166 JOURNAI. OF THE SOCIETY OF COSMETIC CHEMISTS Figure C-4. Figure C-4 shows the corrosion curve for the same shampoo formulation shown in Fig. C-2 except that the container is uncoated. In general the same relationship between the iron and tin exists in that the tin is anodic to the iron however, it can be seen that in the uncoated can, as would be Figure 6.

FORMULATING FOR PRESSURE 167 expected, the tin concentration reaches a higher level. With more tin ex- posed, the current density would be lower, and therefore a lower polariza- tion value would be attained, hence a higher solution rate. Corrosion forces are opposed by the E.M.F. of polarization. Figure 6 illustrates two cans which had been filled with different sham- poos and aged in an oven at 110øF. for two months. The difference be- tween 13 ppm. of tin corrosion and 45 ppm. tin corrosion can just barely be differentiated by the naked eye. The difference shown in the photograph did not show up until the can had remained open for several days. When first opened there was no visual difference detectable except under a lens. These graphs are not intended to simplify the corrosion processes that may occur in tin plate cans, coated and uncoated. On the contrary, they are only offered as an aid in understanding the many complexities involved. In one coated tin plate container, it was possible to demonstrate three definite varieties of corrosion. The valve cup, which was uncoated tin plate, indicated that the iron was anodic to the tin and was corroding rapidly. The coated side of the can, which was drawn tin plate, showed uniform anodic corrosion of the tin. The bottom of the can, which was cupped tin plate and triple coated, showed anodic corrosion of the tin of a localized type characterized by "black spots." These black spots ap- pear at holes in the coating. Table 1 illustrates a corrosion analysis of two different types of hair Sample No. Water Content ppm. • Temperature - Room 110øF. •--(3 Months) ..... (1 Month)------ Fe Sn Fe Sn 23-6-2 23-6-3 23-6-4 23-6-5 23-6-6 23-6-7 23-6-8 23-6-9 23-6-10 23-7-1 23-7-2 23-8-3 23-8-4 23-8-5 23-8-6 23-8-7 23-8-8 23-9-9 23-9-10 1350 1600 1850 2100 235O 2600 2850 3100 735O 1100 1350 1600 1850 2100 2350 2600 2850 3100 7350 Shellac Hair Spray 3.8 2.5 42 38 38 42 42 53 6O ICP Hair Spray 1.0 1.0 1.0 1.0 1.1 1.1 1.0 1.0 1.0 1.0 3.1 3.4 2.3 1.7 1.9 2.4 1.0 1.5 2.0 1.5 1.7 16 10 16 16 15 14 10 10 3.0 3.8 3.4 4.6 4.3 4.5 7.3 4.1 14.6 1.0 1.0 1.0 1.0 1.0 1.0 1.2 1.0 1.5 1.5 8.2 5.2 9.7 23 5 28 9 25 8 18 9 25 9 27 7 23 15 17 19 19 1 0 21 1.5 5.2 3.2