Stress corrosion cracking of nickel aluminium bronze under ammonia-containing artificial seawater

With the growth of urbanization and industries, the seawater near coastal areas has become polluted, and the nickel aluminium bronze (NAB) components around coastal areas are affected by ammonia-containing seawater. Unfortunately, there are some limitations for the evaluation of stress corrosion cracking (SCC) of NAB under ammonia-containing seawater. Firstly, the influence of the ammonia concentration in seawater on the stress corrosion cracking (SCC) of thin NAB components with large plastic zones at the defects (i.e., plane stress condition) has not been evaluated before. Secondly, for a thick NAB component with small plastic zone at the defects (i.e., plane strain condition), some parts of SCC experiment are needed to be improved, i.e., the accurate measurement of crack length and the suitable fracture mechanics parameterat crack front. Under plane stress condition, SCC experiments on NABs under artificial seawater and ammonia-containing artificial seawater were conducted using a 4-point bending technique. The elastic-plastic fracture mechanics parameter (J-integral) was evaluated using finite element analysis (FEA). The J-integral successfully characterized the crack growth rate (da/dt) under the present corrosive environments. SCC was possible under both artificial seawater and ammonia-containing artificial seawater. The threshold J-integral for susceptibility to SCC (JSCC) and fracture toughness ( JC) were the highest for SCC under artificial seawater and decreased as the amount of ammonium hydroxideadded to the artificial seawater increased. Under plane strain condition, the self-deformed specimen (i.e., T-type wedge opening loaded specimen or T-WOL specimen) is used for SCC experiment. Unfortunately, the direct measurement of crack length during SCC experiment is not accurate because of the influences of liquid environment and corrosion product. In the present work, an elastic-plastic finite element analysis was applied to estimate the crack length from the measured crack-opening force and the measured crack-mouth opening displacement. To measure the crack-opening force during SCC experiment, the instrumented reaction pin was numerically designed, constructed, and calibrated. Crack lengths from indirect crack length measurements were compared with those from direct measurements. Both results were in good agreement, and the present indirect crack length measurement can be used for SCC experiment with T-WOL specimen. Moreover, under the combined effects of stress and a corrosive environment, the protective layer at the crack tip of NAB component can be damaged, and the resistance to crack propagation can be reduced. Therefore, the measurement of crack-tip opening displacement (CTOD) should be applied to characterize the crack propagation rate (da/dt) during SCC experiments. In the present work, CTOD of a TWOL specimen with a built-in loading bolt was initially determined using 3D FEA. The influence of stress concentration from side groove on CTOD was clearly evaluated, i.e., the fluctuation of CTOD occurred near the groove-tip plane. However, CTOD became stable on > 90% of thickness of T-WOL specimen. Therefore, CTOD at mid-thickness plane of T-WOL specimen could be applied for SCC experiment.