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HOT DIPGALVANIZERSASSOCIATION
SOUTHERN AFRICA
TheAssociation isa technicalinformation
centreestablished
for the benefit of
specifiers,consultants,
end users andits members
COMPREHENSIVE GUIDELINES FOR THE INSPECTION AND REPAIR OF
HOT DIP GALVANIZED COATINGS
COMPREHENSIVE GUIDELINES FOR THE INSPECTION AND REPAIR OF
HOT DIP GALVANIZED COATINGS
ContentsForeword ........................................................................................ 3
Introduction .................................................................................... 4
Test Sample Selection .................................................................... 4
Coating Thickness and Uniformity .................................................. 4
Thickness Testing ............................................................................ 6
Surface Conditions – Acceptance, Rejection and Responsibility ...... 9
Reference areas .............................................................................. 16
Uniformity Testing .......................................................................... 16
Adhesion of the Coating ................................................................ 17
Testing Adhesion ............................................................................ 17
Appearance .................................................................................... 17
Removal of Wet Storage Stain (White Rust) .................................... 18
Coating Repair Procedures – By the Galvanizer ............................ 18
– Site Repairs ...................................... 19
Applicable Standards ...................................................................... 20
1HDGASA © 2016
The copyright of all material in this catalogue and its supplements, particularly material which 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 written permission of Hot Dip Galvanizers Association South Africa.
Whilst every effort has been made to ensure the accuracy of the information contained in thispublication, no responsibility can be accepted for any errors or for any direct or consequential
loss or damage resulting from such information.
RELIABILITYThe hot dip galvanized coating is formed by a metallurgical reaction betweensuitably cleaned steel and molten zinc. This results in the formation of a seriesof iron/zinc alloys which are overcoated with relatively pure zinc. The process
entails total immersion of components in both pretreatment chemicals andmolten zinc. This ensures uniform protection and coating reliability even on
surfaces which would be inaccessible for coating by other methods.
DEPENDABILITYEase of inspection and dependability in service are beneficial features of a
hot dip galvanized coating. The cathodic protection of steel by zinc ensuresthat 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 durability of a hot dip galvanized coating is determined by the degree ofcorrosion of zinc in a specific environment and the thickness of the coating.Corrosion of zinc is normally uniform, thus durability of a hot dip galvanized
coating is predictable in most applications.
SUSTAINABLEThe enhanced service life of steel and iron components resulting directly fromthe hot dip galvanizing process, contributes to a conservation of resources and
reduced energy consumption, thus promoting a more sustainable future.
2 HDGASA © 2016
Hot Dip Galvanizers Association Southern AfricaHot Dip Galvanizers Association Southern Africa
Bedfordview Office Park, Building Two, Ground Floor, 3 Riley Road, Bedfordview
P O Box 2212, Edenvale, 1610
Telephone: (011) 456-7960 Fax: (011) 450-0728
Email: [email protected]
www.hdgasa.org.za
Design and layout by Sandra Addinall Telephone: (011) 868-3408 Email: [email protected]
Printed by Camera Press Telephone: (011) 334-3815 Fax: (011) 334-3912 Email: [email protected]
The Association is a technical information centre established for the benefit of specifiers, consultants, end users and its members
HDGASA © 2016
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.
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.
3
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 is inspected for compliance withspecifications. Interpretation of in-spection results should be madewith a clear understanding of thecauses of the various conditionswhich may be encountered andtheir effects on the ultimate ob-jective of providing corrosionprotection.
This manual has been designedto assist in testing, inspection,and the interpretation of test re-sults. It deals with numerous sur-face conditions, their origins, andtheir effects on protection fromcorrosion. This manual also con-siders undesirable design andfabricating features as well as un-acceptable hot dip galvanizingpractice. Although it is difficult tocover every condition, the man-ual covers many of the condi-tions frequently encountered inpractice.
To effectively use this manual, in-spectors should remember thatthe main purpose of hot dip galva-nizing is to protect steel from cor-rosion. The length of time thisprotection can reasonably be ex-pected to last is called its “servicelife or time to first maintenance”.This is defined as the time takenfor the appearance on an article of5% surface rust. The service life ofa hot dip galvanized coating is
directly proportional to the thick-ness of the coating. Corrosion pro-tection is greater when the coatingis thicker. Thus coating thickness isthe single most important inspec-tion check to determine the qual-ity of a hot dip galvanized coating.
Coating thickness, however, isonly one inspection aspect. Otherfeatures include the uniformity ofthe coating, coating adhesion andappearance. Possible brittlenessand defects, which arise from in-correct design and fabrication, also need to be assessed by the inspectorate.
While minimum specified stan-dards must be satisfied, their rela-tive importance varies accordingto the end use of the product. Forexample, the aesthetic appear-ance of hot dip galvanized structural steel in an obscured ap-plication is different from thatwhere a product is used as an ar-chitectural feature. An awarenessof the end use of the product andthe capability of the hot dip galva-nizing process is essential forgood 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 become
necessary. Repair procedures andthe recommended repair materi-als are covered in this manual.
The quality standards to whichreference is made in the follow-ing sections are in accordancewith International and SouthAfrican National Standards’ spec-ifications.
Test SampleSelectionTo effectively evaluate hot dipgalvanized coatings, it is essen-tial that randomly selected spec-imens be representative of aninspection lot. An inspection lotis a single order or batch await-ing despatch. For products re-quired to comply with SANS 121/ ISO 1461 the minimum numberof articles from each inspectionlot that forms the control sampleshall be in accordance with table1 of the specification.
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 prod-ucts leave the premises of thegalvanizer.
Coating Thicknessand UniformityThe thickness of the hot dip galva-nized coating is the primary factorin determining its life under givenservice conditions. The thicker thecoating, the better corrosion pro-tection it offers. For most atmos-pheric conditions, the relationshipbetween corrosion protection andcoating thickness is approximately
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
4 HDGASA © 2016
linear, i.e., the service life is dou-bled if the coating thickness isdoubled. There is however a limitto the maximum coating thicknessthat can 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 be-comes thicker. Higher bath tem-peratures and longer immersiontimes will generally producethicker alloy layers.
