1、 Corrosion under scale
Just under the sediment or in the crevice, the oxygen content in the solution is low, and outside the crevice, the oxygen content in a large number of solutions is very high, which establishes a battery. Under the sediment or in the crevice is the anode, and outside it is the cathode. In the crevice of chloride containing medium, pH value decreases and chloride concentrates. This acidic chloride condition results in accelerated corrosion and acts as an automatic mediator.
Then severe localized corrosion occurred. An example of this form of corrosion: occurs when a stainless steel fastener is placed on a stainless steel plate and exposed to chloride containing water. Crevice corrosion can occur when the bolt head or gasket is used as the anode area. Preventing the formation of deposits and scales or using materials with high alloy content will help to reduce crevice corrosion.
2、 Exfoliation corrosion
In this case, a loose and flaky corrosion layer is formed on the metal surface. Even if the flow rate is low, the loose layer of corrosives can be easily removed. As a result, new uncorrosive metal is exposed, and many other lamellar layers are formed. Again, these flakes are easily removed and the process continues. Exfoliation corrosion can be avoided by using alloy which is not easy to react.
3、 Intergranular corrosion
Intergranular corrosion occurs in some special alloys when they are heated to their sensitive temperature zone during welding or heat treatment. When some stainless steel alloys are heated to 425-870 ℃, chromium carbides will precipitate at grain boundaries.
The results show that the chromium poor zone appears near the carbide and the passivation of the grain boundary zone is affected. In special media, such as nitric acid or high temperature water, the corrosion of low chromium zone may occur. Grains appear as a sugar like surface, which can be easily wiped off by a sampler. Intergranular corrosion of stainless steel and nickel alloy can be avoided by using low carbon alloy, adding carbide forming elements such as titanium or niobium, or stabilizing annealing.