Although its chemical composition provides extremely high corrosion resistance, stainless steel is not entirely impervious to corrosion. All grades of stainless steel contain at least 10.5% chromium—it is this element that allows stainless steel to form a thin, self-repairing passive layer of oxide on its surface.
However, certain factors relating to stainless steel passivation can lead to stainless steel rust spotting or staining. Stainless steels with lower chromium content and scratches in the naturally-occurring passive layer that become embedded with particles of carbon steel are common culprits. This naturally occurring cause of stainless steel rust is called rouging.
Stainless Steel Rouging
Stainless steel rouging refers to a form of corrosion found in stainless steel. It can be due to iron contamination of the stainless steel surface due to welding of non-stainless steel for support columns, or other temporary means, which when welded off leaves a low chromium area. There are three classes of stainless steel rouging, they include:
- Class I – The stainless steel surface and the Cr/Fe ratio of the metal surface beneath such deposits usually remain unaltered.
- Class II - Iron particles begin to originate in the unpassivated or improperly passivated stainless steel surfaces. Due to their formation, the Cr/Fe ratio of the metal surface is altered.
- Class III - Iron oxide (or scale) begins to form on surface of high-temperature steam systems. The Cr/Fe ratio of the protective film is usually altered.
Stainless Steel Passivation Process
Stainless steel passivation remains a critical step in maximizing the essential corrosion resistance of parts and components machined from stainless steel. Proper stainless steel passivation can be the deciding factor between acceptable performance and untimely failure. Additionally, incorrectly performed passivation can encourage stainless steel oxidation. The following steps should be taken to ensure that proper stainless steel passivation has occurred.
Part Should be Thoroughly Cleaned
Grease, coolant or other shop debris must be thoroughly cleaned from the surface to obtain the best possible corrosion resistance. Machining chips or other shop dirt can be wiped carefully off the part. A commercial degreaser or cleanser may be used to clean off machining oils or coolants. Foreign matter such as thermal oxides may have to be removed by grinding, or by methods such as acid pickling.
To ultimately prevent stainless steel oxidation the part, or component should be immersed in a passivating acid bath after it has been thoroughly cleaned. This can be accomplished by using any three of the following methods:
- Nitric acid passivation
- Nitric acid and sodium dichromate passivation
- Citric acid passivation
Which bath and composition to use depends on the grade of stainless steel and prescribed acceptance criteria of your part.
Passivation Testing Process
A proper stainless steel passivation test consists of swabbing the part with a copper sulfate/sulfuric acid solution, maintaining wetness for six minutes and observing whether there is any plating of copper. As an alternative, the part may be immersed in the solution for six minutes. Copper plating occurs if iron is dissolved. This test should not be applied to surfaces of parts for use in food processing. Also, it should not be used for the martensitic or lower-chromium ferritic stainless steels of the 400 series because false-positive results are likely.
Note - It is important that the test method be matched to the grade under evaluation. A test that is too severe will fail perfectly good material, while one that is too lenient will allow unsatisfactory parts to pass.
Stainless Steel Passivation Industry Specifications
We passivate stainless steel to numerous industry specifications, including:
- ASTM A967 Nitric 2
- ASTM A967-05
- AMS 2700
- ASTM A967 / A967M-13
- AMS QQ-P-35C
Stainless Steel Rust Prevention & Passivation Services
We offer stainless steel passivation and rust prevention services that accelerate and boost the effects of the material's naturally occurring passive layer of oxide, making it more resilient. Our special nitric acid/sodium dichromate bath facilitates a more rapid formation of the passive layer of film that provides corrosion resistance while removing free iron and other contaminants from the surface without otherwise affecting the material itself.
Upon request, ENS Technology can provide a copper sulfate test prior to processing to determine if passivation is needed.