The thickness of the outer zinclayer of the coating is independentof immersion time. Thickness ofthis layer is determined by the rateof withdrawal from the zinc bathand the degree of drain-off ofmolten zinc. A fast rate of with-drawal causes an article to “drag
out” more zinc. This results in aheavier coating, although the dis-tribution of the zinc layer may beuneven. Wiping of the coatingduring withdrawal materially re-duces its thickness and thus itsprotective value. It is discouragedexcept where necessary for thesmoothness demanded by condi-tions of service such as for smallbore pipe and conduit.
Tubes hot dip galvanized in ac-cordance with SANS 32 / EN10240 utilise steam blowing in-ternally and an air ring externally(the blown process), to removeexcess zinc. Some variation in in-ternal coating thickness is ac-ceptable within 50mm of tubeends. With fabricated articles,local differences in the drain-off,because of the shape of the arti-cle and the angle at which differ-ent surfaces leave the bath, mayalso result in some variation incoating thickness.
Other factors influencing thecoating thickness may be beyondthe control of the galvanizer.
The chemical composition of thesteel plays a major role in deter-
mining the thickness and appear-ance of the final coating. Certainsteel compositions tend to accel-erate the growth of the iron/zincalloy layer so that the hot dip gal-vanized coating may have amatte finish with little or no outerzinc layer. This coating also tendsto be thicker than the typicalbright hot dip galvanized coat-ing. The galvanizer’s control overthis condition is limited. Steelscontaining the elements, phos-phorus in excess of 0.02%, or sil-icon from 0.03% to 0.15% andabove 0.25% or combinations ofboth elements, are particularlyprone to heavier coatings con-sisting mainly of iron/zinc alloys.Due to the iron/zinc alloy growthin these coatings, extending tothe outer surface, the final ap-pearance of the coating can bedark grey in colour distributedevenly or unevenly over the sur-face.
The surface condition of the steelbefore hot dip galvanizing also af-fects the thickness and smooth-ness of the final coating. Steelswhich have been abrasive blastcleaned, or left to weather and rustfor some time prior to hot dip gal-vanizing, can produce substan-tially thicker coatings than thosenormally produced on chemicallycleaned steel.
The mass, shape and degree ofcold working of components tobe hot dip galvanized also influ-ences coating thickness and uni-formity. When a fabricated articleconsists of both heavy and lightsections, a variation in coatingthickness between the sectionsmay result. Immersion time willvary according to the relationshipof the surface area of an item toits mass. The galvanizer has littlecontrol over 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”and “Facts About Hot Dip Galva-nizing – Practical Guidelines).
PROF
ILES
CAST
INGS
TABLE 2
MINIMUM COATING THICKNESS ON ARTICLES THAT ARE NOT CENTRIFUGED
SANS 121 / ISO 1461
Category and thickness Local coating thickness Mean coating thickness (t) mm μm* μm*t ≥ 6 70 85
3 ≤ t < 6 55 70
1.5 ≤ t < 3 45 55
t < 1.5 35 45
t ≥ 6 70 80
t < 6 60 70
* Local coating thickness is defined as the mean of the measurements taken within aspecified 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 to 3mm.
■ Where only one reference area is required according to size of the article, the meancoating thickness within that reference area shall be equal to the mean coatingthickness given in the above table.
5HDGASA © 2016
Since the time to the appearanceof first rusting of the base steel isusually determined by the thinnestportion of the coating, an evalua-tion of galvanizing quality musttake into account both the mini-mum thickness of the coating andits distribution. Specifications forhot dip galvanizing recognisethat variations in coating thick-ness are inherent in the process.The minimum thickness is gener-ally defined as an average ormean thickness of the coating onspecimens tested and/or a mini-mum thickness for any individualspecimen.
When measurements are taken todetermine 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 article whichleaves the bath last will carry athicker coating. This is particularlyso towards the edge, where, atthe time of drainage, the last fewdrops of zinc tend to agglomerateas a result of surface tension.
The minimum coating require-ments specified in SANS 121 /ISO 1461 for different materialthicknesses on samples that arecentrifuged. Table 4 indicates theminimum coating requirementsspecified in SANS 32 / EN 10240for different classes of coating,for steel tubes manufactured bythe blown process.
Specifications do not stipulatemaximum upper coating thicknesslimits, but excessively thick coat-ings on threaded articles are unde-sirable. In order to ensure effectivetensioning, the coating thicknesson fasteners should not exceed amaximum of 65µm, this appliesparticularly, to high strength boltsand nuts.
Variance in coating thickness. Arequirement for a thicker coating(25% greater than the standardin table 2) can be requested forcomponents not centrifuged,without affecting specificationconformity.
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 more brit-tle and may require special han-dling. The efficacy of corrosionprotection of a hot dip galva-nized coating (whether light ordull grey) is approximately pro-portional to coating thickness.
Thickness TestingThere are several methods to de-termine the thickness of the zinccoating on a hot dip galvanized ar-ticle. The size, shape and numberof pieces to be tested, will mostlikely dictate the methods of test-ing. The specified test methodsare either destructive or non-de-structive and are detailed inSANS 121 / ISO 1461. Identicalmethods are detailed in SANS 32/ EN 10240. The most practicaltests are the non-destructivetype, such as gauges utilising theelectromagnetic principle.
