protective coating over the surfaces by oxidation. As shown in Figure 36, 

 corrosion rates for aluminum also approch constancy after extended exposure. 

 Since aluminum alloys do not produce a uniform layer of thick corrosion 

 products over the surface, marine fouling growth, especially barnacles, become 

 firmly attached directly onto the surfaces and are very difficult to remove 

 (Figure 22). Certain types of fouling attachment may even protect metal 

 alloys from corrosion. On the other hand, crevice corrosion does occur under- 

 neath barnacle shells attached to certain stainless steel and monel panels.^'^ 



2. High current velocities. These cause corrosion by removing loose 

 corrosion products and replenishing the cleaned steel surfaces with dissolved 

 oxygen thereby increasing corrosion more rapidly. The effect of tidal currents 

 on the corrosion rates of test panels exposed at the surface was inconclusive 



as shown in Figures 33 and 34. 



3. Temperature. In general, the effect of temperature on the corrosion 

 rate depends on its influence on the factors controlling the corrosion reaction.''^ 

 The corrosion rates of test panels exposed at the surface were not significantly 

 influenced by changes in water temperature at Point Mugu Pier. 



In addition to the various factors mentioned above which control 

 corrosion, a dense growth of Ectocarpus (brown algae) may also have affected 

 the corrosion rates by producing and concentrating a microlayer of dissolved 

 oxygen over the surfaces of test panels during the early exposure period. A 

 dense growth of Ectocarpus was found on panels which were exposed in the 

 sea during the winter. However, such dense growth was not detected on 

 panels exposed during the summer. 



The combined effects of dense marine growth, high dissolved oxygen 

 concentration, current velocities and water temperatures play a significant 

 role in promoting corrosion in the sea. 



The effect of marine fouling on corrosion rates of test panels in the 

 sea (surface) was not definitely established by the experimental methods used 

 in this current study because ( 1 ) bacteria were found inside initially sterile 

 cylindrical chambers containing control panels and their effect on corrosion 

 is not known; and (2) the control and exposed panels were not subjected to 

 identical concentration of dissolved oxygen nor to identical current velocities 

 because deposits of fine silt, debris, and biological growth over the membrane 

 filter prevented the free exchange of fresh seawater between the two environ- 

 ments. It is not definitely known whether the lower corrosion rates experienced 

 by control panels inside the chambers sealed with membrane filters were caused 

 by low dissolved oxygen content or by lack of marine growth. 



37 



