j. Soc. Cosmetic Chemists 20 17-29 (1969) ¸ 1969 Society of Cosmetic Chemists of Great Britain Principles of corrosion of metal containers by cosmetic products D. M. HOWARD* Presented at the symposium on "Technical developments in cos- metics packaging", organised by the Society of Cosmetic Chemists of Great Britain, at Harrogate, Yorks., on 27th March 1968. Synopsis--Mechanisms of corrosion reactions are briefly surveyed, and several experimental methods of finding corrosion rates are summarised. The use of those which have been found to have application to packaging problems are described and methods of approach to such problems are indicated. INTRODUCTION The problems set by the tendency of metal containers to be corroded by their contents are not easy to resolve, for not only are there a variety of different metals and alloys used in packaging, sometimes in contact with each other, and in varying states of purity and stress, but also the sub- stances which they are expected to hold are often mixtures of a complex chemical nature. Our object in initiating work on corrosion was to obtain more knowledge of corrosion processes in general and, in particular, those liable to occur in metal containers, especially aerosol cans. It was hoped that insight into these processes would lead to a reduction in both the size and duration of the storage tests for new products and containers, and would also provide means of solving the corrosion problems which arose. Two steps were necessary for the achievement of this object. Diagnostic experimental techniques had to be developed so that the rate of corrosion of a container *Pack Development Laboratory, Reckitt & Sons Ltd., Hull, Yorks. 17

18 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS by the formulation could be measured fairly quickly, and methods had to be developed to combat corrosion where it was found to be occurring. We have not yet achieved this ideal. However, the object of this paper is to review some of the methods and procedures which have been evolved and which, if used carefully and interpreted in the context of each particu- lar problem, give us some insight into the type of corrosion mechanisms involved and help us to consider more formulation variables than could practicably be screened by using conventional storage test methods. The principles described apply not only to the packaging of cosmetics, but also to many other classes of product. However, cosmetics do not normally present any special problems which do not also occur in other fields, and the general remarks which follow are relevant in most contexts. PRINCIPLES OF CORROSION Most metals are to some extent unstable in their pure state, and tend to revert to a more thermodynamically stable condition by combination with other substances. The most common way in which this occurs is by an electrochemical process, and in this way corrosion products are formed. The corrosion reaction can be considered as consisting of an anodic reaction, in which metal ions are formed from the solid metal M--•M z+ q- ze ..... 1 and a cathodic reaction which uses the electrons formed by the anodic reaction, and could be 2H + q- 2e-• H2 ..... 2 in acid solutions, or 2 H•O d- O• d- 4e--• 4OH- . .... 3 in neutral solutions. These are simple examples, and anodic and cathodic reactions are often of a more complex nature. In order that these reactions take place, there must be electrical contact between the sites on which they occur. When simple anodic reactions occur as in equation 1, the M z+ ions move into solution and the electrons flow from anode to cathode in the external circuit. At the cathode, these elec- trons are utilised in cathodic processes such as those shown by equations oe and 8. It should be noted that for corrosion to take place, the sites must he in electrical contact, and that the total number of electrons involved at the anode and cathode are equivalent. The sites may be large and separated, such as when steel rusts near the waterline when partially immersed in salt