NotesApplication

Metallographicpreparation of zinccoatings

Surface of hot-dip zinc coating, SEMFig.1: Galvalume, etchedwith 1% Nital, 500x

Difficulties during metallographic preparation

Mounting: Shrinkage gaps between sampleand resin result in stains from water andalcohol, rounded edges and trapped grindingdebris. 200 x

Softness of coatings:Scratches from grindingand polishing. 500 x

Solution: Proper mounting, fine grinding with diamond on rigid discs, using waterfree diamondsuspensions and lubricants for polishing and proper cleaning.

Reaction with water:Discoloration and attack of zinc.1000 x

Fig. 2: Colored building facade ofgalvanized steel sheets

The coating of steel or iron withzinc using various processes iscalled "galvanizing". Metallic zincis used for corrosion protection ofcast irons, mild steels and low al-loy steels. The zinc or zinc alloycathodically protects the steel sur-face. In zinc coatings on parts thatare exposed to air and water thezinc forms zinc carbonate that ad-

ditionally protectsagainst corrosion. Thecoated products aremainly used for ex-posed automotive ap-plications, householdgoods, electrical appli-ances and in the build-

ing industry. To increase corrosionprotection or the decorative elementof galvanized steel sheets, organiccoatings such as foils or paint canbe applied onto the zinc coating.

Although general production con-trol of galvanized sheet steel isdone on-line and carried out withphysical tests, metallography is animportant tool for obtaining a vari-ety of information not achievableby other analytical methods. Anunderstanding of forming, weldingand finishing processes of pro-ducts, product research and deve-lopment and failure analysis re-

quire information only provided bymetallographic microstructuralanalysis.

Practical examples: If a zinc coat-ing is too thick problems duringwelding can arise, if during formingthe coating is chafed subsequentpainting and consequently corro-sion protection can suffer. There-fore metallography plays its part inimproving products and advancingcoating technology.

The metallography of galvanizedproducts includes thickness mea-surements of the coatings, micro-structure analysis of coatings andbasemetal, adhesion checks andfailure analysis. Depending on thegalvanizing method, zinc coatingsvary in hardness and thicknessand therefore behave differentlyduring metallographic preparation.In addition, some of the zinc coat-ings react with water, which makesthe preparation particularly difficult.

Production and applicationsof zinc coatings

Fig. 4: Facade of Galvalume sheet withorganic coating

Fig. 3: Pressed floor panel

Hot dipThe steel sheet runs through amolten bath of zinc in a continu-ous process, during which clean-ing, temperature, coating thick-ness etc. are automatically con-trolled. The coating contains morethan 99% zinc and has usually athickness between 7-15 µm, butcan vary according to specifica-tions.Application: primarily for automo-tive applications, construction andhousehold goods, such as wash-ing machines.

GalvanealAfter the hot dip the coated sheetcan be heat treated, changing thezinc coating into a zinc-iron coat-ing, containing approximately 10%

iron. The coating is stronger andhas a higher wear resistance andweldability than the plain hot dipcoating. On its rough surface paintadheres particularly well, whichgives an additional corrosion pro-tection when painted.Application: primarily pressed au-tomotive components that arecoated with organic coatings,outer and inner body panels.

Galvalume™ 1)

This is a hot dip coating from abath containing 55% aluminium,43.4% zinc and 1.6% silicon. Thiscoating has a good heat-oxidationresistance.Application: construction, mainlyfacade coverings, also automotivecomponents.

Galfan™ 2)

This is a hot dip coating from azinc bath containing 5% aluminiumand traces of rare earths.

The Galfan coating has a fine eu-tectic structure (see Fig. 10), whichis suited for bending, deep draw-ing and welding, and has an excel-lent corrosion resistance afterpainting. Application: construction,automotive and household appli-ance industry.

ElectrogalvanizingThis zinc coating is electrolyticallydeposited on the steel sheet andproduces a very thin, uniform layerof pure zinc. The coating thicknessis between 2-6 µm, and it is espe-cially suitable for subsequentpainting. A typical application is forcar automotive bodies.

Fig. 5: Electrolytically deposited zinc coating,polished to 1 µm and final cleaning/polishwith pure alcohol, 1000 x

Pickling Preheat Cooling

Hot Metal bath

Principles ofHot Dip Coating Air knives

1) 2) Galvalume™ and Galfan™ are proprietarygalvanizing methods. Galvalume™ in Canadais a registered trademark of Dofasco Inc andin the USA Galvalume™ is a registeredtrademark of BIEC INC. Galfan™ is aregistered trademark of the International Leadand Zinc Research Organization in USA.

→

The thickness of zinc coatings ofcontinuous galvanized products isusually specified by weight, g/m²or oz/ft2 and can range between6 µm and more than 20 µm de-pending on the application of thefinal product.

For post fabrication hot dip galva-nizing of finished parts, such ascast iron construction parts, thefinished product is immersed intothe molten zinc bath. These coat-ings are much thicker than thecontinuously galvanized coatings(Fig. 6).

