Slow Strain Rate Fracture of High-Strength Steel at Controlled Electrochemical Potentials in Ammonium Chloride, Potassium Chloride, and Ammonium Nitrate Solutions

Earlier investigations using slow strain rate fracture tests at controlled electrochemical potentials demonstrated the susceptibility of AISI 4340 high-strength steel to stress corrosion cracking (SCC) in the combustion product residues of jet engine cartridge ignition starters. X-ray diffraction analyses of the residues revealed significant concentrations of ammonium chloride, potassium chloride, and ammonium nitrate. In the present investigation, slow strain rate testing was conducted to determine the effects of each of these chemical compounds on the fracture process. Test environments included ammonium chloride, potassium chloride, and ammonium nitrate solutions at concentrations of 100, 1000, and 10 000 parts per million by weight and at a pH of 5. The tests were performed at a constant extension rate of 1 × 10^-7 m/s (strain rate of approximately 2.7 × 10^-6/s). Tests were performed at controlled electrochemical potentials, both anodic and cathodic with respect to the open-circuit corrosion potential, to delineate the potential ranges for SCC and hydrogen-induced cracking.

Of the three compounds studied, only ammonium chloride caused SCC of the AISI 4340 high-strength steel. The corrosion potential of about -620 mV versus saturated calomel electrode (SCE) is at the brink of the potential range for stress corrosion cracking. The most severe embrittlement was observed at a potential of -450 mV and at a solution concentration of 1000 ppm ammonium chloride. Hydrogen-induced cracking was observed at -850 mV in the ammonium chloride and ammonium nitrate solutions, but not in the potassium chloride solutions.