| |
|
| Corrosion types |
Many different types of corrosion exist and positive identification of the results of corrosion often pinpoint the cause and consequently a potential remedy. The following are the more generally encountered:
Stress Corrosion The presence of residual stresses within a fabrication, caused by deformation, welding etc. may produce an accelerated form of corrosive attack, especially when the environment contains chloride. This phenomenon is more prevalent in austenitic steels than those with a duplex structure.This type of corrosive attack may be obviated by a stress relieving annealing at a temperature of 880 C or above.
Intercrystalline Corrosion This type of corrosion is to be found fundamentally in the austenitic steels. If an austenitic stainless steel is maintained during a certain period of time at temperatures between 400 and 900 C a precipitation of carbides at the grain boundaries is provoked which depletes the chrome of the adjoining areas. If, in these conditions, the steel is subjected to reactive media, there may be intergranular oxidation known as intercrystalline corrosion. It is essential to avoid slow cooling through the critical temperature range. The cause of the precipitation may be incorrect heat treatment, defective heating or cooling during the transformation of the steel, heating during welding
Selective Corrosion or Pitting When a series of conditions forces the passivity to disappear from some points of the surface of a stainless steel they transform into anodes giving way to the creation of galvanic pairs. In general it is shown by very fine pitting which rapidly develops in depth and in length. A typical case of this corrosion is produced by sea water in almost all steels, and it is dangerous because it is difficult to detect. The chlorides, bromides and hipochlorides are those which present the greatest aggressivity. The composition of the steel and its structure are factors which also influence this type of corrosion. If dirt accumulates on the surface of a stainless steel, the access of oxygen is avoided on the covered areas, pitting forming as a result of the loss of passivity. The austenitic steels with molybdenum show good resistance and this improves if they are copper alloyed. Nitrogen also reduces the tendency towards corrosion by pitting. |
|
|
| Contact Us >> |
Phone: 
Fax: +86-510-68930106
E-mail: info@magvant.com
Technical communication
tech@magvant.com
www.magvant.com |
|
