303 ss has high sulfur or selenium to provide good machinability, etc. It is the sulfur or selenium that gives the apparent problem in passivation. In order to passivate 303 stainless steel you must first remove the sulfides and sulfates from the surface (or selenium in the case of 303Se).

Passivation is a chemical treatment with a specific acid formulation that removes free-iron or other surface contamination from the stainless steel while simultaneously promoting the formation of a passive chromium/nickel oxide layer to act as a barrier to further corrosion.

Passivation is a widely-used metal finishing process to prevent corrosion. In stainless steel, the passivation process uses nitric acid or citric acid to remove free iron from the surface. The chemical treatment leads to a protective oxide layer that is less likely to chemically react with air and cause corrosion.

Per ASTM A967 and A380, stainless parts are shown to be properly passivated if they resist corrosion in salt spray for just 2 hours. Stainless steel performs relatively poorly in salt spray because the constant exposure to a harsh environment rapidly erodes the passivation layer and prevents it from reforming.

Chromate conversion is a common way of passivating not only aluminium, but also zinc, cadmium, copper, silver, magnesium, and tin alloys. Anodizing is an electrolytic process that forms a thicker oxide layer. The anodic coating consists of hydrated aluminium oxide and is considered resistant to corrosion and abrasion.

A passivated stainless steel surface is chrome-enriched, and abrasive blasting will remove that enrichment.

7 The surface must be mechanically polished or lapped prior to passivation to provide the required surface smoothness. The acid/chelant process will not affect the surface finish. Because of the nature of the chemicals used, the organic acid/chelant treatment raises relatively few safety and environmental concerns.

What Passivation Does Not Do? In general, passivation does not discharge existing stains or rust. That requires other methods, such as light abrasion, bead blasting, tumbling and sometimes sanding. Passivation also does not remove weld scale, black oxides and burn marks from welding.

Engineers can use a variety of tests to assess a part’s passive state and overall corrosion resistance including:

1. Salt Spray Testing. The salt spray test is one of the most common corrosion resistance tests.
2. Cyclic Corrosion or Humidity Testing.
3. Copper Sulfate Testing.
4. Ferroxyl Test.
5. When Passivation Isn’t Enough.