Intergranular Corrosion and Weld Decay
Intergranular corrosion is recognised as corrosion that takes place at the grain boundaries in some alloys. It particularly affects some stainless steel grades. At temperatures between 500oc and 800 oc for long time periods the alloys become open to intergraular attack. It is thought that this heat treatment allows the formation of small precipitate particles of chromium carbide by reaction between the chromium and the carbon within the stainless steel. These particles then form along the grain boundaries. The chromium and the carbon both must diffuse to the grain boundaries to form the precipitates which will leave a chromium depleted zone next to the grain boundary. This means that this grain boundary is now susceptible to corrosion.
Intergranular corrosion can be a problem within stainless steels where the metal is heated after welding. During the heating the intergranular corrosion process as discussed above can take place. The location of the weld decay is shown below.
Above: A sketch to show weld decay location (http://www.corrosionclinic.com/types_of_corrosion/weld_decay_weldment_corrosion.htm, last accessed 02/11/14)
Stainless Steels can be protected by intergranular corrosion by taking the following measures;
1) subjecting the material to high temperature heat treatment where all the chromium carbides are redissolved.
2) Lowering the carbon content below 0.03 wt% C so that carbide formation is minimal.
3) Alloying the Stainless Steel with another metal such as niobium or titanium which has a greater tendency to form carbides than does chromium so that the Cr remains in solid solution
(Page 697, Materials, Science and Engineering, William D Callister-David G. Rethwisch, 2011)
Hydrogen Damage
Some steels along with some alloys can experience a reduction in ductility and tensile strength when hydrogen penetrates the material. This is known as hydrogen embrittlement. Hyrdogen embrittlement is a failure within the material. It is the result of a brittle fracture which is very sudden without little warning. To occur, hydrogen diffuses through the crystal lattice within the material. This hydrogen embrittlement is similar to stress corrosion as in a usually ductile material will suffer a brittle fracture due to tensile stresses and a corrosive environment.
For hydrogen embrittlement to occur:
- A source of hydrogen must be available
- The possibility of the formation of its atomic specials must be available.
An example of when these conditions are met include the welding of steels in atmospheres where hydrogen is present (including water vapour). Other examples include pickling steels In sulphuric acid and electroplating. The presence of elements, such as sulphur and arsenic compounds increase the rate of hydrogen embrittlement. These substances retard the formation of molecular hydrogen which increases the residence time of the atomic hydrogen spends on the surface of the material. A common solution which is very bad for initiating this process is hydrogen sulphide which is found in petroleum fluids and natural gas. Martensitic steels are particularly susceptible to hydrogen embrittlement, where bainitic, ferritic and spheroiditic steels are more resistant.
References
Materials, Science and Engineering, William D Callister-David G. Rethwisch, 2011
(http://www.corrosionclinic.com/types_of_corrosion/weld_decay_weldment_corrosion.htm, last accessed 02/11/14)
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