Passivating Stainless Steel
The various processes to which stainless steel products are subjected may alter the metal surface, which will impair the stainless characteristic either temporarily or permanently.
A passive layer of chromium (III)oxide (Cr2O3) will normally protect the metal. This layer is maintained due to a special property of stainless steel. If the metal surface becomes damaged, the passive layer will generally repair itself fairly quickly providing there is sufficient oxygen available. Under certain circumstances, however, this repair will not take place. Various processes can disturb the balance to such an extent that the passive condition disappears, thus producing an active layer. This may occur as a result of processes, such as welding, bending or machining, resulting in an oxygen deficiency which will hamper the repair. The steel will therefore lose its stainless properties and may corrode when exposed to gases or liquids.
In order to prevent this, a method has been developed to reconvert the active layer produced into a passive layer. This usually requires degreasing the processed products and pickling the metal surface with a mixture of nitric acid (HNO3) and hydrofluoride acid(HF), in order to remove any contamination from the metal surface. This may be necessary on welded surfaces or machined objects which have been liquid-cooled.
The actual passivation process involves treating the metal in a bath of nitric acid, as a result of which the passive condition will return due to the layer of chromium oxide having repaired. The underlying metal will therefore recover its corrosion resistance.
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