Organic coatings electrochemical techniques

Metallic corrosion is an electrochemical process associated with the flow of current between surface sites having a difference in electrochemical potential. The assessment and evaluation of organic coatings to prevent metal corrosion has traditionally been accomplished through salt fog testing (ASTM B-117) and long term exposure tests in particular service environments. Electrochemical techniques have often been considered (, but are not routinely employed in practice. 

Because cut edges are crucial areas of coil-coated material and can be used to study the effect of pigments in organic coatings on the corrosion in defects, they have been extensively studied by means of different electrochemical techniques such as electrochemical impedance and scanning vibrating electrodes [102, 139,140]. 

Parallel to these fundamental studies, electrochemical techniques will be further improved with regard to the early detection of failure of organically coated metals and the characterization of macroscopic... 

Among the conjugated polymers, polypyrrole (PPy) is the most representative one for its easy polymerization and wide application in gas sensors, electrochromic devices and batteries. Polypyrrole can be produced in the form of powders, coatings, or films. It is intrinsically conductive, stable and can be quite easily produced also continuously. The preparation of polypyrrole by oxidation of pyrrole dates back to 1888 and by electrochemical polymerization to 1957. However, this organic p>-system attracted general interest and was foimd to be electrically conductive in 1963. Polypyrrole has a high mechanical and chemical stability and can be produced continuously as flexible film (thickness 80 mm trade name Lutamer, BASF) by electrochemical techniques. Conductive polypyrrole films are obtained directly by anodic polymerization of pyrrole in aqueous or organic electrolytes. 

The use of electrochemical impedance spectroscopy (EIS) for the assessment of coatings has been under development for many years now [83]. The first period application of the technique was restricted to the study of the corrosion occurring with poorly protective coatings and coating impedances were evaluated by shape of the Nyquist plots. However, relatively few reports have been published describing results obtained from protective organic coating systems as used commercially [84]. 

Fedrizzi L, Rodriguez FJ, Rossi S, Deflorian F, Di Maggio R (2001) The use of electrochemical techniques to study the corrosion behaviour of organic coatings on steel pretreated with sol-gel zirconia films. Electrochim Acta 46(24—25) 3715-3724... 

AETOC advanced electrochemical techniques for organic coatings AFM atomic force microscopy... 

Other than for electrodes immersed in bulk electrolyte, on electrodes covered by ultrathin layers the electrode potential may differ significantly across the electrode surface. Hence, localised measurements are of interest, being performed by scanning the tip across the sample. This was first applied for organic coated metals where the coating was electrochemically delaminating, driven by corrosion [12-14, 29], Even on the submicron scale the Kelvin probe technique can be applied for such studies, and then based on a modified atomic force microscope, see [34, 35]. Recent developments are the combination of Kelvin probe and SECM [36] and the use of Kelvin probe for hydrogen detection [37]. 

G. Williams, H. N. McMuiray, and D. A. Worsley, Cerium(III) inhibition of corrosion-driven organic coating delamination studied using a scanning Kelvin probe technique , J. Electrochem. Soc., 149, B154 (2002). 

---