AVESTA PICKLING & PASSIVATION
Stainless steel welds present particular problems. The high heat associated with welding has a negative impact on the passive layer of chromium oxide, e. g. chromium depletion in the heat-affected zone. Slag, undercutting, tarnish, iron particles, spatter, uneven surfaces and organic contaminants also have unfavourable consequences.
Post-treatment in practice
Post-treatment should always begin with cleaning and degreasing. Optimally, the process then continues with pickling. This can be done with gels, pastes and sprays or by immersion in a bath. In certain cases, a final treatment with a passivating agent may be necessary. After each step in the process, the object being treated must always be rinsed using water at high pressure. In accordance with good environmental practice, the rinse water must be taken care of as stipulated in relevant laws and regulations.
Stainless steel pickling
Stainless steel are pickled in order to remove certain surface defects and contaminations, such as welding oxide, heat tint or scale, formed during fabrication. If left on the final product surface, these oxides will reduce the corrosions resistance of the stainless steel surface.
When pickling, the metal oxides are dissolved by the combination of hydrofluoric, nitric and sometimes also sulphuric acids.
These acids dissolve the heat tint and the underlying chromium depleted zone.
The clean surface is then re-passivated rapidly by the oxygen in the air, and due to this, the welded object achieves an optimal (for the alloy) corrosion resistance.
This method is technically superior for the surface treatment of stainless steel since it removes through corrosion any areas of inferior corrosion resistance on the surface. The disadvantage is that pickling acids are corrosive and poisonous, and must be handled with care.
Safety is important when pickling
Avesta pickling chemicals are harzardous substances so the user must be protected against fumes and splashes. Follow these general guidelines and safety levels when working with Avesta pickling products.
Safety data Sheet should always be evaluated before chemicals are used.
Passivation is the chemical treatment of a stainless steel with a mild oxidant such as Avesta FinishOne Passivator 630, with the purpose of removing free iron from the surface and speed up the process of forming a protective/passive layer.
The term passivation is commonly applied to several distinctly different operations or processes relating to stainless steels. In order to avoid ambiguity in the setting of requirements, it may be necessary for the purchaser to define precisely the intended meaning of passivation.
Passivation is not effective for the removal of heat tint or oxide scale on stainless steel.
Removal of heat tint or oxide is necessary for passivation to be effective.