What is stainless steel pitting?
To understand this concept, we must first understand the excellent corrosion resistance principle of stainless steel. In fact, it is because there is an invisible but compact oxidation protective film on the surface of the stainless steel plate, which makes the stainless steel material in the passivation state at all times. The formation of this film is due to the corresponding chemical reaction with oxygen when exposed to atmospheric environment, or the reaction with other oxygen environments.
If the passivation protection film is damaged, the stainless steel will continue to be corroded. Unless the passivation protection film is reformed, the corrosion resistance will be restored. Back to the film after the destruction of the situation, most of the time, the stainless steel board will only have a local passivation film is damaged, the corrosion effect, will be formed on the surface of stainless steel plate in small holes or pits, thereby generating a small pit corrosion distribution on the surface of the stainless steel plate.
So what is the specific factor that causes pitting on the surface of the stainless steel plate?
A large number of data show that pitting may be a chloride ion in the surrounding environment of the stainless steel plate with depolarizing agent. The pitting of blunt metals such as stainless steel is often caused by the partial destruction of some erosive anions to the passivation film. The obtuse state with high corrosion resistance usually requires the oxidation environment, and this environment is also easy to produce pitting. The medium for pitting corrosion is Fe3+, Cu2+, Hg2+ and other heavy metal ions in Cl-, Br-, I- and ClO4- solutions or chloride solutions containing Na+, Ca2+ base and alkaline earth metal ions including H2O2 and O2.
The pitting rate increases with the increase of temperature. For example, when the concentration is 4%-10% NaCl solution, the weight loss of stainless steel will be maximized when the temperature reaches 90 degrees, and if the solution is thinner, the maximum value is generally at a higher temperature range.
Look at that much, and how do you avoid pitting? The first is to prevent excessive concentration of halogen ions, followed by ensuring the uniformity of oxygen or oxidizing solution, stirring the solution and avoiding small areas where there is no liquid flow, and also enhancing the concentration of oxygen or removing oxygen elements. Of course, it can also improve the pH value. Compared to neutral or acidic chlorides, obviously alkaline chloride solutions lead to less pitting, or totally no (hydrogen and oxygen ions as an anticorrosion agent).
In addition, the stainless steel plate is best done at the lowest possible temperature. It can also add passivating agents into corrosive medium. Low concentration nitrate or chromate is effective in many media. It is even possible to use cathodic anticorrosion, and the data show that stainless steel with low carbon steel, aluminum or zinc in the cathode can not cause pitting in the sea water.
At the end of the material, austenitic stainless steel with molybdenum 2%-4% can be selected. The material has excellent pitting resistance. It can obviously reduce pitting or common corrosion.
Source: China Stainless Steel Plate Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)