1. Electromagnetic TestingMethod.Instruments, which rely onelectromagnetic principles areprobably the most widelyused devices to determinethe thickness of hot dip galvanized coatings. Electro-magnetic instruments mea-sure coating thickness inspecifically-identified localareas of the article. The aver-age thickness (and there-fore mass) of the coating is cal-culated from measurementstaken at a suitably large num-ber of points distributed over areference area on the surfaceof the article. There are a num-ber of electromagnetic gaugeswhich can be used to give ac-curate measurements of thezinc coating thickness. Thesegauges give reliable thick-ness readings provided theyare properly calibrated andthe manufacturer’s instruc-tions are observed. The mostcommonly used type of gaugeis a portable electronic instru-ment. The gauge uses a tem-
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TABLE 3
MINIMUM COATING THICKNESS ON ARTICLES THAT ARE CENTRIFUGED
SANS 121 / ISO 1461
Category and thickness/ Local coating thickness Mean coating thickness diameter (t) or (ø) mm μm* μm*
ø ≥ 20 45 55
6 ≤ ø < 20 35 45
ø < 6 20 25
t ≥ 3 45 55
t < 3 35 45
* Local coating thickness is defined as the mean of the measurements taken within aspecified 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 to 3mm.
■ Where only one reference area is required according to size of the article, the meancoating thickness within that reference area shall be equal to the mean coatingthickness given in the above table.
6 HDGASA © 2016
perature-compensated mag-netic transducer to measurethe magnetic flux changes thatoccur when the probe (amagnet) is separated from aferrous metal substrate by anon-magnetic coating such aszinc. The output signal fromthe probe is proportional tothe distance of separation and,therefore, to coating thickness.The probe signal is amplifiedand indicated on a meter cali-brated to show coating thick-ness. These battery poweredinstruments have typical accu-racies up to 5% and have theadvantage of not requiring re-calibration with probe orienta-tion. Similar electromagneticinstruments with even greateraccuracies are available for lab-oratory use.
The specification requires thatthe thickness gauges shall (interms of ISO 2178) be -
● Capable of minimizing er-rors in reading caused by
the magnetic permeabil-ity, dimensions, surfacefinish and curvature of thearticle being tested;
● Capable of measuring thethickness of the coatingbeing tested within an ac-curacy of 10% or 1.5µmwhichever is betterA, and
● Calibrated by taking read-ings at zero (or near zero)thickness and at thick-nesses of at least 100µm,on suitable non-ferrousshims placed on accept-able standard pieces ofmetal of composition,thickness and shape simi-lar to those of the articlesunder test.
To avoid possible errors in theuse of magnetic instruments,certain precautions should betaken:
● Follow the manufacturer’sinstructions. The instrument
AWhen curved surfaces are dealtwith, particular care should betaken to ensure that the thick-ness gauge is capable of obtain-ing readings to an acceptablelevel of accuracy.
should be frequently recal-ibrated against non-mag-netic film standards ofknown thickness.
● 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 thickness.
● Readings should not betaken near an edge, hole,or inside corner.
● Readings should not betaken on curved surfaceswithout additional cali-bration of the instrument.
● The test surface should befree from surface contami-nants such as dirt, grease,and corrosion products.
● Test points should be takenin each reference area toavoid obvious peaks or ir-regularities in the 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 have beensubjected to a cutting, rollingor finishing operation thatcould damage the zinc coat-ing. The zinc coating of an ex-ternal standard metric threadthat has not been undercutshall be such as to enable thethreaded part to fit an over-sized tapped nut in accor-dance with the allowancesgiven in table 5.
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 theweld 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 gas andwater installations and ‘B’ for other applications. The number following the quality let-ter 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.
7HDGASA © 2016
ping divided by the surfacearea of the test specimengives the mass of coating perunit area. In the case ofthreaded articles, such as boltsand screws, the determinationis made on an unthreadedportion of the article.
The stripping test gives an ac-curate average coating massof the zinc coating. However,it does not provide any infor-mation on how evenly thecoating is distributed.
To test compliance withSANS 32 / EN 10240, two testpieces shall be taken from thetube to be tested. Theseshould be between 30 and600cm2 in surface area, withlength between 50 and150mm taken at least 600mmfrom the tube end.
4. Metallographic Examination. Where the hot dip galvanizedcoating composition andthickness are of interest, mi-croscopic examination is a re-liable 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 to pro-vide information about the rel-ative thicknesses of the alloyand the free zinc layers whichcomprise the hot dip galva-nized 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 100mlof 95% ethanol or metha-nol).
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.
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
>M24 = M27 0,43
>M27 = M30 0,47
>M30 = M36 0,57
>M36 = M48 0,76
>M48 = M64 1,0
On bolts greater than M24,undercutting of bolt threadsis frequently preferred to onlyoversizing of nut threads.
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 applied wherematerial is inspected after hotdip galvanizing. Since this is adestructive test method, it isgenerally not suitable for theinspection of large or heavyitems unless smaller or repre-sentative specimens can besubstituted for them (see TestSampling). The test specimenis cleaned with naphtha orother suitable organic solvent,rinsed with alcohol, dried andweighed. It is then stripped ofthe zinc coating by immersionin a solution containing 3.5g ofhexamethylenetetramine and500ml concentrated hydro-chloric acid in 1litre of water.The stripping of the coating iscomplete when evolution ofgas ceases. After washing and drying, the specimen isweighed; the differences inmass before and after strip-
8 HDGASA © 2016
Surface Conditions (SC)SC WHAT IS THIS? CAUSE EFFECT / REMEDY / RESPONSIBILITY EXAMPLE
2 ASH DEPOSITSAsh deposits are grey,non-metallic depositsconsisting of zinc oxidethat have been depositedon the hot dip galvanizedcoating.
Although the recommended quantity ofSodium Dichromate is about 0.15 to0.3%, occasionally when topping up, moreis added. This often results in a darkyellow to brown colour on the galvanizedsurface. The darker colour will provideenhanced initial corrosion protection.
This can be accepted since there is noadverse effect on corrosion control.
The galvanizer should maintain theconcentration of Sodium Dichromate atabout 0.15 to 0.3%.
Zinc oxide deposits can take place whenthe component is dipped or when it isremoved from the bath.
This can be accepted or negotiateddependent on functional requirements sincethe coating is normally intact underneaththe ash deposits. If required, ash must beremoved by the galvanizer and the coatingthickness verified for conformance to thespecification requirements.
In the case of liquid conveyance pipes, allash should be removed.
