S-N Fatigue Behavior of Anodized 7050-T7451 Produced in Different Electrolytes

The effect of anodizing layers processed in different electrolytes of chromic acid (chromic acid anodizing [CAA]), sulfuric acid (sulfuric acid anodizing [SAA]), and tartaric acid (tartaric acid anodizing [TAA]) on the S-N fatigue behavior of the 7050-T7451 specimen was examined. The fatigue tests were conducted at an R ratio of 0.1 and at three different stress levels of 200, 220, and 270 MPa. Some specimens were exposed to continuous salt spraying of 5 pct NaCl solution for 336 hours and subsequently fatigue tested at an applied stress of 200 MPa. The detrimental effect of anodizing on the S-N fatigue resistance of 7050-T7451 was observed. The fatigue resistance varied with different anodizing electrolytes particularly at low stress with the greatest resistance for the TAA followed by CAA and SAA. The fractographic and micrographic observations showed the morphological difference in each anodized layer, which seemed to determine the ease of crack initiation and the resistance to S-N fatigue. The resistance to S-N fatigue of anodized 7050-T7451 specimens was, for example, affected strongly by the nature of preanodized layer with different acid-pickling processes. The pits found in the pre-CAA specimens seemed to be slightly irregular and deeper than those in the pre-TAA counterparts, reducing the fatigue resistance of the 7050-T7451 specimens. The CAA, SAA, and TAA specimens showed similar resistance to corrosion plus fatigue.