The reason stainless steel is passivated is not that stainless steel does not rust, but because it corrupts slower than other steel, but everything has two sides. Today, we will look at the advantages of passivation on the stainless steel surface—disadvantages and process principle.
What is stainless steel surface passivation?
Stainless steel surface passivation refers to using an acid solution to remove free iron from the stainless steel surface to prevent rust. When the surface is treated, the other alloy components (mainly chromium, usually also nickel) are left on the underlying steel as a surface layer. When exposed to air, these elements react with oxygen to form an oxide layer that protects the rest of the steel from corrosion. This corrosion-resistant surface can be damaged by mechanical means or thermal or chemical damage. When this happens, the iron is exposed, and the stainless steel can rust again.
Process principle of stainless steel surface passivation
There are details to stainless steel passivation, such as the exact mix of chemicals used, which can vary depending on the steel alloy used. This helps protect one alloy while potentially damaging another, so the process needs to be customized frequently. However, the basic process tends to have the same process steps:
Load the parts to be passivated into another container.
Immerse the container and its contents in a chemical bath (usually a citric or nitric acid bath).
Let the part sit in the chemical bath for some time at the temperature specified for the alloy.
Remove parts from the chemical bath and wipe dry to remove excess chemicals.
Since the stainless steel part is in an acid bath, the acid will begin to strip free iron and other contaminants from the part’s surface. Depending on the alloy being cleaned and the specific chemical bath used, this can enhance the protective oxide layer of stainless steel components, making them more resistant to the chemical attack in some cases.
While the passivation process can significantly alter the rust resistance of a part, it does not considerably change the appearance of the finished piece. This is one of the main differences between passivation and electropolishing. Parts electropolished can be easily identified by their smooth, glossy surface.
Advantages of stainless steel surface passivation
- Improve corrosion resistance.
- Uniform, smooth appearance, and surface.
- Deburring (polished surface).
- High cleanliness.
- Improve and prolong product life.
Disadvantages of stainless steel surface passivation
Before passivating stainless steel parts, some things should be considered, such as:
Welded parts may not be suitable for passivation because the process is not as effective as some other methods in stripping contaminants from the weld area.
Requires custom chemical bath. The temperature and type of acid used in the passivation process chemical bath must be adjusted for the specific steel alloy being passivated. This adds cost and complexity to the method compared to electropolishing.
Some alloys cannot be passivated. Acid baths can damage some stainless steel alloys with low chromium and nickel content. Therefore, they cannot be passivated.
Unlike electropolishing, which strips the surface layer of a part to leave a smooth, shiny surface, passivation does not alter the appearance of the position. Therefore, if the goal is to create a smooth, non-stick surface, the passivation process will not be ideal. The main advantage of stainless steel passivation is that it enhances the rust resistance of stainless steel components, and it is less expensive than electropolishing.