20 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The thermodynamic stability of a metal under various conditions of potential and pH may be seen from Pourbaix diagrams (2). These show that, in general, at a given pH, a metal will corrode, be immune, or be passive, depending upon the potential at which it is held. When a metal is immune, it is at a potential at which solution into ions is thermodynamically impossible, and when passive it is covered by a stable film which prevents further corrosion. There are some corrosion mechanisms which merit special mention. Bimetallic corrosion When two dissimilar metals in electrical contact are placed in a solu- tion, they will take up different potentials, and electrons will flow from the less noble to the more noble metal, leading to corrosion of the former, which becomes the anode of the cell. Concentration cell corrosion If access of solution to an area of the metal is restricted, concentration of the metal ion{s) in this region may be increased, leading to a rise in potential as indicated by the Nernst equation. As the adjacent regions are now anodic with respect to these areas, they will corrode. Differential aeration corrosion This is a special and fairly common case of a concentration cell where, due perhaps to design factors, access of oxygen to an area may be restricted. This then becomes anodic and corrodes. These principles are fully covered in many excellent textbooks, and the above summary is presented to give a convenient reference to terms which may be used later in this paper for those not familiar with corrosion science. CORROSION RATE MEASUREMENTS When a metal surface is uniformly corroded, the rate of corrosion is commonly expressed either as mg weight loss per square decimetre per day (mg dm -• day -•) or inches penetration per year (i.p.y.). A number of methods are available which are helpful in the assessment of corrosion rates, and some of these are summarised below.

PRINCIPLES OF CORROSION OF METAL CONTAINERS 21 Metal pick-up A test pack is made up, and after a short storage period the contents are quantitatively analysed for traces of the metals from which the con- tainer is made. Since extremely small amounts of metal can be discovered by modern analytical techniques, corrosion can be detected before it becomes evident in the can. The results obtained, can, however, be mis- leading if the rate of corrosion varies with time, since only an average value is obtained, such as when, for instance, an initially high rate of tin dissolution occurs on filling which is later stifled, and after which little further corrosion occurs. Conversely, an initially slow reaction may be subsequently accelerated after the dissolution of an oxide or alloy layer. If pitting corrosion is occurring, only a small amount of metal may be found in the contents, yet this may be sufficient to lead to penetration of the wall of the container since the corrosion is limited to a small area. This type of test is, however, useful if cautiously applied as a check to other tests. Immersion Tests Corrosion rates can be found by immersing samples of the metal con- cerned in the liquid, and finding the weight loss of the sample after removal of the corrosion product. Such tests are of doubtful value even as a qualita- tive aid since a number of effects, such as differential aeration, or metal-ion concentration cells, may occur in practice which are not present in the test set-up, and vice versa. Edge effects are also difficult to eliminate. However, as in the case of metal pick-up determination, they can provide information if their limitations are realised. Droplet tests This test is specific for water-based aerosols. It consists of covering a sheet of tinplate with a layer of liquid paraffin, and then injecting the water phase of the concentrate under the film. This simulates the conditions of restricted oxygen access which can occur in these aerosols, and provides a useful preliminary sorting test for concentrates. Metallographic examination This approach is valuable for the diagnosis of the cause of corrosion. The corroded component can be mounted in resin, sectioned, and examined