Welding Through Lubricant on Cold-Rolled-Steel Tube/Cleaning

Question:

I have been asked to clean some parts made from corrosion resisting steel. The grade is S520 or S526, i.e. softened 18/10 Cr/Ni. The parts have been crudely laser (or EB) cut and have scorch marks (bluing) present. The parts also appear generally dirty.

I have tried 5% NaOH, but this does not remove the scorch marks. I have tried 5% H2SO4, this leaves a black smut. Would a passivation treatment (20–25% nitric acid solution with 2–6% weight sodium dichromate added) be a better treatment or is there another treatment which you could recommend? V.A.

Answer:

The sodium hydroxide solution would only be effective in saponifying fatty oils that may be present on the surface. The passivating solution will remove free iron from the surface of the part, but will not do a good job removing the bluing. The passivating solution you reference above generally is recommended for 400 series stainless steels containing less chromium in them than your 18% chromium, austenitic stainless steel.

This discoloration is an oxide that was created from the heat of the laser or electron beam cutting and would have to be removed with a pickling solution or with an abrasive. One effective combination of acids for removing this would be nitric and hydrofluoric acids. The nitric would be used at about 10% by volume and the hydrofluoric at about 1–2% by volume. When heated (about 100–120°F), it should effectively remove this oxide. If heating is not available, room temperature may work as well, but would take longer.