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How to aviod the “blitz” during the metal passivation?


Metal passivation is a method of controlling corrosion in which an acid solution dissolves/corrodes the free iron present on the surface in a uniform and orderly manner. If not handled properly, a phenomenon called "blitz" can occur, resulting in uncontrolled corrosion that darkens and visibly etches the metal surface. So how to prevent this kind of failure from happening?

- Make sure there are no contaminants in the acid solution

Before passivation check the acid solution for other substances that should not be present, the acid environment is very sensitive to impurities, so even a few impurities may cause uncontrolled corrosion. In general, a high purity acid should be used to passivate metal parts to ensure the controllability of the passivation process. This remedy usually involves regularly refilling the acid tank with fresh solution while avoiding contaminants in the acid bath solution. Another recommendation is to use higher grade water, such as RO or DI water, which has relatively little chloride compared to tap water. Therefore, it can also prevent problems such as lightning attacks.

- Clean metal parts thoroughly

Surface contaminants and oxide layers are not conducive to the effect of passivation treatment, affecting the quality and adhesion of the protective layer. Any impurities such as grease or cutting oil on the part can form bubbles that may interfere with the entire process. In this case a degreaser can be used.

Using multiple cleaners alone or replacing current cleaners can also ensure that parts are free of various contaminants. Sometimes, the hot oxides produced by welding or heat treatment may need to be removed by sanding or pickling prior to the passivation process.

At the same time, the metal parts should also be thoroughly rinsed after passivation. After the acid bath, there is not only a residual acidic solution on the surface of the metal parts, but also a certain amount of ions and metal debris, which will cause the risk of corrosion for subsequent exposure and use.

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