Cathodic protection technique

The cathodic protection technique for mitigating corrosion is based on various historical performances and measurable results. Cathodic protection is applied to control the corrosion of a metallic structure in the particular environment to which it is subjected. The key principle of cathodic protection is based on a physical phenomenon where the corrosion of the metal is completely stopped under the effect of electric current. When a measured amount of current is applied, the electrochemical corrosion protection system makes the metal structure a single non-destructive cathode. There is hence no potential available for flow of current, and the metal is protected. This is achieved by applying the current from an outside source. When enough current is applied, the whole structure comes to a uniform potential, thus making anode and cathode sites non–existent.

Experience stipulates that if the steel receives enough current to shift the potential to (–) 0.800 V v/s silver/silver chloride, the corrosion stops completely. In general, a potential between (–) 0.950 V and (–) 1.000 V v/s Ag/Agcl (standards reference electrode against steel) is recommended.

Two systems are used for cathodic protection:

— Galvanic protectors (galvanic system), where more electronegative metal (protectors) are connected to the metallic structures;

— Impressed current cathodic protection system, where the metallic structure is connected to the positive terminal of the direct current (DC) source.

These systems are described in more detail in the following paragraphs.

7.2.2.1. Galvanic system

The galvanic anode system employs more negatively reactive metal as an auxiliary anode that is directly connected to the metallic structure required to be protected. In other words, the auxiliary anode sacrificially protects the structure/piping. The system is therefore also called the ‘sacrificial anode cathodic protection system’. The most commonly used sacrificial anodes include zinc, aluminium and magnesium.

The galvanic anode protective method is considered one of the simplest and most reliable methods, requiring no maintenance during the course of operation. The metals mentioned above are sometimes also alloyed to give these anodes more negative electrolytic potential. Experience has shown that in addition to passive protection methods such as various varieties of paint coatings, the galvanic anode protection system provides more effective corrosion control. The use of a well applied and suitable paint coating increases the effective spread of cathodic protection current. The combination of applying both the protective coating and cathodic protection results in the most practical and economic corrosion protection system as a whole. The best results are obtained with coatings that have high electrical resistance, are continuously well spread, and are strongly adhered to the surface of the component to be protected. The life of the combined protection system is further extended by abrasion resistance qualities of the coating system.

The galvanic protection system can be summarized as follows:

— Simple to install;

— Independent source of DC power not required;

— Suitable for localized protection;

— Neighbouring structures/components are not affected.

Experience to date with such a protection system for isolated fore bay components of the intake water pump house and a short length of buried piping has been satisfactory. The major disadvantage of the galvanic system is the larger size and weight of anodes. The current output of galvanic anodes is also relatively less. This mainly depends on the electrical resistively of the electrolyte, e.g. subsoil, sea water or concrete. The current from the anode is not controllable, hence any change in structural behaviour or in resistivity of the medium during operation, or any general deterioration of the coating, has a larger impact on the consumption of sacrificial anodes. This may also result in slight miscalculations while designing the system.

7.2.2.2. Impressed current cathodic protection

The impressed current cathodic protection system uses the same principle as the galvanic protection system.

Current is directly applied with a polarity that opposes the natural corrosion mechanism and with sufficient magnitude to polarize all the cathodic areas up to open circuit potential of anodic areas to arrest the corrosion. The structure in this system is protected by applying a pre-measured current from the anode. The anode and the structure to be protected are connected by well-insulated wire as a carrier to the current. The primary difference between the galvanic and impressed current system is that a properly designed cathodic protection system with impressed current can completely prevent the corrosion of submerged metallic structures. It is, however, necessary to maintain optimum levels of impressed current at all times, if the cathodic protection system is to be made effective.

Continuous monitoring of the protected structure to soil/sea water potential and automatically adjusting the impressed currents to keep structure-to-soil potential constant becomes imperative, if optimum corrosion protection is desired. The automatic cathodic protection rectifier system accomplishes this by monitoring the protected structure-to-soil potential with reference to a Ag-AgCl saturated half-cell or a zinc reference electrode, and continuously adjusting the impressed current automatically to maintain the metallic structure-to-soil potential at a preset optimum. Thus, optimum cathodic protection can be achieved under varying conditions by use of automatic cathodic protection rectifiers. The galvanic system has to rely on the natural potential difference between the anode and the structure, whereas the impressed current system uses an external power source to derive the current. This system can therefore be regarded as an advancement.

The impressed current system requires:

— An inert anode or a cluster of such anodes connected to the various components to be protected;

— A DC power source;

— Well-insulated and secured conductors between anodes and electrical source with minimum resistance;

— Well-secured connections with the power source and structure with minimum resistance.

The anode material could be a high silicon cast iron, mixed metal oxide or high strontium–chromium-iron electrodes. Non-consumable electrodes made out of titanium, niobium or platinum are considered techno-commercial.

The cathodic protection unit (arrangement shown in Fig. 94) is designed to operate on AC single phase, 50 Hz power supply to a DC rectifier which gives a continuously filtered DC voltage in the range of 0–12 V and current up to 100–150 A. This unit can operate either manually or in automatic mode, depending on the situation.

Automatic protected structure to sea water potential (APSSP) mode and automatic voltage and current control (AVCC) mode are generally used in combination in the automatic impressed current system. It is unlikely that both these automatic modes will fail simultaneously. However, the unit can also be operated in manual mode under such situations, with a built-in alarm mechanism.

It has also been experienced that the impressed current cathodic protection system exhibits far better results if used together with the passive protective coatings, as in the case of galvanic protection.

As an example, it can be highlighted that the Tarapur Project (India) sea coast environment is very hostile.

Most of the fore bay equipment (shown in Fig. 95), such as embedment for isolation gates, coarse screens, trash racks, travelling water screens, circulating pumps, associated pipelines, stairs and handrails is subjected to a continuously hostile saline atmosphere. Use of a good quality paint system along with an impressed cathodic protection system has been found to be very effective against corrosion. In order to verify the effectiveness of the system, sacrificial anodes have been added at a few locations that are most vulnerable to corrosion. Monitoring of the actual loss of weight in sacrificial anodes with regard to the designed value verifies the effectiveness of the impressed current cathodic protection system.

The flow diagram shown in Fig. 96 can be best used to describe the cathodic protection system required.

FIG. 95. Fore bay equipment.

Motor Control Centre

Transformer Rectifier Unit

Anode Distribution Box

Potential Monitoring Station

Cathode Structure

Anode Junction Box

FIG. 96. Cathodic protection system flow diagram.

Advantages:

The major advantages of the impressed current cathodic protection system are:

— Ability to supply relatively large amount of current;

— Provides high DC driving voltage, hence can be used in all types of electrolytes;

— Ability to provide varying output that may accommodate the changing behaviour of the structures being protected;

— Requires fewer electrodes to cover large areas.

Disadvantage:

— This system is, however, more expensive than the galvanic system. Transformer rectifier output can be displayed remotely in the control room and, as an advancement, the cathodic protection system can be monitored remotely.

— As a rule of thumb, if the current requirements are low, the sacrificial anode cathodic protection system is chosen. However, if much larger areas are to be covered, and if the resistivity of the medium is high and current requirements are large, the impressed current cathodic system, shown in Fig. 97, is selected.

Choose type of cathodic system

Review resistivity data

Select anode material, weight, dimension and conducting medium

Number of electrodes required to meet current density limitations

Select layout of electrodes depending on equipment layout

Work out life expectancy and economics

Prepare detailed design and specifications

FIG. 97. Improved cathodic protection system flow diagram.