Cathodic Protection of Offshore Structures

Abstract

INTRODUCTION Cathodic protection---- what's it all about? Many who have been acquainted with cathodic protection have termed it everything from "witchcraft" to "the cure-all for every corrosion problem". Neither description is correct. Most of the engineers involved in the Offshore Industry have some idea as to how cathodic protection works and some are, indeed, very expert. But there are many engineers who have only a vague notion as to what cathodic protection really is. Offshore work involves massive equipment, very sophisticated engineering and technologies quite unrelated to the sciences associated with corrosion control (electro-chemistry and physical metallurgy). Therefore, despite the real importance of cathodic protection, this phase of offshore engineering is frequently passed over with only meager consideration. This should not be the case. The large investments made in offshore structures, extremely high cost for submarine repairs and the financial hazards incurred by shut-down and loss of product dictates a most serious consideration of corrosion control. Use of cathodic protection to insure long, corrosion-free, life of our offshore structures is, indeed, most important. Engineers who specify, buy and operate cathodic protection should, at least, acquaint themselves with the basic principles of corrosion and design of cathodic protection systems. Certainly, this paper, or even a complete textbook on the subject, would be insufficient to instantaneously create "expert" corrosion engineers. However, it is the author's aim to briefly set forth some of the important considerations which may help to avoid failures of cathodic protection systems and prevent catastrophic corrosion damage. NATURE OF CORROSION It has long been known that sub-sea corrosion activity is essentially an electrochemical reaction. This reaction has been correctly, and falsely, given many different names: rust, oxidation, pitting, erosion, chemical attack and electrolysis. Because cathodic protection can be most readily understood as a "reversal of the corrosion reaction", it is essential to have an understanding of the corrosion reaction itself. The action is similar to that which takes place in a common "dry-cell" battery. Basically, a cell is made up of three essential parts: an anode, a cathode and an electrolyte. The container, usually a zinc "can", is the anode (negative terminal). The cathode (or positive terminal) is the carbon rod in the center. The electrolyte is a mixture of ammonium chloride, water and a depolarizing agent such as manganese dioxide, between the zinc and the carbon. When a light bulb is connected between the positive terminal (carbon) and the negative terminal (zinc), an electric current will flow in the conventional manner from plus to minus (from carbon to zinc) because of the electrochemical reaction occurring simultaneously at both electrodes. Within the cell (internal circuit), the current flows from the zinc through the electrolyte to the carbon rod. Corrosion occurs at the anode (zinc) where the current leaves the metal surface and enters the electrolyte. After continued use of a dry cell, the container becomes corroded and the electrolyte leaks out. The current flow is caused, essentially, by the difference in electrical potential between the two dissimilar metals, as zinc is more electro negative (less noble) than carbon, it is anodic to the carbon.