There are several causes of bare spots.These include:•Overdrying: If the time between fluxing
and hot dip galvanizing is prolonged orthe drying temperature is too high, thebarrier protection provided by the fluxmay be lost. This is indicated by a rustyappearance on the ungalvanized article,which can result in coating discontinuitiesafter hot dip galvanizing.
•Excess Aluminium: A conditionsometimes referred to as black spotsmay occur if the aluminium content of abath becomes too high. No troubleshould be experienced if fluxconcentration is correct and thealuminium content of the bath ismaintained below approximately0.007%.
•Further causes are: Blowouts, fluxdeposits, stains and inclusions,mechanical damage, touch marks,uncoated surfaces caused - by surfacecontaminants, scale or sand; welds andweld spatter. See Surface Conditions 5,15, 17, 28, 31, 32, 33 and 34.
Where necessary, such spots may berepaired using one of the specified repairmethods. Gross uncoated areas are acause for rejection.
The galvanizer should avoid overdryingand maintain the correct level ofaluminium content in the kettle.
3 BARE SPOTSAlthough excluded fromSANS 121:2011 (ISO1461:2009), bare spotsof about 5mm2 (2.2 x2.2mm), due to smalllocalised flaws, areadequately protected bythe sacrificial propertiesof zinc and will have verylittle effect on the servicelife of the coating.
1 APPEARANCE OFSODIUM DICHROMATEA small amount ofSodium Dichromate isgenerally added to thequench water bath forpassivation.
4 BLASTING DAMAGESweep blasting (donecorrectly) substantiallyincreases paint adhesionand final coatingappearance. However,done incorrectly it canresult in coating damage.
Incorrect nozzle pressure; nozzle angle;sweeping distance; size of abrasive andrecycling of grit.
This is cause for rejection as a hot dipgalvanized coating will be partially orfully destroyed by excessive blasting.
Refer to the HDGASA Code of Practice.
5 BLOWOUTSStaining and coatingdefects around unsealedweld areas and ventholes. Similar to stainscaused by weeping.
See Surface Condition 26
Pre-treatment chemicals penetratingsealed overlap areas through the requiredvent holes and escaping during immersionin the molten zinc. This effect tends todamage the flux coatings, causinglocalised uncoated areas.
This can be accepted once repaired aftercleaning.
The designer and fabricator should takethis into account during the design andmanufacturing phase of the project.
9HDGASA © 2016
SC WHAT IS THIS? CAUSE EFFECT / REMEDY / RESPONSIBILITY EXAMPLE
7 CLOGGED THREADSThreaded components orattachments have threadsclogged with zinc.
8 DAMAGED COATINGS CAUSED BY WELDING OR NON-CONVENTIONAL FIXING METHODS DURING ERECTION
Molten zinc has a high surface tensionand will not easily drain from holes under8mm in diameter.
This can be accepted once the item hasbeen cleaned if required.
The Designer should make the holes aslarge as possible.
The removal of molten zinc over the bathby the galvanizer will reduce thelikelihood of clogging.
Insufficient centrifuging or poor drainageof threaded attachments on withdrawalfrom the galvanizing bath.
This should be rejected and then cleanedby the galvanizer. The correct centrifugingequipment or post galvanizing threadcleaning by heating and wire brushing oroversize tapping of nuts, will generallyremove clogging.
If necessary, specify delivery of bolts andnuts in nutted up form.
Conventional drilling and bolting after hotdip galvanizing is preferred. Shouldwelding or a non-conventional method offixing be used, which results in damage tothe coating, an approved repair method isnecessary.
9 DISCOLOURATION AFTER HOT DIP GALVANIZING CAUSED BY GRINDING OR OTHER RESIDUES
Material stored in contact with rusty steelor iron filings, can cause surface ruststaining.
This can be accepted as once the causehas been removed the stains willgradually disappear. The Fabricatorshould clean if possible.
The hot dip galvanizing process occurs ata molten zinc temperature of 450°C. Thisis at the lower end of the stress relievingtemperature for treating steel. Thus, anyinherent rolling or welding stresses in thefabrication are likely to be released. Thismay result in a dimensional change, i.e.distortion.
10 DISTORTIONDistortion is theunwanted warping thatoccasionally becomesevident after hot dipgalvanizing.
The Designer has the following optionsavailable: use symmetrical designs; usesections of similar thickness; stiffenunsupported thin wall sections; usepreformed members with the correctminimum bend radii; use balanced orstaggered welding techniques; make use oftemporary braces on thin walled sectionssuch as troughs, cylinders and angle frames.
Stress Relief assembly prior to hot dipgalvanizing.
The galvanizer should avoid quenchingafter galvanizing.
The components can be straightened afterhot dip galvanizing.
11 DRAINAGE SPIKESSpikes and teardrops ofzinc often appear alongthe edge of a componentafter hot dip galvanizing.
The edge most likely to have these spikesis the last to leave the bath onwithdrawal. This applies particularly tocomplex fabrications.
Drainage spikes are easily removed at thebath while still molten but any solidifiedspikes should be removed by carefulfettling by the galvanizer prior to release.
Coating repair can be done by zinc metalspraying or a zinc rich paint or epoxy,providing the product conforms to therequirements of the specification.
6 CLOGGED HOLESZinc film clogging orpartly bridging holes.
10 HDGASA © 2016
SC WHAT IS THIS? CAUSE EFFECT / REMEDY / RESPONSIBILITY EXAMPLE
12 DULL GREY OR MOTTLED COATING APPEARANCEDull grey or mottledcoatings can appear as adark grey circularpattern, a localised dullpath, or may extend overthe entire surface of thecomponent.
13 ENTRAPMENT OF ASHAsh which has not beenremoved from the surfaceof the molten zinc priorto immersion of steel canbe trapped on the steelsurface as it is immersedand result in an uncoatedsurface beneath thetrapped ash.
