XI Международная студенческая научная конференция Студенческий научный форум - 2019

The first practical use of cathodic protection is generally credited to Sir Humphrey Davy in the 1820s. Davy’s advice was sought by the Royal Navy in investigating the corrosion of copper sheeting used for cladding the hulls of naval vessels. Davy found that he could preserve copper in sea water by the attachment of small quantities of iron or zinc; the copper became, as Davy put it, “cathodically protected”.

Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte. In practice its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion.

Principles of cathodic protection

Corrosion in aqueous solutions proceeds by an electrochemical process, and anodic and cathodic electrochemical reactions must occur simultaneously. No nett overall charge builds up on the metal as a result of corrosion since the rate of the anodic and cathodic reactions are equal.

Anodic reactions involve oxidation of metal to its ions, e.g. for steel the following reaction occurs.

Fe > Fe2+ + 2e

(1)

The cathodic process involves reduction and several reactions are possible. In acidic water, where hydrogen ions (H+ ) are plentiful, the following reaction occurs.

2H+ + 2e > H2

(2)

In alkaline solutions, where hydrogen ions are rare, the reduction of water will occur to yield alkali and hydrogen.

2H2O + 2e > H2 + 2OH-

(3)

However, unless the water is deaerated reduction of oxygen is the most likely process, again producing alkali at the surface of the metal.

O2 + 2H2O + 4e > 4OH-

(4)

Methods of applying cathodic protection

Cathodic protection may be achieved in either of two ways. By the use of an impressed current from an electrical source, or by the use of sacrificial anodes (galvanic action).

Protection potentials

In practice, the structure-to-electrolyte potentials are measured using a standard reference electrode based on copper/copper sulphate, silver/silver chloride, or pure zinc. The reference electrode should be very close to the surface whose potential is being measured. For steel in an aerobic electrolyte of nearly neutral pH a commonly accepted protection potential is -850 mV; when exposed to sulphate-reducing bacteria a potential of –950 mV would be required. Both values are referred to a copper/copper sulphate electrode.

Coatings

The provision of an insulating coating to the structure will greatly reduce the current demand for cathodic protection. When first applied, coatings will often contain flaws, and in service, further defects will develop over a period of time. The conjoint use of coatings and cathodic protection takes advantage of the most attractive features of each method of corrosion control. Thus, the bulk of the protection is provided by the coating and cathodic protection provides protection to flaws in the coating. As the coating degrades with time, the activity of the cathodic protection system develops to protect the deficiencies in the coating. A combination of coating and cathodic protection will normally result in the most economic protection system.

Choice of cathodic protection system

In the design of a cathodic-protection scheme, a decision must be made as to whether the scheme should be a sacrificial anode or impressed-current system or a mixture of the two systems.

Sacrificial anode systems have the advantage of being

(a) simple to install,

(b) independent of any source of electric power,

(c) suitable for localised protection,

(d) less liable to cause interaction on neighbouring structures.

Anode resistance

One of the most important parameters in the design of cathodic protection systems is the electrical resistivity of the environment. Resistivities encountered for pipeline environmentsvary from 1 ohm cm for brackish river water to greater than 500,000 ohm cm in non-porous granite. Measurement of the resistivity of the environment and calculation of the electrical resistance between the anodes and the structure due to the electrolyte must be made at an early stage in the design of the scheme to ensure that adequate current output will be obtained from the anodes over the design-life of the structure (by application of Ohm’s Law).

References:

1. A.W. Peabody, Peabody's Control of Pipeline Corrosion, 2nd Ed., 2001, NACE International

2. Ashworth V., Corrosion Vol. 2, 3rd Ed., 1994

3. Roberge, Pierre R, Handbook of Corrosion Engineering 1999

4. NACE International Paper 09043 Coatings Used in Conjunction with Cathodic Protection - Shielding vs Non-shielding Coatings

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