The mechanism of the corrosion reaction is outlined and the influence of external physical and chemical factors on the reaction is briefly mentioned. From this preamble it follows that surface coatings can combat corrosion by one or more of three main factors. These being physical barrier, chemical inhibition and electrical effects. The factors are discussed and correlated with the performance of modern film formers The surface coatings described include alkyd, polyester acrylics, vinyls, silicones, epoxies, urethanes and silicates. The mechanism of film formation of zinc silicates is given and their performance as anti-corrosive coatings is explained.
Combatting corrosion is a major factor in the efficient operation of petrochemical complexes. The purpose of this paper is to:
Define and explain in a general and elementary way the corrosion process.
Explain the ways of preventing corrosion with particular reference to surface coatings.
Corrosion is the electrochemical reaction which leads to the destruction or deterioration of metals. The electrical energy needed for the reaction to proceed may be obtained from two sources:
This occurs when two dissimilar metals are placed in electrical contact with one another.
This occurs when an external source of current supplies the energy for the reaction.
Dealing with these two types of corrosion reactions separately:
Here the reaction which occurs at the anode is:
(Figure 1 available in fullpaper)
The latter reaction occurs if oxygen is absent or when oxygen is present under certain conditions, e.g. at low pH, but the result in both instances is to produce a local preponderance of OH-. At the point where Fe2+ and OH- meet rust is formed and this can take several forms as outlined in Figure 2.
These reactions occur when electrical energy is obtained from a galvanic couple, i.e. when two dissimilar metals are placed in contact with one another there is a potential difference between them and this is a measure of the. driving force needed to cause the anodic reaction to occur. This potential difference can be measured accurately and since these potentials cannot be measured absolutely they are usually referred to a standard which is hydrogen. This leads to the formation of the Electromotive Series shown in Table I.
The potential varies with the concentration according to the well-known themodynamic relationship according to Nernst:
(Equation I and II available in fullpaper)
When delta F is negative then the reaction will occur spontaneously. From this it can be seen that thermodynamic considerations will determine if a corrosion reaction will proceed or not but they will not determine the rate at which the reaction will occur.