Abstract
Some nickel-aluminum bronze (NAB) components exposed to natural seawater environment are known to suffer occasionally from stress corrosion cracking (SCC). In order to understand the overall mechanism of corrosion and mitigate this problem, both galvanic and SCC tests were performed on cast and plate NAB material and AMP10 and AMP46 welding electrodes.
Open circuit potential (OCP) measurements were made for cast and plate NAB and AMP10 and AMP46 weld deposits in ASTM artificial seawater with and without aeration. For galvanic corrosion behavior, several pairs of materials were tested at two area ratios (10:1, 1:1) of cast and plate NAB connected to AMP10 or AMP46 weld electrode material. Tests were done both with and without cathodic protection (CP) in an artificial seawater environment. Currents were measured for galvanic couples and analyzed to predict galvanic trends. The galvanic currents measured were very low (1 to 5 μA) between NAB plate and weld deposits produced using either of the two welding electrodes and tested at an area ratio of 1:1. For galvanic couples with an area ratio of NAB: weld deposit of 10:1, and with cathodic protection and aeration, measured galvanic currents were in the range of 200 μA to 800 μA.
SCC tests were done to determine the susceptibility of NAB plate material to SCC in an ammonia and ASTM seawater mixture and in plain ASTM seawater. Bent-beam SCC tests were conducted in accordance with ASTM G39 on “as-received” and heat-treated NAB plate materials. The SCC tests showed cracking in the “as-received” NAB plate material at stress levels of 90%, 77% and 54% of yield strength in 1:1 (v/v) ammonia and ASTM seawater solution. A heat-treated NAB plate material also showed SCC at a stress level of 77% of yield strength in 1:1 (v/v) ammonia and ASTM seawater solution.
