Intergranular Attacks Are Common
Description
Metals and alloys are composed of grains similar to sand grains in common sandstone. Intergranular corrosion refers to the selective corrosion of the grain boundary regions. This attack is very common in some stainless steels and nickel alloys. Some aluminum alloys can also exhibit intergranular and exfoliation (i.e. corrosion at grain boundary sites parallel to the metal surface where corrosion products force apart the metal).
Prevention or Remedial Action
- Heat treatment of alloy to remove phases from grain boundary regions which reduce corrosion resistance (i.e. solution annealing).
- use modified alloys which have eliminated such grain boundary phases through stabilizing elements or reduced levels of impurities:
EXAMPLE: stainless steels such as AISI 304 or 316 can be "sensitized" by heating or welding in the range 900 to 1500 F. This forms carbide precipitates which reduce corrosion resistance of grain boundaries. The use of low carbon 304L or 316L will increase resistance to inter granular corrosion in welded components. for prolonged service at high temperature stabilized stainless steels (i.e. aisi 321 and 347) will increase resistance to inter granular corrosion.
Standard Test Methods
- ASTM A 262 - practices for detecting susceptibility to intergranular attack in austenitic stainless steels.
- ASTM G-28 - test methods for detecting susceptibility to intergranular attack in wrought, nickel rich, chromium-bearing alloys.
- ASTM G-34 - test method for exfoliation corrosion susceptibility in 2xxx and 7xxx series aluminum alloys (EXCO test).
- ASTM G-66 - test method for visual assessment of exfoliation corrosion susceptibility of 5xxx series aluminum alloys (asset test).
- ASTM G-67 - test method for determining the susceptibility to inter granular corrosion of 5xxx series aluminum alloys by mass loss after exposure to nitric acid (namlt test).
Evaluation of Intergranular Attack
The most common concern for stainless alloys is the influence of welding and/or heat treatment on susceptibility to intergranular corrosion produced by carbide precipitation (i.e. senitization). Therefore, the carbon content is an important metallurgical consideration with lower carbon (and nitrogen) materials or materials that have been stabilized with additions of Ti or Nb showing lower tendencies to intergranular corrosion. In evaluation, the tendencies for intergranular corrosion can vary greatly depending on the severity of the test conditions and environment. Oftentimes, standardized environments are used such as those given in ASTM A262.
Intergranular corrosion various alloys require the use of different environments:
- Aluminum alloys - acidified NaCl/HCl solution or HNO3solution.
- Magnesium alloys - NaCl/HF solution
- Copper alloys - NaCl solution with H2SO4 or HNO3.
- Lead alloys - Acetic acid or HF solutions
