ABSTRACT

Crevice corrosion resistance is one of the main criteria for the selection of a given stainless steel grade for seawater applications. Crevice corrosion resistance can be evaluated using different techniques that are described in standards and in the literature. However, the proposed assemblies are often restricted to plate geometry specimens. Some crevice assemblies for tube geometry have been proposed and used during the last ten years, but they showed a large dispersion of the results due to the difficulty to control crevice geometries on curved surfaces. From this background, an optimized crevice assembly was proposed to allow a better control of the reproducibility of crevice corrosion tests on stainless steel tubes.

Finite element modelling and laboratory testing have shown that the new proposed design provides a better control of gasket pressure and consequently a better reproducibility than previous existing assemblies. The proposed assembly can be adapted and used for fit-for-purpose testing (e. g. evaluation of different gasket materials, different gasket pressures, etc.).

INTRODUCTION

Stainless steels and nickel-based alloys are used for a wide range of chloride-containing electrolytes in different industrial applications. The corrosion resistance of these alloys is due to the formation of a stable passive oxide layer (e.g. passive film) which forms at the surface. However, passive alloys can be sensitive to pitting and crevice corrosion, especially in chloride-containing media such as seawater. In the last decades, the understanding of localized corrosion has received much attention since it is one of the main limiting factors for the use of these alloys in seawater.

Several standards and techniques are used to evaluate the crevice corrosion resistance of stainless steels and nickel based alloys. From previous experience, assemblies for plate geometries as described in ISO 18070 provides satisfying results and good reliability to rank/evaluate the corrosion behavior of stainless steel alloys in seawater 1-4 However, the control of the crevice corrosion tests becomes more difficult when tube geometries are involved. Even if a crevice former for tube specimens is described in ISO 18070, it has been shown that due to the curvature of the tubes and therefore the crevice formers, pressure along the crevice gap may not be constant. The crevice set-up that is described above for tubes could thus be used as a reproducible technique for qualifying tubular stainless steels. However, improvements on the homogeneity of the pressure distribution need still to be done.

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