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Services > Pickling and Passivation
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.
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.Application for:
- Metallic clean surfaces
- Removal of thermal oxide (scale and discolouration)
- Removal of foreign matter
- Uniform visual aspect
- Improvement of corrosion resistance
- Pickling and passivation pastes
- Spray pastes and gels
- Immersion
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.
Rolls Royce - Hood made of Stainless steel 304 passivated by POLINOX Protect B Bio
