Sullivan James, Cooze Nathan, Gallagher Callum, Lewis Tom, Prosek Tomas, Thierry Dominique
College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UK.
Faraday Discuss. 2015;180:361-79. doi: 10.1039/c4fd00251b. Epub 2015 Apr 27.
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 MgZn(2) 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 Al(2)O(3) at the alloy surface retarding the kinetics of cathodic oxygen reduction. The addition of 1 × 10(-2) mol dm(-3) Na(3)PO(4) 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 image analysis. 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 × 10(-2) mol dm(-3) Na(3)PO(4) to the 1% NaCl electrolyte promoted an anodic shift of 50 mV 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.
采用原位延时光学显微镜研究了三种锌镁铝（ZMA）合金涂层钢在pH值为7的1% NaCl溶液中的微观结构腐蚀机制。在所有ZMA合金中均观察到共晶相内MgZn₂薄片的优先腐蚀，随后富锌相溶解。使用延时图像分析和扫描振动电极技术（SVET）估计质量损失来测量的总腐蚀程度和速率，随着Mg和Al合金添加量增加到3 wt% Mg和3.7 wt% Al的水平而降低。这可能是由于合金表面MgO和Al₂O₃的存在增加，阻碍了阴极氧还原的动力学。向pH值为7的1% NaCl溶液中添加1×10⁻² mol dm⁻³ Na₃PO₄对一种ZMA的腐蚀机制有显著影响，通过延时图像分析观察到阳极位点通过磷酸盐沉淀而钝化。还观察到丝状磷酸盐的有趣快速沉淀，据推测这些细丝由于过饱和效应而成核并生长。极化实验表明，向1% NaCl电解液中添加1×10⁻² mol dm⁻³ Na₃PO₄会使ZMA合金的开路电位阳极移动50 mV，阳极电流降低2.5个数量级，这表明它主要作为阳极抑制剂起作用，支持了延时研究的推断。这些磷酸盐添加使SVET测量的估计质量损失降低了98%，证明了磷酸盐抑制剂对该合金体系的有效性。