Difficulties in thepreparation of zinccoatings

Fig. 6: Post-fabrication zinc coating, etchedwith 0.5% Nital, with γ phase on the border tothe base metal and large δ columns in thezinc matrix, 200 x.

Fig. 8: Hot dip coating cleaned with alcohol,500 x

Fig. 7: Hot dip coating cleaned with water,500 x

The main problems of preparingzinc coatings for microscopicobservation are:

1. Shrinkage gaps between mount-ing resin and coated steel sheets.These gaps make it difficult toclean the samples, especially onclamped sheet packs.

Mounting zinc coatings constitutesa major problem because themounting needs to guarantee thatthe resin adheres properly to thesample material, so that no gapsappear. If zinc coatings are pre-pared for production control, aconflict arises between number ofsamples and available time so thatcompromises in mounting are com-mon. This usually results in amounting method that producesshrinkage gaps next to the zinccoating, which makes it difficult toclean the specimens and achievean optimal edge retention. Result-ing water and alcohol stains ob-struct proper thickness measure-ment and structure interpretationon the coating.

2. The softness of zinc and its re-action with water can leave thecoatings with scratches, discolo-red or even etched.

The purer the zinc of the coating is,the softer and the more water-sen-sitive it becomes. Therefore, plainhot dip and electrolytically depos-ited coatings are soft and prone tomechanical deformation and theycannot be cleaned with water (Figs7 and 8). Ethanol or Isopropanolhave to be used for cleaning. Dueto the addition of aluminium, re-spectively heat treatment, GalfanTM,GalvalumeTM and Galvaneal coat-ings are harder than plain hot dipand electrolytically deposited coat-ings. Generally they create fewerproblems during polishing and theirreaction with water is less vehement.

Recommendations forthe preparation ofzinc coatingsCutting steel sheet isnot difficult and canbe done with appro-priate abrasive alu-minium oxide wheels.Sometimes sheets arecut with a guillotine ortin snips, which maybe sufficient for very thin sheets,but can bend the sheet severelyand crack the coating when it isthicker. A subsequent longergrinding time to the undamagedarea of the sheet is necessary.Cutting with an abrasive wheel isthe more economical alternative inthese cases.

MountingThe above-mentioned problem ofshrinkage gaps between mountingresin and coated specimen can beavoided by degreasing the speci-men with acetone before mounting

and using propermounting resins.Slow curing, coldmounting epoxy hasa negligible shrink-age and adheresvery well to thespecimen. Hot com-

pression mounting with a phenolicresin containing carbon fibres(PolyFast) is highly recommendedas it leaves no gaps between resinand coating and the resin has theright hardness to keep the sam-ples flat.

For both mounting methods, indi-vidual sheet specimens are held

upright with plasticclips. Becausethese sample clipscan only hold a fewsheets, these meth-ods are too slow forhigh productionquality control labo-

ratories. In these laboratories themost common method for holdingcoated steel sheets is clampingthem in packs: the cut pieces areput into packs and held togetherby a steel clamp.

This method makes it possible tohold large quantities of sheets,however it does not eliminate theproblem of the gaps between thesheets. The clamps are difficult toclean and dry because of thescrews and parts they contain,and structure interpretation andthickness measurements are ob-structed by alcohol bleeding fromthe gap. Gluing the sheets to-gether with instant glue and then

hot mounting the resulting packsin PolyFast eliminates the gapsand has the advantage of provid-ing a properly mounted, cleansample (see Fig. 9.)

Grinding and polishingGalvanized steel sheets have tradi-tionally been ground with variousgrits of silicon carbide paper, fol-lowed by two or three diamondpolishing steps. Replacing the finegrinding on silicon carbide paperwith a special fine grinding disc(MD-Largo), onto which diamondspray or suspension is applied,can now shorten this procedure.MD-Largo also gives excellentedge retention, which is particu-larly important when observing the

coatings at 1000 x. For the subse-quent polishing, two diamond po-lishing steps with 3 µm and 1 µmare sufficient. With this method,the grinding and polishing stepshave been reduced from six to four.

Although the different zinc coat-ings vary in hardness, it is possibleto grind and polish them automati-cally with the same preparationmethod. When grinding/polishingequipment withoutan automatic dos-ing system isused, diamondspray gives excel-lent results, as itcontains no water.For automatic

Step PG FG 1

Surface SiC-paper, #320 MD-Largo

Suspension DiaProAllegro-Largo

Lubricant Water

Force [N] 30 per sample 30 per sample

Time As needed 4 min.

Polishing

Step DP 1 DP 2 OP

Suspension DP-Susp. A, 3 µm DP-Susp. A, 1 µm

Surface MD-Dac MD-Dur OP-Chem

Lubricant DP-Yellow DP-Yellow Ethanol*

rpm 150 150 150

rpm 300 150

Force [N] 25 per sample 20 per sample 15 per sample

Time 4-6 min. 4-6 min. 15-20 sec.

