178 The Philippine Journal of Science wis 



These have shown great variations in quality. There have been 

 instances in w^hich new sheets showed marked corrosion when 

 brought from storage. Some brands failed completely after 

 from two to five years of service, while some sheets put in 

 place thirty years ago are in good condition and are still 

 rendering satisfactory service. This striking difference in the 

 behavior of old and certain modern corrugated galvanized 

 sheets under actual conditions of weather exposure, together 

 with the fact that the yearly import of this material represents 

 a considerable financial outlay, led the Bureau of Science to 

 study the various factors affecting the corrosion of galvanized 

 iron and the problem of comparing old and modern galvanized- 

 iron sheets.^ 



GENEKAL CONSmERATIONS 



As is well known,* the durability of galvanized iron under 

 normal service conditions depends on various factors, among 

 them thickness, density, purity, continuity, toughness, and ductil- 

 ity of the coating and the purity of the iron base. 



Of the factors mentioned above, thickness of coating is gener- 

 ally considered ^ the most important. It is true that to some ex- 

 tent zinc vdll exert a protective influence on iron with which it is 

 in contact, even after the iron is exposed to ordinary corrosive 

 action, but this influence is generally confined to a very limited 

 area. Furthermore the oxidation products of zinc may be 

 electro-negative to iron, so that corrosion would be accelerated 

 instead of retarded. As zinc is removed at a rate depending on 

 the conditions of service to which it is subjected, it is obvious 

 that, other things being equal, the life of a galvanized sheet de- 

 pends on the amount of zinc per unit area. Actual service tests 

 bear out this view. The actual amount of zinc to be used to give 

 the best service is dependent on economic considerations and 

 on the physical character of the resulting plate. Manufacturers 

 have claimed that a coating heavier than 4.88 grams of zinc 

 per square decimeter (1.6 ounces per square foot) tends to 

 flake off, the result being an imperfect, easily corroded sheet. 

 This claim appears untenable, since the high spelter content of 

 old sheets which have withstood long service exposure has amply 



' In recent years galvanized iron with an exceptionally pure iron base 

 has been brought to the Islands. This has not been on the local market 

 long enough to enable us to obtain comparative data concerning its dura- 

 bility under ordinary service conditions. 



*Cf. Burgess, C. F., Electrochem. & Met. Ind. (1905), 3, 17; Walker, 

 W. H., Proc. Am. Soc. Test. Mater. (1909), 9, 433. 



' Walker, loc. cit. 



