The effects of impurities in corrosive media causing stress corrosion cracking in aisi 304 stainless steel

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Virginia Polytechnic Institute

The influence of impurities in magnesium chloride upon the rate of attack of stress corrosion cracking in AISI 304 stainless steel has been investigated. Previous investigations in this laboratory indicated that different batches of technical grade magnesium chloride produced poor reproducibility of results. These inconsistencies were attributed to the presence of varying amounts of impurities which are commonly found in technical grade magnesium chloride.

In this investigation, certified grade magnesium chloride was used with impurities intentionally added in order to determine how each affected the constants M and C in the following equation, which relates the maximum crack depth D, to the time of exposure to the corrosive medium, t:

log t • D/M + log C

The constant C represents the time of exposure to the corrosive media to nucleate cracking, and M is a function of the rate at which cracks propagate.

It was determined that the impurity ions most commonly found in magnesium chloride were nitrate, sulfate, and calcium. A significant difference in the constants V and M was found to exist with the presence of each impurity. It was not determined, however, if a change in pH resulted from the addition of the impurity compounds. Hence, a further study is required in order to determine the pH of the corrosive media and the effects of its changes.