198 "The Philippine Journal of Science ' i9i6 



Concentrated nitric acid will remove tin without attack on 

 the iron base, although it will entirely destroy a piece of gal- 

 vanized iron. Both tin and zinc are anodic to iron in con- 

 centrated nitric acid, and zinc is anodic to tin. 



The experiments in the preceding tables are of interest, not 

 only because of the apparent "reversals" of potential of tin, 

 but also because of the fact that many of the reactions recorded 

 took place in alkaline solutions in which the metals used (chro- 

 mium, aluminium, lead, etc.) are generally thought to be part 

 of the acid radicals. 



AMOUNT OF IRON IN THE COATING 



The amount of iron alloyed with tin depends on the temper- 

 ature and purity of the plating bath and doubtless varies with 

 different brands of tin plate. It is reasonable to suppose that 

 it should be an important factor in determining the quality of 

 tin plate and its resistance to corrosion. The various brands 

 analyzed by us showed iron contents in the tin coating of about 

 0.05 gram per square decimeter. 



SIZE OP SAMPLES TO BE USED FOR ANALYSIS 



The analytical data previously recorded indicate the lack of 

 uniformity of coating to be expected in commercial tin plate. 

 Although errors are undoubtedly caused by failure to cut samples 

 of tin plate accurately to squares of the required size, it is ap- 

 parent that great discrepancies in uniformity of coating exist, 

 even in samples cut from the same sheet. Obviously more con- 

 cordant results would be obtained if larger samples were used, 

 but in that case the analyst might easily fail to detect local 

 defects in the coating. With the methods outlined, squares 

 5 by 5 centimeters, or at most 5 by 10 centimeters, should be 

 ample for analytical purposes and should yield more reliable 

 data than larger samples. 



STANDARDS FOR TIN PLATE 



The quality of a galvanized iron is determined chiefly by the 

 thickness of the coating, since zinc, under ordinary service 

 conditions, itself corrodes and inhibits the corrosion of iron 

 even after the latter is exposed. Tin, on the other hand, fre- 

 quently accelerates the corrosion of iron ; its protective influence 

 is mechanical. Uniformity of coating is, therefore, an essential 

 requirement for good tin plate, and a test, such as the one 

 devised by W. H. Walker,^^ for the detection of pinholes and 



"Joum. Ind. & Eng. Chem. (1909), 1, 440. 



