Electro-Codeposited Zinc-Nanocomposite Coatings Tailored for Smart Corrosion Protection
Authors: Tran The Hai , Rohwerder Michael 
Published: February 2020

The use of pigments for inhibitor release finds widespread application for enhancing the performance of organic coatings. The drawback of these coating systems is an uncontrollable leaching out of the inhibitors into the environment even without corrosion occurring, resulting in a shortened corrosion protection lifespan and pollution. Gradual environmental and health related concerns of well-known corrosion inhibitors (e.g. CrVI+) as well as corrosion inhibitors currently still in use further contributed to a paradigm change and resulted in intensive research in the field of smart corrosion protection coatings which offer a selective release trigger that is exclusively customized to the corrosion process.

In particular so called self-healing coating systems based on nanocontainers or hollow fibers containing corrosion inhibitors are in the focus of research. Most of these self-healing systems are based on corrosion inhibitor containing nanocontainers stored in organic or inorganic coatings directly applied on metal. Hence, the stability of the nanocontainers and the corrosion inhibitors towards their environment (e.g. oxygen and UV-irradiation that can cause a degradation process of the shell material itself) has to be considered with respect to their long-term corrosion protection capability. Storing nanocontainers that are loaded with corrosion inhibitors in protective metal coatings as they are air-tight seems to be the best choice to keep them active and safe from environmental effects, potentially even for decades.

In this study, pH-sensitive microporous silicon based nanoparticles (MSP) which are surface modified with waterglass and loaded with various corrosion inhibitors such as 2-mercaptobenzothiazole (MBT), 1,2,3-benzotriazole (BTA) or trivalent cerium (CeIII+) are electro-codeposited into zinc using a rotating disk electrode. The advantages of surface modified pH-sensitive MSP are their insolubility and high stability during acidic electro-codeposition conditions and their gradual dissolution at higher pH, thus maintaining their pH-responsive properties even after their incorporation.

The concept is that the MSPs are gradually released during zinc corrosion and then slowly release their loaded content by dissolving at the higher pH caused by the cathodic oxygen reduction reaction (ORR). Prior to corrosion testing the Zn-nanocomposite coatings were coated with a PVB top-coating preventing a global release inorder to simulate a more “real” case situation, such as a defect site in a scratched organic coating. A defect down to zinc with dimensions of 1mmx0,1mm was applied to the specimen and then subsequently covered with a 1M KCl droplet. The corrosion protection performance of the metal-composite-coatings was investigated via the Scanning Kelvin Probe (SKP) technique. The advantage of the SKP lies in its damage-free measurement performance and its capability to monitor the corrosion progress even beneath an organic coating. It will be shown that a significant reduction of the delamination progression could be achieved which was accompanied by an ennoblement of the defect potential.