New experimental approach for intelligent screening of buried metal/oxide/polymer interfaces via local electrochemistry: Example of undamaged model epoxy-coated Zn alloys

Abstract

A new simple experimental approach is proposed for in situ mechanistic studies of the underpaint stability of metal/oxide/polymer interface under immersion. It allows an intelligent screening and fast ranking of conversion coatings on Zn alloys designed for further application of organic coatings. The approach is based on the application of a simple electrochemical aging protocol to the samples preliminary coated with a model epoxy-polymer. The latter is enough thin to allow local electrochemical impedance measurement and stable under this protocol, as revealed by attenuated total reflectance infrared spectroscopy (ATR-IR) and electrochemical impedance spectroscopy (EIS). The interface stability and degradation modes characterization is accessed via combined EIS, local electrochemical impedance spectroscopy and mapping (LEIS and LEIM) and in situ optical imaging.The developed approach allowed to discriminate two Zn alloy substrates, one with intact and another with slightly damaged Cr(III) conversion layer, for which no strong differences in the electrochemical behavior or average surface composition was visible prior to the epoxy-polymer application. The relative stability of two substrates with model thin coating, evaluated in the developed electrochemical test, correlated with the observations obtained for the same substrates coated with thick commercial epoxy primer after 1000 h of immersion. The proposed methodology offers the possibility of a rapid intelligent screening of various formulations of Cr(III)-based surface treatments for Zn based substrates designed for paint applications.

Detection and quantification of defect evolution at buried metal-oxide-polymer interface on rough substrate by local electrochemical impedance mapping

“…Once the sample was placed and held in the spin-coater, the resin was poured in large excess and spread on the whole surface, the spin coating was performed twice with a 15 s relaxation time in between with the following parameters: 3000 RPM rotation speed (achieved by 500 RPM/s linear acceleration) for 35 s. After the spin-coating step, the samples were cured at 50 ± 5 °C for 14-16 h. This procedure allows controlled coating thickness on a flat substrate at ca. 10 ± 1 µm to be obtained [41]. Moreover, the roughness of the substrate in the present work is comparable with the expected polymer thickness.…”

“…Mild steel with electrodeposited Zn-based metallic coating and Cr(III)-based conversion coating, produced as described in [41], was used as the substrate. The coating consisted of approximately 18 ± 2 µm thick layer of electrodeposited Zn-14 at.…”

“…Prior to electrochemical testing, the samples with and without model defect were coated by the model epoxy polymer developed in [41]. It consisted of DiGlycidyl Ether of Bisphenol-A prepolymer and TriEthylene TetrAmine hardener (DGEBA-TETA) used in (1:1) ratio.…”

“…Scanning vibrating electrode techniques (SVET) measures the local currents, which can be helpful to characterize defects in coating but cannot overcome the inherent difficulty in measuring currents underneath high resistance coatings [31,32]. Local electrochemical impedance spectroscopy and mapping (LEIS and LEIM) [33,34,35,36,37,38] are becoming widespread techniques to study buried interfaces for the systems in which the polymer does not have initial defects [39,40,41]. SVET and LEIS measure both the local current density, for the former case, the DC component is measured, whereas for the later, the AC component at a selected frequency can be obtained.…”

“…The main objective of the present work is to propose a methodology for such an evaluation for electrodeposited Zn-alloy substrate with a Cr(III) conversion coating and an epoxy polymer paint. The proposed approach is based on the interplay of the concepts developed in our previous works, namely: 1) a quantitative description of the deadhesion (delamination) advancement from a defect in the polymer coating at metal-polymer interface using LEIM [36] 2) an application of a model epoxy -based coating which allows LEIM measurements underneath without a preliminary defect formation and helps to mimic the behavior of buried interface and distinguish conversion coating with and without defects [41] 3) a methodology able to produce a controlled local nanometer thinning of conversion coating without affecting the roughness of the substrate [44].…”