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

22 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS under a metallurgical microscope. The specimen can be etched to deter- mine whether intergranular or uniform corrosion is occurring. Examination under an electron probe analyser can reveal whether traces of a contamina- ting metal are present in a corrosion pit. The position of alloy layers and the state of oxide films can be seen, and such information can be recorded on photomicrographs. Conductivity measurements It is often of interest to measure the conductivity of the solution under test since any factor which increases this will increase the corrosion rate by lowering the total resistance of the corrosion cell. pH Measurements The pH of a solution is an important factor controlling the rate and type of corrosion. For instance, in solutions inhibited with nitrite or chromate, uniform corrosion Occurs in alkaline solutions, whereas pitting corrosion takes place in solutions below pH 3.0 (3). Potential and pH relations determine whether a metal corrodes, becomes passive, or is immune from corrosion, and, as has been mentioned earlier, diagrams giving these relationships for all common metals have been prepared by Pourbaix (4). Polarity Relations If two different metals are in contact with a liquid they will assume different electrical potentials. If they are coupled electrically, a current will flow, and the anode will corrode whilst the cathode is protected. The current flowing and the potentials of the electrodes in such cells can be easily measured, and although the magnitude of the current flowing gives no quantitative indication of the rate of corrosion in a can, it can provide useful qualitative information on the effect of such variables as aeration of the solution, and quantity and type of inhibitors. In some aerosol cans aluminium, steel, tin and lead are all present, and the polarity relations between these metals will help to assess the probability of corrosion. Special effects such as conditions of restricted oxygen access in the bottom seam of a can may be simulated in specially designed cells.