterial of equal thickness in asample shall be equal to orgreater than the mean coatingthickness values given in table 2or 3 as appropriate.
When more than five articles haveto be taken to make up a reference area of at least 1 000 mm2, a single magneticmeasurement shall be taken oneach article if a suitable area of sig-nificant surface exists: if not, thegravimetric test shall be used.
Within each reference area,which should be at least 1 000 mm2, a minimum of fivemagnetic test readings shall be
taken. If any of the individualreadings is lower than the valuesin table 2 or 3, this is irrelevantas only the mean value over thewhole of each reference area isrequired to be equal to orgreater than the local thicknessgiven in the table.
Thickness measurements shall notbe taken on cut surfaces or areasless than 10mm from edges,flame cut surfaces or corners.
UniformityTestingVariations in coating thicknessare not usually of concern withmaterials hot dip galvanizedafter fabrication provided theminimum coating thicknessrequirements are satisfied,(except for fasteners - see noteon maximum coating thicknessbelow table 3.) Where the coat-ing thickness is not uniform, theservice life of the galvanizedcoating will generally be gov-erned by the amount of zincavailable at the place where thecoating is thinnest rather than bythe overall or average thicknessof the coating.
Standard magnetic measuringinstruments are quick and conve-nient methods for determininglocal coating thickness. By tak-ing a number of readings on ahot 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 thePreece test as a measure of coat-ing uniformity. This test pro-vides limited information as tothe coating thickness, but servesonly to locate the thinnest spot ina zinc coating by chemicallyremoving the zinc.
There are many limitations onthe use of the Preece test; con-sequently, it is no longerreferred to in specifications for
15HDGASA © 2005
BA significant surface can be
defined as a surface whichimpacts on the performance ofthat article.
HDGASA © 2005
standard adhesion tests musttake this into account. Therequirements for transportation,handling and erection should beevaluated against the additionalcorrosion protection afforded bythe thicker coating.
Testing AdhesionTesting adhesion is not necessar-ily a true measure of the adhe-sive strength of the metallurgicalbonding of the hot dip galva-nized coating to the base steel. Itserves, however, as an indicatorof the adhesive properties of thecoating.
Paring Test. This simple but effec-tive test is conducted by cutting orprying the hot dip galvanizedcoating with a sharp knife.Considerable pressure is exertedin a manner tending to remove aportion of the coating. Adherenceis considered satisfactory when itis possible to remove only smallparticles of the coating.
It should not be possible to peelany portion of the coating in theform of a layer so as to expose theunderlying iron or steel in advanceof the knife point. Although notmentioned in SANS 121 / ISO1461, this test has shown practicalsignificance as a test for adhesion.
For compliance with SANS 32 /EN 10240, the most popular testis cold flattening in accordance
with SANS 8492 / ISO 8492. Testpieces not less than 40mm inlength are flattened between par-allel flat platens as shown in table6. No cracking or flaking of thecoating shall occur on the surfaceaway from the cut surface.
AppearanceThe ability of a hot dip galva-nized coating to meet its prima-ry objective of providing corro-sion protection should be thechief criteria in evaluating thecoating’s acceptability.
The basic finish requirements ofthe hot dip galvanized coatingare that it be:
- relatively smooth,
- continuous,
- free from gross imperfections,
- free from sharp points (thatcan cause injury), and
- free from uncoated areasC.
The above points are of particu-lar importance when a subse-quent organic coating (duplexcoating) is to be applied.Smoothness and lack of rough-ness achieved by mechanicallywiped products, such as contin-uously galvanized sheeting orwire, are not to be used as thecriteria for accessing batch hotdip galvanized products.Roughness and smoothness arerelative terms where the end useof the product must be thedetermining factor in setting tol-erances.
The hot dip galvanized coatingshould be continuous to provideoptimum corrosion protection.Handling techniques for hot dip
16
hot dip galvanizing after fabrica-tion. When this test is applied,its scope and limitations shouldbe well understood and noattempt should be made todraw undue inference from thetest results.
