The surface roughness for a #4 finish can vary greatly- from 0.2 to 1. A roughness greater than 0.5 will significantly increase corrosion rates.
 

Corrosion Resistance

Is your stainless steel fabrication really stainless? Simply purchasing stainless steel does not mean that the part or assembly will be corrosion resistant. Critically important steps need to be taken during fabrication to ensure this expensive material maintains its characteristic corrosion resistant properties.
 

Stainless Steel Contamination

Stainless steel can become contaminated throughout the fabrication process.
  • Iron dust can land on the stainless steel
  • Water stains from resting water on the sheets
  • Paint, oil, grease can introduce carbon into the material
  • Scratches from carbon steel forklifts or carbon steel racks
  • Gouges from improper handling
  • Embedded iron particles from machine tooling used on carbon steel and not properly cleaned
Proper storage, handling, and fabrication procedures can avoid many of these problems. QC test procedures will catch any manufacturing defects. Proper chemical cleaning can repair effected areas.
   
 

Corrosion Resistant Welds

Welding stainless steel requires strict planning, training, and execution. Potential pitfalls include:
  • Mixing carbon steel and stainless steel weld consumables
  • Grinding welds with equipment previously used on carbon or galvanized steel
  • Improper cleaning of weld area
  • Blast cleaning with contaminated media
  • Improper restoration of the chromium depleted layer in the HAZ
  • Finishing only the front side of the weld and leaving the back untreated.

Any of these mistakes will result in a weld that is not as corrosion resistant as the base material.

   
 

Pickling, Electropolishing, and Passivation

Passivity is the state in which stainless steels are chemically inert due to the chromium oxide film that naturally forms on the surface of the metal. During fabrication, this oxide layer can be broken and the steel can be contaminated.

Passivation is defined by ASTMA380 as "[the] removal of exogenous iron or iron compounds from the surface of a stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself." Sometimes oxidants are used to promote the regeneration of the oxide layer after the removal of the free iron. Passivation will not restore the corrosion resistance of welded areas or areas where carbon steel has become embedded into the stainless steel.

Picking is a chemical cleaning process that removes the surface layer of the stainless steel. Electropolishing is an electrochemical process for removing the surface layer of the stainless steel. Electropolishing is often thought of as electroplating in reverse. Both pickling and electropolishing can be used to remove the heat scale associated with welding, and will remove the chromium depleted layer under the heat tint. After pickling or electropolishing, the oxide layer can be regenerated naturally or passivation can be specified.

   
     
     
 
 
   
 
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