ABSTRACT

Cathodic Protection is a unique technique to control corrosion of metal by making the subject metal as the cathode of an electrochemical cell. Here the metal to be protected, referred to as cathode, is electrically connected to an easily corroded metal, referred to as sacrificial metal or galvanic anode. The chemical reaction taking place in an electrochemical cell causes the cathode to be protected and anode to disintegrate. For larger structures and where electrolytes have high resistivity, the potential difference between cathode and galvanic anode is not enough to generate sufficient current required for the protection of cathode. In such cases impressed current cathodic protection (ICCP) systems are used instead of galvanic anode systems. ICCP systems makes use of an external steady state direct current (DC) power source or a transformer rectifier (TR) to provide the required cathodic protection current. ICCP systems with steady state DC is used since 1950’s. This paper presents the methodology, observations and analysis of using pulse width modulated (PWM) DC power source rather than a steady state DC power source on a buried pipeline sample. An experimental setup is made to study corrosion rate, anode life, interference mitigation, etc., when the pipeline is subjected to both steady state and pulsed DC output.

INTRODUCTION

When the surface area of the metal to be protected is large or when highly resistive electrolyte is present, cathodic protection using galvanic anodes is not feasible. This is due to the fact that the galvanic anodes cannot economically provide the required amount of current for the protection levels to be achieved. This gave the path to the introduction of impressed current cathodic protection system (ICCP). ICCP system unlike galvanic cathodic protection system consists of a direct current (DC) source, whose positive terminal is connected to the anode and the negative terminal is connected to the structure to be protected. Unlike galvanic anodes specially designed anodes are used for ICCP systems, which provide more life and adequate protection current to the structure. A transformer rectifier capable of stepping up or stepping down the input alternating current (AC) into direct current (DC) is commonly used for ICCP systems. This paper presents the experimental procedure and results of an ICCP system developed, which makes use of pulse width modulated (PWM) signal rather than the traditional steady state DC signal. PWM systems consumes much less power when compared with existing transformer rectifier systems.

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