ABSTRACT

Crevice corrosion, pitting or stress corrosion cracking (SCC) of steel, stainless steels, nickel base alloys, or other metallic materials such as titanium can occur in aerated solution due to a differential oxygen (O2) concentration cell (OCC). An O2 concentration gradient exists in solution between the bulk and the occluded region and within the occluded region itself. Since O2 concentration in the occluded region varies significantly in a non-linear manner in the direction perpendicular to the metal wall(s), it is a challenge to model the corrosion in one-dimension (1D) by considering O2 reduction at the metal wall(s). There is a need for a method that can quantitatively describe this O2 concentration in 1D. This work serves to derive simple algorithms to fulfill the above purpose. Discussion of the validity boundaries of the simple algorithms is provided.

INTRODUCTION

In natural corroding environments, a potential gradient exists between inside and outside of, and within, an occlusion such as a crevice, a pit or a crack (Figure 1). In aerated solution, this potential gradient may highly depend on the significance of an OCC that creates and drives a positive ionic current to flow, through solution, from tip of a pit or crack, or from inside a crevice, to the rest of the occlusion surface and to the metal surface outside of the occlusion. When cathodic polarization is present at the occlusion mouth in the case of steel pipeline crevice corrosion under disbonded coatings,[1-4] the polarization could fully remove this OCC and the positive ionic current rather flows from the mouth into the occlusion. Regardless of the current flowing directions, this ionic current links the electrochemical reactions inside the occlusion to those outside. Depending on the solution resistivity and can travel over within the occlusion solution varies. Through this current, O2 reduction at the metal surface outside of the occlusion may control the pit or crack growth rate or the crevice corrosion rate, although in the case of pipeline steel corrosion under a disbonded coating,[1-4] with little steel surface exposure to bulk solution and often this surface covered by a calcareous deposit and thus inactive, O2 reduction inside the crevice is more significant.

Although the effect of O2 reduction within a crack on the crack growth rate was in an earlier model[5] treated substantially in both 2D and 1D, the assumptions made in the model presented it with deficiencies. The assumptions included that potential invariant along the crack depth, O2 concentration averaged without considering the significant variation of O2 concentration gradient across the crack gap, and ferrous ion oxidation treated without considering the possible limitation of this oxidation rate (limited by the generation rate of ferrous ions from corrosion or crack growth). In addition, the simplified formats of the model were still relatively complex. In other literature,[6-7] the effect of O2 reduction within an occlusion on the pit or crack growth rate or crevice corrosion rate was simply neglected. Even though this neglecting of O2 effect has been justified by quoting that O2 is unable to penetrate deep into the occlusion, no clear quantitative verification has been made to compare the corrosion rates obtained when the in-occlusion O2 reduction is or is not considered.

This content is only available via PDF.
You can access this article if you purchase or spend a download.