Stainless Steel Passivation Testing
We talked about the passivation of stainless steel alloys and how this fundamental property is due to an invisible protective film mainly composed of chromium oxides and hydroxides of the type Cr2O3 and Cr(OH)3.
The commonly accepted model to explain the passivation is focused on the presence of 3 layers: the first one is the base metal (stainless steel, containing Iron, Chromium, Molybdenum, Nickel, carbon, phosphorus, sulfur, etc.).
The second layer is the area in which oxygen is adsorbed; it will lead to the formation of the passivation layer of stainless steel and is composed mainly of Iron, Chromium, Nickel, Molybdenum.
The third is defined as the real passivation layer of stainless, made by oxides and hydroxides of chromium. In addition, there can be iron oxides and contaminants / pollutants that must be removed by a pickling or electrochemical pickling of stainless steel.
The last layer has a thickness between 1 and 5 nanometers, so the use of measuring methods of the passivation layer by interaction with light is not possible: stainless steel passivation is totally invisible, even at very high magnifications.
For that reason, only techniques based on electron interactions or chemical reactivity will produce a tangible result that could lead to an analysis of the passivation layer of stainless steel.