Question

34) pitting corrosion occurs due to
formation of
a pits and holes​

Answer 1

Explanation:

Pitting corrosion of stainless steel and nickel base alloys is characterized by the existence of a CPT, which is defined as the lowest temperature at which the growth of stable pits is possible. At temperatures lower than CPT no stable pitting can occur and only transpassive corrosion takes place. Brigham and Tozer78 introduced the concept of the CPT as a criterion for ranking resistance of different austenitic stainless steels to pitting corrosion: the higher the CPT, the more resistant the alloy is to pitting.

CPT can be determined by exposing steel samples to an oxidized chloride solution such as FeCl3 and increasing the temperature in a sequence of test periods until pitting is observed.156

The CPT can also be determined electrochemically, either by varying the potential at different constant temperatures or by varying the temperature at different constant potentials.156–159 At temperatures lower than CPT, passivity breakdown is observed at extremely high anodic potentials, corresponding to transpassive dissolution. Just above the CPT, pitting can occur and breakdown potentials far below those required for transpassivity are measured. The potential independent CPT is then determined as the temperature at which a sudden decrease in breakdown potential from the transpassive range to the pitting range is observed . The CPT of many stainless steels falls in the range of 10–90 °C.78,159