Principle:
Under the action of corrosive medium, a kind of corrosion phenomenon between the grains of stainless steel is called intergranular corrosion.
The stainless steel that produces intergranular corrosion, when subjected to stress, will fracture along the grain boundary and the strength will almost completely disappear, which is one of the most dangerous forms of damage to stainless steel. Intergranular corrosion can occur in the heat-affected zone (HAZ), weld or fusion line of the welded joint, respectively. The intergranular corrosion generated on the fusion line is also called knife line corrosion (KLA).
The necessary condition for stainless steel to have corrosion resistance is that the mass fraction of chromium must be greater than 10~12%.When the temperature increases, the diffusion rate of carbon inside the stainless steel grain is greater than that of chromium. Because the solubility of carbon in austenite at room temperature is very small, about 0.02% to 0.03%, and the carbon content in general austenitic stainless steel exceeds this value, so the excess carbon is continuously transferred to austenite crystals. The grain boundary diffuses and combines with chromium to form chromium carbide compounds between grains, such as (CrFe) 23C6 and so on.
The data show that the activation capacity of chromium diffusion along the grain boundary is 162-252KJ/mol, while the activation energy of chromium diffusion from within the grain is about 540KJ/mol, that is, the diffusion rate of chromium from the grain is smaller than that of chromium along the grain boundary, and the internal diffusion rate of chromium is smaller than that of chromium along the grain boundary. The chromium does not have time to diffuse to the grain boundary, so the chromium required for the chromium carbide formed in the intergranular mainly does not come from the interior of the austenite grain, but from the vicinity of the grain boundary, resulting in a large amount of chromium near the grain boundary. When the mass fraction of chromium at the grain boundary is as low as less than 12%, the so-called "chromium-poor zone" is formed. Under the action of the corrosive medium, the chromium-poor zone will lose its corrosion resistance, resulting in intergranular corrosion.
Qualified:
(1) The structures of grains and grain boundary regions are different, so there are significant differences in electrochemical properties. - internal cause
(2) The difference between grains and grain boundaries can only be revealed in an appropriate environment. - external cause
Stainless steel sensitization:
Unstable austenitic stainless steel with a carbon content of more than 0.03% (ie, 0Cr18Ni9 stainless steel without titanium or niobium) is prone to intergranular corrosion in some environments if the heat treatment is improper. Intergranular segregation occurs when these steels are heated between 425-815°C, or slowly cooled through this temperature range. Such heat treatment causes carbides to precipitate at the grain boundaries (sensitization) and causes chromium in the immediate vicinity. Depletion makes these areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent localized corrosion in the weld heat affected zone.
The most common method for checking the susceptibility of stainless steel is the 65% nitric acid corrosion test method. During the test, the steel sample was placed in a boiling 65% nitric acid solution for 48 hours as a cycle, a total of 5 cycles, and the weight loss was measured in each cycle. It is generally stipulated that the average corrosion rate of 5 test cycles should not be greater than 0.05mm/month.
