Why Do Stainless Steel Castings Rust?

When brown spots are found on the surface of the stainless steel casting, people are surprised and believe that “stainless steel cannot rust. This must not be stainless steel. There must be a problem.” In fact, due to the lack of understanding of stainless steel, this is a side view. In some cases, stainless steel can also rust.

Reasons for stainless steel casting to rust

Stainless steel has the ability to resist atmospheric oxidation, that is, corrosion resistance. Stainless steel relies on the extremely thin, strong, dense, and stable chromium-rich oxide film (protective film) formed on its surface to prevent the infiltration and oxidation of oxygen atoms, thus obtaining rust resistance. However, the degree of resistance to distortion varies with the chemical composition of the steel itself, processing state, use conditions, and the type of environment medium. Once for some reason, the oxide film on the surface of the stainless steel casting is destroyed, oxygen atoms in the air or liquid will continue to penetrate, or iron atoms in the metal will continue to precipitate ions, forming loose iron oxide, and the metal surface will continue to be corroded. Taking 304 steel as an example, it has excellent corrosion resistance in dry and clean atmosphere, but when it is moved to a seaside area and placed in seawater mist containing a large amount of salt, it will quickly rust. However, when it comes to 316 steel, it performs well in this situation. Therefore, there is no stainless steel that can resist corrosion and rust in any environment.

Common causes of rusting of stainless steel casting

Chemical corrosion:

Surface pollution: Oil, dust, acid, alkali, salt, etc. attached to the surface of the workpiece are converted into corrosive media under certain conditions, and chemically react with certain components in the stainless steel casting, resulting in chemical reaction. Corrosion and rust.

Surface scratching: Various scratches will damage the passivation film, reduce the corrosion resistance of stainless steel, and easily react with chemical media, thereby causing chemical corrosion and rust.

Cleaning: After pickling and passivation, the cleaning agent cannot be thoroughly removed, resulting in residual liquid, which directly corrodes stainless steel parts. For example, during the pickling process, the acidic liquid enters the shrinkage cavity of the casting (as shown below), and then reacts with the air to produce yellow spots on the casting surface.

Electrochemical corrosion:

Carbon steel pollution: Scratches caused by contact with carbon steel parts and corrosive media will form primary batteries and cause electrochemical corrosion.

Cutting: Adhesion of rusting materials such as slag and splashes and the formation of primary batteries by corrosive media cause electrochemical corrosion.

Heating: The composition and metal structure changes unevenly in the flame heating area, forming a corrosive medium primary battery, and causing electrochemical corrosion of the stainless steel casting.