Corrosion Resistance
Information from Materials Standards for PM Structural Parts, 2018 Edition
published by Metal Powder Industries Federation
The corrosion resistance of PM stainless steel is dependent on the corrosive media, the composition of the alloy, and the processing conditions used to manufacture the material. Some generalities can be stated pertaining to the corrosion resistance of PM stainless steels.
The corrosion resistance of PM stainless steels may be evaluated by immersion in a 5% NaCl solution in accordance with ASTM B895 or via salt-spray testing following the procedures in ASTM B117.
Method 1 (time to first rust) of B895 is good for evaluating new materials/processes. Method 2 (% rust ranking) is really only suitable for the PM stainless steels that have relatively good corrosion resistance such as SS-316L and SS-316H. Nitrogen-alloyed versions of the same grade have lower corrosion resistance.
Salt spray testing is really only appropriate for the PM stainless steels such as SS-316L and SS-316H that have relatively good corrosion resistance. For salt spray testing it is not possible to use the percentage rust ranking approach of Method 2 from ASTM B895 because the test bars are inclined for the test, and rust stains spread down the test bars making a percentage rust judgment inaccurate. Salt spray testing is, however, suitable for determining the time to the first sign of rust. At least ten test specimens should be used for the test.
The resistance of various PM stainless steel alloys to corrosion was tested in accordance with ASTM G31. Standard transverse rupture specimens were immersed in a 2% sulfuric acid solution at room temperature for 24 hours. Three replicates were tested. The mass loss was determined and then converted into a mass loss per surface area per day factor. The alloys were rated using the following system (1):
The results are summarized in the following table:
NOTE: Stainless steel grades with modified chemical compositions have demonstrated an improved corrosion rating compared with the standard 300 series stainless steel alloys. Resin impregnation also improves the corrosion resistance.