Hot Dip Galvanizing is the process of applying a zinc coating to steel by dipping it into a molten zinc bath to form a metallurgical bond to the steel. The process consists of three steps: 1. Surface Preparation (cleaning), 2. Galvanizing and 3. Final Inspection.
The most effective step in the galvanizing process is to prepare the steel to be immersed in the zinc bath. Molten zinc will not react properly or metallurgically bond to steel properly if it is not completely clean. Cleaning of galvanizing materials consists of three solutions and rinses. The first of the three is called a caustic bath. Caustic is a hot alkaline solution that cleans surface dirt, some sign paints, grease and oil. The next step is a hot sulfuric acid bath, more commonly called pickle. Pickling removes mill residue and rust on the surface. Pickling time is determined by the cleanliness of the steel. Sandblast material significantly reduces pickling time and generally encourages faster turnaround times. The last step of the cleaning process is flux. The flux tank consists of an aqueous zinc ammonium chloride. Flux is eliminated and prevents other oxides from forming on the steel surface before galvanizing. Materials from the powder are air dried to prevent the zinc from splashing out of the kettle. Between each step in the cleaning process, the material is quickly rinsed with water to avoid cross-contamination of the different solutions.
After the material is properly cleaned, it is ready to be galvanized. The materials are immersed until they reach bath temperature (842); then they begin to pull back the material at a slow speed to allow the excess zinc to exit the holes. During this step in the process, all preparations pay off. Convenient holes allow the zinc to flow through the material and do not have the appearance of exhaustion and solidification of a deposit. Threaded materials are passed through a centrifuge and excess zinc is returned from the material that cleans the teeth.
After the materials are galvanized, they are quenched in water or air cooled for use and final inspection. Coating thickness and appearance are two of the top concerns for property inspectors and customers. Materials are galvanized according to CSA (Canadian Standards Association) and ASTM (American Society for Testing and Materials).