Characteristics & Protective Measures of Crevice Corrosion of SS Flanges

1. Characteristics of crevice corrosion of stainless steel flanges are as follows:
(1) The corrosion of the metal in the crevice is caused by the existence of foreign matter or structural reasons on the metal surface, which makes the migration of corrosion related substances in the solution in the crevice difficult, and is referred to as crevice corrosion. Studies have shown that almost all corrosive media can cause crevice corrosion of stainless steel, and there is no specific medium, but crevice corrosion in environments containing Cl is the most common to be seen; crevice corrosion has certain requirements for the size of the crevice; not only the material should be difficult to migrate between the solution inside and outside the gap, and the solution should be allowed to enter the gap. The width of the crevice corrosion of stainless steel is generally in the range of 0.025 to 0.1mm. Corrosion morphology is usually broad and shallow, mostly pitting or ulcer-like in pieces.
 
(2) Unified mechanisms of crevice corrosion of stainless steel: In the initial stage, a certain degree of corrosion occurs uniformly on the entire metal surface where the stainless steel is in a passivated state, including the surface inside crevice corrosion. The charge generated by the anodic reaction, that is, oxidation, M → M++ e is balanced by the cathodic reaction, that is, reduction, O2 + H2o + 4e → 4OH-. After some time, the oxygen consumed by the cathodic reaction cannot be supplemented due to the poor flow of the solution in the gap, so that the cathodic reaction in the gap can gradually terminate. The anodic reaction in the gap continues. In order to balance this charge, negatively charged anions especially Cl migrates to the gap. OH ions and others can also migrate from the outside, but their mobility is not as good as Cl-, so the migration is much slower, which in turn causes the Cl concentration in the gap to increase; the metal chloride, that is M+Cl- formed in the crevice is hydrolyzed into insoluble hydroxide and free acid: M+Cl- + H2o → MOH + H+Cl-, resulting in the pH value in the crevice decreased, and acidity increased. This low pH value and high acidity environment in the crevice accelerates the anodic dissolution process of the metal, and the concentration of metal cations increases, which further causes Cl to migrate to the crevice. In this way, an autocatalytic process is formed, which accelerates and strengthens the crevice corrosion process with time.
 
2. Preventive measures for crevice corrosion of stainless steel flanges
(1) Structural design avoiding the formation of gaps
In the process of tightening flanges, the force is balanced to avoid gaps, and appropriate gaskets should be selected. Improve the design and welding process to eliminate the arc transition area between the flange sealing surface and the pipeline. Avoid the transition arc of the flange sealing surface from forming a gap of a certain size after the flanges are connected, which induces the occurrence of crevice corrosion.
(2) Reducing the chloride ion content in the pressure test water
Change the pressure test circulating water in time. Reduce the stopping time of pressure test water and empty it in time.
(3) Reasonable selection of corrosion-resistant materials
Adding corrosion-resistant alloying elements to reduce the influence of harmful elements is the fundamental measure to control crevice corrosion. For example, increasing the content of molybdenum, nickel and chromium elements in stainless steel can obtain excellent crevice corrosion resistance. Reducing carbon content and sulfide impurities in steel can also improve its crevice corrosion resistance.
(4) Adding corrosion inhibitor
Suitable corrosion inhibitors can be selected to help prevent and control the occurrence of crevice corrosion. Commonly used corrosion inhibitors include nitrates and chromates.
(5) Electrochemical protection
The impressed current method or sacrificial anode method are used for cathodic protection to prevent crevice corrosion. As can be seen from Figure 6, in the microstructure near the inner wall of the pipeline, no secondary microcracks of stress corrosion and traces of intergranular corrosion were found, thus eliminating the occurrence of stress in the pipeline This eliminates the possibility of stress corrosion and intergranular corrosion in the pipeline. It can be seen from Figure 7, the metallographic structure of the pipeline is a large amount of ferrite and a small amount of pearlite, and the grain size reaches 7; the metallographic structure is fine, and not many non-metallic inclusions are found. The metallographic structure is normal. There is no defect in the material of the pipeline, which is not a factor causing serious partial corrosion.
 
3. Conclusion
The rust spots on the flanges of the factory were caused by crevice corrosion. The unbalanced tightening of flanges and the formation of gaps by the arc surface at the joint are the prerequisites for crevice corrosion. The high concentration of chloride ions in the pressure test water and stagnant media aggravated the degree of crevice corrosion. Improving the structural design of flange sealing connections, selecting higher grade stainless steel such as 316L stainless steel containing Mo with the same material as the pipeline, and controlling the chloride ion content in water quality can help reduce crevice corrosion of stainless steel flanges. In addition, protective measures such as adding corrosion inhibitors or electrochemical protection to the medium can also be used.