OFFSHORE CATHODE PROTECTION

What is it and how does it work?

Why does steel corrode in sea water?

To fully understand the importance of cathode protection , the mechanism of corrosion itself must be understood  in the first place. There are three factors that underlie the occurrence of corrosion:

1. two dissimilar metals
2. one electrolyte (water with any type of salt or  salts dissolved in it)
3. a conducting path between the metals

The two different metals can be entirely different alloys such as steel and aluminum. However, it is usually a question of microscopic and macroscopic metallurgical differences on the surface of one and the same piece of metal.

If this is the case ( let us bear in mind that steel does not corrode evenly), then these will be places with increased activity and the following reaction will take place ( two iron ions plus four free electrons): 

2Fe => 2Fe⁺⁺+ 4e^-

The free electrons pass through the metal moving towards less active sites, where the following reaction takes place- oxygen in the form of gas combines with four free electrons and turns into an oxygen ion , which in turn combines with water to form hydroxyl ions:

O₂+ 4e^-+ 2H₂0 => 4 OH^-

Recombination of these ions on the active surface produces the following reaction, which lead to the formation of iron-corrosion product  –  ferrous  hydroxide.

2Fe + O₂+ 2H₂O => 2Fe (OH)₂

This reaction is more commonly described as current  flow through the water from the anode ( the more active site) to the cathode ( the less active site).

How does cathode protection stop corrosion?

Cathode protection prevents corrosion  by converting all anode  sites (active) on the metal surface into cathode sites ( passive) by means of electric current ( or free electrons) coming from an external source of electricity. This is usually in the form of galvanic anodes, which are more active than the steel. This technique is also known as a cathodic protection system by means of sacrificial anodes, i.e. the  galvanic anodes are sacrifice themselves to protect the steel structure  or pipeline from corrosion. 

In the case of alumina anodes the reaction on the surface of alumina is the following – 4 alumina ions plus 12 free electrons.

4Al => 4AL⁺⁺⁺+ 12 e^-

On steel surfaces oxygen in the form of gas, turned into oxygen ions , combines with water to form hydroxyl ions:

3O₂+ 12e^-+ 6H₂0 => 12OH^-

As long as the current ( free electrons) reaches the cathode ( steel) faster than the oxygen,  no corrosion will occur.

Figure 1: Sacrificial anode system in seawater  

Basic considerations when designing  sacrificial anode system

The electric current an anode discharges is controlled by  Ohm’s Law:

                       I = E/R

I is the current flow measured in Amps /A/

E is the difference in potential between the anode and the cathode measured in  Volts/V/

R total circuit resistance measured in Ohms /Ω/

Initially , the electric current will be high, because the  difference in potential between the anode and the cathode is big. But the flow of electric current onto the cathode will make that difference smaller, thus, resulting in gradual decreasing of the current due to the polarization of the cathode.

The circuit resistance  includes any water path ,  metal path,  and any cable in the circuit. The dominant value in this case is the resistance of the anode to the seawater. For  most applications, the resistance of the metal is so insignificant  as compared to that of water that it can be easily ignored ( this does not refer to sleds or long pipelines  protected from both ends).

In general, the long, thin anodes have less resistance than the short, thick ones. They will discharge more electric current but they are not that durable. This is the reason why a designer of a cathodic protection system must be capable of determining the size of the anodes so that they would have the right shape and surface area to discharge enough   current to protect the structure. In addition, the anodes must have the appropriate weight so as to endure the desired lifetime when discharging that current.

A general rule: The length of the anode determines how much current they will produce. Therefore, this will determine how many square meters of steel will be protected. The cross section ( weight) determines how long the anode will be capable of sustaining  this level of protection.

Impressed current Cathodic protection systems

Due to the high  currents involved in many of the systems with sea water, impressed current systems are commonly used. These systems use a type of anodes, which are not easily dissolved into metallic ions. This causes the following alternative reaction: oxidation of dissolved chlorine ions.

            2Cl^-=> Cl₂+ 2e^-

Power is supplied by an external  DC power unit.

Figure 2 : Cathodic protection system with injected current

How do we know that we have  efficient cathodic protection?

To make sure that the current value is sufficient, we have to compare the potential ( pressure) in the steel against the potential of a standard reference electrode, usually silver or silver chloride (Ag/AgCl) sea water, or sometimes  zinc (Zn sea water).

The current flow in any metal changes its potential in the negative direction. It has been  proved by experience that if the steel receives  enough current so as to change the potential to – 0,800V vs silver/silver chloride, corrosion is actually stopped.

Due to the nature of the films formed, the minimum pressure ( -0.800V) is rarely achieved, so the designers try to achieve a potential between – 0,950 V and  - 1,000V vs  Ag/AgCl sea water.

Figure 3: Measuring the potential of cathodic protection

( Unprotected – on the left / protected- on the right)

Sleds with anodes for impressed current cathodic protection

Protection of metal rabbets in harbours and shore structures with static metal surfaces by impressed current is a challenge in terms of anchoring the anodes ( iron, silica or titanium anodes). The sleds are designed for placing anodes on the seabed or riverbed.

The anode bed is held in place by counterweights , and in some cases anchors are added for better fixation, especially in cases of moving shore masses.