Effect of pH Variation on Passive Film Stability and Pitting Initiation in Selectively Laser Melted AlSi7Mg Alloy

Abstract

ABSTRACT This study investigates the pH‐dependent corrosion response, passive‐film stability, and pitting initiation behavior of selectively laser melted (SLMed) AlSi7Mg alloy in naturally aerated 3.5 wt.% NaCl solution at pH 2, 6, and 12. The alloy was evaluated using open‐circuit potential (OCP), potentiodynamic polarization (PDP), cyclic potentiodynamic polarization (CPP), chronoamperometry (CA), electrochemical impedance spectroscopy (EIS), and post‐corrosion SEM/EDS and XRD analyses. The electrochemical response was strongly controlled by solution pH and by the SLM‐induced cellular α‐Al/Si‐rich intercellular network. At pH 6, the alloy exhibited the lowest apparent corrosion current density (11.8 × 10 −6 A cm −2 ), the lowest calculated corrosion rate (0.129 mm year −1 ), and the largest CPP separation between Ecorr and Epit (approximately 470.17 mV), indicating the strongest resistance to stable pit propagation and the most effective repassivation. At pH 2, the current response was characterized by frequent transients and localized attack, confirming metastable‐to‐stable pit transition at electrochemically heterogeneous sites such as melt‐pool boundaries and Si‐rich cell boundaries. At pH 12, the polarization curve showed an extended passive‐like region at relatively low current density; however, alkaline exposure promoted hydroxide‐assisted dissolution of the aluminum oxide/hydroxide surface layer and produced more uniform surface degradation rather than deep localized pits. EIS further confirmed the corrosion‐resistance order pH 6 > pH 2 > pH 12, with polarization resistance decreasing from 3160 Ω cm 2 at pH 6 to 2370 Ω cm 2 at pH 2 and 255 Ω cm 2 at pH 12. The results demonstrate that solution pH controls the transition between stable passivation, localized chloride‐assisted pitting, and alkaline film dissolution in SLMed AlSi7Mg alloy.