Comments *Ethanol denatured with isopropyl alcohol

Preparation Method

Grinding

Fig. 9:Galfan coating,etched with 0.5%Nital. Primarydendritic structure,500 x

Fig.11: Galvanealcoating etchedwith 1% Nital,1000 x

Fig.10: Samecoating as Fig. 9,higher magnificationshows lamellareutectic structure,1000 x

dosing systems, waterfree dia-mond suspensions and lubricantsare necessary to avoid staining ofthe coatings. Silk cloths keep thesamples flat although not com-pletely scratchfree. Polishing withsoft, napped cloths should be avoi-ded as it can result in some relief.

Using automatic grinding/polishingequipment with automatic dosing,such as the RotoSystem, has theadvantage of giving reliably, verygood and reproducible results. Thepreparation method shown on theprevious page, for 30 mm mountedindividual samples or multiple sam-ples in holders, has successfullybeen used for galvanized and elec-trolytically deposited zinc coatings.

CleaningZinc reacts with water, whichmakes the cleaning of the samplesdifficult. During grinding the effectof the reaction with water is negli-gible, but between diamond pol-ishing steps swabbing and rinsingwith ethanol denatured with iso-propyl alcohol is recommended,followed by drying with clean com-pressed air.For final cleaning, a very briefpolish with ethanol denatured withisopropyl alcohol on a MD-Chempolishing cloth, followed by rinsingand drying, has given the best re-sults. If an automatic dosing sys-tem is used, a large dose of water-free lubricant can be programmedto follow after the final seconds ofpolishing. This will make the sub-sequent cleaning easier.

Etching and InterpretationThe most common etchant for zinccoatings is a 0.5%-2% alcoholicnitric acid. Etching times are verybrief (seconds!) and over-etchingoccurs very easily, and each coat-ing reacts differently.

Fig. 9 shows a Galfan coatingetched with 0.5% Nital. It shows aprimary dentritic structure.

Fig.10, Galfan coating as fig. 9,showing a zinc rich solid solutionα and a lamellar eutectic structureconsisting of α and the aluminiumrich phase, β.

Fig.11 shows a Galvaneal etchedwith a 1% Nital. A thin, iron richdiffusion layer γ shows betweensteel base and zinc coating. Thestructure of the coating is zinc-ironwith varying concentrations de-pending on the distance from thebase metal. Some of this structurecan be revealed using a more so-phisticated etchant containing pic-ric and nitric acid. (For the detailsof this etchant please, see theStruers e-Metalog.)

SummaryTo make the preparation of zinccoatings easier and more repro-ducible, and to improve the im-ages for structure interpretation,the following recommendations for

automatic grinding and polishingcan be made:

- Despite varying hardness, allgroups of zinc coatings can beautomatically prepared using thesame method.

- The main prerequisite for a suc-cessful polish is a good mountwithout shrinkage gaps betweenresin and sample.

- Fine grinding with diamond onrigid disc gives excellent flatness.

- Waterfree diamond suspensionsand lubricants keep the zinccoatings from staining.

- Silk cloths keep the coatings flatduring polishing.

- All cleaning steps duringpolishing have to be carried outwith ethanol denatured withisopropyl alcohol.

The given preparation method canbe used as a base for further cus-tomising the grinding and polishingprocedure in individual laborato-ries, depending on the variety ofcoatings to be polished.

Struers A/SValhøjs Allé 176DK-2610 RødovrePhone +45 36 70 35 00Fax +45 38 27 27 01e-mail: struers@struers.dkwww.struers.com

05.02 / 62140102Printed in Denmark byJørgen Larsen Grafisk

ApplicationNotes

Metallographic preparation ofzinc coatings

Elisabeth Weidmann,Struers A/S, Copenhagen

Credits:We wish to thank DOFASCO Inc., andSTELCO Inc., Hamilton, Canada,Fa. Julius Blum GmbH, Höchst, Austria,Rautaruukki Steel, Hämeenlinna, Finland,for generously supplying sample materialand related information, and VoestalpineStahl GmbH, Linz, Austria, andZinkberatung Ingenieurdienste GmbH,Düsseldorf, Germany, for contributinggeneral information on zinc coatings.We thank Stahl-Informations-Zentrum,Düsseldorf, Germany for givingpermission to reproduce the followingimages: Washing machine and Fig. 4from Charakteristische Merkmale 095and Fig. 3 from Dokumentation 535.We thank Rautaruukki Steel, Hämeenlinna,Finnland for giving permission toreproduce the following images: Fig. 2and harvest combine, and SEM imageson the first and last page.

Bibliography:Charakeristische Merkmale 092„Schmelztauchveredeltes Band undBlech“, 1998Charakteristische Merkmale 095„Elektrolytisch verzinktes Bandund Blech“Dokumentation 535 „Weiterverarbeitungvon verzinktem und beschichtetemStahlblech“, 1997All: Stahl-Informations-Zentrum,Düsseldorf, Germany

Surface ofGalvanealed coating

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