14 FLAKING OR DELAMINATION OF COATINGNo adhesion of zinc to thesteel surface or a thick,rough coating.
This appearance indicates the presence ofextensive iron / zinc alloy phase growth,caused by steels with high reactive levelsof Silicon and Phosphorus.
Although not as aesthetically pleasing, adull grey coating provides similar orbetter corrosion protection.
Inadequate skimming of ash from themolten zinc surface prior to dipping.
On removal of entrapped ash, smalluncoated surfaces shall be repairedaccording to the requirements of SANS121: 2011 (ISO 1461:2009) by theGalvanizer.
Large defects greater than 0.5% of totalsurface area or single spots over 10cm2
are a cause for rejection and requirestripping and re-galvanizing.
High phosphorus content greater than0.02% can cause the entire coating todelaminate from the steel.
The Designer should use a steel that has aphosphorus content of lower than 0.02%.
Flux deposits or stains may occur as aresult of excessive “ dusting” withammonium chloride on withdrawal fromthe molten zinc. Flux inclusions can occurwhen a surface flux blanket is applied tothe zinc surface (wet galvanizing). Fluxblankets are normally only used forspecialised processes, e.g. galvanizingtubes and fasteners.
Flux deposits or stains should be removedby the galvanizer and the underlyingcoating measured to determine whether itconforms to the minimum requirements ofthe specification.
Inadequate repair of a damaged surfaceon the hot dip galvanized coating prior tothe application of a paint coating.
16 DISCOLOURATION OF THE PAINT COATING OVER HOT DIP GALVANIZING AFTER EXPOSURE TO THE ENVIRONMENT
It is the installers responsibility to ensurethe correct repair materials andapplication procedures are used whentouching up cut or welded hot dipgalvanized components and prior topainting. Where corrosion control hasbeen compromised the job should berejected.
17 MECHANICAL DAMAGEMechanical handling ortransport damage canoccur, particularly withextremely thick coatings.
The use of chains, wire ropes, dragging ordropping of the component onto a hardsurface, can cause mechanical damage.This is particularly relevant with thickcoatings.
This can be accepted and repaired by thegalvanizer or builder if necessary.Warning labels highlighting possibledamage if manhandled, should beattached by the galvanizer before thecomponent is transported. The use ofnylon lifting slings is recommended.
15 FLUX DEPOSITS, STAINS AND INCLUSIONSFlux deposits or stains fromthe galvanizing processmay adhere to the steel orbecome included in thecoating. Flux residues areblack, brown, grey oryellowish non-metallicdeposits consisting mainlyof ammonium chloride.
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18 OXIDE LINESLight aluminium oxidefilm lines on a hot dipgalvanized surface.
19 PIMPLES OR BLISTERSPimples or blistersformed during hot dipgalvanizing are usuallyassociated with surfaceimperfections such asdross inclusions.
20 REACTIVE AND NON-REACTIVE STEELS, WELDED TOGETHERVariations in coatingthicknesses can arisewhen reactive and non-reactive steels are weldedtogether. Efforts toincrease coating thicknesson the less reactive steelmay result in anundesirably thick andbrittle coating on themost reactive steel.
Due to the shape and / or drainageconditions of some components, the hoistcrane has stopped and started uponwithdrawal of the items from the moltenzinc.
This can be accepted as it has no effect oncorrosion resistance, with the overallappearance becoming uniform in time.
Dross pimples result from agitation of thedross layer at the bottom of the bath orfrom dragging material through the drosslayer. They appear as small, hard lumpson an otherwise normal galvanizedsurface. Blisters may be formed byhydrogen, which is absorbed duringpickling and diffused at galvanizingtemperatures.
This can be accepted since dross pimplesrepresent minor disturbances in coatinguniformity and do not affect corrosionresistance. Smooth if sufficiently sharp tocreate the risk of injury.
The galvanizer should avoid disturbingthe dross layer at the bottom bycontrolling immersion depth and drossingregularly.
This difference in coating thickness isbrought about by a combination of amore reactive silicon killed steel and / orhigh phosphorus, resulting in a thickercoating and a less reactive aluminiumkilled steel, resulting in a coatingthickness sometimes below that requiredin the specification.
Should the galvanizer be asked toregalvanize in accordance with thespecification, the resultant coatingthickness on the reactive steel will beexcessively thick, resulting in a brittlecoating more susceptible to damage.
The fabricator should select the same steelfor fabricating on a component.
If need be, accept a concession request bythe galvanizer when the thinner coating ispossibly below specification.
21 REMOVAL OF ZINC COATING BY EXCESSIVE CLEANINGUnless otherwise agreed,the galvanizer will limitcleaning of the finalcoating by mechanicalmeans to that required inthe specification.
Excessive cleaning of the coating bymechanical methods can result inuncoated areas.
Care should be exercised by thegalvanizer to avoid over cleaning. Ifcorrosion control has been compromised,i.e. exposed areas greater than tolerance;reject.
Alternatively repair in accordance withstandards.
Steel may occasionally includelaminations, laps, folds and non-metallicimpurities, which result in slivers rolledinto the metal surface. Defects of this typeare sometimes detected before or afterpickling, but may only become apparentafter hot dip galvanizing.
22 ROLLING DEFECTS IN STEELThese defects may bebroadly classified assurface discontinuities inthe steel that have beenelongated during rolling.
This can be accepted, as minor surfacedefects will not adversely influencecoating life.
Surface flaws in the base material may beremoved by local grinding after hot dipgalvanizing followed by repair of theaffected surface.
23 ROUGH COATINGS, CAUSED BY STEEL SURFACE CONDITIONS
Rough surfaces, typical of coatings oncorroded steel surfaces, can be hot dipgalvanized satisfactorily. The coating willhowever, reflect the texture of thesubstrate. Other causes of rough surfacesinclude uneven cold working, overpickling, a high galvanizing temperatureand / or extended immersion in themolten zinc.
This can be accepted as the roughersurface will produce a thicker coating andresult in a longer service life.
