When stainless steel is TIG welded, the intense heat generated causes chromium depletion, so an iron rich zone is left around the weld.
The high concentration of surface iron, together with the high welding temperatures, causes the iron to immediately oxidize. This results in the characteristic blue and a brown discolouration around the weld. The discoloured area corrodes easily, as chromium isn’t present in high enough concentrations to protect the metal.
The TIG Brush has been designed to rapidly remove the discoloured oxides and passivate the underlying surface, without altering the look and finish of the metal. The features which enable the TIG Brush to do this quickly and in a safe manner are explained below.
2. TIG Brush Features
The TIG Brush comprises a brush through which an electric current is passed. The brush is dipped into a cleaning fluid and then applied to the areas to be cleaned. The fluid is designed to remove the iron oxide marks which result from welding.
The brush is designed to convert the electric current to high temperatures, especially at the end of the bristles. While the fluid is quite benign at room temperatures, the brush heats the fluid to an elevated temperature, when it becomes much more re-active. This results in an accelerated chemical action at the surface of the Stainless Steel.
Unlike Pickling Paste, the active ingredients of the cleaning fluid are not inherently dangerous to operator health, and if used correctly the fluid is non-toxic to the environment.
3. The Chemical Action
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.
4. Surface Improvement (passivation)
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”.
Chromium Oxide is very good at protecting the underlying metal from contamination, and will limit corrosion and rusting. The Chromium Oxide layer is clear, so the original surface texture and appearance of the metal is maintained.
5. The Alternatives
Two traditional methods for cleaning stainless steel are Pickling Paste and mechanical abrasion. Both methods suffer from problems when used to clean stainless steel.
Pickling Paste comprises two very strong acids, which eat away the surface of the stainless steel. This process is called “etching”, which damages the surface of the stainless steel. Although Pickling Paste removes iron oxide discolouration, the metal surface is left without an oxide layer, which can allow contaminants to cause corrosion to take place. Therefore a separate “passivation” operation has to be performed. Due to the type of acids used, Picking Paste poses a very serious health risk to the operator, and is dangerous to the environmental.
Mechanical abrasion, by definition, wears or scrapes away the surface of the stainless steel by linishing or other mechanical methods. The process does remove iron oxide layers, but also changes or damages the metal finish. Once again this method does not passivate the metal, so further processes are needed in order to protect the surface of the stainless steel.
6. Conclusion
The TIG Brush is a safe and effective way to clean the surface of stainless steel, particularly after welding. It has been designed to quickly remove blue and brown iron oxide marks without altering the surface finish of the metal. The TIG Brush also passivates the metal in the same operation, producing a complete result very quickly. The fluid used with the TIG Brush is activated at high temperatures, and is therefore relatively safe for the operator and the environment under normal conditions.
