The Effect of Microstructural Refinement on the Localized Corrosion of Model Zn-Al-Mg Alloy Coatings on Steel

A combination of in-situ Scanning Vibrating Electrode Technique (SVET) and time-lapse immersion optical microscopy (TLM) is used to investigate the effect of microstructural refinement on patterns of localized corrosion affecting zinc-aluminum-magnesium (ZAM) galvanized coatings on steel. Model ZAM coatings comprising Zn-2.7 wt% Al-1.5 wt% Mg are produced on 0.7 mm mild steel sheet by hot dipping, and the resulting coating microstructure is systematically refined by increasing the cooling (solidification) rate from 5°C.sec−1 to 1000°C.sec−1. The intact ZAM coated surface is immersed in 0.17 M aqueous NaCl, and SVET and TLM are used to follow the resulting localized corrosive attack. TLM shows that corrosion initiates preferentially within MgZn2 and spreads laterally over the ZAM surface by preferentially following MgZn2 rich phases. In coarse microstructures, large primary zinc grains tend to deflect and constrain lateral spreading whereas in fine microstructures the smaller primary zinc grains do not. Consequently, lateral spreading rate increases with microstructural refinement. SVET shows that global corrosion rates are similar for all the ZAM coatings but that increased lateral spreading results in lower rates of through-coating penetration for the refined microstructures. These findings are explained in terms of the lateral diffusion of aggressive anolyte species

In situ monitoring of corrosion mechanisms and phosphate inhibitor surface deposition during corrosion of zinc–magnesium–aluminium (ZMA) alloys using novel time-lapse microscopy

In-situ time-lapse optical microscopy was used to examine the microstructural corrosion mechanisms in three zinc-magnesium-aluminium (ZMA) alloy coated steels immersed in 1% NaCl pH 7. Preferential corrosion of MgZn2 lamellae within the eutectic phases was observed in all the ZMA alloys followed by subsequent dissolution of Zn rich phases. The total extent and rate of corrosion, measured using time-lapse image analysis and scanning vibrating electrode technique (SVET) estimated mass loss, decreased as Mg and Al alloying additions were increased up to a level of 3 wt% Mg and 3.7 wt% Al. This was probably due to the increased presence of MgO and Al2O3 at the alloy surface retarding the kinetics of cathodic oxygen reduction. The addition of 1 x 10-2 mol/dm3 Na3PO4 to 1% NaCl pH 7 had a dramatic influence on the corrosion mechanism for a ZMA with passivation of anodic sites through phosphate precipitation observed using time-lapse. Intriguing rapid precipitation of filamentous phosphate was also observed and it is postulated that these filaments nucleate and grow due to super saturation effects. Polarisation experiments showed that the addition of 1 x 10-2 mol/dm3 Na3PO4 to the 1% NaCl electrolyte promoted an anodic shift of 50mV in open circuit potential for the ZMA alloy with a reduction in anodic current of 2.5 orders of magnitude suggesting that it was acting primarily as an anodic inhibitor supporting the inferences from the time-lapse investigations. These phosphate additions resulted in a 98% reduction in estimated mass loss as measured by SVET demonstrating the effectiveness of phosphate inhibitors for this alloy system

An investigation into the atmospheric corrosion of brass using sodium chloride-containing water droplets

Imitating gold at a fraction of the cost is an attribute of brass that has driven watchmakers and minters to utilise this alloy, providing their products with desirable aesthetics while being commercially feasible. Unfortunately, tarnishing under atmospheric conditions reveals the imitator all too soon. This study aims to simulate and measure a fundamental corrosion mechanism on brass.As a NaCl water droplet becomes deficient in oxygen, a differential aeration cell is established whereby cathodic oxygen reduction becomes preferable at the droplet circumference. This region draws sodium cations to counteract the electronegativity associated with an accumulation of hydroxide anions producing a secondary spreading phenomenon – originating at the droplet perimeter and proceeding radially outside its boundaries.A combination of time-lapse photography and height-regulated scanning kelvin probe (SKP) potentiometry is used to study cathodic secondary spreading phenomenon, following exposure to NaCl containing water droplets. The rates at which these highly alkaline electrolytic films proceed from the droplet edge are measured and are shown to follow parabolic kinetics. NaCl droplet concentrations ranging 0.1 – 5 mol/dm3 exhibit significant rates of secondary spreading as shown using time-dependent Ecorr vs distance plots, though visual analysis was inadequate in measuring similar rates at lower NaCl concentrations. Cl– sequestering corrosion product under the droplet coupled with migration of Na+ ions from the droplet bulk lower the solute concentration causing progressive evaporation under constant humidity, shown to be most extreme at lower NaCl droplet concentrations