Corrosion is the gradual erosion of a stainless metal by a chemical, affecting its mechanical, appearance and sealing properties. Although stainless steels are preferred for their resistance to corrosion, they cannot resist it. Stainless steels are generally corrosion resistant and perform satisfactorily in most chemical environments. Corrosion resistance is influenced by the structure of the stainless steel and the environment.
Wet Corrosion
Wet corrosion is corrosion in liquid, vapor and atmospheric media. It is an electrochemical process involving + [anode] – [cathode] and an electrolyte liquid medium. The metal begins to corrode at the anode. At the cathode, oxygen or hydrogen electrons are reduced. Prevention of corrosion is achieved by stopping this mechanism.
Wet corrosion forms are as follows:
- Pitting Corrosion
- Crevice Corrosion
- Stress Corrosion Craking
- Atmospheric Corrosion
- Intergranular Corrosion
- Pitting Corrosion
This type of corrosion is highly localized and occurs in a discrete area of the stainless steel surface. If the protective film is damaged or weakened, pitting corrosion begins. This small weak zone becomes the unprotected area of the protective layer and is anodic. The undamaged main body is cathodic.
- Crevice Corrosion
As the name suggests, this type of corrosion occurs in cracks and hidden gaps. Cracks can occur during production, during use or during repair. Since oxygen is limited in this area, the protective layer is weakened and a cavity forms. Subsequently, dissolved metal ions in the cavity lower the PH level and chlorine ions move into the cavity. Finally, the protective film layer is destroyed and the reducing environment initiates the corrosion attack.
- Stress Corrosion Cracking SCC
Like pitting and crevice corrosion, stress crevice corrosion occurs mostly in chlorine-containing environments. It can be characterized by the proliferation of cracks within or between grains. It occurs for three main reasons:
- Tensile stresses in the material
- Corrosive media, especially chloric or sulfuric based.
- Use of materials susceptible to this corrosion.
- Intergranular Corrosion IGC
Previously, this type of corrosion was a serious problem for grades with high carbon content (0.05 – 0.15%). However, with modern steelmaking methods, it is no longer a serious problem. Intergranular corrosion can occur mostly during heat treatment and welding (550 – 850 °C). Chromium carbides precipitate at the grain boundaries with slow cooling. The area adjacent to the precipitation is chromium-poor and becomes susceptible to corrosion.
- Atmospheric Corrosion
It is a type of corrosion caused by contact with water, carbon dioxide, water vapor, sulfur and chlorine components coming from the atmosphere.
