28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (5) Stern, M. and Geary, A. L. J. Electrochem. Soc. 104 56 (1957). (6) Stern, M. and Weiserr, E. D. Proc. Am. Soc. Test. Mater 59 1280 (1959). (7) Iamm, G. G. and Willey, A. R. Corrosion 17 77 (1961). (8) Hancock, P. and 3/layne, J. E. O. J. Appl. Chem. Lond. 7 700 (1957). (9) Tomashov, N. D., 3/l[kha[lovsky, Y. N. and Leonov, V. V. Zh. fiz. Khim 85588 (1961). (10) Antropov, L. I. First International Congress on Metallic Corrosion 147 (1962). (Butter- worths, London). (11) Brasher, D. M. ibid. 156. (12) Rosenf'ld, I. L. ibid. 243. (13) 3/lindowicz, J. and Puchalska, D. Corros. Sci., 4 179 (1964). (14) Tomashov, N. D. Corrosion 9,0 7 (1964). (15) Koehler, E. L. Corrosion 17 93 (1961). (16) Legault, R. A. and Walker, 3/1. S. Corrosion 19 222 (1963). (17) Antropov, L.I., Corros. Sci. 7 607 (1967). (18) Neufeld, P. Corros. Sci. 4 245 (1964). Introduction by the lecturer The polarisation resistance method described is quite useful for assessing the probability of corrosion of metal wads in a product. We have used it to decide whether wads faced with tin foil or aluminium foil would be suitable for a product, and also to compare the effect of different formulations on the facing material. We have not used, or done very little work on, potentiostatic polarisation. This is acknowledged to be a powerful method for the study of corrosion problems. One reason is that potentiostats are rather expensive instruments, and another that they are mainly used for studying active/passive systems, such as the corrosion of stainless steel. They are very useful in such contexts but stainless steel is used very rarely as a packaging component, and the metals which are used in packaging usually do not exhibit active/passive behaviour in the environments in which they are used. DISCUSSION MR. E. K. CLARKE: I gather that most of your work has been done in essentially aqueous systems. How have you made measurements in non-aqueous systems, in particular alcohol and propellant? T•E L•CTUR•R: This is certainly a difficult subject, and the measurement of potential and pH is not easy. A very high impedance voltmeter has to be used for measuring potential and the one which we have has an input impedance of over 10 ' 5 ohms. We also have to be very careful in screening all our leads to prevent drift from stray current. In this manner we are able to take measurements of a compara- tive nature, and we do not pretend to be able to measure the absolute potentials of metals. What we do is to compare several different solutions and say whether one is better than the others. DR. J. J. MAUSNER: Can you suggest a quick way of finding pin-holes in metal containers ? THE LECTURER: Pin-holing of tinplate occurs when one has a solution in which tin is cathodic to iron. Under these conditions the only solution is usually reformulation. Luckily, although tin is usually above iron in the electrochemical series, due to the

PRINCIPLES OF CORROSION OF METAL CONTAINERS 29 complexing nature of the tin ion, it is usually effectively below. One approach to this is to put something into your solution which will complex tin and lower its potential. For aluminium containers the question is somewhat more complex, because it is hard to forecast at which site of the container one will obtain pin-holes. They occur at active points, and we do not really know what causes these: it could be impurities in the aluminJure, proximity of another metal, or perhaps other effects. MR. J. M. BLAKeWAY: We have experienced corrosion in aerosol cans with hair- spray formulations and I wonder to what extent your tests can be used as a test for screening possible passivators, or possible antioxidants which might work in relatively small portions in the formulations. Is it a possible approach to the problem? T• L•CTUR•R: A formulation (such as hairspray) can be taken and small amounts of various inhibitors added. These can be organic or inorganic, but the former is the most likely for a hairspray. Corrosion rates caused by different types, and levels, of inhibitor can then be compared, and the most favourable selected from the point of both bimetallic corrosion and direct attack. The effect of other variables, such as water concentration, can also be assessed. MR. T. A. BRocK: Some years ago I was associated with a hair spray where the container which was intended to be used was perfectly satisfactory in the preliminary screening tests. When the product went into bulk manufacture a very high per- centage of the cans developed pin-hole corrosion after a very short time. At that time, the container manufacturer suggested that perhaps the cans that were used in the test had aged, and an oxide film had developed on the surface of the aluminium, whereas the ones used in production were straight off the fabricating lines, and there- fore did not have the chance to develop this protective oxide film. Would you like to comment? T• Luc'ruR•R: This argument can be applied both ways. It can also be said that if a large area of oxide film was present this would act as a large cathodic area and pin-hole corrosion could occur at the places where the film was not complete. A more likely explanation of this would be based on the difference in the oxygen content of the test and production cans. An aerosol storage test is not realistic unless it can be filled on a production line, and you get the sanhe purging conditions as you will in practice. In the laboratory these conditions could be quite different, and they are usually better than on the production line. If the purging conditions in the laboratory were very good and there was more oxygen in the production cans this might be enough to cause corrosion. MR. E. K. CLARKe: HOW do you measure potentials and currents in an aluminium container? If one is thinking of using an aluminium container with an aluminium cup a single metal system results. T}m L•CTUR•R: We have done much less work on aluminium than tinplate, mainly because we use far more tinplate containers than aluminium. A relevant test in this case is to look at the direct attack on aluminium by polarisation resistance measurements. Unfortunately this does not take into account the possibility of pin- hole corrosion of aluminium which is rather a special problem, as the origin of active anodic sites leading to the incidence of pin-holes is sometimes uncertain.