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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 the 300 series PM stainless steels is superior to that of the 400 series PM stainless steel alloys when processed under the same conditions.
  • Inefficient delubrication may lead to chromium carbide precipitates at the grain boundaries during cooling; making the sintered parts susceptible to intergranular corrosion.
  • High-temperature sintering in a pure-hydrogen atmosphere or in vacuum results in the best corrosion resistance, which is directly related to the resulting low oxygen and nitrogen contents.
  • Corrosion resistance usually improves, in nitrogen containing atmospheres, with high-temperature sintering >2100 ºF (>1150 ºC).
  • With nitrogen-containing sintering atmospheres, fast cooling after sintering is essential to prevent additional nitrogen absorption and precipitation of chromium nitrides.
  • Many instances of the development of rust spots, in atmospheric exposure or in other relatively mild environments, may be accounted for by contamination with iron or low-alloy particles. Isolation of the equipment used to process PM stainless steel is a way to solve this problem.
  • Improved corrosion resistance of PM stainless steels may be obtained by modifying the chemical composition of the material. Some proprietary compositions are commercially available.
  • In acidic environments, corrosion resistance increases with increasing part density.
  • Corrosion resistance may decrease with increasing density in chloride environments. Higher density results in smaller pores and these are more prone to crevice corrosion.

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.

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