Pitting, one of the surface defects of steel. It is characterized by an uneven and rough surface, also known as pitted surface. It is often continuous and patchy, but can also be localized or periodically distributed. Spots are permissible defects, the depth of which does not exceed the range of the thickness deviation of the product and does not affect its us On metal surfaces, microcells are formed in areas of electrochemical inhomogeneity due to the inhomogeneity of the organization, chemical composition, etc. For stainless steel, the surface of the oxide film due to scratches, microparticle pressure injury, active anion damage was damaged and lost its integrity, thus forming a membrane pore cell, the membrane for the cathode, the metal under the pore for the anode. Incomplete surface, relative to the oxide film is very small, the anode area is much smaller than the cathode area, the electron by the direction of the oxide film to the pore convergence, accelerated corrosion, on the other hand, the cathode to electrons continue to passivate the state, resulting in corrosion to the pore to the migration, resulting in the formation of small pits, which is what we see as the ” pitting “.
In order to solve this problem, we must first find out how this problem has arisen, and there are three reasons for its emergence.
1. Material problem, stainless steel is formed by iron ore, metal element materials (different materials add different composition and proportion of elements), smelting, ingot, and need to go through several processes such as cold rolling or hot rolling to form, in these processes, may accidentally add some impurities, and these impurities are very small and integrated with the steel, from the surface can not be seen, after grinding and polishing, these impurities show up to form a very obvious pockmarks! After grinding and polishing, these impurities show up and form very obvious pitting! (Generally, the material that produces this situation is mostly 2B material, that is, matte material, after grinding, the brighter the surface, the more obvious the pockmarks) The pitting produced by this material problem, there is no way to get rid of it!
2. The use of substandard polishing wheels, if the polishing wheel problems, then the problem is not only numbness, and may produce abrasive head flower! Machinery above the polishing wheel is too much, to find out where the problem is, need to polish master one by one to check the replacement, if the quality of the polishing wheel is not off, then all need to be replaced! If the quality of the polishing wheels is not good, then all of them need to be replaced! There is also an imbalance of the polishing wheels, which makes the material unevenly stressed, and these problems will also occur!
3. There are small particles inside the machine (this is the surface of the stainless steel in the grinding process is ground off), general mechanical polishing machinery are filtered above the small particles of the filter, but if you do not change the filter from time to time, the small particles will flow with the flow of water and flow to the plate, and this small particles will mirror the stainless steel plate material causing great damage, may produce pitting scratches and other problems, there is a problem! This situation produces problems, usually need to carry out a second grinding! Secondary grinding not only costs a lot of labor and materials, but also a lot of time! We also need to take precautions to avoid this problem by changing the filters frequently!
The occurrence of pitting is inextricably linked to the environment, and pitting behavior under extreme conditions will show its special characteristics. The effects of ambient temperature and pressure on corrosion are often different, and the process of material pitting is more complex at high temperatures and pressures. With the development of science and technology, the industry puts forward higher requirements on the performance of stainless steel, and the development of the research on stainless steel pitting under special environment is an important direction in the future.
