Practical Considerations for Installing ICCP Systems

Anodes made from materials such as listed in Table 10.5 are capable of supplying high current densities and it would be possible to protect a structure with a few large anodes supplied with a high current. However, in practice anodes are usually distribute at regular intervals over the whole structure. This is because: -

The high current density that would be present in the immediate vicinity of a single anode could damage paint surfaces and possibly cause embrittlement as previously discussed.

o The use of more anodes reduces the current density for each one and reduces the probability of this type of damage

Offshore structures have a reasonably complicated geometry that makes it difficult for corrosion engineers to predict the total distribution

potentials. Therefore it is prudent to use more anodes, each one

protecting a smaller area thus minimising the areas at risk of inadequate protection

Tuition Notes for 3.1U Course Chapter 10

o When designing the system should the corrosion engineers have any doubts about protecting any particular area of the structure sacrificial anodes may be installed to work in conjunction with the ICCP system

The ICCP system installed on the Claymore platform was designed to provide 160 mA m^-2 utilising 55 platinum-iridium anodes and 12 reference electrodes.

Also the Murchison platform uses 100 anodes and 50 reference electrodes. In general in the North Sea the most common anode materials are Platinum sheathed Titanium and Lead/Silver alloys.

It is vitally important that the power supply is connected with correct polarity.

The negative terminal must be connected to the structure and the positive terminal must be connected to the anode. Should these connections be reversed the structure would corrode catastrophically. Figures 10.6 and 10.7 refer.

Figure 10.6

Diagrammatic Layout of an

Impressed Current Cathodic Protection System

Tuition Notes for 3.1U Course Corrosion Protection

Figure 10.7

Impressed Current Cathodic Protection Distribution of Anodes and Dielectric Shield

As indicated in paragraph 2.2.1 the actual distribution of the anodes on any structure may be either: -

Platform Based

Here numerous anodes are attached to the structure at intervals around it in similar fashion to sacrificial anodes but ensuring that they are insulated from the structure. Figure 10.7 refers.

o Two problems are associated with this method. One is the possibility of ‘shadow’ areas where inadequate protection is provided. This problem can be solved by the use of sacrificial anodes complementing the ICCP system as indicated earlier. The second problem is the possibility of current flowing directly from the anode to the adjacent structure. This could cause

embrittlement as discussed earlier and to avoid this dielectric shields are employed to insulate the structure electrically. Also the current is limited by design because each anode is positioned to provide adequate protection for the local area only. This limits as well the possibility of embrittlement and coating damage. See Figure 10.7

o There is also a diver safety consideration in that these anodes are at about 80 V potential with some 1000 A current. If divers are employed adjacent to any of the anodes they should be isolated from the system

Remote from the Structure

A number of anodes may be placed on the seabed at a designated distance from the structure.

Tuition Notes for 3.1U Course Chapter 10

o This method avoids the possibility of current flowing directly from the anode to the adjacent structure but there being fewer anodes the current density is higher and therefore there is still a possibility of coating damage and embrittlement.

o As discussed in paragraph 2.2.1 design considerations generally favour more anodes distributed around the structure.

o There is a safety issue with divers but as the anodes are some distance away from the structure it may be possible to ensure safety by imposing an exclusion zone around the anode. See Figure 10.8

Figure 10.8

Diagram of ICCP System with Anodes Remote from Structure 2.2.2 Reference or Control Electrodes

These electrodes are commonly zinc, silver/silver-chloride (Ag/AgCl) or (SCC) or copper/copper-sulphate (CSE). CSE is favoured in applications with

reinforced concrete.

Reference or control electrodes are vital components of any ICCP system. They determine the current required from the power source, without these items the system cannot provide a quantifiable degree of protection. Figure 10.9 refers.

Tuition Notes for 3.1U Course Corrosion Protection

Figure 10.9

Zinc Reference Electrode Installed On an Offshore Structure 3 Using Coatings to Protect the Structure

Coatings form a barrier between the electrolyte and the surface of the protected structure. They may be paints, organic films, varnishes, metal coatings or

enamels and even sheathing. It is surprising how effective coatings can be when consideration is given to the thickness of a typical paint coat. This may be only in the order of 25 to 100 microns thick for some applications.

3.1 Paints

When paint is applied to a metal surface it presents a barrier to air, moisture and ions aggressive to the metal. However, paint cannot provide a complete barrier to oxygen or water. In time these will penetrate through to the surface of the metal. Any paint system used underwater must have a strong bond onto the metal surface and therefore high quality metal surface preparation is required such as SA 3. The bonding between successive coats must also be strong and the topcoats must provide as impervious a barrier to the electrolyte as is

possible. This last is achieved by ensuring the constituents making up the topcoats have very low water absorption and transmission coefficients.

Coal Tar Epoxides are used extensively on offshore structures. They consist of coal tar and epoxide resin for the binder. These coatings are highly impermeable to water and resistant to attack by most chemicals and hydroxyl ions (that are produced by the cathodic reaction)

Tuition Notes for 3.1U Course Chapter 10

Zinc coatings utilising a combination of zinc dust and complex silicates with a solvent-based self-curing binder give good protection to steel surfaces. These coatings are frequently over-painted by another system and are used on components such as ladders in a marine environment Concrete is used to provide a protective coating to pipelines where it provides a passive environment for the steel pipe as well as adding weight.

Metallic coatings such as galvanising, using zinc impose a continuous barrier between the metal surface being protected and the surrounding environment. These coatings may be applied in a number of ways.

Electroplating utilises a bath of salts as an electrolyte. The component and rods of the plating metal are immersed in the electrolyte and a potential is applied between the component and the rods. The

component becomes the cathode and the rods the anode so metal ions of the plating material deposit from the solution onto the component Hot dipping involves the component being immersed in a bath of molten coating metal. Galvanising is accomplished by this method. See Figure 10.10

Figure 10.10 Galvanising

Spray coats utilise a specialised torch that is fed with wires of the coating metal that are melted and blown out by it. The molten metal is expressed in the form of droplets travelling at 100 to 150 m s^-1 that flatten and adhere on impact with the component

Cladding uses metal skins laminated onto the component. The skin can be applied by

o Rolling

o Explosive welding

o Buttering (building up a welded coat on the surface to be protected)

o Sheathing

Tuition Notes for 3.1U Course Corrosion Protection

Aluminium roll-bonded to duralumin is marketed as Alcad.

o Some offshore risers are sheathed with Monel (cupronickel) See Figure 10.11

Figure 10.11 Monel Cladding on

an Offshore Riser

Diffusion requires the component to be heated to just below the melting point of the coating metal in the presence of the coating in powder form and in an inert atmosphere. The component is allowed to ‘baste’ for several hours and the coating diffuses into the surface of the component.

4 Inhibitors (Controlling the Electrolyte)

Remember the Pourbaix diagram indicates three methods for preventing corrosion: -

Making the electrode more positive Making the electrode more negative Changing the electrolyte pH

This section will outline methods for changing the electrolyte.

Also remember there are four processes in metal corrosion: - The anodic reaction

The cathodic reaction

Ionic conduction through the electrolyte Electron conduction through the metal

Tuition Notes for 3.1U Course Chapter 10

Only the first three are affected by the electrolyte, electron conduction through the metal is not considered here. The properties of the electrolyte that can be affected by using inhibitors are: -

The conductivity of the electrolyte The pH of the electrolyte

The interaction of the electrolyte with the metal surface, attacking or strengthening passive films

As an example of how this can be achieved consider steel in seawater. If distilled water is substituted for the seawater the conductivity and pH of the electrolyte is reduced and a passive film will form on the surface of the steel.