INTRODUCTION

Corrosion, simply viewed, is the process of the earth reclaiming from man that which man has taken from the earth. Through the ages man has learned to extract ores and, through various smelting, refining and alloying techniques, to produce metals to serve his needs. Many of these metals were used to construct vessels, pipelines, structural configurations of many shapes and forms and all sorts of machinery. Metals such as steel, cast iron, wrought iron, and copper have commonly been used by the gas industry to provide piping systems with sufficient strength to contain internal pressure and resist external loads.

Through corrosion mechanisms, the earth continuously recovers its losses, at the expense of man. In the case of ferrous pipe, the end product of corrosion is usually a form of iron oxide (rust). Corrosion occurs when four required components are present: an anode, cathode, electrolyte, and an electrical connection between the anode and cathode. When a metal is placed in soil, current flows between its anodic and cathodic areas. Metal ions go into solution at the anode (oxidation) while hydroxyl ions are formed at the cathode (reduction). The greater the potential difference between an anode and cathode, the greater the tendency for corrosion to occur.