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POLIGRAT PICKLING of Stainless
Steel
Why Pickle?
Pickling cleans and restores the corrosion resistance of components
fabricated from Stainless Steel and thus considerably influences their
service life and usefulness.
The corrosion resistance of Stainless Steels is primarily based upon
the presence of at least 13% chromium in the alloy. In combination
with oxygen, the chromium forms a dense chemically resistance passive
layer of chromic oxide which protects the surface against corrosion.
Unfortunately, every mechanical treatment will damage the surface
layer by contamination with ferritic matter, changes to the microstructure,
development of stresses and a reduction of chromium content. In addition,
any heat treatment, such as annealing and welding, will lead to scaling
and discolouration. These aspects not only diminish the visual appearance
of the component, but in particular reduce its corrosion resistance.
A dense passive layer of chromium oxide can only form on a metallically
pure surface.
Professionally pickled Stainless Steel surfaces and welding seams:
are metallically pure, free of scale and discolouration
have full corrosion resistance
have a decorative satin metallic appearance.
How to Pickle
Depending upon size and shape, the components that are to be pickled
are either immersed in a pickling bath or the pickle solution is applied
to the metal surface. The process is allowed to take effect for a
few minutes up to a few hours at room temperature and then fully rinsed
with water.
The active chemicals consist of mixed acids that cause a removal of
the surface layer of the component from 1 to 3 microns. Oxide scale
and discolouration, ferrites and surface contamination are preferentially
attacked and removed. The final passive film is formed while rinsing
with water and upon exposure to air.
Applications
Virtually all 300 Series stainless steel components are suitable for
pickling and indeed should be pickled as a matter of routine, especially
any welded fabrication.
POLINOX Protect Passivation for stainless steel
POLINOX Protect is a new product from POLIGRAT formulated to optimise
the corrosion resistance of austenitic stainless steel. It works by
significantly improving the behaviour of the surface passive layer.
Treatment with POLINOX Protect can increase the corrosion resistance
to a degree, which until now could only be achieved by the use of
much more expensive processing.
The application of POLINOX Protect results in an ideal through-depth
improvement of the passive layer, which shows an optimum chrome/iron
ratio and full corrosion resistance. Free iron will be substantially
eliminated from the passive layer.
On Site Processing
For structures too large to move or those already fixed into position,
Poligrat have an experienced team of technicians who can carry out on
site cleaning, pickling and passivation of stainless steel components. |
