Tirnc, days FIG. 5 Oiirdoor csposrire test resrrlts for deposits frow tire stairtioils srrlpltare bath The curves in descending order, d r r 10 un~inncd r~ccl and steel carrying 2, J, 6.8, 10, 12, 16,10,27,36,53 and 133 02. per basis box Time, dnsa FIG. 6 Oiitdoor esposirre tesr resitlrs .for deposits Jrorri the sodiiiirr stairtiate hark The curves in descending order, rekr to untinned steel ond ateei carrying 2,4,6, 8, 10, 12, 16, 20,27,36, 33, 133 UL. per busit box the effect of this correction becomes appreciable only with the heavier deposits exceeding about 27 0%. per basis box. It is seen that the z oz. and 4 02. coatings show a steady rate of weight
increment, but with the thicker coatings there is an initial period of slow corrosion followed by an increase in the rate of attack. The pxiod of slow rusting becomes more marked as the coating thickness increases and is greater for the sodium stannate bath deposits than it is for the stannous sulphate deposits. The existence of this initial period of slow corrosion is 3f practical significance as it implies that even thinly coated electro-tinplate can be reasonably resistant to rusting for a limited period. It is clear that long-term resistance to weathering is only afforded by the thickest coatings used in these tests.
Conclusions
A standard method of preparing the steel for plating was used throughout this section of the investigation, and under these conditions the various tests of quality have revealed no significant difference between deposits obtained from the stannous sulphate and from the sodium stannate bath. It will be shown in Part I11 of this series that the method of preparation has an important influence, not necessarily the same for both types of electrolyte, on the quality of electro-tinplate. All of the tests applied show a deterioration in the quality of the deposits as their thickness diminishes. The curve for the thiocyanate, hot-water porosity, humidity, and salt-spray tests are similar in form, indicating a progressive increase in the rate of deterioration as the thinner deposits are approached.
Acknowledgcments
This work forms part of a programme of research on the properties of tin and tin alloy coatings being carried out in the laboratories of the Tin Research Institute, to whom the authors are indebted for permission to publish. Approval for publication has been granted by the Chief Scientist, Ministry of Supply.