The Correlation Between Substrate Mass Loss and Electrochemical Impedance Spectroscopy Data for a Polymer-Coated Metal

Abstract

Research has been carried out to evaluate the correlation between substrate mass loss and data generated by electrochemical impedance spectroscopy ͑EIS͒. Experiments were carried out as a function of exposure time for both polymer-coated and bare samples. Mass loss was determined in situ and in real time with a vibrating sample magnetometer ͑VSM͒. In all cases, the sample was held potentiostatically at the open circuit potential ͑OCP͒ while EIS data were collected. Mass loss data calculated from electrochemical impedance ͑EI͒ spectra were then compared to equivalent data generated on the VSM. Although previous research has investigated the correlation for bare samples ͓W. J. Lorenz and F. Mansfeld, Corros. Sci., 21, 647 ͑1981͔͒ establishing this relationship for a coated substrate is more convoluted. The difficulty results from errors associated with solution uptake within the polymer, and corrosion product entrapment. Such errors obviate the use of traditional gravimetric techniques and, until now, the relationship between substrate mass loss and EIS has not unequivocally been established for polymer-coated samples. Although the current research indicates that EIS can accurately define the mass loss for a bare sample, the correlation for coated samples has, generally, not been as good.One methodology for reducing corrosion damage is painting or coating with an organic, and one of the most widely employed techniques for evaluating such polymer-coated systems is electrochemical impedance spectroscopy ͑EIS͒. Although the association between EIS data and mass loss for noncoated metals was previously described, 1 the analogous relationship for polymer-coated metals was not accurately defined prior to the current research. The difficulties in substrate mass loss determination are associated with solution uptake within a polymer, and corrosion product entrapment at the coating/metal interface. Such errors obviate the use of traditional gravimetric techniques. During the current study, the metallic mass loss for a polymer-coated metal was assessed and correlated to equivalent data produced by EIS. This was accomplished by utilizing a vibrating sample magnetometer ͑VSM͒ to magnetically quantify the mass loss for a polymer-coated cobalt substrate. These data were then correlated to corresponding mass loss data assessed by EIS.While degradation depends on a number of variables, 2 coatings isolate the substrate from direct contact with the environment and by retarding the arrival of reactants at the interface. Such coatings, however, tend to contain pores, defects, and virtual pores ͑Fig. 1͒, and, ultimately, underfilm corrosion is still likely to develop.When a material corrodes, it experiences a change in mass as a function of exposure time, and mass loss is, in many respects, the reference measurement against which all other techniques are assessed. During an electrochemical reaction, the proportionality between the current ͑I͒ and mass reacted ͑m͒ is defined by Faraday's lawwhere m is the change in mass, I is the curren...

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“…Electrochemical Impedance Spectroscopy (EIS) is a useful technique for ranking coatings, assessing interfacial reactions, quantifying coating breakdown, and predicting the lifetime of coating/metal systems. It has been used, for example, to monitor the underfilm corrosion of polymercoated cobalt [26,27]. Advantages of this technique over dc techniques include the absence of any significant perturbation to the system, its applicability to the assessment of lowconductivity media, and the existence of a frequency component that may provide mechanistic information.…”