Metallographic preparation of zinc coatings

Zinc coatings are widely used for corrosion protection of steel and iron. But zinc coatings vary greatly, and metallographic preparation can be a challenge. This application note demonstrates proven and optimized methods that will help with the preparation of zinc coatings in a quick and reliable manner, resulting in high-quality images for structure interpretation.

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The main characteristics of zinc coatings

The coating of steel or iron with zinc, a process known as galvanizing, is mainly used to protect cast irons, mild steels and low alloy steels against corrosion. Zinc coatings provide cathodic corrosion protection, i.e., when exposed to air and water, the zinc forms zinc carbonate, which protects the steel beneath against corrosion.

To increase corrosion protection or the decorative element of galvanized steel sheets, organic coatings such as foils or paint can be applied onto the zinc coating. As a result, zinc coated products are commonly used in many sectors, such as the automotive, household goods, electrical appliance and construction industries.

The metallography of zinc coatings

The metallography of zinc is an important tool in product research and development, quality control and failure analysis. When working with zinc coatings, metallography is mainly used to:
  • Failure analysis
  • Measure the thickness of zinc coatings
  • Analyze the microstructure of the coating and the base material
  • Check adhesion

Zinc Coatings
Fig. 1: Galvalume, etched with 1 % Nital, 500x

Zinc Coatings
Fig. 2: Surface of hot dip zinc coating, SEM

Production: the coating of steel with zinc

Zinc Coatings
Fig. 3: Principles of hot dip coating 


The zinc coating is electrolytically deposited on the steel sheet, which produces a very thin, uniform layer of pure zinc.

Hot dip

The steel sheet runs through a molten bath of zinc in a continuous process. The resulting coating usually has a thickness between 7-15 µm, but can be as thick as 20 μm. After fabrication, some hot dip galvanized parts may be given further treatment, including heat treatment or static immersion into a molten zinc bath.

There are a number of very specific galvanizing methods. You can read about these methods in more detail in the full application note.

Zinc Coatings
Fig. 4: Electrolytically deposited zinc coating, polished to 1 μm. Final cleaning/polishing with pure alcohol, 1000x

Zinc Coatings
Fig. 5: Post-fabrication zinc coating, etched with 0.5% Nital, with γ phase on the border to the base metal and large δ columns in the zinc matrix, 200x

Challenges when preparing zinc coatings for metallographic analysis

As zinc coatings vary in hardness and thickness, they behave differently during metallographic preparation. In addition, some zinc coatings react with water, which makes preparation particularly challenging.

Challenges during mounting
Mounting of zinc coated specimens can be a challenge, especially when time is an issue. To avoid shrinkage gaps, proper adhesion between the mounting material and the specimen must be ensured.

Zinc Coatings
Fig. 6: Shrinkage gaps between the specimen and resin can cause water and alcohol stains, as well as edge rounding and trapped grinding debris, 200x 

Challenges during grinding and polishing steps
Zinc coatings become softer and more water sensitive depending on the purity of the zinc in the coating, especially plain hot dip and electrolytically deposited coatings, which have a high zinc content. This makes them soft and prone to mechanical deformation. They cannot be cleaned with water.

Zinc Coatings
Fig. 7: Soft coatings, visible scratches from grinding and polishing steps, 500x

Zinc Coatings
Fig. 8: The reaction with water leads to discoloration and zinc attack, 1000x

Recommendations for cutting zinc coatings

Cutting steel sheets is relatively simple and can be done with the appropriate abrasive aluminum oxide wheels. Cutting with a guillotine or tin snips can bend the sheet severely and crack the coating.


Recommendations for mounting zinc coatings

The biggest challenge when mounting zinc coated specimens is avoiding shrinkage gaps between the mounting resin and the coated specimen. The solution is to degrease the specimen with acetone before mounting and use the appropriate mounting resins.

The following mounting resins are recommended:
  • Slow-curing cold mounting epoxy that has negligible shrinkage and adheres very well to the specimen.
  • Hot compression mounting with a phenolic resin that contains carbon fibers (PolyFast) is highly recommended as it leaves no shrinkage gaps and has the appropriate hardness to keep the specimens flat.
How to mount large volumes of Zinc coatings
When mounting sheet specimens, it is recommended to hold the specimens upright with plastic clips. However, specimen clips can only hold a few sheets, which is too slow for high volume quality control laboratories. Therefore, the following process is recommended:
  • Glue the multiple sheets together with instant glue.
  • Hot mount the glued sheets in PolyFast.
You can find a more detailed description of how to mount zinc coating specimens in the full application note.

