Now that you have a better understanding of the corrosion problem, let’s explore one solution, hot-dip galvanizing. To understand how galvanizing can minimize the corrosion problem, one must first understand the process and characteristics of the coating. Let’s start with the galvanizing process. There are three basic steps to the galvanizing process; surface preparation, galvanizing, and inspection.
The metallurgical bond between zinc and steel is ensured by thoroughly cleaning the steel before immersing it in the molten zinc. As part of the galvanizing process, steel goes through a series of cleaning steps.
The first cleaning step, degreasing, is often a hot alkali solution that removes organic contaminants like dirt, water-based paint, grease and/or oil. After degreasing, the article goes through a water rinse. (Any epoxies, vinyls, or asphalt coatings must be removed by mechanical means (grit-blasting, etc.) before steel is taken to the galvanizer.)
Next the steel is moved to the pickle bath which consists of an acidic solution that removes iron oxides and mill scale from the surface of the steel. After pickling, the steel is rinsed again.
Finally, the steel moves into the pre-flux tank. The flux serves two purposes; first it’s a lightly acidic solution that cleans any remaining iron oxides, and second it provides a protective layer to prevent any iron oxide formation prior to immersion in the galvanizing kettle.
The true “galvanizing” phase of the process consists of completely immersing the steel in a minimum 98% pure zinc bath. The bath temperature is maintained at 815°F or higher (435°C +). The steel is lowered at an angle by crane hoist. This allows air to escape from tubular shapes or pockets that may be within the design of a fabricated piece and of course permits the molten zinc to displace the air. Approximately 5 – 7 minutes after complete immersion (depending on the size of the articles), the steel reaches the bath temperature and the metallurgical reaction is complete.
The last phase of the process is the final inspection. A very accurate determination as to the quality of the galvanized coating can be accomplished through a visual inspection of the material. As stated earlier, if the steel surface is not properly and thoroughly cleaned, the zinc will not adhere to the steel. Additionally, a magnetic thickness gauge can be used to determine the thickness of the coating to ensure compliance with specification requirements.