I understand that Stainless Steel is a low carbon steel having minimum 10.5% chromium content in it there r 60 different type of stainless steel. Kindly informa) Is this info correct?????b) Which stainless steel can get rusted or can have superficial rust????
Stainless steel is a generic term for a whole range (there are more than 60 types) of Fe-Cr alloys. They all contain some amount of C and some alloys include a good number of other alloying elements. In almost all cases, the materials have corrosion resistance due the Cr atoms that are in solid solution with the Fe matrix. Under corrosive conditions, the Cr forms a thin tight coherent layer of Cr-oxide. This Cr oxide coating prevents further corrosion so long as it is not chemically or physically removed. If the SS has been subject to mechanical abrasion, the corrosion resistance can sometimes be restored by a light exposure to nitric acid which passivates the surface, restoring the Cr-oxide layer. The carbon content is an important factor in corrosion preformance but so is the heat treatment. Under the wrong conditions and in different environments, all SSs can rust. One of the most common corrosion problems is sensitization which occurs in the heat affect zone (HAZ) around welds. If the C content is high enough and the heat is high enough, the Cr atoms in solid solution form Cr-carbides and are no longer available to form a protective Cr-oxide. The corrosion resistance can be restored by appropriate heat treatment.
