ABSTRACT

When assessing coatings for corrosion protective performance properties, conventional electrochemical impedance spectroscopy (EIS) and electrochemical noise measurements (ENM) are well established methods available in the laboratory. However, there remains a need in the protective coatings industry for techniques to evaluate the corrosion-protective properties of anti-corrosive coatings in-service. A portable electrochemical noise measuring device has recently been developed at the University of Northampton (UK) for this purpose. In this paper, this device has been used to evaluate electrochemical noise resistance characteristics on coated substrates during exposure in an accelerated test. These values have been compared with conventional ENM data, and with EIS results for the same coating systems. Overall, the information generated by the portable corrosion meter showed good correlation with the equivalent conventional EIS and ENM data obtained. Additionally a preliminary assessment using the portable ENM instrument has been made on a coated test sample exposed on an exterior exposure test rack for a period of 1-year, which demonstrates the potential utility of the portable electrochemical noise measurement method.

Organic coatings provide protection of metallic structures from corrosion. Corrosion is inhibited in three ways: barrier protection, use of active corrosion inhibitors/inhibitive pigments, and by electrochemically active cathodic protection, often followed by long term barrier properties. In barrier protection, the coating serves by blocking corrosive chemical species such as water, oxygen and particularly ions from reaching the metal substrate.1,2 It also intersperses a high ionic barrier between anodes and cathodes.

Laboratory-based assessment methods to determine corrosion protective properties of coatings intended for atmospheric service operating conditions have been under continuous development over many years. One of the most widely accepted test methods in current use, described in ASTM D5894 3, is a procedure for exposing corrosion protective coatings to combined cycles of wet/dry corrosion conditions, whilst also incorporating weathering factors, primarily UV light degradation and water condensation cycles. After exposure in the accelerated corrosion test, coated panels are often rated visually as per industry standards for the degree of corrosion, blistering and scribe creepage. However, these visual methods give essentially qualitative information and are subjective evaluations prone to error.4

There has long been the need for quantitative assessments of coatings corrosion performance properties, and since corrosion is an electrochemical process, the science of electrochemistry and associated electrochemical measurement methods can provide such quantitative evaluations. These values can be directly related to the coating degradation and corrosion processes taking place at the coating/metal interface. Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise Measurement (ENM) are the two most commonly-used non-destructive electrochemical techniques for assessing corrosion protection of a coating. These methods have been used extensively in this laboratory and by others over many years. 5-12

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