The nature of the corrosion process.

Corrosive attack is the result of chemical reaction at the interface between the material and the associated environment. At its simplest it can be regarded in terms of a normal bulk reaction, with the free energy for the reaction, and the thermodynamic activity (i.e. effective concentration) of the reactants providing the driving force for the process, i.e. determining the stability of the system. The actual rate at which the corrosion process occurs, i.e. the reaction kinetics, is controlled by the rates at which transport mechanisms operate within the reactants at a common interface and within the corrosion product developing between them.

The corrosion reaction is dictated by the chemical nature of the environment and the effective concentration of reactive species, whether major or minor. In some cases of corrosion by acids, the presence of oxygen is required and the degree of aeration of the system, i.e. the oxygen concentration, can be controlling in determining whether corrosion will occur or not.

As regards the material being corroded, the overall composition alone does not necessarily indicate the activity of individual elements in solution. The structure of an alloy, for example, may be very heterogeneous, with several different phases present, each of differing composition and distributed in different forms. Individual phases within metals are themselves nonuniform, the more reactive sites being associated with disorder, such as grain boundaries and structural defects produced by mechanical deformation (dislocations).

Whilst corrosion is sometimes considered only in the context of metallic materials, in the more general sense of deterioration of materials through reaction with an environment it also includes the behaviour of glasses, ionic solids, polymers, concrete, etc. in a range of environments, including electrolytes and nonelectrolytes, molten metals and gases.

Types of corrosive attack

It is difficult to classify the various types of corrosive attack. Traditionally, a broad division into ‘wet’ and ‘dry’ corrosion reactions has been employed, determined by the presence or absence of water or an aqueous solution. A more rational classification for metals is as follows:
(1) Film-free chemical interaction in which there is direct chemical reaction of a metal with its environment. The metal remains film-free.
(2) Electrolytic systems:
(a) Inseparable anode/cathode type. The anodes and cathodes cannot be distinguished by experimental methods although their presence is postulated by theory, i.e. the uniform dissolution of metals in acid, alkaline or neutral aqueous solutions, in non-aqueous solution or in fused salts.
(b) Separable anode/cathode type. Certain areas of the metal can be distinguished experimentally as predominantly anodic or cathodic, although the distances of separation of these areas may be as small as fractions of a millimetre. In these reactions there will be a macroscopic flow of charge through the metal.
(c) Interfacial anode/cathode type. One entire interface will be the anode and the other will be the cathode. Thus a metal/metal oxide interface might be regarded as the anode and the metal oxide/oxygen interface as the cathode.

In general, 2(a) and 2(b) include corrosion reactions which are normally classified as ‘wet’ while 2(c) includes those which are normally designated ‘dry’.

Chromic acid anodising and sulphuric acid anodising

Aerospace anodising methods produce a tight but porous layer which then requires a sealing process to close the pores and prevent natural attack on the aluminium. Sealing is carried out by immersing the component in near boiling de-mineralised water, which, if required, can have small quantities of other chemicals, such as Nickel acetate or sodium dichromate to further enhance the corrosion resistance of corrosion resistant aircraft components.

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