What Does Intercrystalline Corrosion Mean?
This corrosion attacks the surface grain layers or crystallites and is known to penetrate metal objects along the grain boundaries. This results in destruction of the metal articles along the grain boundaries.
It arises from the precipitation of chromium carbide at the grain boundaries. This happens when chromium-nickel austenitic stainless steels are welded in the presence of various corrosive media. This results in an improvised chrome in the surrounding area and loss of the passivation effect.
Intercrystalline corrosion can also be referred to as intergranular corrosion or weld decay.
Corrosionpedia Explains Intercrystalline Corrosion
Intercrystalline corrosion occurs when steels are heated with a temperature of about 450°-800°C. Chromium carbides precipitate at the boundaries of the affected grains. This results in the weakening of the chromium protection on the peripheries of the grains at the zones of the weld.
There are many test procedures that can be used in the determination of intercrystalline corrosion. These tests include the Strauss test, the improved Strauss test, the Streicher test and the Huey test. All these tests are based on the specimen's aging in a holding time, definite temperature and definite solution. The mass lost, or the depth of the attack after the specimen has been bent is measured.
Electrochemical potential-dynamic reactivation (EPR) is the most common method that is applied. This method describes the susceptibility of intercrystalline corrosion of the rolled austenitic strip. The potential range between the passivation region and the free potential is applied. This method reactivates voids within the passive layer. The current density from the reactivation and passivation loop is used for data interpretation. Also necessary is the interpretation of corrosion attack in the case of weldments.
Intercrystalline corrosion can be prevented when the correct grade of steel is used. The best steels are those with a low carbon content of not more than 0.03%. One can also use niobium- or titanium-stabilized steels. Intercrystalline corrosion is also dependent on the nature of the surface. A smooth and homogeneous surface will result in better corrosion resistance. A post weld and heat-treated material can overcome this type of corrosion.