JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS CORROSION Such is the range of metals at your disposal and only the question of corrosion remains to influence your choice. Each of our C.T.M.A. members is ready to advise and assist in selecting a suitable tube for any given product. More often than not, the time available in which to make a decision is limited and we have established procedures for assessing corrosion behaviour fairly quickly. Even so, let me state quite definitely that there is no real substitute for the ordinary shelf-life test over a period of 6 months, or more, at room temperature. In all other methods an element of risk is present although it gets progressively smaller as our experience accumulates. We are accus- tomed to receiving jars of a given product, and in this connection I would say "Don't be mean" 2 ounces (yes, I have had 2 ounces quite recent]y) comes near to being a waste of time, as it is our practice to fill 4 tubes of each type. Half of the specimens are set aside for storage at room temperature whi]st the remainder go into an incubator controlled at 100 ø F. Already,' two points of some importance have arisen. First, occluded air can play an important part where the product is potentially corrosive. Spooning a product from a jar to a tube, and tapping the latter on a bench to pack it down, is hardly the equivalent of machine-filling from a hopper containing freshly prepared product, often fairly freely-flowing due to being slightly warm. Secondly, this matter of 100 ø F. has to be considered carefully. In my opinion, anything higher is not justified, and any product which suffers considerable change in physical form at 100 ø F., due to emulsion breakdown or melting of its fatty content, from which it does not recover on cooling, should be treated with suspicion, whatever the corrosion result. After three or four weeks at 100 ø F., tubes can be opened and any resulting damage to the metal surface observed. How long does this represent under normal. conditions ? Some say six months, but I think the real answer is that it doesn't represent any particular time at all. Any visible corrosion must mean the rejection of that particular system, though a negative result is notl to be taken as the green light signal to go straight ahead, but as an ambe rl signal to proceed with caution whilst further circumstantial evidence can be::' obtained sufficient to reduce the risk to acceptable proportions. I hav e personally encountered a case where no corrosion resulted during three:! weeks at 100 ø F., but severe pitting corrosion occurred at room temperature!ii after only five weeks. What other steps can be taken if the product must be?•i marketed before the expiry of a proper shelf-life test ? In the first place, individual responsible for formulating the product must make full use of background knowledge concerning the behaviour of similar products ingredients in common with the new one. Actual pH determinations be made, bht their value should not be overrated to the extent of any safe range nonsense. I once encountered an artist's colour which was..11 three-component system, one of which was water and another an 112

THE COLLAPSIBLE TUBE pigment, which had a pH of 7 and yet it was through the wall of a pure tin tube within a month. Finally, there are the electrochemical investigations which can be quite revealing in view of the fact that such a high proportion of metallic corrosion is electrolytic in origin. Simple potential measurements between the metal and a calomel electrode in the product have little or no value. Extremely useful information can, however, be obtained by setting up a cell with the product in between a Noble metal electrode and the metal of the tube wall. A microammeter, preferably in the form of a recorder, placed across this cell, will reveal its current-producing characteristics, and these in turn are a measure of the product's corrosive powers in a rather highly magnified form. I have found it convenient to utilise a Cambridge Recording Thermo- couple instrument in conjunction with a shunt assembly which can be switched to give 0-60, 0-120, 0-300 microamp ranges whilst keeping the total circuit resistance constant. The recording is continued over a period of four to five days and it has been possible, by comparing the records of a large number of products of well-known behaviour, to interpret the results with considerable accuracy. There are at least four distinct types of curve, certain irregularities, and an overall current value to guide the assessment. Once again, we do not claim 100 per cent accuracy because there is the inevitable, somewhat ill-defined, dividing line between safe and unsafe. The method is particularly good for comparing the effectiveness of experimental corrosion inhibitors since a result is obtained so quickly. It is beyond the scope of this lecture to go into greater detail, but as tube suppliers it is our practice to submit to this test any product which our customers care to send along. PROTECTIVE COATINGS If, by one means or another, you have discovered that your product is corrosive and you have been unable to inhibit this corrosion, it is necessary to fall back upon protective coatings. In this event, one thinks almost exclusively in terms of the aluminium tube. To apply such a coating on a tin tube is really a case of "gilding the lily" and a waste of good money. In the case of the lead, or tin-plated lead tube, the most effective coatings cannot be used owing to their high-storing temperatures, and I have already commented upon the adhesion and flexibility difficulties common to a lead surface. A coating of microcrystalline wax is probably the best suggestion which can be offered in these cases, although one must remember that such a coating is subject to hydrolysis, or even actual solution, when in contact with some products. For the alurainium tube there are three distinct types of protective coating: (a) The "High Bake," fully polymerised and therefore insoluble resin- based lacquers, which include Araldite, Epikote and phenolic resins. 113