INTERGRANULAR CORROSION OF SINGLE PHASE ALUMINIUM AS A PITTING PHENOMENON

Abstract

— The characteristics of the intergranular fissuring phenomenon long known for aluminium in hydrochloric acid were reviewed. Electrochemical and morphological studies of pitting corrosion in sulfuric acid containing sodium chloride were described. Intergranular fissuring was reproduced in these solutions and an interpretation given of the roles of chloride ion and copper content in producing this type of attack. The intergranular corrosion phenomena which are exhibited by high purity aluminium exposed to hydrochloric acid have received attention from a number of researchers since the first report in 1934 [1]. Most of the interest apparently originated in the hope of obtaining insight into metallurgical questions — the properties of grain boundaries, possible segregation phenomena, the properties of aluminium. The present work takes a more electrochemical approach. The rapid intergranular fissuring which occurs under certain conditions is a type of pitting phenomenon, and it was sought to interpret this in terms of an electrochemical model. One difficulty was that in hydrochloric acid the incidence of pitting corrosion severely limits the range of electrochemical potential . over which studies can be made. The intergranular fissuring question was therefore set aside for the moment in favor of ongoing electrochemical and electronmicroscopic studies of the passive state and of pitting in solutions based on sulfuric acid, to which NaCl could be added to produce pitting corrosion. Some information has now been developed on electrochemical behavior in these solutions, and the intergranular fissuring phenomenon has been reproduced in them, so that it is now possible to give a preliminary interpretation of this phenomenon. This report presents a review of the relevant information developed in the hydrochloric acid studies, a brief account of pertinent results for sulfuric acid and application of these to the interpretation of this type of attack. 1. Classification of intergranular attack. — A scheme for discussing the relation of pitting to intergranular attack is given in figure 1. In Type 1, there is no pitting but only preferential attack reflecting the special structure and perhaps composition of the grain boundary which gives a rate of penetration vb somewhat greater than the rate of uniform attack of the adjacent grain surfaces, vg. A broad shallow groove is formed. In Type 2, after a certain amount of Type 1 corrosion, pits nucleate preferentially at the boundary and grow rapidly but without a preferred growth direction. The pit will then be twice as broad as deep, or approximately hemispherical. In Type 3, pits nucleate (at the same time) at the grain boundary and within the grain, but there is for some reason a much higher rate of pit growth parallel to the boundary, Vb, than normal to it, Vg. Vg represents the rate of pit growth in the grain body and determines the dimensions of the pits within the grains. 2. Observations in HC1. — A number of workers in this system have noted intergranular fissuring, which can cause a coarse grained sheet sample to FIG. 1. — Schematic of forms of intergranular pitting attack. 1. Intrinsic attack — no pitting. 2. Preferential pit nucleation, nonpreferential pit growth. 3. Preferential pit growth along boundary to form fissure. JOURNAL DE PHYSIQUE ColloqueCA, supplement au n° 10, Tome36, Octobre 1975, page C4-387 Resume. — Les donnees relatives a l'attaque intergranulaire fissurante dans l'aluminium pur par l'acide chlorhydrique sont rappelees. Des etudes electrochimiques et morphologiques de la corrosion par piqures dans l'acide sulfurique contenant du chlorure de sodium sont decrites. Une attaque intergranulaire fissurante a ete produite dans ce milieu et une interpretation est presentee concernant les roles joues par le chlorure et par la teneur du metal en cuivre, dans ce mode d'attaque. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975439