PRINCIPLES OF CORROSION OF METAL CONTAINERS 23 Polarisation Resistance When a metal is placed in a solution, it assumes an electrical potential with respect to the liquid. if a current is now passed between the metal and a counter electrode, the potential of the metal can be changed by a small amount. Stern and Geary (5) have shown that the magnitude of the current required to polarise the metal by a small amount (5-10 mV) is proportional to the rate of corrosion of the metal. A convenient form of the relation is given by:-- A I 2.3 Icorr ([la + AE where is the "polarisation resistance", although this should not be AI thought of as a real resistance, as it is only so called since it has the appropriate dimensions. [la and [•c are the slopes of the logarithmic local anodic and cathodic polarisation curves. These are generally unknown for a particular case, but according to Stern and Weisert (t3) the majority of [I values lie between 0.0t3 and 0.12 V, xvhich would provide a value for the corrosion current, I corr, to within about 20%. This view has been con- firmed by Neufeld (18). Corrosion rates in mg dm q day -• or i.p.y. can quickly be calculated from the corrosion current for any particular metal. This method provides a rapid and convenient means of finding the corrosion rate. A determination will take no more than I{0 s, and so any change of corrosion rate with time can be found. Measurements can be made in glass apparatus under controlled conditions with electrodes made from appropriate metals, or in some cases electrodes can be mounted in the actual containers themselves. it is difficult to make polarisation resistance determinations in some non-aqueous liquids since their low conductivity makes potential measure- ments difficult. The electrodes described by Neufeld (18) are helpful in this instance since the liquid path between reference and working electrode is small, and there is no risk of contamination of the solution by electrolyte from the reference electrode. Other tests A number of other test methods have been evaluated for use in corrosion testing in packaging, but which we do not use for various reasons, such as excessive cost of apparatus, difficulty and length of experimental methods,

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS restricted application, and uncertainty in the interpretation of results. Such techniques include the alloy-tin couple test of Kamm and Willey (7), the anodic polarisation method of Hancock and Mayne (8), the over-voltage- intercept method, potentiostatic polarisation, electrical capacity deter- minations as used by Tomashov, Mikhailovsky and Leonov (9), and zero charge potential studies as described by Antropov (10), and Brasher (11). The equipment needed for the above methods is not complex. The main requirements are a high impedance millivoltmeter, a microammeter, an adjustable low current source such as that described by Tomashov et al (9), an accurate balance, and a metallurgical microscope. CORROSION IN THE VAPOUR PHASE Little is as yet known of the mechanism of corrosion in the vapour phase, although research is being pursued (12 - 14). Unless the atmosphere is quite dry, a thin film of moisture is always present on the surface of metals at room temperature. The thickness of this film varies according to humidity, but is usually about one micrometre thick. It is, of course, very much thicker in atmospheres of 100% RH when physical condensation occurs. Electrochemical corrosion processes occur in these thin films, but they are somewhat different from those occurring in bulk solutions of the same composition, since such controlling factors as gas diffusion, diffusion coefficient of dissolved substances, and dielectric constant are considerably modified by action of surface forces on the water molecules, and other intermolecular reactions. Inhibitors for vapour phase corrosion are known, although their mode of action has not yet been established. Such an inhibitor must have an appreciable vapour pressure so that it is transferred to the metal surface, which it must then protect. One theory is that the inhibitor undergoes hydrolytic dissociation, the products of which evaporate and recombine in the water film (13). Electrochemical processes in thin water films (160-350 [•m) have been studied by Rosenf'ld (12) and thinner films (about 1 Mm) by Mindowicz and Puchalska (13) using a microelectrode. This technique is as yet not well developed, but may eventually lend itself to evaluation of vapour phase corrosion in a metal container. APPLICATION OF METHODS No single test will provide adequate information for all corrosion