Radial Spreading of Localized Corrosion-Induced Selective Leaching on α-Brass in Dilute NaCl Solution

The radial spreading of selective leaching on an α-brass alloy, Cu20Zn, in diluted sodium chloride (NaCl) has been followed in situ by light optical microscopy. The corrosion process initiates through localized dissolution of micrometer-sized, Zn-rich areas. These areas initially act as anodes, where the Zn will dissolve as Zn²⁺. The continued growth is a highly dynamic process, involving dezincification that results in local ennoblement of initial anode areas and changes in the local chemistry of the solution. These changes drive the anodic area outward to form a ring-shaped anode with an increasing diameter. At a sufficiently high local Cl⁻ ion concentration, Cu dissolves as CuCl₂⁻, whereby the radial growth continues. At critical minimum local Cl⁻ ion concentration, the radial growth is terminated.