Passivating Stainless Steel

Question:

Recently, we had to resurface a small section on a piece of stainless steel equipment. The grinding wheel that was used was made of iron. This was a very small section on a very large part so we would like to know if there is a way of removing any iron contamination without having the whole piece passivated. If it must be passivated, is it possible to do a “spot passivation” and will that be sufficient? M.T.

Answer:

That section should be passivated if you want the stainless steel to maintain its corrosion resistant properties. You are correct in that you only need to concentrate the treatment in the area that was ground with iron. Would it be possible to dip only that section, or possibly brush the area several times with the passivating solution (usually about 30% nitric acid at room temperature)? The brushing idea may start to get messy since several applications may be necessary. You may also want to contact your chemical supplier regarding a passivating product that can be used on vertical surfaces. It is essentially nitric acid with a thickener added so that the product is like a gel (see my column from July 2000 for a product similar to this). I would be interested in hearing from other readers that are required to passivate welds on large structures made from stainless steel..