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What is Passivation ?

To render a metal less susceptible to corrosion by coating the surface with a substance, such as an oxide.

Passivation also is accomplished by Electropolishing.

Electropolishing is an electrochemical process that is a SUPER PASSIVATOR of stainless steel and results in a more passive surface.

This is typically done to remove any "free-iron" from the surface of the metal, which enhances the corrosion resistance of the stainless steel surface and accelerates the re-formation of the passive layer.

In years gone by, the process specified was to "pickle and passivate" - a two step method - in order to create a surface on stainless steel that would be resistant to corrosion.

Passivation of stainless steel is done to make the surface more passive and corrosion resistant.

The passivation of stainless steel is a process performed to make a surface passive, ie a surface film is created that causes the surface to lose its chemical reactivity.

Stainless Steel passivation unipotentializes the stainless steel with the oxygen absorbed by the metal surface, creating a monomolecular oxide film.

Passivation can result in the very much-desired low corrosion rate of the metal.

  • Steels containing more than 12% Chromium are capable of forming an invisible, inert or passive, self-repairing oxide film on their surface. It is this passive layer that gives stainless steels their corrosion resistance.
  • If a stainless steel surface is scratched, then more Chromium is exposed which reacts with oxygen allowing the passive layer to reform.
  • However, if a particle of carbon steel is embedded in the scratch then the passive layer cannot reform and corrosion will occur when the metal is wetted or exposed to a corrosive environment.

Because during handling and processing such as rolling, forming, machining, pressing, tumbling, and lapping, particles of iron or tool steel or abrasive particles may be embedded in or smeared on or into the surfaces of stainless steel components.

If allowed to remain, these particles may corrode and produce rust spots on the stainless steel.

This is due to the formation of a galvanic couple between two dissimilar metals that can promote a corrosive reaction.

To dissolve the embedded or smeared iron and prevent this condition, as well as restore the original corrosion-resistant surface, semi-finished or finished parts are given a 'passivation' treatment.

Passivation remains a critical step in maximizing the essential corrosion resistance of parts and components machined from stainless steels.

It can make the difference between satisfactory performance and premature failure. Incorrectly performed, passivation can actually induce corrosion.

 

Reasons for Passivating in Welding

  • Increase  corrosion  resistance  of  fabricated  parts.
  • Removal  of  weld  discolouration.
  • Restoration  of  protective  oxide  layer.
  • Contaminants  are  potential  corrosion  sites  which,  if  not  removed, result  in premature  corrosion  and  ultimately  result  in  deterioration  of  the component.
  • Especially in welded areas.

                

 

              

                                                                                                          

            

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