What is Hot Dip Galvanizing?
Hot-dip galvanizing has been utilized for over 250 years to protect steel and iron from destruction by corrosion. The process, which has evolved over the years, entails dipping fabricated steel into a kettle of molten zinc. The iron in the steel reacts with the zinc to form a tightly bonded alloy coating which protects the steel from corrosion.
Our Hot Dip Galvanizing Process
The “galvanizing” occurs when items are completely immersed in a zinc bath (kettle) of 99.95% pure molten zinc maintained at a temperature of approximately 840 F (449 C). The items are lowered at an angle best suitable for each shape allowing air to escape any pockets or gaps that may be within the design of the item permitting the molten zinc to displace the escaped air.
Once the fabricated item reaches bath temperature articles are slowly withdrawn from the zinc bath. Excess zinc is removed by draining, vibrating, and/or centrifuging depending on the item. The metallurgical reaction continues even after withdrawal from the zinc bath as long as the item remains near bath temperature. Articles are cooled off by immersing in a passivation solution and water and then left in open air or hot plate for some time to dry (ASTM 123 and 153).
Coating Thickness Test
Coating thickness refers to the thickness of the final hot-dip galvanized coating. Two different methods are used to measure the coating thickness of hot-dip galvanized items.
Hot-dip galvanized coatings is evaluated with an electronic gauge by randomly selecting specimen representing every batch. The inspection quantities are determined by the lot sizes as detailed in the ASTM specifications A123/A123M, A153/A153M, and A767/A767M or as per customer request. The ASTM specifications contain detailed information, but a couple of universal practices are recognized e.g.:
Service Life Estimate Test
Salt Spray Test
The purpose of salt-spray is to estimate the service life a particular coating might have in a specific environment. The salt spray test (or salt fog test) is a standardized and popular corrosion test method, used to check corrosion resistance of materials and surface coatings. We conduct Salt Spray Tests as per ASTM B117 and G85 specifications from time to time to determine thickness, uniformity, adherence, and appearance to ensure the coating life.
The official name of the Preece Test is Standard Test Method for Locating the Thinnest Spot in a Zinc (Galvanized Coating on Iron or Steel Articles by the Preece Test (Copper Sulfate Dip) (ASTM 239-95). It is used primarily for hardware items such as nuts and bolts. It is impractical for structural steel items because of the testing solutions and handling that would be required.
Rejected items are separated and QA team inspects each rejected item and decides action items which could be repair for minor defect by wire brushing, touching etc. or re-galvanizing for defects of considerable size.