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24 ROUGH, HEAVY COATINGS CAUSED BY A ROUCH SURFACE AND/OR THE CHEMICAL COMPOSITION OF THE STEEL – “TREE BARK EFFECT”
25 ROUGH HEAVY COATINGS CAUSED BY INSUFFICENT CENTRIFUGING
26 STAINS CAUSED BY WEEPING
Hot dip galvanized components showingmarkedly rough surfaces. This can includecoatings that have a generally rough sur-face and, in some cases, groove type sur-face configurations, “ tree bark effect”caused by variations in surface steelanalysis.
The thicker coating produced will providegreater corrosion protection; except whenthe coating tends to flake off ordelaminate (see surface condition 14).
Efficient centrifuging will generallyremove excess zinc and provide a smoothand attractive exterior.
Provided the steel / casting surface isreasonably smooth, correctly centrifugedarticles will provide an acceptable finish.
Should the surface not be reasonablysmooth; it should be rejected.
The salts from acid or flux that havepenetrated porous welding or betweencontact surfaces during pickling, can weepafter hot dip galvanizing and waterquenching, providing a stained area.
Weld seepage stains are not a cause forrejection. The stains can be easilyremoved by means of bristle brushing.Should the component be destined for acorrosive area, the crevice may be sealedafter cleaning.
27 TIGHTHLY ADHERENT LUMPS OF ZINC ON THE INSIDE OF HEAVY WALLED STEEL PIPING
Heavy walls and thick flanges used in themanufacture of piping can act as a heatsink when immersed in molten zinc. Thiseffect considerably lengthens theimmersion time. Occasionally thegalvanizer will remove the pipes beforeall the zinc has melted from the inside ofthe pipe.
The inspector should reject this. Thegalvanizer should ensure all zinc hasbeen removed from the inside of the pipeby longer immersion times.The item can be cleaned or repaired ifacceptable to the customer.
Articles entering the galvanizing bathshould not be in tight contact with eachother. Jigging wire should also be looselyattached to eliminate wire marks. Wherea component has been resting on jiggingor dipping equipment, an uncoated areaor touch mark could appear.
28 TOUCH MARKSThe zinc in thegalvanizing bath shouldhave free access to allcomponent surfacesotherwise small uncoatedor damaged areas canresult.
The galvanizer should minimise contactbetween components and jig connections(loosen jigging wire). Small componentscan be centrifuged.
These areas should be repaired if withinallowable limits.
29 TYPICAL SPANGLED HOT DIP GALVANIZED COATINGA typical hot dip galva-nized surface is shown inthe example. The surfaceis silver grey in colourand not necessarily, butoften has, a spangledeffect (zinc crystals) in arange of sizes.
Surface appearances may vary accordingto the chemical composition of the steel.Cooling rate has a direct effect on thesurface brightness and spangle size.Faster cooling usually results in a brightercoating with a smaller spangle size.
Not rejectable if coating thickness withinallowance limits.
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30 UNEVEN DRAINAGEUneven drainage resultsin an uneven or lumpyarea on which zinc buildup has occurred.
31 UNCOATED SURFACES CAUSED BY STEEL SURFACE CONTAMINANTS OR ENTRAPPED AIR
32 UNGALVANIZED SURFACES CAUSED BY SCALE OR SAND
This can occur over the entire surface or inisolated areas. Uneven drainage alsoincludes drips on the ends of parts, runsnear holes. A cause may be highwithdrawal speed and / or thegalvanizing temperature being too low.
This condition does not adversely affectcoating performance and is acceptable.
However, protuberances and lumps whichinterfere with mating surfaces areunacceptable.
Residues (such as oil based paint, grease,oil or labels) on the steel surface orincorrectly positioned vent holes, canresult in localised ungalvanized areas inan otherwise continuous galvanizedcoating. Uncoated areas often manifestthemselves as black or very dark colouredspots.
To avoid uncoated surfaces, ensure allpaint or grease is removed prior to hotdip galvanizing. Make use of suitablemarking pens for temporaryidentification. Correctly positionadequately sized vent holes.
Uncoated areas within the limits of 0.5%of total area or single areas of 10cm2 orless can be repaired. Larger areas arerejectable.
Sand on cast iron or scale on the steelsurface is generally caused by the processused to form or roll the product. Alocalised ungalvanized area in anotherwise continuous coating can occur ifscale or sand from the moulding or rollingis not removed by acid pickling orabrasive blasting.
These ungalvanized areas may occur in alinear pattern on angles, channels orother rolled products. They can alsoappear on cast iron products.
Uncoated areas within the limits of 0.5%of total area or single areas of 10cm2 orless can be repaired. Larger areas arerejectable.
33 UNGALVANIZED AREA IN THE VICINITY OF A WELD
A localised ungalvanized area near a weldcan be caused by weld slag deposit, weldporosity or weld undercut. Oxide depositsand residues from welding are resistant tonormal pickling acids and must beremoved before the work is pickled andhot dip galvanized.
Weld slag deposits should not be acceptedprior to galvanizing and must be removedby the fabricator by means of abrasiveblast cleaning. The deposit can also beremoved by proper chipping or wirebrushing.
Shielded arc welding as opposed to stickwelding is preferred for componentswhich are to be hot dip galvanized.
Since corrosion control is compromised,this is rejectable; but may be repairedafter negotiation.
Weld spatter is caused by weld poolexplosions when improper weldingparameters are used, or if the material isdirty or contaminated.
34 WELD SPATTER Weld spatter is oxidised,normally sphericalexpelled weld metal, thatis fused (or not) onto thesurrounding materialduring welding.
Loosely adherent weld spatter should beremoved by the fabricator prior to hot dipgalvanizing. Although not acceptable interms of the specification, the presence oftightly adherent weld spatter after hot dipgalvanizing will not affect the corrosionresistant properties of the coating.
35 WET STORAGE STAIN OR WHITE RUSTWet storage stain orwhite rust (as it iscommonly called) is awhite voluminous depositthat is occasionally foundon the surface of afreshly galvanizedcoating.
Wet storage stain (zinc hydroxide) isformed on freshly galvanized surfaceswhich are in close contact in the presenceof moisture.
