The influence of steel surface treatment by a novel eco-friendly praseodymium oxide nanofilm on the adhesion and corrosion protection properties of a fusion-bonded epoxy powder coating

What makes epoxy-phenolic coatings on metals ubiquitous: Surface energetics and molecular adhesion characteristics

HYPOTHESIS

Wetting characteristics of epoxy and phenolic resins on metals depend on the molecular interactions between resins' functional groups and metal surface. Those interactions affect the practical adhesion strength of epoxy-phenolic coatings on metals. Estimation of the theoretical adhesion energies can reveal this system's microscopic adhesion mechanisms.

EXPERIMENTS

Adhesion is estimated theoretically based on resins' wettability on metals, and experimentally through pull-off adhesion testing of cured coatings. The effect of various functional groups on adhesion is decoupled using epoxy and phenolic resins with different functionalities. To assess the impact of the metal passivation on adhesion, tinplated and tin-free steel substrates are used. Differences in their surface chemical composition and polarity are investigated using XPS.

FINDINGS

Theoretical adhesion results reveal a superior adhesion of epoxy compared to phenolic resins. Moreover, epoxy resins having a higher content of epoxide-to-hydroxyl groups show improved theoretical and practical adhesion. The importance of epoxides in driving resins' initial adhesion on metals is attributed to the formation of direct chemical bonds with active hydrogen on metal surfaces. The adhesion of coatings on tin-free steel is found to be higher than on tinplated steel. This is associated to the increased hydroxyl fraction on tin-free steel surface leading to more hydrogen bonds formation.

Epoxy coating anti-corrosion properties enhancement via the steel surface treatment by nanostructured samarium oxide-poly-dopamine film

Nowadays, the rare earth element-based conversion coatings (REE-based CCs) are a potential eco-friendly alternative for hazardous and carcinogenic Cr-based CCs. These coatings have morphological defects that impair their performance; therefore, they need to be surface modified. In this study, for the first time, the steel surface was coated with an eco-friendly Sm-based CC and then post-modified by poly-dopamine based biopolymer. The air-exposed based self-polymerization and oxidant-induced polymerization are two protocols which have been utilized for poly-dopamine synthesis. The SEM/EDS analysis and Raman spectroscopy have been employed for the treated steel surface characterization. In addition, the electrochemical impedance spectroscopy (EIS) analysis and salt-spray test (SST) were carried out to investigate the epoxy (EP) coating corrosion protection performance. The R_t values of the EP applied on the Sm-PDA modified steel, subjected to a 3.5 wt. % NaCl solution, are respectively 2550 GΩ.cm² and 100 kΩ. cm² before and after the creation of scratch. These values are about 94000-fold and 21-fold more than the R_t of the defected/un-defected EP coatings applied on the unmodified steel. In addition, the EP applied on the Sm-PDA modified steel showed lower corrosion and less disbonding in SST and higher resistance against CD than the EP applied on the unmodified steel.