Pickling
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.