Metallographic preparation of zinc coatings - Struers.com

Production: the coating of steel with zinc

Fig. 3: Principles of hot dip coating 

Electrogalvanizing

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.

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

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.

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.

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

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

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.

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

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, x

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, x

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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