Adhesion of the CoatingAdhesion is concerned with thepractical conditions of transporta-tion, erection and service. Thehot dip galvanized coating shouldbe sufficiently adherent to with-stand handling consistent with thenature and the thickness of thecoating in normal use of the arti-cle, without peeling or flaking.Bending or forming, other thanmild straightening after hot dipgalvanizing, is not considered tobe normal handling.
When certain grades of steel orvery heavy steel sections are hotdip galvanized, coatings mayoccur, which are thicker thanusual. The galvanizer has limitedcontrol over the development ofthicker coatings. Thick coatingsare a function of the chemicalcomposition of the steel or thelonger immersion time requiredfor massive items. Heavy hot dipgalvanized coatings, usuallygreater than 250mm thick, maybe more brittle than a typicalcoating. 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 > 48.3 ≤ 76.1mm 75% of outside diameter
Round > 76.1 ≤ 114.3mm 70% of outside diameter
Round > 114.3mm 65% of outside diameter
CTo be essentially free from
uncoated areas is best describedin SABS 763 4.3.2 b). “The area ofan individual bare spot or thinarea shall not exceed 5mm2. Thecombined area of bare spots orthin areas shall not exceed25mm2 per metre of length or persquare metre of surface of an arti-cle.”
galvanized articles may require theuse of chain slings, wire or otherholding devices to immerse thematerial into the galvanizing bath ifsuitable lifting fixtures are not pro-vided on the item. Chains, wireand special jigs used to handle theitems may leave a mark on the hotdip galvanized item. These marksare not always detrimental andreason for rejection. Should thesemarks, be greater than 5mm
2per
chain mark and expose the baresteel, suitable repair should be car-ried out using the procedures indi-cated in SANS 121 / ISO 1461.See also “Coating RepairProcedures”.
Differences in the lustre andcolour of hot dip galvanized coat-ings do not affect corrosion resis-tance and the presence orabsence of spangle has no effecton coating performance. Thewell-known spangled effectfound on some hot dip galva-nized surfaces is simply a factor ofprimary crystallisation. It ischiefly dependant upon the zincbath chemistry, the rate of cool-ing, the method of pickling, thesteel chemistry, and the thicknessof the coating. In fact, dull greyor patchy matte grey hot dip gal-vanized coatings give servicelives equal to or greater thanbright or spangled coatings.Variations in coating appearanceor finish are important only to theextent that they will affect corro-sion performance or the intendeduse of the article. The primaryfunction of a hot dip galvanizedcoating is corrosion protection.Specific requirements beyond thestandard set out in SANS 121 /ISO 1461, shall be communicat-ed to the galvanizer in writing ordiscussed at the contract review,prior to the work being hot dipgalvanized.
The information supplied mayinclude –
a) The composition and anyproperties of the metal thatmay affect the quality of thehot dip galvanized coating.
b) Identification of significantsurfaces. See page 15
Bfor
definition.
c) A visual standard should beestablished if a special finishis required.
d) Any particular treatmentsthat are required or notrequired before or after theprocess.
e) A variance in coating thick-ness. See notes below table 3.
f) The acceptable method ofrepair, if this is found to benecessary – see also “CoatingRepair Procedures”
Removal of WetStorage Stain (White Rust)Although in extreme cases theprotective value of the coatingmay be impaired, wet storagestain attack is often superficialdespite the relative bulkiness ofthe corrosion product. Wheresurface staining is light andsmooth without growth of thezinc oxide layer as judged bylightly rubbing fingertips acrossthe surface, the staining willgradually disappear and blend inwith the surrounding zinc surfaceas a result of normal weatheringin service.
When the affected area will notbe fully exposed in service orwhen it will be subjected to ahumid environment, wet storagestaining must be removed, evenif it is superficial. This is essentialfor the basic zinc carbonate filmto form. The formation of thiszinc carbonate film is necessaryto ensure long term service life.
Light deposits can be removed bycleaning with a stiff bristle (notwire) brush. Heavier deposits canbe removed by brushing with a5% solution of sodium or potassi-
um dichromate with the additionof 0.1% by volume of concentrat-ed sulphuric acid. Alternatively, a10% solution of acetic acid can beused. These solutions are appliedwith a stiff brush and left for about30 seconds before thoroughlyrinsing and drying.
