TYPES OF CORROSION IN STAINLESS STEELS
Uniform corrosion is considered an even attack across the surface of a material and is the most common type of corrosion.
- Uniform loss of metal over entire surface
- Stainless steel with pH value of less than 1 are more prone to general corrosion
This type of corrosion typically occurs over relatively large areas of a material’s surface.
Pitting is one of the most destructive types of corrosion, as it can be hard to predict, detect and characterize.
- Localized corrosion resulting in cavities or holes
- Occurs when stainless steel is exposed to environments containing chlorides
Pitting is a localized form of corrosion, in which either a local anodic point, or more commonly a cathodic point, forms a small corrosion cell with the surrounding normal surface. Once a pit has initiated, it grows into a “hole” or “cavity” that takes on one of a variety of different shapes. Pits typically penetrate from the surface downward in a vertical direction. Pitting corrosion can be caused by a local break or damage to the protective oxide film or a protective coating; it can also be caused by non-uniformities in the metal structure itself. Pitting is dangerous because it can lead to failure of the structure with a relatively low overall loss of metal.
Crevice corrosion is also a localized form of corrosion and usually results from a stagnant microenvironment in which there is a difference in the concentration of ions between two areas of a metal. Crevice corrosion occurs in shielded areas such as those under washers, bolt heads, gaskets, etc. where oxygen is restricted.
- Localized corrosion at the crevice between two joining surfaces
- Formed between two metals or between metal and non-metal
These smaller areas allow for a corrosive agent to enter but do not allow enough circulation within, depleting the oxygen content, which prevents re-passivation. As a stagnant solution builds, pH shifts away from neutral. This growing imbalance between the crevice (microenvironment) and the external surface (bulk environment) contributes to higher rates of corrosion. Crevice corrosion can often occur at lower temperatures than pitting. Proper joint design helps to minimize crevice corrosion.
An examination of the microstructure of a metal reveals the grains that form during solidification of the alloy, as well as the grain boundaries between them. Intergranular corrosion can be caused by impurities present at these grain boundaries or by the depletion or enrichment of an alloying element at the grain boundaries.
- Corrosion where boundaries of crystallites are more likely to corrode than inside surfaces
- Occurs after heating austenitic stainless steel at approximately 842–1562°F
Intergranular corrosion occurs along or adjacent to these grains, seriously affecting the mechanical properties of the metal while the bulk of the metal remain intact.
In stainless steels, during these reactions, carbon “consumes” the chromium, forming carbides and causing the level of chromium remaining in the alloy to drop below the 10.5% needed to sustain the spontaneously-forming passive oxide layer. 304L and 316L are enhanced chemistries of 304 and 316 stainless that contain lower levels of carbon, and would provide the best corrosion resistance to carbide precipitation.
STRESS CORROSION CRACKING (SCC)
Stress corrosion cracking (SCC) is a result of the combination of tensile stress and a corrosive environment, often at elevated temperatures. Stress corrosion may result from external stress such as actual tensile loads on the metal or expansion/contraction due to rapid temperature changes.
- Growth of crack formation in corrosive setting
- Tensile stresses in combination with corrosive environmental conditions lead to cracking
It may also result from residual stress imparted during the manufacturing process such as from cold forming, welding, machining, grinding, etc. In stress corrosion, the majority of the surface usually remains intact; however, fine cracks appear in the microstructure, making the corrosion hard to detect. The cracks typically have a brittle appearance and form and spread in a direction perpendicular to the location of the stress. Selecting proper materials for a given environment (including temperature and management of external loads) can mitigate the potential for catastrophic failure due to SCC.
GALVANIC CORROSION(or bimetallic corrosion)
Galvanic corrosion is the degradation of one metal near a joint or juncture that occurs when two electrochemically dissimilar metals are in electrical contact in an electrolytic environment; for example, when copper is in contact with steel in a saltwater environment.
*Electrochemical process where one metal corrodes preferentially compared to another in the presence of electrolyte.
Large engineered systems employing many types of metal in their construction, including various fastener types and materials, are susceptible to galvanic corrosion.
SKETCH OF VARIOUS TYPE OF CORROSIONS