The blue and brown marks which result from welding are ferrous and ferric oxides of iron. Iron oxide layers are quite weak, and the temperature at which the TIG Brush® operates quickly breaks down the iron oxides.
The resulting ferric ions enter into solution in the fluid, where they are sequestered (held so they cannot form oxides again). When the TIG Brush® is removed from the work and the fluid wiped away, the surface of the stainless steel is left free from ferric ions and oxides.
Iron Oxides are not very good at protecting the metal surface from contamination, and for this reason any discoloured areas start to corrode soon after welding, even after they have been cleaned. To stop this corrosion the metal surface needs to be “passivated”. When the TIG Brush® removes the iron oxide layer, chromium rich metal underneath is exposed. The high temperature of the TIG Brush® causes the chromium to rapidly oxidize, forming a Chromium Oxide layer in the place of the Iron Oxide layer. The process of forming the Chromium Oxide layer is called “passivation”.