The medium causes the erosion of the metal. If the corrosion products have a compact structure, it would form a thin and invisible film, which cover metal’s surface. To be honest, it chances the metal’s surface situation, and makes the electrode potential to the positive position. Therefore, it is corroded. For example, the standard potential of Fe→Fe＋＋ is －0.44 V.
But when it is corroded, it becomes ＋0.5～1 V and shows the precious metal property, which is anti-corrosion. And the thin film is called steel passivation coating. The chromate solution on the metal’s surface to produce trivalent and hexavalent chromium layer is called steel passivation or chromizing. Sometimes, the chemical reaction of the steel would produce the chromizing layer, but it is rarely used alome.
Generally, people match it with the parkerising to seal its holes. And it makes the bare steel to passivate to resist the corrosion in the phospating accelerator and enhances its protective capability. When it passivated, people often put the metal into the protective capability (2-4 g/L, or 1-2 g/L) with 80-90 degrees for 2-3 minutes. And then it should be washed.
Effects: to enhance the great corrosion resistance and adhesive power.
Galvanized steel sheets are divided into the greased plates and passivation plates. The greased galvanized sheets is oiled obviously and if people use the napkins to clean it, you can see it. The surfaces of passivation plates only have little oil. And if people clean it, they can not find it. The galvanized steel without oil must be passivated. If it is not passivated, the zinc layer must be pale.
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