J.D. Thirkettle,
MICorr, MNACEG. Clapp,
MA, MScRevised by J.D. Thirkettle,
MICorr, MNACE(Senior Corrosion Technologist, ACEL/NECE)
INTRODUCTION
Cathodic protection is a dynamic electrochemical process for the prevention of metallic structures which are buried, immersed or otherwise in contact with an elec- trolyte. Structures to which cathodic protection may be typically applied include pipelines, tank farms, plant pipework, steel work, well casings, sheet and tubular pile systems, jetties, piers, offshore facilities, production platforms, and ships. In addition to the above, cathodic protection is now commonly used for corrosion prevention of above ground structural steel and reinforcing steel in concrete.
Standards and normative documentation
Cathodic protection in the UK is generally carried out in accordance with the appro- priate British Standards. These are:
•
BS 7361, Cathodic protection: Part I: Code of practice for land and marine applications•
BS 5493: Code of practice for the coating of iron and steel structures against corrosion.Other normative documentation is available from:
•
Institute of Corrosion (UK)•
National Association of Corrosion Engineers (USA)•
Society for the Cathodic Protection of Reinforced Concrete (UK)•
European Standards•
Corporate standards issued by operating organisations.European Standards are currently in draft or more advanced stages of preparation.
These are likely, in some cases, to replace or supplement British Standards. The British Standards Institute will advise whether European standards will replace British Standards or Codes of Practice.
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Theory of corrosion
Corrosion of a metallic substrate in contact with an electrolyte is fundamentally an electrochemical process. The corrosion process involves anodic areas (anodes) at which metal is converted to positive ions and cathodic areas (cathodes) at which an oxidising species, usually oxygen, is reduced. An electric current flows from the anode to the cathode through the electrolyte, and the species formed at the anodic and cathodic areas combine to form the corrosion product. The circuit is completed by current flow within the metal between the cathodic areas and the anodic areas.
A particular area of metal is determined as anodic or cathodic by the reactions taking place on it. The following must be present for corrosion to occur: anode, cathode, electrolyte and return current path.
Electrochemical and galvanic series
When two different metals are in metallic contact and in the same electrolyte, in general one metal will have a greater tendency to go into solution than the other.
The electrochemical and galvanic series are attempts to make this predictable. The two series are the potentials adopted by different metals in an electrolyte. The elec- trochemical series comprises the potentials adopted by metals when in a solution of their own ions, thus it does not have much practical application. The galvanic series comprises the potentials adopted by metals when in seawater or soil, thus cor- responding to a real situation. It is only possible to measure potential differences, as absolute values of potential cannot be obtained. In order to standardise poten- tial measurements a particular potential value is taken to be zero. One such stand- ard is the potential of the standard hydrogen electrode.
When two metals are in metallic contact in the same electrolyte the metal with the more negative potential in the galvanic series will be made more anodic and suffer accelerated corrosion; the other metal will become more cathodic and have reduced corrosion. Tables 8.1 and 8.2 show the electrochemical and galvanic series of some metals of interest in corrosion control and cathodic protection work.
The two series do not indicate the magnitude of the change in corrosion rate which will occur when two metals are connected together in the same electrolyte, but only the direction of any change.
Corrosion cell
There are two basic cell arrangements in which corrosion takes place, known as galvanic and electrolytic cells.
Galvanic cell
A galvanic cell comprises anode, cathode, electrolyte and return current path where the corrosion current flows spontaneously. The anode is negative with respect to the cathode. Galvanic cells exist not only when dissimilar metals are in contact but may be caused by one or more of the following conditions: dissimilar surface conditions;
differences in heat treatment; differences in local stresses within metal; differences in concentrations of ionic species; differences in oxygen concentrations; differences
in soil conditions. Examples where galvanic cells are created by dissimilar surface conditions and differential aeration are shown in Fig. 8.1.
Electrolytic cell
An electrolytic cell comprises anode, cathode and electrolyte, where the corrosion current flows as a result of the application of an external electromotive force. The external potential causes current to flow into the electrolyte from the anode and
Table 8.1 Electrochemical series of some metals.
Table 8.2 Galvanic series of metals. This table provides a practical guide to galvanic couples when subject to immersion in the same electrolyte.
* Typical potential, normally observed in neutral soils and water, measured with respect to a copper/copper sulphate reference electrode.
back to the power source through the cathode. The anode is made positive with respect to the cathode by the externally applied e.m.f.
This reversal of polarity of the external terminals of an electrolyte cell, as com- pared with a galvanic cell, is a source of confusion; this confusion can be avoided if it is remembered that in all types of electrochemical cells, current always enters the electrolyte from the anode and leaves the electrolyte at the cathode.
PRINCIPLES OF CATHODIC PROTECTION
In all corrosion cells, corrosion normally occurs only at anodes or anodic sites. There are, however, exceptions; these include copper and aluminium, which are ampho- teric. If an entire structure buried in the soil, or immersed in water, is made the cathode of an electrochemical cell so that the entire surface receives current from the electrolyte, the structure will not corrode. Under these conditions, the structure is said to be cathodically protected.
The cell, of which the protected structure is the cathode, may be either galvanic, with current entering the electrolyte from the galvanic anodes more electronega- tive than the protected structure, or electrolytic, with the anode electrically con- nected to the positive terminal of an external power source. In any cathodic Fig. 8.1 Galvanic corrosion.
protection system, the structure to be protected is made more negative with respect to the surrounding medium so that it receives current from the electrolyte, and this current is conducted through an external circuit to an anode system where it enters the electrolyte. The principles of protection by the galvanic (sacrificial) and elec- trolytic (impressed current) system of cathodic protection are illustrated in Figs 8.2 and 8.3.
Mechanism of cathodic protection
To understand cathodic protection one needs to consider both general and electri- cal concepts.
General concept
On the surface of any corroding structure there are cells consisting of anodic and cathodic areas. Current is discharged from the metal surface to the electrolyte at Fig. 8.2 Sacrificial cathodic protection system.
Fig. 8.3 Impressed current cathodic protection system.