PRINCIPLES OF CORROSION OF METAL CONTAINERS 25 problems occurring in packaging. Each case has its own requirements, for which relevant tests must be selected. The results must be interpreted both with a knowledge of the limitations of the test, and in the light of informa- tion gained from other tests. The amount and reliability of corrosion data which can be deduced will vary considerably from one case to another, depending on the complexity of the system under review. We have found that the best approach in the first instance is to establish the polarity relations between the metals occurring in the system. This is conveniently done by using a cell in the form of an inverted T. Metal plates can be sealed at either end of the cross-piece, and the 'stem' is used to fill the cell, and to give access for a reference electrode, if required. The magni- tude and direction of the current flowing between the plates can then easily be measured. A similar cell has been used by Koehler (15) for potentio- static studies. If a reference electrode is used, polarisation resistance measurements may be made at the same time. When this basic information has been obtained, together with conductivity and pH measurements, other tests can be applied if considered necessary. Potentially dangerous situations occur when a large cathodic area is in contact with a small anodic area, for under these conditions anodic attack is intensified due to the disproportionate sizes of the areas. For instance, a large area of oxide film on a metal can act as a cathode, although the forma- tion of such films is less likely in organic solutions. Tinplate consists of large areas of tin in contact with small areas of tin/iron alloy or steel at flaws in the tin coating, and if these flaws are anodic, pinhole corrosion will occur. However, although tin is more noble than iron in the electrochemical series, it is fortunately anodic to steel in many solutions, and consequently there is only slow solution of the tin since there is so small an area available for the equivalent cathodic reaction. If the presence of tin in the product is undesirable it may be considered necessary to apply a lacquer coating, but this should be done with caution since in some cases corrosion could become more acute. Lacquer should not be used as a protection if the tin is cathodic to steel, since most com- mercially applied lacquer systems contain flaws to a greater or lesser extent, and pinhole corrosion could still occur. Given a new product and container combination it is not usually possible to state whether or not it will be satisfactory in practice only from these tests. Their main value lies in providing comparative results, from which the most promising of a number of formulations may be selected. In

26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS some cases comparisons may be made with packs already known, by experience, to be satisfactory. PREVENTION OF CORROSION This is certainly the most rewarding aspect of corrosion studies, but also the most difficult. A corrosion reaction can be controlled by inhibiting either the anodic or cathodic reactions or both. This can be effected either by modifications to the pack or the product or both. Different materials or protective coatings may be used for the pack, and less aggressive chemicals used in, or inhibitors added to, the product. The choice of material for the container is often only between tinplate and aluminium, but a wide variety of inhibitors is available. Corrosion of aluminium occurs in solutions of high acidity or alkalinity. Chlorides, especially if a depolariser such as hard water is present, will cause pitting corrosion in neutral solutions particularly if these have a low electrical resistance. These aggressive halides are sometimes produced by the hydrolysis of the chlorofluorinated hydrocarbon propellants often used in aerosol systems. Traces of other metals should be avoided, especially copper, and also salts of heavy metals, since these readily cause bimetallic corrosion. Lower alcohols and phenols are sometimes safe to use only with a critical range of water content, above or below which corrosion will occur. Tin also corrodes in solutions of high acidity or alkalinity. Local cor- rosion in neutral solutions can occur in salt solutions which do not form insoluble salts with tin. The corrosion of tinplate is not straightforward due to such effects as the inhibiting action on mild steel of dissolved tin, and the reduction of tin ion concentration by complex formation. The action of many inhibitors in preventing corrosion is not yet perfectly understood, but they can be broadly classified into two categories, viz. organic and inorganic. Organic inhibitors are often specific to the metal they protect {17). They are thought to function by adsorption on to the metal surface, providing a protective layer which greatly reduces the corrosion rate, although the corrosion potential of the metal is little changed. Inorganic inhibitors stimulate the formation of a passive oxide film, which stops metal dissolution, and usually causes considerable changes in the corrosion potential until a passive potential is reached. The amount of inhibitor added is critical since in some cases (e.g. chromate and aluminium) if insufficient is used, corrosion may actually be accelerated. A very large number of corrosion inhibitors is known, and the choice depends on both