186 'rhe Philippine Journal of Science isie 



evidence of wetting by sea water, which, as is well known, is 

 corrosive toward galvanized iron; in some cases sheets showed 

 imperfections in galvanizing, such as pores, ragged edges, and 

 buckles, which are conducive to the formation of centers of 

 corrosion. There were sheets, however, that had imperfections 

 yet which did not show corrosion, and there were cases of ac- 

 celerated corrosion which imperfections in the coating and 

 insufficiency of spelter did not explain. For example, in one in- 

 stance galvanized sheets which had arrived apparently in good 

 condition were stacked one over the other without shelter and 

 were thus exposed to sun and rain for about three weeks. The 

 majority of the sheets examined at the end of that time were in 

 an advanced stage of corrosion. Some of the badly attacked 

 sheets were covered by other sheets and were still moist. The 

 average thickness of spelter was 6.41 grams per square decimeter 

 (2.10 ounces per square foot) . 



There seemed to be but one fact to which the immediate cause 

 of the damage may properly be attributed — that is, the con- 

 tinuous presence of moisture between sheets stacked one upon 

 another, during exposure to weather. This explanation is borne 

 out by the fact that sheets of the same brand that happened to 

 be used for temporary roofs on sheds were in much better con- 

 dition after nine months' exposure than the damaged sheets of 

 even later arrival. 



In a supplementary experiment six pairs of sheets were taken 

 from different brands of the old and modern sheets. The edges 

 were covered with paraffin, and the sheets were placed one on 

 top of the other in pairs. The unexposed sides were kept moist 

 by sprinkling with water once a day. In a few days signs of 

 corrosion were noted and at the end of three weeks all sheets 

 were in an advanced stage of corrosion. 



Galvanized sheets undergo accelerated corrosion when the 

 surface is kept moist either by actual wetting with water or 

 by the presence of damp air. It is believed that similar con- 

 ditions greatly contributed to the corrosion of sheets that arrived 

 in damaged condition or which rusted during storage. The 

 uniformly high temperature and humidity of most parts of the 

 Philippines probably offer exceptionally favorable conditions for 

 corrosion. 



PAINT PROTECTION 



The frequent failure of modern galvanized sheets in com- 

 paratively few years led to the necessity of applying paints, of 

 which red lead, graphite, asphaltum, and oxide of iron are 



