Stainless Steel Streaking After Passivation

After passivating 347 stainless steel per AMS700 Type VI there is a stain pattern. On inspection at 10x magnification, I see that the surface is pitted. Process steps are alkaline degrease at 140°F, two rinses, final rinse in reverse osmosis water, nitric strike at 5 percent by volume, then two rinses with the final reverse osmosis (water quality 200 ppm). The stains or discolorations are in a straight line pattern. Any suggestions?

Q. After passivating 347 stainless steel per AMS700 Type VI there is a stain pattern. On inspection at 10x magnification, I see that the surface is pitted. Process steps are alkaline degrease at 140°F, two rinses, final rinse in reverse osmosis water, nitric strike at 5 percent by volume, then two rinses with the final reverse osmosis (water quality 200 ppm). The stains or discolorations are in a straight line pattern. Any suggestions? C.T.

A. Your comment about the surface being pitted is probably the most significant. You need to walk your process to understand at what point the pitting of the stainless steel components is occurring. As I am sure you are aware, stainless steel has excellent corrosion resistance and would generally be considered immune from attack when in contact with the chemicals you mention.

The first step of your process is to review a certification from the person supplying the parts to insure they are indeed 347 stainless steel. As a quick verification, you could place a magnet on the surface of the part to see if there are any magnetic properties. Austenitic stainless steels like the 347 (or any other 300 series) have little to no magnetic response. (A very small degree of magnetism may exist, depending on the previous processing.) If your part is magnetic, you can be sure it is not the 347 stainless steel you think it is, and what you are witnessing is much more likely, considering the alternatives. A ferritic stainless steel (400 series) would be more likely to have the response you are describing (pitting with associated streaking from the localized corrosion).

If it is indeed an austenitic stainless steel, then you need to literally check parts or material as received (before subjecting them to any of your manufacturing steps) for signs of corrosion or pitting. Then inspect and note any changes at each stage of the process. Inspect before the cleaning and passivating and at every individual step along the way to check for pitting and to verify if streaking is occurring concurrent with the pitting corrosion observed. The point in the process that you find corrosion and pitting would be the process step you need to isolate to better understand the root cause of the problem.