An Environmentally Assisted Cracking Evaluation of UNS C64200 (Al–Si–Bronze) and UNS C63200 (Ni–Al–Bronze)

Abstract

A recent failure of a union nut, UNS C64200, made of Al–Si–Bronze (ASB) in a breathing air system of a marine platform has highlighted the need for environmentally assisted cracking (EAC) data for both ASB and Ni–Al–Bronze UNS C63200 (NAB) components in environments relevant to marine use. In addition, the possibility of exposure to ammonia environments via cleaning agents or biological processes warrants consideration because of the known susceptibility of bronze to EAC in ammonia environments. A displacement-controlled, rising step load (RSL) technique was employed on precracked compact tension specimens to quantify and compare the threshold stress intensities for EAC in air, seawater (SW), and SW + ammonia environments for wrought ASB and NAB materials. These results are compared to calculations of the stress intensity in service to determine the probability of EAC initiation. ASB was found to be susceptible to subcritical intergranular EAC initiation in laboratory air, SW, and SW + ammonia environments. NAB was immune to EAC under the conditions tested in laboratory air and SW, but was susceptible to intergranular EAC in SW + ammonia solution. The threshold stress intensity in SW + ammonia was found to be similar for both ASB and NAB; however, the subcritical crack growth rate for NAB was found to be 2–3 times faster than ASB. Calculations of stress intensity indicate that, in the air system applications where the installation torques are higher, the likelihood of subcritical cracking in ammonia environments is high. Stress intensities approach the ASB threshold values for subcritical intergranular cracking in air when the defect depth approaches half the wall thickness of the nut.