Why should cathodic protection and a coating be used together to protect against corrosion?

Applying a protective coating to metallic surfaces is the most effective method to mitigate corrosion damage. According to a National Association of Corrosion Engineers (NACE) report, more than $100 billion dollars is spent in the U.S. to control corrosion by the application of either organic or metallic coatings. Fifty percent of the corrosion damage cost is preventable with 85 percent of that related to coatings. Thus, coatings play an important role in protecting metallic materials against corrosion.

The Impact of Coating Defects

However, coatings are not perfect. In reality, no coating is entirely free of defects even when freshly applied. The presence of defects is considered inevitable. Some of these defects appear during coating production and some of them stem from handling and improper application. (Learn more about these in the article 5 Coating Defects That Can Be Avoided By Adhering To Coating Specs.) Also, some other defects form during the coating's service lifetime, which is called “coating degradation.” This degradation can be caused by mechanical stress (e.g., soil expansion and contraction during wet and dry conditions), chemical reactions (presence of aggressive ions such as chloride ions), UV exposure, bacterial attack (such as anaerobic sulfate-reducing bacteria (SRB) or cathodic disbonding due to over-cathodic protection.

Table 1 shows how coatings fail with time. Coatings always have some small defects when they are applied. The density of defects will increase over time.

Table 1. Guide to coating breakdown for offshore structures.

Source: L.L. Sheir, R.A. Jarman, and G.T. Burstein; “Corrosion”, Volume 2: “Corrosion Control”, 3rd edition, Butterworth Heinemann, ISBN 0-7506-1077-8, PP 10.3 – 10.28

Here, the question is whether we can produce a coating with zero defects. The answer is no in practical application but we might have a defect-free coating in theory. Producing a defect-free coating is impossible or at least not feasible on a mass production scale. Figure 1 shows how the cost of coating increases when we try to reduce the defect density.

Figure 1. Illustrates the relationship between the cost to protect a structure by cathodic protection (brown line), protective coatings (blue line), or combined methods (black line) for various coverage areas. There is an overall benefit when a good coating application is combined with cathodic protection (optimized situation).

Source: L.L. Sheir, R.A. Jarman, and G.T. Burstein; “Corrosion”, Volume 2: “Corrosion Control”, 3rd edition, Butterworth Heinemann, ISBN 0-7506-1077-8.

It is obvious that highly effective coatings cost more. When the coverage of coating is less than approximately 80 percent there is a linear relationship between cost and percent of coverage. However, if a high degree of coverage is demanded, this becomes a logarithmic relationship. It means that the cost required to increase the efficiency of the coatings from 98 to 99 percent (one percent) can be 10 times more than when we increase the efficiency of coatings from 65 to 66 percent (again one percent). Higher cost is a result of thicker coatings, the use high-tech inspection techniques and repairing individual defects. Therefore, it is not possible to eliminate all defects in reality.

Adding Supplementary Cathodic Protection

Sometimes it is advantageous to use another protection method to supplement coatings, which is why we use cathodic protection. (Learn about The Basics of Cathodic Protection.)

Let’s see the issue from a different perspective. Can we protect a bare structure with only cathodic protection?

Theoretically it is possible. However, cathodic protection of a bare structure is usually not cost effective. Even in a near neutral pH environment a considerable electric current is required to protect a structure. Table 2 shows the required current densities to protect a steel pipeline buried in various soil types. As shown, the required current density to protect a steel pipeline will be reduced more than five hundredfold when a protective coating is applied.

Table 2. Required current densities to protect buried steel structures in different conditions.

Source: L.L. Sheir, R.A. Jarman, and G.T. Burstein; “Corrosion”, Volume 2: “Corrosion Control”, 3rd edition, Butterworth Heinemann, ISBN 0-7506-1077-8.

The required current density is a function of the efficiency of the coating that is applied. Table 3 shows the required current densities to protect 10 miles of 36-inch diameter pipeline with coatings of varying quality. (Note: The required current is measured according to 0.3 V voltage drop criterion). The results show that a current of 500 A is required to protect a bare structure, whereas this current can be reduced to approximately 0.5 A (the same as a two-cell flashlight bulb) when the pipeline is coated with a fairly strong coating.

Table 3. Required current densities to protect the same coating under various conditions.

Source: "Peabody's Control of Pipeline Corrosion," 2nd Edition

When the required current density is rather high, designing a cathodic protection system based on sacrificial anodes is usually not recommended or even practical. It generally requires many large anodes.

Furthermore, cathodic protection with a high current is expensive because it requires high capacity rectifiers, non-consumable anodes, higher grades of connections and more power.

Returning to Figure 1, we see that we can reduce the cost of cathodic protection by reducing expenses. For that reason, cathodic protection is usually applied on coated structures. Buried pipelines, ships and the undersides of liquid storage tanks are usually coated. (See An Intro to Pipeline Corrosion and Coatings for more information.) Only rarely is it preferable to omit protective coatings, such as on offshore oil production platforms in the North Sea. In these cases, since the required current to protect cathodically is very low, sacrificial anodes are used and their design is based on a continuous current. When a coating is used the required current cannot be constant, because the coating deteriorates and leads to increasing the current density.

No coating is flawless, so cathodic protection is an effective complementary method to protect the points where a defective coating exposes structures to a corrosive environment.