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The susceptibility of aerospace bolting alloys to stress-corrosion cracking has been of continuing interest especially since strength levels have by far exceeded 200 000 psi, the threshold above which failures are much more common. The search for a stress corrosion resistant material has progressed through a number of alloys with strength levels up to 300 000 psi.
All of the high-strength steel alloys exhibted a similar trend where time to failure decreases as strength level increases. However, different levels of immunity were observed for the different alloys. Martensitic stainless steels were found to be much more resistant to stress-corrosion cracking but were very sensitive to hydrogen embrittlement from plating or galvanic corrosion reactions.
The use of a MIL-STD-1312 test procedure to evaluate coated or plated alloy steel bolts is sanctioned because of the short time to failure required, the reproducibility of results, and the duplication of service failures. The same is not true for the more resistant alloys where times to failure are in thousands of hours and results are scattered. A more aggressive test is warranted.
Crevice corrosion and pitting at interfaces are the most common initiators of stress-corrosion cracking failures for high-strength fasteners. Since this is unavoidable with fastened joints, it behooves the consumer to realize all of the advantages and disadvantages of various materials so that he may select the best one for his needs.
stress corrosion, coatings, high strength steels, bolts, environmental tests, corrosion, electroplating, pitting, crack propagation, corrosion resistant alloys
Corrosion engineer, Standard Pressed Steel Co., Jenkintown, Pa.