“Does Stainless Steel Rust?” This question sometimes arises in every material engineer’s mind whenever they learn about stainless steel. It is well-known that stainless steel is specifically prepared to provide it in-build corrosion resistance by adding a sufficient amount of chromium. So, it is believed that stainless steel is rust-resistant. In this article, we will find out the answer to the most popular question “Does stainless steel rust or corrode?”
Rust resistance mechanism of Stainless Steel
The mechanism of stainless steel’s corrosion resistance is as mentioned below:
Stainless steel contains more than 10.5% chromium. Surrounding oxygen of air reacts with this chromium and form a thin stable chromium oxide layer that musk the steel surface. This chromium oxide layer works as a passive barrier film that does not allow the air and water to access the underlying metal and thus protects the surface from rusting. This is the reason that stainless steel is rust-resistant and this alloy is preferred for countless manufacturing applications.
Certain specific alloying additions increases this corrosion resistance properties of stainless steel. For example, molybdenum. Because of the presence of molybdenum grade 316 stainless steel is more corrosion resistant as compared to grade 304 stainless steel.
Does Stainless Steel Rust?
Yes, Stainless steel do rust or corrode in certain situations. There are various factors that can corrode or develop rust in stainless steel.
Pitting Corrosion in Stainless Steel:
Certain types of stainless steel can rust when exposed to strong chlorides. For example, the corrosive nature of chlorine in seawater or pool water can cause the stainless steel to rust or corrode. This is known as pitting corrosion in stainless steel. This type of rusting of stainless steel can be prevented by:
- Using the superior grade of stainless steel (for example SS316 is resistant to chlorides).
- Applying a specialized coating to prevent direct contact with chloride environment.
- Proper cleaning of the surface.
Galvanic Corrosion due to Dissimilar Stainless Steel Alloys:
Sometimes, two different grades of dissimilar alloys are welded together to prepare some custom products. This creates a galvanic current flow between them and the anode starts to corrode. This phenomenon is known as galvanic corrosion. The weld filler can also cause galvanic corrosion. This type of stainless steel corrosion or rusting can be prevented by:
- avoiding joining dissimilar metals.
- adding a coating to seal the metal off for electron flow.
General corrosion of Stainless Steel:
When the stainless-steel surface comes into contact with an acid-based material, general corrosion may occur in stainless steel. It creates uniform metal loss from the entire surface. Stainless steel components having a pH value of lower than 1 have a greater tendency to be attacked by general or uniform corrosion.
Transplanting of Plain Iron or Steel onto Stainless Steels:
Sometimes residue from iron or steel parts is transferred to stainless steel components. For example, while cleaning stainless steel utensils or cookwares using steel wool or wire brush. If these residue particles are exposed to humid air or moisture, rusting starts. This type of stainless steel rust can be prevented by thorough cleaning of the surfaces to avoid deposition of iron or normal steel over the SS surface.
Applying Temperature Extremes to Stainless Steel:
When stainless steel is exposed to extreme temperatures (750 to 15500F) the rust-resistance capability reduces. For example, during welding (or heat treatment) of stainless steel, high heating and cooling generate a process called sensitization where carbon and chromium bond together to form carbides. Due to these, the amount of chromium reduces in stainless steel making it prone to rusting. To prevent stainless steel rusting due to temperature extremes the product must be operated in its operating temperature range.
Chemical Corrosion in Polluted Air
In the prolonged exposure to polluted air (containing larger amounts of sulfides, oxides, hydrogen oxide), in the case of condensed water, sulfuric acid, nitric acid, acetic acid liquid spots are formed on the stainless steel surface causing chemical corrosion.
Removing Rust from Stainless Steel Surfaces
There are various methods by which the rust from the stainless steel surface can effectively be removed. For example, By dissolving the rust or iron oxide using weak acids like phosphoric acid or acetic acids can help substantially to get rid of the rust. These acids do not react with the parent metal and simply dissolved the rust to provide a cleaner surface.
Preventing Stainless Steel from Rusting
The best effective way to prevent stainless steel from rusting is to powder coat the stainless steel surface by depositing a dry-colored powder using electrostatic charges. Upon heating the surface, the powder coating cures to a hard finish, which results in a full coating of the bare stainless steel in an attractive and protective layer. Also, stainless steel passivation will reduce the rust or corrosion in stainless steel material.