XI, A, 4 Argiielles: Galvanized-iron Roofing 179 



demonstrated that a much heavier coating can successfully be 

 applied. 



In hot-galvanizing practice considerable difficulty is en- 

 countered in trying to attain high purity of spelter not only 

 because the original spelter may contain such impurities as iron, 

 lead, and other materials which will add impurities to the coat- 

 ing, but also because the iron sheets dipped in the molten bath 

 are slightly dissolved, leading to the formation of zinc-iron 

 alloys on the surface of the iron. 



Very few data are available regarding the amounts of im- 

 purities permissible in spelter coating.® Cowper-Coles ^ stated 

 of the impurities in the coating of hot-galvanized iron : 



It has been found that iron above 13% makes the zinc too brittle to 

 bend. Lead up to 1% is harmless, but above 1.5% will not dissolve, and 

 the excess collects and forms weak spots. 



V. Vegesach,* W. Guertler,^ and Arthur and Walker " have 

 made elaborate studies of the complex zinc-iron alloys formed 

 during the process of hot galvanizing. Cushman " has called 

 attention to the fact that the use of zinc chloride or ammonium 

 chloride as flux, sometimes in conjunction with hydrochloric 

 acid, is a strong corrosive mixture and that small portions of 

 the reaction products formed become embodied in the zinc 

 coatings and tend to accelerate corrosion. 



Porosity is a defect in zinc coating, since it accelerates the 

 electrolytic corrosion of zinc in contact with iron and thus 

 shortens the life of galvanized sheets under weather exposure. 

 Although "hot galvanized" iron is generally free from pinholes 

 and cracks, "wet galvanized" material, especially when thinly 

 coated, is often extremely porous.^^ Increasing the thickness 

 of the spelter tends to eliminate pinholes and thereby offers 

 less chance for corrosion to start. 



Toughness and ductility of coating are important factors, as 

 they determine the resistance of galvanized iron to bending 

 stress and abrasion. 



In a well-galvanized sheet the iron base is rarely brought to 

 test under weather exposure until after many years of service. 



' Burgess, loc. cit., 17. 



' Industries and Iron (1898), quoted by Burgess, loc. cit. 



" Zeitschr. f. anorg. Chem. (1907), 52, 30. 



» Metallographie (1911), 353. 



^"Journ. Ind. & Eng. Chem. (1912), 4, 398. 



^^Proc. Am. Soc. Test. Mater. (1909), 9, 129. 



" Walker, W. H., op. cit., 430. 



