Passivate Stainless Steel

Question:

I have been told that it is not necessary to passivate 316 S.S., but that 303 and 304 must be to provide a corrosion resistant surface.

Answer:

The 316 stainless steel has better pitting corrosion resistance than either the 303 or 304 stainless steel through addition of about 2-3% molybdenum. This allows the 316 to withstand chloride exposure better than either the 303 or 304 stainless steel. However, even with the addition of the molybdenum, the 316 would have the same need for passivation as the 303/304 material.

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Stainless steels derive their corrosion resistance through the development of a thin chrome oxide layer on the surface. This chrome oxide layer makes the material relatively inert to many different environments through the development of a chrome oxide layer. If this layer is damaged or incomplete in some way, it provides a site for the initiation and propagation of corrosion on any of the stainless steels.

The oxide layer can be breached by a number of industrial manufacturing practices including welding, brazing, cutting and grinding. In addition to the breakdown of the oxide layer, other metal chips or fines may become embedded into the surface. These would also serve as initiation sites for corrosion. To eliminate this, the parts are passivated. With austenitic (300 series stainless steels), this usually involves immersion or application of nitric or citric acid to the surface of the parts. This will help to dissolve any embedded foreign material and re-establish the chrome oxide layer to the surface.