ABSTRACT

Corrosion inhibitors are required to provide adequate protection in CO2 and/or H2S environments operated under high wall shear stress conditions. Jet impingement test methodology is used in this paper to study the flow-accelerated corrosion under extremely high shear stress conditions. This study is focused on the methodology aspects of using the jet impingement coupled with a new localized corrosion monitoring (LCM) technique for the selection of corrosion inhibitors for high shear applications with emphasis on localized corrosion. LCM technique and its data analysis program were employed to identify the occurrence of localized corrosion events as well as its magnitude, duration, frequency and distribution. A number of sulfur-containing corrosion inhibitors were evaluated using the new test methodology. Keywords: jet impingement, localized corrosion, corrosion inhibitors, localized corrosion monitoring

(LCM), wall shear stress, sulfur compounds.

INTRODUCTION

The use of corrosion inhibitors to control internal corrosion of carbon steel structures in oil and gas production facilities is a well established industrial practice adopted by the industry. The typical approach in selecting a corrosion inhibitor for a specific application is via series of laboratory screening tests to determine the inhibition capability of candidate chemicals under a specific set of test conditions. The preliminary performance tests undertaken may include wheel test, kettle tests and partitioning tests1- 3. Additional tests may then be conducted to simulate more closely the field operating conditions, such as high pressure, high temperature and high wall shear stress, etc. In such cases, more specialized tests can be performed using more complex equipment, such as rotating cylinder electrode (RCE), high shear autoclave (HSAT), flow loop and jet impingement equipment4-6, etc.

The evaluation of corrosion inhibitor performance is traditionally based on weight loss measurements and/or electrochemical monitoring, such as linear polarization resistance (LPR) measurements, which provide information on the general corrosion rate. With coupon exposure tests, localized corrosion information may also be obtained, provided the duration of the exposure test is sufficient long to enable pits to be fully developed, which may take weeks or even months depending on the test conditions.

In high flow rate high shear stress operating environment, a number of corrosion inhibitors containing sulfur based compounds have been developed for this type of applications. Two of such sulfur based compounds are thioglycolic acid (TGA) and mercaptoalcohol (MA), which were reported to give good performance in high shear tests7, 8. Although corrosion inhibition performance tests on both types of chemistry suggested that the overall general corrosion rate was low, there was also some experimental evidence suggesting that the presence of TGA might lead to localized attack in RCE tests; whereas MA containing inhibitors exhibited superior pitting corrosion inhibition behavior9.

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