Abstract
The galvanic corrosion of the alloy UNS(1) C95800, referred to as Nickel Aluminium Bronze (NAB) has been investigated in natural seawater under the influence of a magnetic field (MF). NAB was coupled to the austenitic stainless steel UNS S31603, referred to as 316L, in area-ratios of 2:1, 1:1, 2:1 and 1:8, NAB:316L, respectively. The tests were performed in two parallels, where one parallel was exposed to a MF of 0.15-0.2 Tesla. Samples of freely exposed NAB and 316L were also tested, for comparative reasons. Upon immersion of the galvanic couples, the mixed potentials were close to that of freely exposed NAB for all galvanic couples, and no significant galvanic current was measured. After 5-12 days the mixed potential increased about 180 mV, and the galvanic current increased simultaneously. The increase in mixed potential and galvanic current was due to the formation of a biofilm on the 316L sample, enhancing the cathodic properties of the 316L surface. The galvanic corrosion was under cathodic control, as the galvanic current density on 316L was close to 120 mA/m2 for all galvanic couples, while the galvanic current density on NAB increased linearly with increasing cathode area. The galvanic corrosion of NAB was first in the form of selective phase attack, and secondly in the form of local corrosion, as a crevice environment developed beneath initially formed corrosion products. The local corrosion caused wide and shallow corrosion craters on the NAB surface. The effect of a MF was found to be negligible in an overall aspect, but results indicate a minor influence on the slopes of measured potentials and galvanic currents vs. exposure-time.
Introduction
Ni-Al Bronze (NAB) is one of the preferred alloys for casted, heavy-duty components working under conditions where cavitation, fatigue and corrosion are important factors. This alloy has a complicated microstructure, consisting of an alpha-matrix and several intermetallic phases known as kappa-phases. The alpha-matrix is a ductile face centered cubic (FCC) structure with Cu and below 9 wt. % Al in solid solution. The kappa-phases contribute to the strength of the alloy and vary in composition, distribution and size, see Table1.1 Exposure to fresh, natural seawater causes the formation of a well-adherent and protective Al- and Cu-oxide on the NAB surface. The oxide is reported to be about 1000 nm thick, and consists predominantly of Al-oxide on the metal/oxide interface, and of Cu-oxide in the oxide/electrolyte interface.2