Find out more

Recommendations for grinding & polishing zinc coatings

Galvanized steel sheets have traditionally been ground with various grits of silicon carbide foil or paper, followed by two or three diamond polishing steps. To shorten this procedure, replace fine grinding on silicon carbide foil or paper with fine grinding using a special fine grinding disc (MD-Largo).

Preparation method

The following preparation method can be optimized according to the process requirements.
Zinc Coatings

The five-step grinding and polishing method consist of:
  1. Plane grinding with SiC Foil or SiC Paper #320.
  2. Fine grinding with MD-Largo and DiaPro Allegro 9 µm/DiaPro Largo 9 µm suspension.
  3. Diamond polishing with MD-Dac and DP suspension A, 3 µm suspension with yellow lubricant.
  4. Diamond polishing with MD-Dur and DP suspension A, 1 µm suspension with yellow lubricant.
  5. To avoid problems with surface cleaning, an additional step with ethanol denatured with isopropyl alcohol on an MD-Chem polishing cloth is recommended.
General recommendations for grinding and polishing of zinc coatings:
  • For automatic dosing systems, water-free diamond suspensions and lubricants will eliminate staining.
  • When using grinding and polishing equipment without an automatic dosing system, diamond spray gives excellent results as it contains no water.
  • Silk cloths keep the specimens flat (although not completely scratch free).
  • Polishing with soft, napped cloths should be avoided as it can result in some relief.

Cleaning and drying zinc coatings

As zinc reacts with water, cleaning the specimens can be difficult. The effect of water is irrelevant during grinding, but water can cause discoloration of the coating in subsequent polishing steps that might affect analysis.

How to avoid discoloration:
  • Do not use water to clean between diamond polishing steps. Instead, swab and rinse the specimen with ethanol denatured with isopropyl alcohol. Dry it with clean compressed air.
  • For final cleaning, give a very brief polish with ethanol denatured with isopropyl alcohol on a MD-Chem polishing cloth. Rinse and dry the specimen.
  • If using an automatic dosing system, program the dosing system to give a large dose of water-free lubricant immediately after final polishing. This will make subsequent cleaning easier.

Zinc Coatings
Fig. 9: Hot dip coating cleaned with water, 500x

Zinc Coatings
Fig. 10: Hot dip coating cleaned with alcohol, 500x

Find out more

Recommendations for etching zinc coatings

The most common etchant for zinc coatings is 0.5-2 % alcoholic nitric acid. Etching times are very brief, just seconds. Over-etching occurs very easily, so etching should be done with care.

Zinc Coatings
Fig. 11: Galfan coating, etched with 0.5 % Nital. Primary dendritic structure, 500x

Zinc Coatings
Fig. 12: Same coating as Fig. 9. The higher magnification shows a zinc-rich solid solution α and a lamellar eutectic structure consisting of α and the aluminum rich phase, β, 1000x

Zinc Coatings
Fig. 13: Galvaneal coating etched with 1% Nital. A thin, iron-rich diffusion layer γ shows between the steel base and zinc coating. The structure of the coating is zinc-iron, with varying concentrations depending on the distance from the base metal, 1000x

Delve deeper

For a more detailed description of the challenges and recommendations when preparing zinc coated specimens for materialographic analysis.

Download the full application note


The coating of steel with zinc is commonly done in the automotive, electrical appliance and building industries to protect cast irons, mild steels and low alloy steels from corrosion.

Depending on the galvanizing method, zinc coatings vary in hardness and thickness. Therefore, they behave differently during metallographic preparation. In addition, some zinc coatings react with water, which makes preparation particularly challenging. In general, zinc coatings require:
  • A reliable mounting technique with the appropriate mounting resin.
  • Fine grinding with diamond suspension on a rigid disc (MD-Largo).
  • Polishing and thorough cleaning with ethanol denatured with isopropyl alcohol.
  • Very short etching times.

Get insight into other materials

If you would like to learn more about the materialography of other metals and materials, check out our materials page.

Get the full application note

Want to read the full application note on the metallographic preparation of Zinc Coatings? Download it here.

Holger Schnarr

All images by Ólafur Ólafsson, Application Specialist, Denmark.
For specific information about the metallographic preparation of Zinc Coatings, contact our application specialists.