The surface galvanizing of steel plates is hot-dip galvanizing

Hot dip galvanizing is a development of the older hot-dip galvanizing method, which has a history of 170 years since France applied hot-dip galvanizing to industry in 1836. However, the hot-dip galvanizing industry has experienced large-scale development in the past thirty years with the rapid development of cold-rolled strip steel.
The production process of hot-dip galvanized sheet mainly includes: original plate preparation → pre plating treatment → hot-dip coating → post plating treatment → finished product inspection, etc. According to tradition, hot-dip galvanizing processes are often divided into two categories based on different pre-treatment methods: offline annealing and in line annealing.
a. Off line annealing
Before hot-rolled or cold-rolled steel plates enter the hot-dip galvanizing production line, they are first subjected to recrystallization annealing in a bottom drawn annealing furnace or a cover type annealing furnace, so that there is no annealing process for the galvanizing line. Before hot-dip galvanizing, the steel plate must maintain a clean, pure iron active surface free of oxides and other dirt. This method involves first removing the annealed surface oxide scale by acid washing, and then coating it with a layer of zinc chloride or a solvent composed of a mixture of ammonium chloride and zinc chloride for protection, in order to prevent the steel plate from being oxidized again.
b. In line annealing
It is directly provided by the cold rolling or hot rolling workshop as the raw material for hot-dip galvanizing, and gas protected recrystallization annealing is carried out in the hot-dip galvanizing production line
① Principle
The hot-dip zinc layer is formed by dividing zinc into three steps in liquid state, and the iron based surface is dissolved by the zinc liquid to form a zinc and iron alloy phase layer; The zinc ions in the alloy layer further diffuse into the matrix to form a zinc iron miscible layer; The surface of the alloy layer is wrapped with a zinc layer.