ABSTRACT

When exposed to concentrated sulfuric acid, stainless steel exhibits unique electrochemical behavior. This behavior can be observed as an oscillation in open-circuit potential between the active and passive states. The transient nature of the corrosion behavior under these conditions results in a distinct challenge for measuring and predicting corrosion rates. Using a series of commercial alloys with various nickel contents, this paper outlines the utilization of electrochemical experimentation to refine the prediction of corrosion rates. The paper also discusses some of the difficulties associated with many traditional electrochemical techniques such as potentiodynamic scans when used for characterizing systems which undergo oscillations in open-circuit potential.

INTRODUCTION

Stainless steels (SS) are commonly used in the production, transport, and storage of sulphuric acid 1. These alloys exhibit excellent corrosion resistance, but they are not stably passive when exposed to concentrated sulphuric acid. The 300-series grades of stainless steel are known to exhibit oscillations in potential between the active and passive states. Figure 1 shows the open-circuit behavior of Type 309 stainless steel (UNS S30908) in 93.5% sulfuric acid at 60 °C. The oscillations in open-circuit potential for 300-series grades are known to be related to alloying with nickel, since nickel-free 400-series grades do not exhibit this phenomenon2

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This phenomenon is more then a scientific curiosity, as it greatly complicates the measurement and prediction of weight loss associated with these alloys. Although previous work has been done to document the corrosion resistance of many of these alloys1-9 often insufficient attention is paid to the electrochemical behavior exhibited during exposure. This paper will outline how the accuracy of predicted weight loss may be improved by the use of continuous online monitoring of the open-circuit potential. As stated above, nickel additions are known to have strong influence oscillation behavior; thus a range of nickel concentration will be examined to show how variations in the open-circuit behavior impact on the corrosion resistance of the alloys. An additional complication is that preliminary results indicate that it is of questionable validity for this range of alloys to utilize potentiodynamic techniques for the acquisition of anodic polarization data. Current theory which has been developed to explain the source of the open-circuit oscillations may also be utilized to understand the source of these inconsistencies observed in the polarization data

EXPERIMENTAL PROCEDURE
Materials and Experimental Setup:

Cylindrical test samples were prepared from a series of commercial alloys selected to exhibit a variation in nickel content with a relatively constant content of chromium and other additions. Molybdenum lean grades of stainless steel were selected in order to keep the contribution of other alloying elements to a minimum. Alloy compositions were determined by glow discharge optical emission spectroscopy (GDOES). Cylindrical test samples were machined with an outer diameter of 1.2 cm, thickness of the samples was dictated by the thickness of the steel plate; for most samples 6mm thickness was used. All measurements stated below have been normalized by surface area. TABLE 1, shows the measured compositions of the alloys tested.

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