What is stainless steel?
A steel can be called stainless when it contains at least 12% chromium. Currently there are around 200 different alloys that are classified as stainless steels. New or modified alloys are being developed all the time. In addition a large number of elements are added to give the stainless steel specific properties. Other elements that are added include nickel (Ni), manganese (Mn), molybdenum (Mo), carbon (C), titanium (Ti), copper (Cu), sulphur (S), nitrogen (N) and selenium (Se). Various types of stainless steel only contain chromium as the major alloying element. We call these chromium steels or ferritic stainless steels. To a large extent chromium steels have the same properties as ordinary structural steel, but with the big difference that they are passive, and remain passive as long as highly corrosive environments are avoided. Due to their limited corrosion resistance their applications are also limited.
Why and when stainless steel
Especially because of its specific, corrosion resistant properties, stainless steel is widely used. These properties are caused by the stainless steel’s natural ability to form a protective oxide layer. This oxide-skin however, is extremely thin and therefore very vulnerable. The many handlings that stainless steel is undergoing during construction phase and sometimes during transportation, can lead to damage of this oxide layer. The final result is that the stainless steel looses some of its corrosion resistant properties and consequently (corrosion) damages may occur, depending on application and usage.
Influences which can lead to corrosion damage
– Heat input, i.e. during welding processes, where a discoloration occurs and a chromium depleted
– Free iron contamination, f.i. through tools containing iron/steel elements or through the surroundings
– The presence of chlorides and salts in the environment or medium
– Transformation of the material during bending and forging at which a change of structure in the
material is caused
Pickling after the construction phase will remove the chromium depleted zone and free iron, fully renews the corrosion resistance and brings it back entirely to the specifications of the original raw material