Corrosion is defined in different ways, but the usual interpretation of the term is “an

attack on a metallic material by reaction with its environment.

Corrosion of metallic materials can be divided into three main groups.

1. Wet corrosion, where the corrosive environment is water with dissolved species.The liquid is an electrolyte and the process is typically electrochemical.

2. Corrosion in other fluids such as fused salts and molten metals.

3. Dry corrosion, where the corrosive environment is a dry gas. Dry corrosion isalso frequently called chemical corrosion and the best-known example is hightemperaturecorrosion.

In general, the development of modern society and industry has led to a strongerdemand for engineers with specialized knowledge in corrosion. There are a number of reasons for this:

a) The application of new materials requires new corrosion knowledge.b) Industrial production has led to pollution, acidification and increased corrosivity of water and the atmosphere.c) Stronger materials, thinner cross-sections and more accurate calculation ofdimensions make it relatively more expensive to add a corrosion allowance tothe thickness.d) The widespread use of welding has increased the number of corrosion problems.e) The development of industrial sectors like nuclear power production andoffshore oil and gas extraction has required stricter rules and control.f) Considering the future, it should be noticed that most methods for alternativeenergy production will involve corrosion problems.

Cathodic ProtectionThe basic principle of cathodic protection (CP) is a simple one.Through the application of a cathodic current onto a protected structure,anodic dissolution is minimized. Cathodic protection is oftenapplied to coated structures, with the coating providing the primaryform of corrosion protection. The CP current requirements tend to beexcessive for uncoated systems. The first application of CP dates backto 1824, long before its theoretical foundation was established. Thischapter deals mainly with CP related to buried pipelines, an importantapplication field. Other common CP installations include buriedtanks, marine structures such as offshore platforms, and reinforcingsteel in concrete.

Measurement of instant-OFF potentials, by interrupting the CP current supply(schematic).

Sacrificial Anode CP Systems

Cathodic protection can be applied by connecting sacrificial anodes toa structure. Basically, the principle is to create a galvanic cell, with theanode representing the less noble material that is consumed in the galvanicinteraction . Ideally, the structure will be protected asa result of the galvanic current flow. In practical applications a numberof anodes usually have to be attached to a structure to ensure overallprotection levels. The following advantages are associated withsacrificial anode CP systems:No external power sources required.Ease of installation (and relatively low installation costs).

Principle of cathodic protection with sacrificial anodes (schematic).

Anodes for the protection of underground structures are buried at intervals along the structure.

They are installed in an upright position, if possible, deep enough to be in permanent moist soil.

For pipelines, the top of the anodes will be usually be approximately level with the bottom of the pipeline.

For underground protection, the anode may be packaged in a cotton bag with backfill around the anode.Alternatively, the backfill may be placed as a slurry around the anode during burial. The backfill ensures a uniform consumption of the anode and promotes ahigher current supply.Anodes used to provide protection in water should be distributed as evenly as possible over the surface of the structure. They are mounted on brackets welded orbolted to the structure, suspended on galvanized cables, or placed on sea bottom alongside the structure.

The spacing between anodes used to provide pipeline protection may varyfrom one anode every 3m to one anode every few miles depending on currentrequired by the pipeline.The normal distance from the structure at which the anodes are placed isapprox. 1-1.5m.

The design of a galvanic ground bed issimilar to impressed ground bedThe driving voltage available to forcecurrent from anode to electrolyte is theopencircuit potential less the polarized pipelinepotential.

In impressed current systems cathodic protection is applied by meansof an external power current source (Fig. 11.7). In contrast to the sacrificialanode systems, the anode consumption rate is usually muchlower. Unless a consumable “scrap” anode is used, a negligible anodeconsumption rate is actually a key requirement for long system life.Impressed current systems typically are favored under high-currentrequirements and/or high-resistance electrolytes. The followingadvantages can be cited for impressed current systems:

High current and power output range

Ability to adjust (“tune”) the protection levels

Large areas of protection

Low number of anodes, even in high-resistivity environments

May even protect poorly coated structures

The limitations that have been identified for impressed current CPsystems are

Application:1. Current from out side source is impressedon the pipeline by using of a groundbedand a power source.Advantage:1. It can be used in high soil resistivity withlarge current demand..Limitations:1. Need power source and moremaintenance work.2. Corrosion interference can be a problem.Anode materials:1. Silicon iron2. Mixed metal oxide3. Platinized titanium etc.

When impressed current is used for protection of the buried structure, the anodebed is buried at some distance from the structure. The positiveterminal of the power source is connected to the anodebed and the negative terminal connected to the structure. The resulting current is from anodethrough the soil to the structure.Transformer/rectifier is usually used to supply thedirect current.T/R: convert AC to DC and supply power to the CP system.Anodebed: Transfer the current to the environment:Reference cells and cables.

The apparatus is usually asilicon rectifier.Its output voltage depends onelectrical resistance of thecathodic protection circuit.According to requirements, itcan work under constantvoltage; constant current,potential controlled output.

Polarization: The deviation from the corrosion potential of an electrode resulting from the flow of current between the electrode and the electrolyte Polarized

Potential: The potential across the structure/electrolyte interface that is the sum ofthe corrosion potential and the cathodic Polarization

Reference Electrode: A reversible electrode with potential that may be considered constant under similar conditions of measurement. (Examples:saturated copper/copper sulfate, saturated calomel, and silver/silverchloride.)

Three primary criteria for CP of underground or submerged steel or cast iron piping are listed in Section 6 of NACE RP-169-96:-850mV CSE with CP applied without IR drop.A polarized potential of -850mV CSE100mV of polarization

CATHODIC PROTECTION OF STORAGE TANKS

Prepared by: Fatih ÖKSÜZOĞLUErgil GruopChemical Engineer(Cathodic Protection Engineer)