PICKLING AND PASSIVATION
Pickling
The stainless steel properties are obtained by a unique composition of the alloy
in which chromium plays the leading role. Chromium combines with oxygen to produce
an extremely fine impenetrable film of chromium oxide, which protects the underlying
material. When this film of chromium oxide is present the stainless steel is corrosion
resistant in which case we may call the metal passive.
It is during construction (welding, heat treatment, grinding) and handling
that this layer may become damaged, by iron, paint and grease. In those places
in which the oxide layer is damaged and other forms of contamination prevent
the passive film from reforming naturally, then corrosion can take place.
Pickling after the construction phase will restore the corrosion resistance
to the same level as of the basic material. This is achieved by stripping the
contaminated oxide layer via etching and allowing a new layer to form under
controlled conditions, passivation.
Any sized object can be pickled, from food processing plants to containers.
Items which are impractical to transport can be serviced on-site and all chemical
waste carefully disposed of.
Passivation:
There are quite a number of misunderstandings with regard to the passivation of stainless steel. After pickling and complete removal of oxide scale/contamination, stainless steel will passivate spontaneously when exposed to air. Stainless steel will after rinsing be completely passivated after 24 hours.
We accelerate this process and ensure the oxide layer reforms free from any contaminants. This process usually leaves the stainless steel with a white matt appearance.
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Pickling and passivating
A pickling treatment is the only type of post-treatment capable to restore the
corrosion resistance of the stainless steel at the weld and other surface
damage, to what it was prior to welding. This is true irrespective of the type
of stainless steel and there is no difference in effectiveness between immersion
in a pickling bath or application of a pickling paste [Practical
recommendation No. LM.94.04 NIL (Dutch Institute for Welding Technology), TNO
(Applied Scientific Research)– metal institute]. The corrosion resistance is
restored during the pickling treatment because iron dissolves quicker during
pickling than other metals, whereby the surface is enriched with chromium (see
figure 5) and the corrosion resistance of the stainless steel increases. VECOM
has been treating a variety of materials, including stainless steel, for over 50
years. Pickling and passivating this material after processing restores the
corrosion-protective oxide layer. We cannot stress the importance of this
post-treatment of stainless steel often enough when it comes to preventing
various types of corrosion, with all the associated consequences.