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The initial stages of hot-salt stress corrosion cracking of titanium alloys were studied using hot-stage microscopy and cinematography. An incubation period for cracking was observed, the duration of which depends on exposure temperature, salt composition, and alloy composition. For Ti-8Al-1Mo-1V exposed to sodium chloride (NaCl) the incubation period decreased from about 96 hr at 475 F, to 20 hr at 500 F, to 1½ hr at 650 F, to only 10 min at 850 F. This same alloy cracked severely in only 20 min at 475 F when exposed to tin chloride (SnCl2) indicating that the apparent “threshold temperature” for cracking is strongly influenced by chemical environment. At 650 F the incubation period for NaCl cracking of four alloys containing aluminum increased with decreasing aluminum content. Cracks appeared abruptly and propagated rapidly for a short distance, then apparently paused for additional corrosion-embrittlement to occur. Moisture was demonstrated to be a vital ingredient in hot-salt attack, and the presence of absorbed hydrogen in salt-corroded areas has been demonstrated using radiotracer tritium (H3). These results support the hypothesis that absorption of corrosion-produced hydrogen promotes embrittlement and crack initiation.
titanium alloys, stress corrosion, hot-salt cracking, hot-stage microscopy, crack initiation
Savannah River Laboratory, E. I. du Pont de Nemours and Co., Aiken, S. C.