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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Fig. 1: Galvalume, etched with 1 % Nital, 500x
Fig. 2: Surface of hot dip zinc coating, SEM
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.
Fig. 4: Electrolytically deposited zinc coating, polished to 1 μm. Final cleaning/polishing with pure alcohol, 1000x
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
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.
Fig. 7: Soft coatings, visible scratches from grinding and polishing steps, 500x
Fig. 8: The reaction with water leads to discoloration and zinc attack, 1000x
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.
Fig. 9: Hot dip coating cleaned with water, 500x
Fig. 10: Hot dip coating cleaned with alcohol, 500x
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, 1000x
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
All images by Ólafur Ólafsson, Application Specialist, Denmark.
For specific information about the metallographic preparation of Zinc Coatings, contact our application specialists.
