In the simplest terms, passivating stainless steel is a way to prevent it from rusting/corroding. In many people's minds stainless steel is a wonder metal that doesn't corrode. This is not true - some grades of stainless steel are much less corrosion resistant than others and others, even 316 stainless steel (often referred to as marine grade or surgical steel) can corrode - especially if contaminated
What is stainless steel passivation?
Despite the already high corrosion resistance of stainless steel, some uses demand even more resistance. One of the key issues with corrosion (other than the corrosion itself) is that it is autocatalytic. This means that once the corrosion has started, even if the corrosion site is very small, it will continue to spread and at an ever increasing rate. Therefore, it is very important to prevent any kind of corrosion in the first place. Passivation is a popular way to reduce pitting corrosion in stainless steels. Without passivation, it is not uncommon for lower chromium content stainless steels to show red rust if they are not passivated. Higher chromium content stainless steels will naturally self-passivate in the right environment, but this process is slow (it can take months), uncontrolled and can be disrupted. Our passivation process forms the passive layer in controlled conditions, in a very short period of time.
Stainless steel passivation works by removing any free iron (exposed iron on the surface of the part). This leaves a thin, passive layer usually comprised of chromium and its oxides. Since chromium is so corrosion resistant, this layer provides greater protection to the already resistant base metal.
Metal Finishings Limited, offers nitric and nitric/dichromate passivation. These are both industry standard. For large orders, we would be prepared to introduce other passivation processes such as citric acid passivation. Parts with scale will require mechanical treatment or pickling before passivation.
- ASTM A967 Nitric 1, Nitric 2 or Nitric 3
- AMS 2700 Type 2, Type 6 or Type 7 (AMS 2700 replaces QQ-P-35 and AMS QQ-P-35)
- Def Stan 03-2 Method M (Method M1) / AMS 03_2 Process M (Method M1)
- TS112 F2A
We can work to many other specifications, but the above are our most popular. Please contact us with your requirements.
Information for Designers
When to passivate
The passive film on stainless steel is very thin, so it is important to passivate after any cutting or abrasion. It is common practice to passivate after welding.
As a production control, copper sulphate spot testing or humidity testing can be used.
Salt spray testing can be used as a destructive test.