A Review of Corrosion and Cathodic - Protection Principles From An Electrochemical Point - of-View

Abstract

Abstract The corrosion of steel in an aqueous environment proceeds by an electrochemical process, usually controlled by the speed of the cathodic reaction. Over the pH range from 5.0 to 9.0, the cathodic reaction is commonly the reduction of dissolved oxygen. Electrons consumed in the cathodic reaction are supplied by the anodic reaction. Electronic current flowing from anodic to cathodic areas, the "corrosion current", can be calculated from the corrosion rate. In cathodic protection enough current must be supplied to "satisfy" the cathodic reaction. This amount of current is usually about 20 per cent higher than the corrosion current. The anodic and cathodic areas may be microscropic in size and practically superimposed, or they may be physically separated by distances up to many feet. Cathodic protection is applicable to both sets of conditions provided there is a continuous electrolyte and provided the geometry is favorable. For highest efficiency, the cathodic-protection current should be directed to the cathodic areas of the system, rather than to the anodic or corroding areas as one might expect. Before applying cathodic protection one should look for some way to minimize or stifle the cathodic reaction, such as by removing dissolved oxygen. Cathodic protection is normally achieved when the potential of a structure has been depressed to –0.85 v referred to a Cu-Cu SO4 reference electrode. There may be some cases, where the cathodic reaction is provided by bacterial activity, in which the structure/water potential may need to be depressed more than –0.85 v to achieve protection. The potential required for protection is the open-circuit potential of the anodic areas. In a stable system, corrosometer probes can be used to determine whether cathodic protection has been achieved. An impressed-current system should generally be used, where feasible, because such a system is more flexible than a sacrificial-anode system. Introduction Both external and internal corrosion often plague secondary-recovery water systems. Externally, the problem is corrosion of underground pipe; once leaks occur, the soil is likely to become contaminated with brine which, in turn, makes the corrosion problem even more serious. Internally, the problem is corrosion in water-handling equipment heaters, filters, etc. as well as corrosion inside the water distribution lines themselves. The actual loss of metal may not be so serious as the plugging which results from accumulation of corrosion products. Cathodic protection has a place in combating both external and internal corrosion in water-injection systems. On the one hand, applications of cathodic protection to external corrosion are numerous. The procedures are well established, and so they will not be discussed in detail here. However, much of what will be said will apply in principle to cathodic protection against external corrosion. On the other hand, applications of cathodic protection to internal corrosion are less obvious and are not so well developed. The objective of this paper is to discuss the theory or principles of cathodic protection as they might be applied internally in a water-injection system. To accomplish this objective, the nature of aqueous corrosion will be reviewed, and the relationship between corrosion and cathodic protection will be discussed. The key role of the cathodic reaction in the corrosion process will be emphasized because, in one sense, the study of aqueous corrosion and cathodic protection is the study of what goes on at the cathodic part of corrosion cells. The electrochemical point-of-view will be used throughout. Too often, even in the technical literature, cathodic protection is discussed in the language of the electrical engineer. Positive current is pictured as moving through metallic conductors, and cathodic protection is said to -drain" corrosive current from a corroding structure. Concepts such as these make it difficult to understand the processes involved. The fact is, corrosion as it occurs in common experience is electrochemical in nature; and so, to understand corrosion and the related cathodic protection, one must look to electrochemical explanations. JPT P. 967^