Pickling and Passivation
It has be to understood that once a stainless steel surface has been heated beyond 400°C then it will start to oxidise and exhibit heat oxides colours. Sources of heat includes; welding operation, straightening operations, thermal heat treatments such as annealing or stress relieving.
These areas are both unsightly and potential corrosion sites.
The HAZ around the area can be removed either mechanically or chemically.
Pickling is a very useful operation since it's quick and can deal with complicated shaped objects.
The methods of pickling include;
- Immersion pickling in a bath or vat
- Circulation pickling using a liquid around a pipe system - closed loop type
- Spray pickling using a GEL pickle product (see Enerjex product range) which will stick to the surface once applied by spray applicator
Pickling can be done either in-house at our 60,000ft² undercover Manchester facility or on your site anywhere in the world.
Pickling explained A term used to describe the etching or dissolution of the stainless steel surface (a controlled dissolving of the surface) a relatively small loss from the surface is witnessed in the order of µm values typically 5 - 10µm are removed from the surface thus rendering the "new" surface a virgin surface and free from contamination from fabrication, manufacturing or product from processing.
Before pickling is commenced the surface is degreased of organic matter since this can inhibit the pickling.
Passivation explained This term is used to describe the "artificial" re-building of the passive layer or passive film that renders the surface of stainless steel passive or corrosion resistant (it is the low level of electrical conductivity across this film that helps maintain corrosion resistance) The surface will passivate naturally in an atmosphere containing oxygen, this is termed auto-passivation.
ASTM A30 and ASTM A967 have a number of interpretations of passivation, this being SRL's preferred take on the subject.