Probing Coating Degradation on AA2024‐T3 Using Local Electrochemical and Chemical Techniques

Abstract

The service life of an organic coating on a metal substrate is determined by variables associated with the environment, the coating, the substrate, and the interphasial chemistry that develops between the substrate and the coating. Although our basic understanding of the corrosion and materials issues within each of these regions is increasing, our understanding of the interactions among them and how these interactions lead to failure is limited. This gap in understanding is, in part, associated with the need to understand the origins of failure. Because corrosion failures on coated alloys initiate and grow from a local site, it is essential to know whether heterogeneities within the coating material, substrate alloy, or both are controlling the initiation of corrosion events. An understanding of the localized breakdown events of a coated alloy substrate will require a description of the local chemical and electrochemical events that take place within the coating and at the polymer/metal interface.Whereas the characterization of local electrochemical events on bare metal surfaces has been widely investigated, 1-7 the examination of local events on coated alloys has been less extensive. [8][9][10][11][12] It has been only recently that investigations of local defects of coated alloys has moved from the characterization of extrinsic (i.e., man-made) defects to the characterization of intrinsic 10 (i.e., naturally occurring) heterogeneities. The use of local electrochemical impedance methods has enabled the identification and monitoring of local breakdown processes of coatings on certain alloys. However, a more detailed understanding of defect origins and growth characteristics requires information on substrate microstructure and surface chemistry, as well as the underfilm solution chemistry that develops at a local site.Prior research on the analysis of underfilm chemistry has been limited to either pH determinations [13][14][15] or to more detailed chemical analyses of blisters on coated steel under cathodic conditions. [16][17][18][19][20] Acquiring information on the chemistry within these late stage defects represents an important step in the evolution of coatings research. Nonetheless, it would be of greater benefit to be able to sample the solution chemistry of an underfilm corrosion site in its early stages of development. Capillary electrophoresis (CE) is a technique that has shown feasibility of sampling and analyzing extremely small volumes of solutions (e.g., ca. 30 nL) from occluded corrosion sites to speciate and quantify the ions present. 21-23 Its potential application to analyzing the underfilm chemistry beneath early stage blisters has been explored in the present study.This investigation was an initial venture into understanding the breakdown of organic coatings on an aerospace alloy, AA2024-T3, through the characterization of the local electrochemical and chemical changes at the local sites. Information on the location and electrochemical nature of these defects along with the interfacial chemist...

Local electrochemical surface analysis

“…An analysis of the distribution of intermediate diffuse layers showed that their growth followed a parabolic law previously described by Frenkel and Sergeev. The degradation of coatings on the AA2024-T3 alloy was studied in [47], and the composition of the wearing products of multilayer metal coatings was determined in [48]. Electrochemical criteria for assessing the quality of protective coatings were considered in [49], such as the surface distribution of the thickness and composition of coatings, the protective capacity and porosity of coatings, and the integral characteristic of the quality of coatings.…”

Microelectrochemical Studies of Pit Initiation on High Purity and Ultra High Purity Aluminum

“…[12,13] In order to evaluate the critical factors that trigger initiation and propagation of localized corrosion local techniques are frequently applied to gain a better insight in the mechanisms of pitting. Several groups used in situ scanning techniques to track the evolution of the surface topography during pitting, [14,15] local pH values, [16,17] local impedance, [16] and local current densities. [18] The sulfate and chloride accumulation in the passive film during and after the breakdown of the passive film was also studied with radiotracer techniques.…”

Infrared microspectroscopic studies on the pitting of AA2024‐T3 induced by acetone degreasing

“…It can be seen clearly that the aliphatic carbon is found in high concentrations in the region surrounding the pit. The hydrocarbons could be due to acetone (CH 3 COCH 3 ), acetic acid (CH 3 COOH) or acetate (CH 3 COO ) as in copper acetate (Cu(CH 3 COO) 2 ) and aluminum acetate (Al(CH 3 COO) 3 ). They could be due also to the sum of all these components.…”