Effects of Chromate and Non-Chromate Coating Systems on Environmentally Assisted Fatigue of an Aluminum Alloy

Abstract

Fatigue crack growth testing of 2024-T3 Aluminum plate was performed using compact tension (CT) specimens with chromate and non-chromate primer paint systems to evaluate the effects of the coatings on fatigue crack growth rates. The tests were conducted in lab air and sea water environments for each of the coating systems. Standard E399 CT specimens were tested to determine the influence level of environmentally assisted cracking (corrosion fatigue) on crack growth rates and cyclic count to prescribed precrack and final crack lengths. Increasing stress range (DK) tests were conducted at 10 Hz in the range of 6.5 to 26.5 MPa ffiffiffiffi m p . It was determined that the coated specimens exhibited a 12% shorter total life, on average, than the bare specimens for the lab air cases. In the case of salt water exposure, the coated specimens exhibited approximately 10% life increase over the bare specimens. The number of cycles to the 2.54 mm pre-crack length for the coated specimens was all less than the cycle count for the bare tests. In each case (coated or bare), there was an increased growth rate at the lower stress ranges in the salt water environment, with the chromate system case displaying the smallest change (increase). It can be concluded that the coated specimens initiate cracks and propagate faster than the bare specimens for short cracks at low stress range, but the environmental influence on the specimens is quickly overshadowed as the cracks elongate and the rate of growth increases. The coated specimens exhibited a higher total life cycle count to final crack length for this testing.

Effect of pH on Stress Corrosion Cracking of 6082 Al Alloy

“…It has been clearly shown that the majority of aluminum alloys are susceptible to SCC [4][5][6][7][8][9][10][11][12]. For this reason, many researchers have tried to improve the SCC resistance of aluminum alloys by utilizing different approaches such as thermomechanical [13] and post heat [14] treatments, as well as surface modifications [15] and deposition of coatings [16].…”

“…Despite the many existing studies on the SCC behavior of aluminum alloys [4][5][6][7][8][9][10][11][12][13][14][15][16], very little is actually known about the SCC susceptibility of the widely applied 6082 aluminum alloy. In addition, most of the previous work has been performed under neutral pH environments, neglecting the significant effect of pH on the occurring mechanisms.…”