November 28, 2011

What is Corrosion ?

Corrosion is the inadvertent electrochemical destruction of metal, proceeding from surface, by the effect of chemical or electrochemical reactions.

Most metallic materials in general use are unstable towards their environment, and have a marked tendency to revert to their original, non-metallic state. This revision would occur more frequently from the energy point of view. If it were not inhibited by various natural and artificial, kinetic restraints.

In the event of failure of any one of these inhibiting factors, however, the metal reverts freely to its non-metallic state with the release of electrons.

Cathodic protection consists in the counteraction of this process of electrochemical destruction of the metal by means of an electron flow of at least equivalent voltage and strength (impressed current). Cathodic protection is not therefore a form of electrolysis, in which metal already in solution are deposited, but rather the compulsory maintenance of the existing metallic state. From the physical point of view, every electrochemical process requires:
  1. An electrolyte
  2. Two electrodes connected by a metallic conductor
  3. A potential gradient between the electrodes

The current flow in these galvanic corrosion cells is dependent on the potential gradient, the resistance at electrodes, the polarization of the electrode surfaces and the resistance of both the electrolyte and the metallic conductor connection.

Processes At The Electrodes.

When iron corrodes in water, the metal dissolves as a divalent and releases two electrons:

This shows that there is a current flow (electron flow) during corrosion, although the metal appears currentless. The electron current flows inside the metal from the micro anode to the micro cathode which is connected to it by short circuit. With uniform surfaces the anodic and cathodic current densities are the same.

Acceleration or retardation of the process at the anodes will affect the process at the cathode and vice versa. Conditions encouraging corrosion could be:
  • An increase in the salinity of the water, which is equivalent to a decrease in electrolyte resistance and increase in conductivity.
  • An increase in the temperature and in the current velocity in the water.
  • A surface with a non-uniform covering of mill scale or paint or a combination of both.
  • Thermally induced changes in the microstructure in the vicinity of the weld seams.
  • Mechanical distortions of fastenings and connections with electrochemically nobler metals.
The primary process at the anode, the dissolution of the iron, is accompanied by simultaneous acceptance at the cathode of the ions which have migrated towards it, by means of an “electron acceptor”

This acceptor can be either the hydrogen ion of the water only, in which case molecular, gaseous hydrogen will be produced via the atomic hydrogen according to the reaction:

Or the oxygen dissolved in water, which reacts as follows:

In both cases, the action of OH increase the alkalinity at the cathode. If oxygen present, reaction will be preferred. With no oxygen present, or after oxygen has been consumed. The availability of oxygen at the cathode of a corrosion cell and the rate of electron transfer determine the dissolution processes at the anode.

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