Evaluation of Selective Phase Corrosion Thickness on Nickel Aluminum Bronze Valves using Transient Eddy Current

Nickel Aluminum Bronze (NAB) is a material with marine environment applications. Under certain conditions the material can undergo selective phase corrosion (SPC), which involves the removal of particular elements resulting in a loss of material strength. Additional changes in material properties include a change in conductivity and permeability in comparison with the parent material. Transient eddy current (TEC) was evaluated for determination of SPC thickness on a NAB valve section with access from the surface corroded side. TEC signal characteristics were correlated with SPC thickness variations, observed along the sample’s edge and averaged over 5 mm, the estimated sensing diameter of the probe. A primarily linear response of TEC amplitude, up to the maximum available SPC thickness of 4 mm, was observed for a 20 mm thick valve wall. Variability in observed TEC signal response was attributed to SPC thickness variations on surface length scales less than 5 mm and the anticipated limits to depth-of-penetration of the sensing field. Elemental analysis, using scanning electron microscopy of corroded and non-corroded regions of NAB showed a reduction in elemental Fe and Ni within the corroded phase, suggesting a corresponding reduction in magnetic permeability would occur. Conductivity was also lower by 77% relative to the base NAB material. The combination of reduced conductivity and permeability was used to explain the observed sensitivity of TEC to SPC thickness variations.