Can noncorrosive steel do rust?
Considering the noncorrosive stainless steels with a Cr-content of over 10.5% the formation of rust cannot automatically be excluded. Even austenitic stainless steels with Cr-contents exceeding 20% and Ni-contents of over 8% can rust in case of incorrect handling and processing or in case of constructive deficiencies.
The passive layer
Just as normal steels, stainless steels react with oxygen and form an oxide film. However, regarding standard steel the oxygen reacts with the existing iron atoms and forms a porous surface that allows the reaction to proceed. This can proceed until the complete "rusting out" of the workpiece. Regarding noncorrosive stainless steel the oxygen reacts with the chromium atoms of the steel that exist in a relatively high concentration. The chromium and oxygen atoms form a close oxide film that prevents the reaction from proceeding. Due to its inert behaviour towards the environment this oxide film is also decribed as passive layer. The characteristic respectively durability of the passive layer primarily depends on the alloy composition of the steel.
There are two reasons for the formation of rust in case of "noncorrosive" stainless steels:
- the passive layer could not form itself, or
- the passive layer was damaged
Only through a high degree of cleanliness it can be avoided that the passive layer ist not formed. In principle, all surfaces worked have to be cleaned from all residues.
The types of corrosion described in the following start from a subsequent destruction of the passive layer:
Erosive surface corrosion
Erosive surface corrosion describes an even wear of the workpiece surface. This type of corrosion only occurs when acids or intense alkaline solutions act on the workpiece surface. Is the yearly wear rate under 0.1 mm it is spoken about a sufficient resistance of the workpiece to surface corrosion.
Pitting corrosion occurs when the passive layer is penetrated locally. Responsible for the breaking open are chloride ions that withdraw the chromium atoms, necessary for the formation of the passive layer, from the stainless steel at presence of an electrolyte. Holes in the form of pinpricks develop. The existence of residue deposits, external rust, slag residues or discoloration (blueing) leads to an intensification of the pitting corrosion.
Intercrystalline corrosion can appear when by effect of heat chromium carbides are deposited on the grain edge and pass over into solution at availability of an acid medium. This happens at the following temperatures:
- austenitic steels 450° - 850°C
- ferritic steels at more than 900°C
In case of proper material selection intercrystalline corrosion is irrelevant today.
Galvanic corrosion occurs when different metallic materials are in contact with each other and are moistened by an electrolyte. The less precious material is corroded and passes over into solution. Noncorrosive steels are precious compared with most of the other metallic materials.