Effects of environmental factors on key kinetic parameters relevant to pitting corrosion

Abstract

Experiments using stainless steel artificial pit (leadin-pencil) electrodes in ferric chloride and lithium chloride solutions were performed in order to determine the effects of key environmental factors such as chloride concentration and pH of the bulk solution on the central parameters utilized to characterize the pitting phenomenon-the repassivation potential E rp and the pit stability product under a salt film (i· x) saltfilm . For all the stainless steel alloys studied, a relative independence of the E rp to the pit depth was observed once sufficient anodic charge had been passed. The pit stability product under a salt film was seen to be largely insensitive to the pH of the bulk solution. E rp , on the other hand, was fairly independent of bulk pH only at the lower chloride concentrations of both lithium chloride and ferric chloride solutions. The two parameters were affected differently by variation in the chloride concentration of the bulk solutions. Increasing the chloride concentration resulted in a decrease in the value of (i·x) saltfilm for all alloys in both solutions. In ferric chloride, the value of E rp increased with increasing chloride concentration for Custom 465 and the austenitic steels, whereas it decreased across the same range for 17-4 pH. These trends were explained qualitatively using solution conductivity and alloying composition arguments. Finally, the results obtained from this study allowed for a rationalization of the phenomenology, enabling a method of measurement of the diffusion coefficient and the concentration at saturation of the Bstainless steel cation^within the pit, both of which agreed well with values obtained from the existing literature.

Geometric Evolution of Flux from a Corroding One-Dimensional Pit and Its Implications on the Evaluation of Kinetic Parameters for Pit Stability

“…37 Furthermore, Ernst and Newman, 29 Jun et al, 36 and Woldemedhin et al 27 have shown that modeling the transport characteristics of the 1-D pit corroding under a salt film purely as a diffusion-limited process with the value for D as reported by Gaudet et al 14 25 to be independent of pit diameter. Also, studies by Novakovski and Sorokina, 38 and later by Isaacs 18 have shown that the majority of this potential drop occurs across the salt film, and the current in the pit is primarily regulated by diffusion.…”

“…Furthermore, the data from deep pits reflected pit stability kinetics isolated from effects of the bulk chloride environment, thus affording better agreement of experimental measurements with the theoretical basis of the Galvele stability criterion. Finally, the results of this study provide a scientifically defendable rationale for the empirical observation of a plateau in the measured repassivation potential for deep pits, 27,28,32,33 thereby assisting in the development of a quantitative framework 30,34 that connects electrochemical parameters critical to both pit stability and repassivation. …”

“…26,27,29,36 Also, the assumption of constant diffusivity has been shown by Gaudet et al 14 to effectively approximate the combined effects of variable diffusivity and electromigration on the cation flux.…”

“…Variants of this procedure to measure i L have been employed in other pitting studies as well. 27,28 This dissolution current density was converted to a flux value J as shown in Equation 1.…”

“…The presence of the salt film results in diffusionlimited dissolution conditions that permit the study of the corroding system in a quasi-steady state. The pit stability product under a salt film, (i·x) saltfilm can then be extracted 13,14,[25][26][27][28] from measuring the diffusion-limited current density at different depths. The critical pit stability product can then be expressed as a fraction of this measured pit stability parameter (i·x) saltfilm .…”