Pitting corrosion of stainless steel, causes and prevention methods
TIME：2016-12-10 AUTHOR： SOURCE： VIEWS： 582
Stainless steel pitting
As mentioned earlier, the excellent corrosion resistance of stainless steel is due to the formation of invisible oxide film on the surface of the steel, so that it is a blunt state. The formation of the passive film is due to the reaction of the steel when exposed to the atmosphere with oxygen, or due to contact with other oxygen containing environments. If the passive film is damaged, the stainless steel will continue to corrode. In many cases, the passivation film is only localized in the local surface of the metal, the role of corrosion is to form a small hole or pit, the surface of the material to produce irregular distribution of small pit corrosion.
Two, cause stainless steel pitting factor
There is likely to exist with pitting depolariser compound chloride ion. Pitting corrosion of stainless steel, such as stainless steel, is mainly due to the partial destruction of passive film by some aggressive anions. It is usually necessary to protect the environment with high corrosion resistance, but this is also the condition of pitting corrosion. Pitting corrosion medium is in CI-, Br-, l-, C104-, Cu2+ and Fe3+ existed in solution containing H2O2 or Hg2+ and other heavy metal ions, such as O2 Na+, Ca2+ of alkali and alkaline earth metal ions chloride solution.
The pitting rate increases with the increase of temperature. For example, in the concentration of 4% to 10% sodium chloride solution, at a temperature of 90 degrees to achieve the weight loss caused by pitting large; for a more dilute solution, the super high temperature.
Three, stainless steel corrosion prevention method
1, avoid halogen ion concentration.
2, to ensure the uniformity of oxygen or oxidizing solution, mixing solution and avoid the solution in static.
3, increase the concentration of oxygen, or oxygen.
4, increase pH. Compared with neutral or acid chlorides, less pitting caused by chloride solution was alkaline, or no (hydroxyl ion corrosion inhibitor plays a role).
5. Work at the lowest possible temperature.
6, in the corrosive medium by adding passivation agent. The low concentration of nitrate or chromate is effective in many medium (inhibitory ions preferentially adsorbed on the metal surface, thus preventing the corrosion caused by chloride ion adsorption).
7, the use of cathodic corrosion. There is evidence that the combination of cathode protection and low carbon steel, aluminum or zinc electric corner of stainless steel will not cause pitting corrosion in seawater.
Austenitic stainless steel containing 2% to 4% molybdenum has good pitting corrosion resistance. The use of molybdenum austenitic stainless steel can significantly reduce pitting corrosion or general corrosion, corrosive media such as Sodium Chloride Solution, seawater, sulfurous acid, sulfuric acid, phosphoric acid and formic acid.