Wet storage stain ceases when the cause iseliminated, i.e. the objects are separatedand dried. Once it has been removed (witha nylon bristle brush) an evaluation ispossible. If the coating thickness at theaffected area is equal to, or greater thanthe minimum required in the specification,it is not a cause for rejection. The customeris to exercise caution during transport andstorage to eliminate the susceptibility to wetstorage stain.
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36 ZINC METAL SPRAYED REPAIR APPLIED TO INADEQUATELY BLASTED SURFACES OR NOT WIRE BRUSHED AFTER APPLICATION
37 ZINC SPLATTERSplashes and flakes ofloosely adherent zinc,caused by moisture onthe steel surface whenhot dip galvanizing.
In order for zinc metal spraying to adhereon applications, the damaged galvanizedsurface must be adequately roughened bysweep blasting or other approvedmethods. As it is difficult not to overspray,excess zinc metal spray loosely adheres tothe surrounding coating.
The fabricator or galvanizer must preparethe surface for repair by roughening thesurface by sweep blasting or some otherapproved method. Loosely applied zincmetal sprayed coating at the perimeter ofthe repair should be removed by wirebrushing. If not removed, there is nocompromise in the corrosion resistance.
When hot dip galvanizing on unusuallydeep fabrications by double dipping,moisture on the surface of the steelcontacts with the molten zinc causingsplashes of zinc to loosely adhere to thealready hot dip galvanized surface.
The loosely adherent zinc splashes areeasily removed and should be prior torelease. An experienced galvanizer canensure the coating overlap on double enddipped surface, is not visible.
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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 significantsurfaceB area greater than 2m2,at least three reference areasshall be taken on each controlsample.
b) For articles with a total signifi-cant surface area greater than2m2, manufactured from ma-terial of several different thick-ness’, at least three referenceareas shall be taken on eachspecific material thickness.
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 and in-cluding 2m2 inclusive, manu-factured from material ofseveral different thickness’,each different material thick-ness, representing the controlsample shall have at least onereference area, per materialthickness.
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 articles
tested equals the number ofarticles required to provideone reference area multipliedby the appropriate numberfrom the last column of table 1related to the size of the lot (orthe total number of articlesgalvanized if that is less).Alternatively, sampling pro-cedures selected from SANS2859-1 / ISO 2859-1 shall beused.
Note: 2m2 is typically 200cm x 100cm and 1 000mm2 is typically 10cm x 1cm.
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 articleshave to 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 ofsignificant surface exists: if not,the gravimetric test shall beused.
Within each reference area,which should be at least 1 000 mm2, a minimum of fivemagnetic test readings shall betaken. If any of the individualreadings is lower than the valuesin table 2 or 3, this is irrelevant asonly the mean value over thewhole of each reference area is
required to be equal to or greaterthan the local thickness given inthe table.
Thickness measurements shall notbe taken on cut surfaces or areasless than 10mm from edges,flame cut surfaces or corners.
UniformityTestingVariations in coating thickness arenot usually of concern with mate-rials hot dip galvanized after fabri-cation provided the minimumcoating thickness requirementsare satisfied, (except for fasteners -see note on maximum coatingthickness below table 3.) Wherethe coating thickness is not uni-form, the service life of the galva-nized coating will generally begoverned by the amount of zincavailable at the place where thecoating is thinnest rather than bythe overall or average thickness ofthe coating.
Standard magnetic measuring in-struments are quick and conve-nient methods for determininglocal coating thickness. By takinga number of readings on a hotdip galvanized surface, unifor-mity as well as actual thicknesscan 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 provideslimited information as to thecoating thickness, but servesonly to locate the thinnest spot ina zinc coating by chemically re-moving the zinc.
There are many limitations on theuse of the Preece test; conse-quently, it is no longer referredto in specifications for hot dip gal-vanizing after fabrication. Whenthis test is applied, its scope andlimitations should be well under-stood and no attempt should bemade to draw undue inferencefrom the test results.
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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 or prying the hot dip galvanizedcoating with a sharp knife. Con-siderable pressure is exerted in amanner tending to remove aportion of the coating. Adhe-rence is considered satisfactorywhen it is possible to removeonly small particles of the coat-ing.
It should not be possible to peelany portion of the coating in theform of a layer so as to exposethe underlying iron or steel in ad-vance of the knife point. Al-though not mentioned in SANS121 / ISO 1461, this test hasshown practical significance as atest for adhesion.
For compliance with SANS 32 /EN 10240, the most popular testis cold flattening in accordancewith SANS 8492 / ISO 8492. Testpieces not less than 40mm inlength are flattened betweenparallel flat platens as shown in table 6. No cracking or flaking ofthe coating shall occur on thesurface away from the cut sur-face.
AppearanceThe ability of a hot dip galvanizedcoating to meet its primary ob-jective of providing corrosionprotection should be the chiefcriteria in evaluating the coat-ing’s acceptability.
The basic finish requirements ofthe hot dip galvanized coatingare that it be:
- relatively smooth,
- continuous,
- free from gross imperfec-tions,
- free from sharp points (thatcan cause injury), and
- free from uncoated areasC.
The above points are of particularimportance when a subsequentorganic coating (duplex coating)is to be applied. Smoothness andlack of roughness achieved bymechanically wiped products,such as continuously galvanizedsheeting or wire, are not to beused as the criteria for accessingbatch hot dip galvanized prod-ucts. Roughness and smoothnessare relative terms where the enduse of 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 dipgalvanized articles may requirethe use of chain slings, wire orother holding devices to immersethe material into the galvanizingbath if suitable lifting fixtures arenot provided on the item. Chains,wire and special jigs used to han-dle the items may leave a mark on
Adhesion of the CoatingAdhesion is concerned with thepractical conditions of trans-portation, erection and service.The hot dip galvanized coatingshould be sufficiently adherentto withstand handling consis-tent with the nature and thethickness of the coating in nor-mal use of the article, withoutpeeling or flaking. Bending orforming, other than mildstraightening after hot dip gal-vanizing, is not considered tobe normal handling.
