ABSTRACT

The oil industry has many high value hydrocarbon applications requiring corrosion rate monitoring for real-time problem solving and control. Electrochemical techniques are capable of high resolution, instantaneous determination of corrosion rate in aqueous (conductive) environments, but if incorrectly applied in the presence of hydrocarbons, their performance is limited, generally problematical and often unreliable.

The industry also seeks improved working life of sensors without significantly compromising performance. Metal loss technology exists today that provides either high sensitivity but with short life, or conversely, long life but with substantially reduced sensitivity.

The paper discusses the limitations of existing on-line technologies and describes the performance of a new, emerging, technology which the authors believe promises unrivaled sensitivity combined with long sensor life.

INTRODUCTION

A range of measurement technology exists today to determine the rate, type and mechanisms of corrosion. These technologies may be grouped into two general categories, namely: metal loss and electrochemical methods.

Metal loss techniques include weight loss coupons, electrical resistance, electric current field mapping, inductive resistance and ultrasonic. The corrosion rate is derived by measuring the rate of reduction of thickness of the sensor elements or wall thickness. The capabilities and performance of these metal loss techniques tend to vary between excellent thickness resolution with a short sensor life (e.g. thin element electrical resistance, inductive resistance) and low thickness resolution with long life (e.g. electric current field mapping and ultrasonics).

The electrochemical methods, including Linear Polarisation Resistance (LPR), Electrochemical Impedance Spectroscopy (EIM) and Electrochemical Noise (EN), relate to the electrochemical reaction thermodynamics and kinetics of the corrosion process which can reveal uniform and localized corrosion together with the associated corrosion mechanisms. EIM and EN tend to be interpretive requiring specialist knowledge, although when correctly applied are capable of providing quantitative corrosion rate indications in low conductivity hydrocarbon streams. LPR is used extensively to detect instantaneous corrosion rates but its performance is limited in non-aqueous environments. However, unlike metal loss methods, electrochemical techniques are unable to detect metal loss due to erosion.

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