THE USE OF ELECTROCHEMICAL NOISE METHODS (ENM) TO STUDY THICK, HIGH IMPEDANCE COATINGS

Thick, high impedance organic coatings are those class of coatings use d to provide corrosion protection to naval vessels, pipelines, gasolin e storage tanks, and other large structures such as bridges and plant structures. These coatings, especially the newest generations now bein g used in practice, can provide exceptional protection and lifetime of performance such that properly and accurately assessing and different iating among competing coatings is a very difficult task. The standard protocol of salt fog testing (ASTM Bl17),immersion testing, and outdo or exposure in a corrosive environment with subjective evaluation of a coating's performance during and after testing, does not adequately r ank and predict coating lifetimes for new coating systems, especially for the environmentally compliant coating systems such as powder coati ngs (especially the thick, fusion bonded epoxy (FBE) coatings used for pipelines), two component epoxy and urethane coatings and waterborne coatings. New, objective test methods are desperately needed by users and manufacturers of coatings. A relatively new electrochemical test p rocedure, electrochemical noise methods (ENM), as developed by Skerry and Eden, has been shown in our laboratory to be very successful in th e ranking and prediction of relative coating performance. We have used the method successfully on naval ship coatings, several pipeline coat ings and other related systems, and Skerry has used them successfully on industrial maintenance coatings. We have used these methods in conj unction with electrochemical impedance spectroscopy, d.c. resistance m easurements and cyclic salt fog testing of the Prohesion(TM) type. In our studies of pipeline coatings, we needed to investigate thermal eff ects because of their extended range of use temperature. In these stud ies, we have discovered that electrochemical methods can be used for a n in situ measurement of the T-g of coatings in electrolyte immersion. Further, the 'plasticizing' effect of aqueous electrolyte absorption as well as its relative irreversibility has been shown. For all coatin gs studied, ENM provided useful, objective, numerical data which rapid ly ranks coatings and provides useful information on the relative life time prediction of coatings which may provide up to 30 years of servic e.