Process of galvanizing
In the process of hot dip galvanizing steel under goes rigorous preparation i.e physical and chemical treatment of the steel. When sending steel through the Hot Dip Galvanizing process, it undergoes work, chemical treatment to remove impurities before finally dipping into a kettle of molten zinc at a temperature of around 450°C. When exposed to the atmosphere, pure zinc reacts with oxygen to form zinc oxide, which further reacts with carbon dioxide to form zinc carbonate, a dull grey, fairly strong material that stops further corrosion in many circumstances, to deliver a reinforced metallurgically alloy bond that protects the steel from corrosion.
Depending upon the project requirement treatment of the steel is decided, by selecting the most effective corrosion protection system, and your analysis of each method should include such things as the durability, maintenance schedule, coating’s service life, and initial and life-cycle costs.
Hot-dip galvanizing, with its superior durability, life-cycle cost, and maintenance schedule make it suitable for most applications.
Surface Preparation is the most important step in the application of any coating. In most instances where a coating fails before the end of its expected service life, it is due to incorrect or inadequate surface preparation.
The surface preparation step in the galvanizing process has its own built-in means of quality control in that zinc simply will not react with a steel surface that is not perfectly clean. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the molten zinc. Any areas that were not properly prepared will remain uncoated. Immediate corrective action is taken.
Surface preparation for galvanizing typically consists of three steps: degreasing cleaning, acid pickling and fluxing.
Degreasing– A solution often is used to remove organic contaminants such as dirt, markings, grease and oil from the metal surface. Epoxies, vinyls, asphalt or welding slag must be removed before galvanizing by grit-blasting, sandblasting or other mechanical means.
Pickling – Scale and rust normally are removed from the steel surface by pickling in a dilute solution of hydrochloric acid.
Rinsing - To remove acid and iron salt.
Fluxing - Steel is immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the bath of molten zinc. The item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized.
Galvanizing In this step, the material is completely immersed in a bath consisting of minimum 99.9% SHG pure molten zinc. Fabricated items are immersed in the bath long enough to reach bath temperature. The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by draining.
The chemical reactions that result in the formation and structure of the galvanized coating continue after the articles are withdrawn from the bath as long as these articles are near the bath temperature. The articles are cooled in either water or ambient air immediately after withdrawal from the bath.
The two properties of the hot-dip galvanized coating that are closely scrutinized after galvanizing are coating thickness and coating appearance. A simple physical test may be performed to determine thickness, uniformity, adherence and appearance.