Cathodic driven coating delamination suppressed by inhibition of cation migration along Zn|polymer interface in atmospheric CO2

The degradation of the Zn|polymer interface is inhibited by CO2 gas in a humid environment. The inhibition mechanism varies greatly for different polymer matrices and depends on the affinity of the polymer to CO2. Coatings based on polymers with high affinity to CO2 such as polyacrylamide show high delamination rates due to the fast uptake of water. In this case, the cation transport that causes the initial pull down of potential for initiating the oxygen reduction reaction occurs via the polymer. Here CO2 decreases water uptake due to competitive absorption into the polymer matrix, inhibiting the delamination rate. CO2 can quickly reach the interface of polymers with functional groups with a low affinity to water and CO2, such as polyvinyl butyral and polyvinyl alcohol. In this case, the inhibition of the delamination rate is achieved by a strong decrease in cation migration rate at the Zn|polymer interface accompanied by the formation of mixed hydrozincite/absorbed CO2 layers on the ZnO surface underneath the polymers. Further experiments showed that the presence of CO2 accelerates anion migration, suggesting an influence of CO2 on the surface charge at the Zn|coating interface, thus affecting ion migration. Inhibition of cation migration has never been reported before and should be taken into account into the mechanism of cathodic-driven delamination on Zn under atmospheric conditions. Polymer-coated metals: CO2 inhibits degradationCO(2) stops delamination of metal-polymer interfaces in corrosive conditions, but how it does it depends on polymer composition. A team led by Viacheslav Shkirskiy and Michael Rohwerder from the Max Planck Institute for Iron Research in Germany monitored different polymer-coated zinc systems immersed in corrosive conditions under a CO2 atmosphere to establish how CO2 inhibits degradation and delamination. They found that hydrophilic polymers with a high CO2 affinity, such as polyacrylamide, preferentially absorbed CO2 and retarded the delamination rate; in contrast, CO2 readily penetrated and adsorbed onto the zinc-polymer interface in polymers with a low affinity for CO2 such as polyvinyl alcohol, creating carbonate complexes which inhibited cation migration. This effect was seen at CO2 concentrations that can occur atmospherically, and may help to better understand metal-polymer wear in service.