Abstract
A method is described for producing an occluded cell in a large chamber, which contains a small anode surface coupled to a cathode located in an adjoining cell. By separating the two cells with an agar plug, it was possible to deaerate and then chemically reduce the anode cell while maintaining aerated water of essentially original composition in the cathode cell. By thus simulating a metal surface containing a localized deposit (i.e. tubercle) in an open water system, it is possible to study proposed chemical and electrochemical effects generated by bacteria under a tubercle, and the subsequent affects on corrosion under that tubercle. It is demonstrated that, while removal of oxygen under a tubercle does increase corrosion rates at that site, the reaction quickly becomes anode diffusion limited. Introduction of free sulfide into this deoxygenated site cause no increase in the rate of corrosion. It is concluded that factors other than the oxygen differential effect with or without sulfide production are necessary to produce the high corrosion rates seen on steel under tubercles. Further, it is seen that Type 304 stainless steel is unaffected by both oxygen differential and free sulfide, and remains passive even when coupled to a large external cathode.
