Experimental and Modeling Studies of Single Pits on Pure Aluminum in pH 11 NaCl Solutions I. Laser Initiated Single Pits

Abstract

During localized corrosion, a large number of phenomena exert themselves over wide ranges of length scale. During the growth of a corrosion pit, for example, surface and homogeneous reactions, equilibration among many species, transport by migration, diffusion and convection, and three-dimensional interactions among adjacent pits are all important simultaneously. The large number of phenomena as well as the large range of length scales over which they occur places stringent demands on the design of experimental techniques for measuring localized phenomena. For example, the chemical environment in a corrosion pit is much different than the chemical environment in which the metal is initially immersed. Measurements of the various critical chemical species in the pit micro environment represents an extreme challenge for experimentalists. Mathematical models therefore prove useful because they allow for the testing of hypothesis and, in addition, can direct experimental efforts to complex regions of operation that can easily confound one's intuition.In the present investigation (Part I), experimental measurements were carried out with use of an in situ laser initiation technique that allowed for the precise control of the location of the initiation site as well as electrochemical control of single-pit dissolution rate. These data were compared with several mathematical models in order to determine the transport mechanism that best described controlled dissolution in the experimental conditions. Several chemistries were considered in mathematical models, although all were related to the hypothesis of Wong and Alkire 1 that for Al pits grown in basis NaCl solutions, the pit contains aluminum oxychloride species. The testing of several chemistries was important for two reasons: (i) to investigate the effect of chemistry on the mechanisms that control dissolution and (ii) to determine which chemistry was to be used in Part II of this series. In Part II, one of the mathematical models of Part I was selected for further refinement to consider transient behavior, and to test hypotheses of the local chemical conditions required to keep a corrosion pit growing.Newman et al. 2 solved Laplace's equation for the potential field in the concave hemisphere geometry subject to the boundary condition that the surface dissolved at constant current density which is obviously a good assumption for conditions that yield smooth, hemispherical pits. 3 Using the results of the model, Newman 3 derived the following equation to describe the variation of current density with radiusEquation 1 predicts that the current density varies as r Ϫ1 .Beck and Alkire 4 developed a simple mathematical model to describe the relationship between current density and radius, and current density and time for a convex hemisphere. They derived the expression for the flux (reaction rate) of a species present on the surface of a sphere at constant concentration into a medium of lower concentration [2] In order to determine an expression for the variation of curren...

New insights into the cathodic dissolution of aluminium using electrochemical methods

“…Aluminium corrosion in aqueous neutral solutions has been widely investigated in the anodic range where a passive oxide/hydroxide layer provides a passive behaviour to aluminium [1][2][3][4][5][6]. Many experiments have also been performed on aluminium alloys (for instance AA 2024-T3 alloy) in which galvanic corrosion occurs between an aluminium matrix acting as anode and copper rich intermetallic particles acting as cathodes [7][8][9][10].…”

Pitting corrosion in presence of inhibitors and oxidants

“…Some attempts to access weaker currents have been published. They are based either on the study of microelectrodes (Kobayashi et al , 2000), or on amplification of the anodic current (Burstein et al , 2004), or directly on the provocation of just one dissolution site (Verhoff and Alkire, 2000). Investigation of the exact limit of the material resistance becomes more complicated when it comes to observing the influence caused by minor variations in the chemical composition of a weak solution.…”

Stress distribution in the neighborhood of a corrosion pit on the outer surface of an elastic spherical shell

“…[7], [9], [13], [23]. In this paper, pitting geometry used is of a cylindrical shape with a hemispherical bottom (as shown in fig.…”

“…As a result, it significantly reduces the life of structures. Many researchers have investigated pitting initiation and growth as well as local corrosion protection (for example [1], [5]- [9]). The stress concentration factor considerably increases near such defects as notches, cracks, cuts or corrosion pitting [10], [11], which may lead to premature failure of mechanical structures.…”