When the stainless steel is being welded a scale is often formed on the either side of the weld and such side is a heat affected zone will vary in color from blue to black depending on the welding condition. Such area is enriched with metal oxides that consist of very little corrosion resistance and readily allows corrosion to commence. Such effect extends way beyond the areas that are affected by the change of color. This area is very reactive and will easily rust in only a short period of time. Since this is not fully understood, it’s normal to see severely corroded areas wherein the stainless steel can be found on site. This is most unfortunate because the method of avoiding such corrosion is easy and is in fact, inexpensive and this is by means of Pickle Passivation.
Pickling is the process of removing a thin surface layer off the stainless with the use of an acid solution which is often a mixture of hydrofluoric and nitric acid. In order for the Pickling to be effective, the surface should be clean and be clear of oils and greases. The process of pickling can help to remove embedded iron, heat tint, surface iron contamination, as well as weld scale. Pickling can also help to produce a dull matt and uniform grey finish during the process of Pickle Passivation.
A major cause of the corrosion on service is the iron contamination and this could arise from a wide variety of sources. These will include using tools that are made of constructional or mild steels, abrasives that contains iron and any tools that have been previously used on steel that are non stainless. A cross contamination can also take place from debris and grindings as a result of working near the steel either on site or in the workshop. Furthermore, iron contamination is not always obvious. If it is expected, it can be detected with the use of the ASTM A380 or ferroxyl test. This is a type of rapid test where a solution will turn to blue during the presence of iron. Pickle Passivation is an effective process of taking off the iron contamination off the surface.
Heat Tint and Weld Scale
The heat tint is produced through the welding process and is not only unsightly but the thicker oxide layer will include chromium coming from the metal surface, which lowers its corrosion resistance. Pickle Passivation both removes the oxide layer and will trigger color tints and a thin layer of underlying metal that restores the original properties.
The stainless steel has owed its corrosion resistance from the formation of the chromium oxide surface layer and is then being called as passive. This happens naturally and it spontaneously provides enough oxygen. Even the aerated water will provide enough oxygen for such process to occur. The material that is supplied by the producing mills fully passivated and further Pickle Passivation is rarely needed. But if the oxide is stripped off, perhaps by means of picking, then the oxide layer will take a short time to be able to reach its full thickness.