Combination of FTIR Reflection Absorption Spectroscopy and Work Function Measurements for In Situ Studies of Plasma Modified Passive Films on MgZn₂

Abstract

The chemical and electronic properties of passive films on a ZnMg alloy coating can strongly influence the cathodic de‐adhesion process of an applied polymeric coating. Chemical composition of the atmospheric passive film on MgZn2 was compared to the surface state after sequences of Ar/H2 and oxygen plasma treatments by means of in situ IRRAS and ex situ XPS. It could be shown that the chemical composition of the passive film and its electronic properties as displayed by its work function could be significantly changed by the applied plasma processes. As analyzed by Kelvin probe measurements the surface potential could be shifted cathodically by the reductive plasma and anodically by the oxidative plasma. The very negative potentials led to polymer/oxide/metal interface potential more negative than the free corrosion potential of MgZn2. Thereby, a cathodic de‐adhesion process could be perfectly inhibited.

First-Principle Calculation of Volta Potential of Intermetallic Particles in Aluminum Alloys and Practical Implications

“…36 Interestingly, for Al 2 CuMg, the Volta potential of (100) surface is 109 mV, in agreement with experimental data listed in Table III. However, other crystal faces have negative Volta potentials, and the average Volta potential (−242 mV) indicates overall an active behavior relative to pure Al.…”

“…15,44 A Volta potential value of −1400 mV vs. Ni reference was measured on a deposited MgZn 2 thin film, 41 another value of −1100 mV was measured on synthesized MgZn 2 vs. SHE. 36 Therefore these data are not directly comparable with the calculated Volta potential of MgZn 2 relative to pure Al.…”

Adhesion and corrosive delamination of epoxy films on chemically etched ZnMgAl‐alloy coatings

“…The addition of Mg to ZnAl‐based coatings has been shown to result in significant improvements in the corrosion resistance compared to conventional hot‐dip galvanized ZnAl‐coatings . Even though the exact mechanisms are still not fully understood, there is solid evidence suggesting that the semiconducting properties of surface oxides , the pH buffering effect of Mg 2+ ions , and the formation of corrosion products with increased stability contribute to the higher corrosion resistance of Zn–Mg–Al alloys .…”

“…Especially in the case of underpaint corrosion, high Mg‐containing Zn–Mg alloys have been reported to possess an inherently high resistance to cathodic delamination, which was explained by the inversion of the potential difference between the corroding defect and the intact polymer/oxide/metal interface . The occurrence of potentials at the polymer/oxide/metal interface which are more negative than the actively corroding metal defect leads to the polarization of the interface and thereby to the suppression of electron transfer and thus the cathodic delamination .…”

“…It has been shown that Ar/H 2 plasma treatment led to a significant increase of magnesium ion concentration and to a decrease of the hydroxide to oxide ratio in the passive film without affecting the oxide thickness. The resulting negative shift of the Volta potential of the intact interface to levels below the defect potential and corresponding Fermi level alignment led to a depletion of electrons at the interface and thus to an effective inhibition of oxygen reduction .…”