subjected to deep-ocean bacterial slime growth were slightly higher than for 

 the control panels exposed at the surface of the sea. The slime-covered 

 exposed aluminum alloy panels which were placed near the deep-ocean floor 

 also corroded at a higher rate than the control panels which were exposed 

 both at the surface and in the deep ocean, with low dissolved oxygen concen- 

 tration present in all three cases. 



On the deep-ocean floor where the concentration of dissolved oxygen 

 is low, the exposed aluminum panels corroded at a faster rate than the exposed 

 carbon steel panels (Figures 35 and 36). 



Corrosion Rates — Surface Versus Deep-Ocean 



At the surface, materials submerged in the sea soon become heavily 

 fouled with animal and plant growth. The principal fouling animals are 

 barnacles, microorganisms, tunicates, hydroids, mussels, tubeworms, and 

 bryozoans; the principal plants are algae and l<elp. In the deep-ocean environ- 

 ment, it has been found that a large number of bacteria inhabit the sediment, 

 and these microorganisms have destroyed test samples of organic materials 

 placed on the seafloor. These microorganisms also form heavy slime coatings 

 over the surfaces of metallic as well as nonmetallic materials placed on the 

 seafloor. It has also been found that attachment of other fouling organisms 

 on materials at great depth is minimal. However, there are many marine wood 

 boring molluscs present, even at depths of 6,800 feet, and these borers have 

 destroyed untreated wood test specimens within a very short period. ^^■''^'^^'''^ 



As shown in Tables 4 and 7, the seawater environment at the surface 

 of the sea is considerably different from that at great depths. The corrosion 

 rates of carbon steel and aluminum alloy test panels have varied significantly 

 when panels have been placed in these different environments. 



After 189 days in the sea, the corrosion rate of exposed 1010 carbon 

 steel panels exposed at the surface of the sea was 8.5 mpy (estimated from 

 Figure 35) as compared to 1 .34 mpy at depth. Reinhart''^ obtained compa- 

 rable corrosion rate data for 1010 carbon steel exposed at the surface of the 

 sea and at a depth of 6,000 feet. For 7178-T6 aluminum alloy, it was 8.2 mpy 

 at the surface (estimated from Figure 36), compared to 3.65 mpy at depth. 

 Since aluminum alloys corrode by the pitting rather than by the uniform 

 surface corrosion, the prediction of performance by the extrapolation of 

 corrosion rates calculated from weight loss data alone is not entirely satisfactory. 



49 



