Corrosion
Corrosion is one of the most concerning issue in industry. Unlike any carbon steel (AISI: American Iron and Steel Institue/SAE: Steel Grade System, SA516), stainless steel (SA304 or Germany DIN, 1.4301/316 or 1.4401)can survive and thrive, oxidation free, in corrosive or humid environments, because stainless steel has a high chromium content which acts as a protective layer against corrosion and rust, while carbon steel (St. 35.8) cannot stand it.
Chromium and nickel oxidise much slower than iron and also their oxides are not an obviously different colour and do not flake off like iron oxides, exposing even more underlying iron. Chromium and nickel oxides protect the underlying metal much more than does iron oxide.
Corrosion reactions are listed in the following:
2Fe → 2Fe2+ + 4e- : oxidation(loss of electron)
O2 + 2H2O + 4e- → 4OH-: reduction (gain of electron)
2Fe2+ + 4OH- → 2Fe(OH)2: formation of iron hydroxide
Galvanization: There is classifications according to the industry air and atmosphere.
Water can be corrosive to most metals. Pure water, without dissolved gases (e.g., oxygen, carbon dioxide, and sulfur dioxide) does not cause undue corrosion attack on most metals and alloys. In other words, water without dissolved gases in a cycle will not be problematic.
Corrosion allowance is not only determined by the designer, but also and especially by a state or local agency. The latter often have years of experience with local conditions and especially the weather conditions in a relevant area. The corrosion allowance is usually reported in mm
Pitting Corrosion
It is a form of extremely localized corrosion depassivating and creating small holes in the metal. It occurs typically in alloys with passivating oxide film such as stainless steel in the presence of chloride with -1 charge whereas chlorine is neutral. Chloride is heavy and it goes down and creates a hole. Pitting corrosion occurs when the cathode (damaged coating) is large and the anode (exposed metal) is small.
Cl (atom of chlorine) Cl2 (molecule of chlorine): Chlorine is a yellow green gas at room temperature/ 17 electrons
Cl-: Chloride has 18 electrons
Stress Corrosion Cracking (SCC)
It may proceed along the grain boundaries (intergranular), interface between two crystallites, or through the individual grains (transgranular).
Stree corrosion cracking occurs through the following conditions:
- Susceptible material like low alloy steel and mild steel cracks
- Corrosive environment such as high temperature water and salt solution
- Tensile stress, a force to pull apart or stretch a material, due to bending, welding,…
Hydrogen affects the electrochemical kinetics of the metal, which subsequently lead to pitting and accelerate the rate of intergranular corrosion. The interaction of hydrogen and stress can significantly escalate the stress corrosion cracking susceptibility of steel, especially in the welded joints. One atom of hydrogen will with another atom of hydrogen.
This causes a big volume and stress.
- Cracks can be the result of the thermal expansion not corrosion.
- Mechanical damages can be the cause of thermal shock or corrosion