Unless present prior to shipmentfrom the galvanizer, the develop-ment of wet storage stain is notthe responsibility of the galvaniz-er. The customer must exerciseproper caution during trans-portation and storage to protectagainst wet storage staining.
Coating RepairProceduresBY THE GALVANIZER
In terms of SANS 121 / ISO1461 a galvanizer may repair acoating by either zinc metalspraying or zinc rich epoxy orpaint. The latter method mustconform to certain requirementsin the specification. The pre-ferred method of repair is byzinc metal spraying. Repair willonly be necessary if bare spotsare present, usually caused byinadequate cleaning, air entrap-ment or if mechanical damagehas occurred.
The total uncoated areas for ren-ovation by the galvanizer shallnot exceed 0,5% of the total areaof the component.
For articles equal to an area of2m2; 0,5% represents a maxi-mum area of 100cm2 or 100mmx 100mm.
For articles equal to an area of 10 000mm2; 0,5% represents amaximum area of 50mm2 or7mm x 7mm.
No individual repair area shallexceed 10cm2 or 10mm x100mm.
If uncoated areas are greaterthan 0,5%, the article shall be
17HDGASA © 2005
regalvanized, unless otherwiseagreed between the purchaserand the galvanizer.
Zinc Metal Thermal SprayedCoatings
Method
The damaged area is to be lightlyblasted using preferably a pencilblasting nozzle or the surround-ing coating should be masked inorder to limit damage.
A zinc metal sprayed coating isthen applied to the abrasivelyblasted surface to a thickness atleast 30µm greater than theminimum specified zinc coatingthickness, or equal to the sur-rounding coating thickness,whichever is the greater. Therepaired area is then wirebrushed (preferably stainlesssteel) to remove loosely adher-ing over sprayed zinc. Wirebrushing provides the addedbenefit of sealing the pores thatmay be present in the sprayedcoating.
Zinc Rich Epoxy or Zinc RichPaint
Method
The defective area shall be blast-ed as above or abraded withabrasive paper (roughness 80grit). All dust and debris mustbe completely removed. In theevent 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 zincin the dry film (53% by volume),should be applied to a thick-ness, 30µm greater than theminimum specified for the gal-vanized coating or equal to thatof the surrounding galvanizedcoating, whichever is greater.The paint coating should over-lap the surrounding zinc by atleast 25mm.
The preferred product is a twoor three component zinc richepoxy.
SITE REPAIRS
The preferred method of repair isby zinc metal thermal spraying.Due to the remoteness of mostsites, however, and the unavail-ability of metal spraying equip-ment, repairs by zinc rich epoxyor zinc rich paint have to dategenerally been more popular.
Site repairs should be limited tosmall coating defects and areasthat have been cut or welded onsite.
Should excessive amounts ofgrease or oil be present at theaffected area, it should beremoved by means of anapproved solvent. All residuesare to be thoroughly removed bywashing with clean water.
The affected area should then beabraded 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 outusing an approved product.
Single pack zinc rich paints aregood materials and can easily beapplied. They, however, requireseveral coats to achieve a rea-sonable repair. Multiple coatswill also necessitate longer dry-ing times between coats.
Site Repairs by “Zincfix”
Until recently, the approvedproducts for repair were onlyavailable in large containers.Due to the large quantitiesinvolved and short pot lifewhen mixed, the productsproved to be expensive andwasteful.
A product is now available in athree component, solvent freeform, packed for conveniencein handy, easy to use squishpacks. The repair product iscalled “Zincfix” and isapproved by and available from
the Hot Dip GalvanizersAssociation of Southern Africaand all of its members.
The product has been testedagainst a number of reputableproducts and has performedexceptionally well.
Zincfix is available in a 100gsquish pack and will coat anarea of about 0,25m2 to a DFT(Dry film thickness) of 100 to150µm in a single application.
The contents are easily mixedin accurate proportions.
