Carl E. Locke

Principles of Anodic Protection

An understanding of the electrochemical principles on which the application of anodic protection is based is beneficial for anyone who believes that the method may be useful in controlling corrosion of his equipment. This is particularly true when its use must be technically justified or when decisions must be made about alternative proposals, equipment, or operational modes.

Anodic Protection of Metals—A Technique Whose Time Has Come

The twin compulsions of materials scarcity and escalating costs have brought about a marked increase in interest among designers, materials specialists, and engineers in effective methods to control wear and corrosion. This interest has resulted in the most intense studies of materials properties that has ever been experienced in the industrial nations of the world. Additional impetus has been given to this trend by the impending necessity to find and develop ways in which fuels can be produced to take the place of diminishing supplies of petroleum and natural gas, not to mention the financial drain of buying petroleum from abroad at escalated prices.

Economic Evaluation of Anodic Protection

The task of making an economic study to determine the desirability of using anodic protection does not differ from that in evaluating any other engineering decision. Cost factors to be considered are listed in Table 5-1. Not all of the factors listed must be considered for every evaluation. It is virtually impossible to quantify other, perhaps equally pertinent, factors such as effects of extended delivery times for certain types of equipment, availability of materials at any price, and the cost of liability insurance covering one type of installation as compared to another.

Laboratory Tests and Procedures

So that respective plant processes and parameters influencing prospective application of an anodic protection system can be understood, specific laboratory tests must be conducted. These tests should be designed to include all pertinent conditions involved in the plant system under consideration. The data which result from these tests provides an overall corrosion-rate profile for the specific metal/electrolyte system over the preselected potential range. The tests can be conducted using either the potentiostatic or potentiodynamic mode. The potential range of interest can be scanned very slowly, or by rapid-scan techniques, the choice depending upon the system in question. Further, the tests should be designed so that the resulting data can be used to (1) provide a basis for quality control, (2) establish performance parameters, and (3) serve as a comparison standard for improved developments. In view of the diversity of natural conditions, very serious consideration should be given to laboratory testing to improve the reliability of, and confidence in, the resulting information.

Anodic Protection of Industrial Equipment

Many methods of corrosion control were first applied in an empirical manner, often many years before the scientific principles underlying the methods were identified. Anodic protection, however, is an exception to this procedure in that an understanding of the basic mechanisms was developed in the laboratory first. These discoveries were followed by a period of pilot-scale development, which included several trial installations in chemical plants. Finally came the commercial development of the technique. This development procedure is very satisfying to the scientist and engineer.

Design, Operation, and Maintenance of Anodic Protection Systems

Anodic protection installations must be planned carefully. As discussed in Chapters 2 and 6, this method of corrosion control is successful only in systems that exhibit an active-passive anodic-polarization behavior. Protection by anodic polarization also has some inherent dangers in that it is possible to accelerate corrosion by incorrect application. However, if correctly designed, operated, and maintained, this technique of corrosion control can be a very powerful tool in the hands of the corrosion engineer.

Selected Examples of Anodic Protection

Among major decisions to be made in establishing the design and materials parameters of equipment that will be exposed to corrosive solutions is the choice of a material. For some materials sufficient data based on experience is available to permit unequivocal approval of one or more candidates. Thus the materials engineer must weigh the availability, design, and fabrication aspects of alternates concurrently with an economic analysis. Frequently these decisions are further complicated by consideration of the possibility that one or more of the candidates is amenable to electrochemical protection, or that their attributes with respect to inhibitors need also be taken into account. Many of these considerations are discussed elsewhere herein.

Equipment for Anodic Protection

A typical anodic protection installation in a tank is sketched in Figure 2–9. Figure 2–12 illustrates a heat exchanger that is built with anodic protection as an integral part. Both diagrams show the equipment necessary for anodic protection: cathode and reference electrodes, potential controller, and power supply. This chapter describes the characteristics of this equipment and surveys the systems discussed in the literature, as well as some in the personal experience of the authors.

Synthesis and Thermal Analytical Characterization of Chlorinated Polyethylene-g-styrene

Chlorinated polyethylene has been reported to be an excellent modifier for blends of polyethylene and poly(vinyl chloride) (1,2). Properties of blends of polyethylene-polystyrene and poly(vinyl chloride)-polystyrene have been improved but not to the extent possible for the polyethylene-poly(vinyl chloride) blends. The improvement in blend properties was attributed to the blocklike structure of the slurry-produced chlorinated polyethylene (3, 4).