Effect of acidic etching and fluoride treatment on corrosion performance in Mg alloy AZ91D (MgAlZn)

Fluoride activation of Mg AZ91D die-cast alloy surfaces by ammonium fluoride pre-treatment is an important procedure in industry. The procedure forms a fluoride rich layer on the surface enabling the formation of coatings in a uniform manner during further treatments. The present study explores the time dependence of the sequential two-step pre-treatment process – acid etching in H₂SO₄ (first step) and steeping in NH₄F (second step) – on the corrosion properties of AZ91D alloy. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) show the α phase to be selectively etched via a “peeling-off” mechanism that eventually leads to the undercutting and undermining of the β-phase network. After ammonium fluoride treatment, variations in structure and chemical composition of the fluoride rich layer were shown to depend on the underlying phase of the alloy surface by SEM and time-of-flight secondary-ion-mass-spectroscopy (ToF-SIMS). After step 1 (acid etching), electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution indicates greater charge transfer resistance (R_t) for samples of greater exposed β surface area. This greater R_t increases after step 2 (NH₄F steeping). Furthermore, the corrosion protection properties of F-coated AZ91D can be significantly enhanced by optimizing the durations of the two pre-treatment steps. Indeed, over extension of the duration of either step 1 or step 2 leads to deterioration of the overall corrosion resistance.