When certain grades of steel orvery heavy steel sections arehot dip galvanized, coatingsmay occur, which are thickerthan usual. The galvanizer haslimited control over the devel-opment of thicker coatings.Thick coatings are a function ofthe chemical composition of thesteel or the longer immersiontime required for massiveitems. Heavy hot dip galva-nized coatings, usually greaterthan 250mm thick, may bemore brittle than a typical coat-ing. Consequently, applicationand interpretation of the stan-dard adhesion tests must takethis into account. The require-ments for transportation, han-dling and erection should beevaluated against the additionalcorrosion protection affordedby the thicker coating.
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
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CTo be essentially free from un-coated areas is best described inSABS 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 orper square metre of surface of anarticle.”
the hot dip galvanized item. Thesemarks are not always detrimentaland reason for rejection. Shouldthese marks, be greater than5mm
2per chain mark and expose
the bare steel, suitable repairshould be carried out using theprocedures indicated in SANS 121/ ISO 1461. See also “CoatingRepair Procedures”.
Differences in the lustre andcolour of hot dip galvanizedcoatings do not affect corrosionresistance and the presence orabsence of spangle has no ef-fect on coating performance.The well-known spangled effectfound on some hot dip galva-nized surfaces is simply a factorof primary crystallisation. It ischiefly dependant upon the zincbath chemistry, the rate of cool-ing, the method of pickling, thesteel chemistry, and the thick-ness of the coating. In fact, dullgrey or patchy matte grey hotdip galvanized coatings giveservice lives equal to or greaterthan bright or spangled coat-ings. Variations in coating ap-pearance or finish are importantonly to the extent that they willaffect corrosion performance orthe intended use of the article.The primary function of a hot dipgalvanized coating is corrosionprotection. Specific requirementsbeyond the standard set out inSANS 121 / ISO 1461, shall becommunicated to the galvanizerin writing or discussed at thecontract review, prior to thework being hot dip galvanized.
The information supplied may in-clude –
a) The composition and anyproperties of the metal thatmay affect the quality of thehot dip galvanized coating.
b) Identification of significantsurfacesB.
c) A visual standard should beestablished if a special finishis required.
d) Any particular treatments thatare required or not requiredbefore or after the process.
e) A variance in coating thick-ness. See notes below table 3.
f) The acceptable method of repair, 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 superficial de-spite the relative bulkiness of thecorrosion product. Where surfacestaining is light and smooth with-out growth of the zinc oxide layeras judged by lightly rubbing fin-gertips across the surface, thestaining will gradually disappearand blend in with the surroundingzinc surface as a result of normalweathering in 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 removedby cleaning with a stiff bristle(not wire) brush. Heavier de-posits can be removed by brush-ing with a 5% solution of sodiumor potassium dichromate withthe addition of 0.1% by volumeof concentrated sulphuric acid.Alternatively, a 10% solution of
acetic acid can be used. Thesesolutions are applied with a stiffbrush and left for about 30 sec-onds before thoroughly rinsingand drying.
Unless present prior to shipmentfrom the galvanizer, the develop-ment of wet storage stain is notthe responsibility of the galvanizer.The customer must exerciseproper caution during transporta-tion and storage to protect againstwet storage staining.
Coating RepairProceduresBY THE GALVANIZER
In terms of SANS 121 / ISO 1461
a galvanizer may repair a coatingby either zinc metal spraying orzinc rich epoxy or paint. The lattermethod must conform to certainrequirements in the specification.The preferred method of repair isby zinc metal spraying. Repair willonly be necessary if bare spots arepresent, usually caused by inade-quate cleaning, air entrapment or if mechanical damage has oc-curred.
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 of 2m2;0,5% represents a maximum areaof 100cm2 or 100mm x 100mm.
For articles equal to an area of 10 000mm2; 0,5% represents amaximum area of 50mm2 or7mm x 7mm.
No individual repair area shall ex-ceed 10cm2 or 10mm x 100mm.
If uncoated areas are greater than0,5%, the article shall be regalva-nized, unless otherwise agreedbetween the purchaser and thegalvanizer.
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BA significant surface can bedefined as a surface whichimpacts on the performance ofthat article.
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.
The coating thickness on the ren-ovated areas shall be a minimumof 100µm, unless the purchaseradvises the galvanizer otherwise.The coating repair should overlapthe surrounding zinc by about5mm. The repaired area is thenwire brushed (preferably stain-less steel) to remove loosely ad-hering 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 blastedas above or abraded with abrasivepaper (roughness 80 grit). All dustand debris must be completely re-moved. In the event of moisturebeing present, all surfaces are tobe properly dried.
A zinc rich paint or zinc richepoxy conforming to ISO 3549should be applied. The coatingthickness on the renovated areasshall be a minimum of 100µm,unless the purchaser advises thegalvanizer otherwise. The paintcoating should overlap the sur-rounding zinc by about 5mm.
The preferred product is a two orthree component zinc rich epoxy.
SITE REPAIRS
The preferred method of repairis by zinc metal thermal spray-ing. Due to the remoteness ofmost sites, however, and theunavailability of metal sprayingequipment, repairs by zinc richepoxy or zinc rich paint have todate generally been more pop-ular.
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 the af-fected area, it should be re-moved by means of an approvedsolvent. All residues are to bethoroughly removed by washingwith 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.
Until recently, the approvedproducts for repair were onlyavailable in large containers. Dueto the large quantities involvedand short pot life when mixed,the products proved to be ex-pensive and wasteful.
Products are now available in twocomponent form, packed for con-venience in handy, easy to usesquish packs. Two of these prod-ucts are approved and availablefrom the Hot Dip GalvanizersAssociation of Southern Africa andall of its members.
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APPENDIX A
Applicable Standards
SANS 121 / ISO 1461 Hot dip galvanized coatings on fabricated ironand steel articles - Specifications and testmethods
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.
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