Fatigue behavior of a high strength steel in vacuum, in air and in 3.5% NaCl solution under cathodic protection

Fatigue resistance of the high strength steel SE702 (A517 type) in air and in 3.5% NaCl solution under cathodic protection is compared with the fatigue resistance in high vacuum in order to identify the effects of the environment on the fatigue damage processes. Fatigue lifetimes, obtained in air and in vacuum, reveal a strong influence of the gaseous environment and a significant effect of the test frequency in air. Experiments performed in aqueous environment under cathodic protection exhibit an additional reduction of the fatigue resistance relative to results obtained in air. Interrupted fatigue tests show that both ambient air and NaCl solution reduce initiation and propagation stages compared with vacuum. The gaseous environment tends to accelerate transgranular cracking whereas the NaCl environment strongly favors ‘intergranular’ propagation, enhancing the crack growth rate relative to air. In some conditions, the air appears to be more detrimental than the NaCl solution with cathodic protection on the initiation stage. This work confirms the key role of hydrogen in NaCl solution and the main role of water vapor in air. But it also shows that the role of oxygen can not be neglected in air and especially for the crack initiation stage.