【Corrosion Technology Edition】Daily Question 20170721: What are the different types of local corrosion?

Local corrosion is divided into main types such as galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, wear corrosion, stress corrosion, fatigue corrosion, and selective corrosion.

Intergranular corrosion, subsurface corrosion, pitting corrosion, film pore-type corrosion

Intergranular corrosion, subsurface corrosion, pitting corrosion, film pore-type corrosion, etc

1 Pitting corrosion Pitting corrosion is also known as pit corrosion or hole corrosion. 　　 2 Crevice Corrosion: In an electrolyte, narrow gaps are formed between metal surfaces or between a metal and a non-metal surface. The movement of substances within these gaps is hindered, resulting in a concentration cell that causes local corrosion; this type of corrosion is known as crevice corrosion. 3 Stress corrosion: The brittle cracking that occurs in a material within a specific corrosive environment, under static tensile stress (including residual stresses caused by external loads, thermal stresses, cold working, hot working, welding, etc., as well as wedge-shaped stresses from rust products in cracks), and which results in a strength level below the material’s limit, is known as stress corrosion cracking. 　 4 Corrosion fatigue Corrosion fatigue occurs under the combined action of a corrosive environment and cyclic stress. This reduction in corrosion fatigue resistance caused by corrosive agents is known as corrosion fatigue. The stress value for fatigue failure is below the yield point; fatigue failure occurs only when the stress exceeds a certain critical cyclic stress value (the fatigue limit, or fatigue life). Corrosion fatigue, on the other hand, can cause failure even under very low stress levels, which makes it very dangerous. 　 5 Intergranular Corrosion Intergranular corrosion is a type of localized corrosion damage in which metal materials are corroded along the boundaries between their grains in specific corrosive environments, resulting in a loss of cohesion between those grains. 6 Uniform corrosion Uniform corrosion refers to corrosion that occurs at almost the same rate across the entire metal surface in contact with the environment. 7 Wear corrosion (erosion) The material degradation process resulting from the combined action of wear and corrosion is called wear corrosion. 8 Hydrogen embrittlement: Once metal materials, especially titanium, absorb hydrogen, brittle hydrides are formed, which reduces their mechanical strength

Stress corrosion cracking (SCC): This is a general term for the failure of alloys under stress in corrosive environments, as a result of the propagation of cracks. Stress corrosion cracking presents a brittle fracture pattern, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking are the presence of tensile stress (whether residual or applied, or both) along with a specific corrosive environment. The formation and propagation of cracks occur roughly perpendicular to the direction of the tensile stress. The stress level required to cause stress corrosion cracking is much lower than the stress level needed for the material to break in the absence of a corrosive environment. At the microscopic level, cracks that run through the grains are called transgranular cracks, while those that propagate along the grain boundaries are known as intergranular cracks. When stress corrosion cracking progresses to a certain depth (at which point the stress on the cross-section of the loaded material reaches its breaking stress in air), the material breaks in the usual manner, through cracks that arise from the aggregation of microscopic defects. Therefore, the cross-section of a part that fails due to stress corrosion cracking will contain areas characteristic of stress corrosion cracking, as well as “toughness pits” associated with the aggregation of such microscopic defects. Pitting corrosion: This is a form of localized corrosion. Intergranular corrosion: The interfaces between grains are regions where grains with different crystallographic orientations are misaligned; thus, these areas are conducive to the segregation of various solute elements in steel or the precipitation of metal compounds such as carbides and the δ-phase. As a result, it is not surprising that these grain boundaries may be corroded first in certain corrosive environments. This type of corrosion is known as intergranular corrosion, and most metals and alloys can suffer from intergranular corrosion in specific corrosive environments. Crevice corrosion: This is another form of localized corrosion that can occur in crevices where liquids remain stagnant or within shielded surfaces. Such crevices can form at the joints between metals or between metals and non-metals, for example, at points of contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms.

Local corrosion includes: pitting corrosion, galvanic corrosion, crevice corrosion and its special case of filiform corrosion, intergranular corrosion and its special case of weld corrosion, selective corrosion, stress corrosion cracking, hydrogen embrittlement, hydrogen-induced cracking, hydrogen blistering, hydrogen corrosion, corrosion fatigue, wear corrosion, erosion corrosion and its special case of cavitation corrosion, abrasive wear and its special case of fretting corrosion.

Intergranular corrosion, stress corrosion, galvanic corrosion, pitting corrosion, crevice corrosion, etc

pitting corrosion, crevice corrosion, intergranular corrosion, selective corrosion, stress corrosion, turbulent corrosion, etc

Stress corrosion cracking, pitting corrosion, intergranular corrosion, crevice corrosion

Intergranular corrosion, subsurface corrosion, pitting corrosion, film pore-type corrosion, etc