18 HDGASA © 2005
HDGASA © 2005
APPENDIX A
Applicable Standards
SANS 121 / ISO 1461 Hot dip galvanized coatings on fabricated ironand steel articles - Specifications and testmethods
SABS 763 Hot dip (galvanized) zinc coatings (other thanon continuously zinc coated sheet and wire).
SANS 32 / EN 10240 Internal and/or external protective coatings forsteel tubes – Specification for hot dip galva-nized coatings applied in automatic plants.
SANS 14713 / ISO 14713 Protection against corrosion of iron and steel instructures – Zinc and aluminium coatings –Guidelines.
SANS 2178 / ISO 2178 Non-magnetic coatings on magnetic substrates -Measurement of coating thickness – Magneticmethod.
ISO 1460 Metallic coatings – Hot dip galvanized coatingson 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 Associationand Industrial Galvanizers Corporation (Pty) Ltd of Australia, for permission to include some of the contentsof their publication on inspection after hot dip galvanizing.
19
HDGASA © 2005
The Hot Dip Galvanizers Association ofSouthern AfricaIntroduction:The Hot Dip Galvanizers Association of Southern Africa was founded in1965 and its membership represents over 90% of the available hot dipgalvanizing capacity of Southern Africa.
The Vision:To position the Hot Dip Galvanizers Association of Southern Africa,comprising all its Members and other interested parties, as aprofessional organization serving the interests of all parties dependantupon the hot dip galvanizing industry.
Mission Statement:To develop and expand the demand for hot dip galvanizing, identifyand develop new market opportunities for the benefit of Members andother stakeholders.
Strategic Objective:To convince users and specifiers to use hot dip galvanized steel inpreference to other coatings and alternative materials, where suitable.This is carried out in three ways:
1. Through the use of general promotion activities.
2. Through focused technical marketing support.
3. Through training and education programmes.
Delivery Activities:� 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, fabricatorsand end users while also recommending alternative protectivemethods where appropriate.
� Identifying and investigating potential new applications for hot dipgalvanizing.
� Participating in development projects on behalf of industry byproviding assistance in the form of technical consulting, practicalrecommendations and assistance with the preparation of designspecifications.
� Providing assistance with quality control during fabrication and hotdip galvanizing.
� Disseminating technical knowledge by providing a consulting andtraining service as well as the publication of technical literature.
� Providing training and education for member companies to ensurea high standard of quality and service throughout the hot dipgalvanizing industry.
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PublicationsAvailable fromthe Association� Steel Protection by Hot
Dip Galvanizing andDuplex Systems.
� Practical Guidelines forthe Inspection and Repairof Hot Dip GalvanizedCoatings.
� Specification for thePerformanceRequirements of DuplexCoating Systems.
� Code of Practice forSurface Preparation andApplication of OrganicCoatings Applied to NewUnweathered Hot DipGalvanized Steel.
� Guidelines for the Use ofHot Dip GalvanizedProducts in the MiningIndustry.
� Wall Chart - Design forHot Dip Galvanizing.
� Hot Dip GalvanizingIndustry Buyers’ Guide.
HOT DIP GALVANIZING HAS BEEN USED TO PROTECT STEEL FROM CORROSIONFOR MORE THAN 160 YEARS. APPLICATIONS FOR WHICH HOT DIPGALVANIZING IS SUITABLE ARE NUMEROUS AND VARIED AND THE DEMANDCONTINUES TO INCREASE.
RELIABILITYThe hot dip galvanized coating is formed by a metallurgical reactionbetween suitably cleaned steel and molten zinc. This results in theformation of a series of iron/zinc alloys which are overcoated withrelatively pure zinc. The process entails total immersion ofcomponents in both pretreatment chemicals and molten zinc. Thisensures 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 byzinc ensures that corrosion of the underlying steel cannot occur aslong as zinc is present. Even at discontinuities on the coating,corrosion creep under the surrounding zinc is not possible.
PREDICTABILITYThe durability of a hot dip galvanized coating is determined by thedegree of corrosion of zinc in a specific environment and thethickness of the coating. Corrosion of zinc is normally uniform, thusdurability of a hot dip galvanized coating is predictable in mostapplications.
ISSUE DATE: NOVEMBER 2005
