Developments of the electrochemical noise method (ENM) for more practical assessment of anti-corrosion coatings

Abstract

The electrochemical noise method (ENM) has particular attractions because of its non-intrusive nature, quickness in gathering data and ease of interpretation. The electrode arrangement for the standard (“Bridge”) method of conducting ENM requires two separate working electrodes, e.g. two painted Q panels and a reference electrode. Although satisfactory for laboratory use, it is not so suitable for monitoring or quality control. An improved experimental configuration is the single substrate (SS) method but this still requires the metal to be connected to the measuring instrument. This is avoided in the most recent development which needs no connection to substrate (NOCS). Results will be given for immersed low VOC samples monitored using the ENM NOCS arrangement and compared with the standard (“Bridge”) method and DC resistance. Results will also be presented for work done using several different electrodes (platinum, calomel and silver/silver chloride). It is accepted that, because of the very small voltages and currents involved, ENM data can sometimes be affected by extraneous signals (although normally the results are changed by only a factor of two or less) and it may be that NOCS is more sensitive to interference of this type than the standard bridge arrangement. A simple data analysis package checking on the Gaussian nature of data enables the operator to have confidence in the R_n value. This has been applied to NOCS data. Further work is required to make ENM attractive enough to be employed as the electrochemical method of choice by users, specifiers and producers of organic anti-corrosive paints.

Introduction

The pressure on industry due to increasingly stringent legislation to produce products with lower volatile organic content (VOC) has led to increased development and production of coatings that use water-based solvent. Market pressures combined with this mean that a new or modified coating needs to be tested and proven as rapidly as possible before it can be marketed with the guarantees expected of today's high performance products. Traditionally accelerated cabinet tests have been used such as ISO 11997 (cyclic – prohesion) which determines resistance to cyclic wet/dry/humidity and ultra-violet light conditions. Other cabinet tests use only salt spray or alternate wet dry cycles [1], [2], but the aim of the tests is essentially the same in that they attempt to accelerate the ageing mechanism of the coating under test. The assessment of coated panels from these tests is usually by eye, assessed against a standard or measured in the case of coating under-cut at a scribe and it is here that electrochemical techniques can be of great assistance. Not only is a numerical value obtained which can be directly related to expected corrosion protection, but also by monitoring the performance of the coating with time, different coatings/formulations can be compared. Some workers claim to be able to predict life times by extrapolation of electrochemical data [3], and good correlation has been seen between immersion testing (using ENM to monitor) and Cyclic Prohesion testing with the less aggressive Harrison's solution [4].

There are numerous electrochemical techniques in use for measurement of corrosion activity and some of these have been adapted to coatings applications with great success. Each has its advantages and disadvantages and as such in isolation one method can be viewed as part of a toolbox of available techniques. The choice of which “tool” depends much on the particular requirements at the time. Some common electrochemical techniques used for coating assessment are DC resistance, impedance spectroscopy (EIS), electrochemical noise monitoring (ENM) [5], [6], [7] and the DC transient (current interruptor) method [8]. This paper concentrates on electrochemical noise monitoring, which at one level can be used as a fast indicator of coating condition and used by a low skilled operator, but by contrast has many complex mathematical treatments available that can satisfy the most ardent theoretical electrochemist [9].

In particular this work concentrates on how the ENM technique has been adapted to enable a more practical application both as an assessment/screening tool in the laboratory. In this it builds on previous work conducted by two of the authors [10], [11] and ultimately as a technique that can be applied in the field for condition monitoring. Some previous attempts have been made using ENM or EIS looking at coatings on ships [12], on bridges [13], and on transmission leg towers [14]. Electrochemical noise method has also been applied to monitoring uncoated chemical plant [15] and the adaptions described in this paper could have application to that type of situation as well.