Correlation of electrochemical behavior and nanoscale interfacial processes in TiZr-pretreated polyester coil-coatings: Role of an adhesion promoter revealed by EIS and in situ ATR-FTIR

Highlights

• A multi-scale approach was used to probe the metal/polymer interface in TiZr-pretreated systems.
• Polyester coatings on HDG panels were studied by EIS, while primer-coated Zn surfaces were analyzed by in situ ATR-FTIR.
• The TiZr system with adhesion promoter showed less interfacial water accumulation and Zn(OH)2 formation.
• The adhesion promoter led to a more uniform Ti distribution and improved interfacial stability.

Abstract

The role of interfacial water in adhesion performance was investigated for coil-coated hot-dip galavanised steel pretreated with TiZr-based conversion layers, with and without an adhesion promoter. The polymer coating was a polyester/melamine system. Laboratory-prepared Zn model surfaces were used to perform in situ spectroscopic investigation of the metal/polymer interface. A multi-scale approach combining electrochemical impedance spectroscopy (EIS), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), electron probe microanalysis (EPMA), X-ray fluorescence (XRF), and in situ ATR-FTIR spectroscopy in the Kretschmann geometry was employed to correlate macroscopic electrochemical behavior with nanoscale interfacial processes. Water uptake was estimated between 0.5 to 0.8 vol.% independently of the surface pretreatment and regardless of the presence of the topcoat. The topcoat provided effective barrier properties independent of the surface pretreatment. When only the primer was applied, differences appeared. EIS analysis revealed earlier adhesion loss and/or corrosion initiation in absence of the adhesion promotor. In situ ATR-FTIR analysis and optical simulations of the IR spectra showed greater interfacial liquid-like water accumulation for TiZr, accompanied by early formation of Zn(OH)2 species. Surface analyses showed that the presence of the adhesion promoter led to a more uniform Ti distribution, reducing Ti-deficient regions. These results demonstrated that the adhesion promoter improved coating performance by stabilizing the metal/polymer interface and by limiting interfacial water accumulation. This work established a robust multi-scale methodology for probing interfacial processes in coil-coated systems.

Keywords

• TiZr conversion layers
• Organic adhesion promoter
• EIS
• In situ ATR-FTIR
• SEM-EDS
• “Buried” interface