Metallographic preparation of zinc coatings The coating of steel or iron with zinc using various processes is called "galvanizing". Metallic zinc is used for corrosion protection of cast irons, mild steels and low alloy steels. The zinc or zinc alloy cathodically protects the steel surface. In zinc coatings on parts that are exposed to air and water the zinc forms zinc carbonate that additionally protects against corrosion. The coated products are mainly used for exposed automotive applications, household goods, electrical appliances and in the building industry. 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. Fig.1: Galvalume, etched with 1% Nital, 500x Although general production control of galvanized sheet steel is done on-line and carried out with physical tests, metallography is an important tool for obtaining a variety of information not achievable by other analytical methods. An understanding of forming, welding and finishing processes of products, product research and development and failure analysis require information only provided by metallographic microstructural analysis. Fig. 2: Colored building facade of galvanized steel sheets Practical examples: If a zinc coating is too thick problems during welding can arise, if during forming the coating is chafed subsequent painting and consequently corrosion protection can suffer. Therefore metallography plays its part in improving products and advancing coating technology. Application Notes Surface of hot-dip zinc coating, SEM The metallography of galvanized products includes thickness measurements of the coatings, microstructure analysis of coatings and basemetal, adhesion checks and failure analysis. Depending on the galvanizing method, zinc coatings vary in hardness and thickness and therefore behave differently during metallographic preparation. In addition, some of the zinc coatings react with water, which makes the preparation particularly difficult. Difficulties during metallographic preparation Mounting: Shrinkage gaps between sample and resin result in stains from water and alcohol, rounded edges and trapped grinding debris. 200 x Softness of coatings: Scratches from grinding and polishing. 500 x Reaction with water: Discoloration and attack of zinc. 1000 x Solution: Proper mounting, fine grinding with diamond on rigid discs, using waterfree diamond suspensions and lubricants for polishing and proper cleaning.
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