During recovery of the cylinders covered with nylon screen after 

 23 days of immersion, the seawater flowed out. However, seawater was 

 retained inside those cylinders which were exposed for 43 days. The screen 

 was covered with marine growth, silt, and fine detritus. Inside the cylinders 

 exposed for 23 and 43 days, about a 3/4-inch-thick layer of fine silt was 

 found trapped at the bottom. The bottom portion of the fine, gray silt had 

 turned black, indicating that hydrogen sulfide was being produced by sulfate- 

 reducing bacteria in an anaerobic environment. The presence of sulfate-reducing 

 bacteria in the sediment samples was confirmed by bacteriological tests using a 

 nutrient medium employed by Morita and ZoBell.''^ Only a small number of 

 calcareous tubeworms were found growing on the surface of these test panels. 

 Other varieties of marine animals were absent. 



The corrosion rates of the 23- and 43-day panels are presented in 

 Table 5 and are shown graphically in Figures 33 and 34. Information on 

 weight losses incurred by individual panels during this period is presented 

 in Table 6. 



The corrosion rates of carbon steel and aluminum alloy test specimens 

 for the three exposure tests have been combined and are shown graphically 

 in Figures 35 and 36. The corrosion rates for both carbon steel and aluminum 

 alloy panels exposed in cylinders covered with (1) nylon screen, and (2) in 

 cylinders with ends left open have been extrapolated for the period between 

 92 days and 268 days of exposure. Exposure tests under these conditions 

 were not conducted during these periods. 



In general the steel and aluminum alloy test panels which were 

 directly exposed to various destructive agents of the ocean environment 

 including marine fouling organisms corroded at a much faster rate than those 

 test panels which were protected inside cylindrical chambers. 



Some other factors which affect and control corrosion besides marine 

 fouling organisms (barnacles, hydroids, bryozoans, mussels and microorganisms) 

 and their metabolic and decay products are: 



1. Dissolved oxygen concentration. The corrosion rate of steel in 

 seawater is affected by the amount of oxygen supplied to the metal surfaces.'"-^'''^ 

 After prolonged exposure, the carbon steel starts to produce and accumulates 

 a thick layer of rust which protects the underlying metal surfaces from coming 

 in contact with fresh seawater. The corrosion rates of such panels approach 

 constancy after extended exposure as shown in Figure 35. The fouling orga- 

 nisms are usually attached to the layer of rust over the metal surfaces and their 

 foothold on these steel panels depends upon the adhesion of the corrosion 

 products to the metal surfaces. Aluminum alloys, on the other hand, are 

 usually protected by dissolved oxygen which aids in the formation of a thin 



36 



