180 The Philippine Journal of Science me 



Spellers " found that the base of galvanized roofing taken from 

 Panama, which had seen service for over twenty-five years and 

 was still in good condition, "proved to be steel, but a larger 

 proportion is wrought iron, and often of inferior quality." 



It is apparent that a well-coated sheet with an inferior iron 

 base will last as long as sufficient spelter remains to prevent 

 air and moisture from acting on the iron beneath. It is believed 

 that a pure iron base, per se, does not greatly add to the durability 

 of galvanized sheet in the absence of a liberal amount of coating. 

 The outer layer of the iron base is no longer of original purity, 

 being replaced by a series of iron-zinc alloy layers of different 

 solution tensions. Under the action of weathering this in time 

 offers a field for galvanic action between parts of different elec- 

 trical potential, until the pure iron base itself is exposed and ulti- 

 mately becomes corroded. In a corroding solution (Table IV) 

 dezincked pieces from old iron sheets show no greater resistance 

 to corrosion than modern sheets of very pure iron, although the 

 former have given long and satisfactory service. 



EXPERIMENTAL PART 



The samples used for experimental purposes have been of two 

 kinds: (1) Those that are known to have been under exposure 

 to weather for thirty years and upward and (2) various brands 

 of corrugated sheets recently introduced. The former were 

 taken from roofs in the towns of Batangas and Lipa, representing 

 seacoast and interior highland towns, respectively; the latter 

 were unused specimens of representative brands obtained from 

 the Bureau of Supply or in the open market. The old corrugated 

 sheets were selected from houses whose ages are definitely known. 

 Among the new galvanized sheets are included brands to which 

 belong sheets known to have succumbed to corrosion in from 

 two to five years. 



AMOUNT OP SPELTER 



Determinations of the quantity of spelter have been made of 

 old corrugated sheets known to have been under weather exposure 

 for more than thirty years. Whenever possible, the unexposed 

 laps as well as exposed portions were analyzed to get a general 

 idea of the amount of spelter originally present. The zinc was 

 determined from pieces 7.62 by 10.16 centimeters (3 inches by 

 4 inches), these being large enough to give reliable results, yet 

 small enough to enable the detection of marked local irregular- 



"Proc. Am. Soc. Test. Mater. (1909), 9, 440. 



