Influence of pH and composition of the electrolyte on Zn-Mg coated steel corrosion

In automotive industry, zinc coated steel is largely used thanks to its high corrosion resistance and sacrificial protection. Several studies showed that we are going to run out of zinc in several years if its consumption remains so high. A large number of new coated steel formulations have consequently been studied for many years in order to reduce the use of zinc and its cost in coatings. Moreover, some alloys such as Zn-Mg coated steel are more and more investigated and seem to present superior corrosion resistance than conventional hot-dip and galvanized coatings. However, the corrosion resistance of Zn-Mg coated steels markedly depends on corrosion conditions. The objective of this work is to study the behaviour of the Zn-Mg alloys during dipping tests in aqueous solutions.
Different immersion environments were studied. The composition, especially the presence of chlorides or not, and the pH (natural, acid and basic) were changed. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the composition of baths after different immersion times. Electrochemical impedance spectroscopy (EIS) and open circuit potential measurements were carried out in order to study the mechanisms of corrosion in each electrolyte. Scanning electron microscopy (SEM) was used to analyze the alloy surface after immersion. The localized corrosion in the different environments was investigated by using the scanning vibrating electrode technique (SVET). Conductivity and pH of the electrolytes were checked according to the immersion times.
The results indicate that the environment is very important for the alloy behavior. It was also shown that Zn-Mg coated steel has a significant effect on the immersion environment, especially on the pH and compositions. Results particularly show that the alloy has a very good corrosion resistance in alkaline environment.