corrosion protection

4.1 FACTORS THAT CONTROL THE CORROSION RATE

Certain factors can tend to accelerate the action of a corrosion cell .

These include :

(a) Establishment of well-defined locations on the surface for the anodic and cathodic reactions. This concentrates the damage on small areas where it may have more serious effects, this being described as “local cell action”. Such effects can occur when metals of differing electrochemical properties are placed in contact, giving a “galvanic couple”. Galvanic effects may be predicted by means of a study of the Galvanic Series which is a list of metals and alloys placed in order of their potentials in the corrosive environment, such as sea water. Metals having a more positive (noble) potential will tend to extract electrons from a metal which is in a more negative (base) position in the series and hence accelerate its corrosion when in contact with it. The Galvanic Series should not be confused with the Electrochemical Series, which lists the potentials only of pure metals in equilibrium with standard solutions of their ions.

Galvanic effects can occur on metallic surfaces which contain more than one phase, so that “local cells” are set up on the heterogeneous surface. Localised corrosion cells can also be set up on surfaces where the metal is in a varying condition of stress, where rust, dirt or crevices cause differential access of air, where temperature variations occur, or where fluid flow is not uniform.

(b) Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to prevent the formation of protective oxide films on the metal surface and thus increase corrosion. Sodium chloride is encountered in marine conditions and is spread on roads in winter for de-icing.

Quite small concentrations of sulphur dioxide released into the atmosphere by the combustion of fuels can dissolve in the invisibly thin surface film of moisture which is usually present on metallic surfaces when the relative humidity is over 60-70%. The acidic electrolyte that is formed under these conditions seems to be capable of stimulating both the anodic and the cathodic reactions. In practical terms it is not usually possible to eliminate completely all corrosion damage to metals used for the construction of industrial plant. The rate at which attack is of prime importance is usually expressed in one of two ways:

(1) Weight loss per unit area per unit time, usually mdd (milligrams per square decimeter per day)

(2) A rate of penetration, i.e. the thickness of metal lost. This may be expressed in American units, mpy (mils per year, a mil being a thousandth of an inch) or in metric units, mmpy (millimetres per year). Taking as an example the corrosion of heat exchanger tubes in industrial cooling water a typical corrosion rate in untreated water would be 40-50 mpy (210-260 mdd); the use of a corrosion inhibitor could reduce this to less than 5 mpy (26 mdd). The mild steel tubing used in heat exchangers is a maximum of 200 thousandths of an inch thick, thus with corrosion rates of 40-50 mpy in untreated water, severe problems might be expected within four or five years. If suitable water treatment with corrosion inhibitors is used a life of at least twenty years might be expected. This, of course, is ignoring the fact that at some time before the metal corrodes away the tubing may have thinned to a point where its required mechanical strength is not attained. When designing equipment for a certain service life engineers often add a “corrosion allowance” to the metal thickness, permitting a certain amount of thinning before serious weakening occurs. In a cooling water system the factors influencing the rate of attack are:

(a) the condition of the metal surface Corrosion debris and other deposits - corrosion under the deposits, with a possibility of pitting (severe attack in small spots)

(b) the nature of the environment pH - in the range of 4-10 corrosion rate is fairly independent of pH, but it increases rapidly when the pH falls below 4. Oxygen content - increase in oxygen concentration usually gives an increase in corrosion rate. Flow rate - increased water flow increased oxygen access to the surface and removes protective surface films, so usually increases corrosion, but can sometimes improve access for corrosion inhibiting reactants. Water type - very important, in general low corrosion rates are found with scale-forming (hard) waters. Aggressive ions which accelerate corrosion are Cl^- , SO₄^2- but quite complex interactions may occur between the various dissolved species in natural waters. number of such methods have been developed, and they are set out in Table 1. The table shows a variety of different concepts by which the surface reaction rate can be reduced. Each of these has given rise to a number of technologies, the majority of which are represented in New Zealand industry. In some cases these industries are on a very large scale. For example paint manufacture is a major chemical industry which consumes large quantities of solvents, resins and pigments. Most paint products in New Zealand are used in corrosion protection. Other major industries involved in corrosion control include electroplating, anodising, galvanising and the production of corrosion resistant alloys. In this article we will concentrate on two important methods of corrosion control used in New Zealand industry, namely cathodic protection and chemical inhibitors. Other types of corrosion control technology, such as electroplating and surface coatings, are covered elsewhere.

Table (4-1) Corrosion protection techniques

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