Pacific N.W. Section Hears Paper On Marine Cathodic Protection

The Pacific Northwest Section of The Society of Naval Architects and Marine Engineers met recently at Sir Winston's Restaurant in Portland, Ore., for a presentation on marine cathodic protection. The meeting was opened by John Mitchell of Northwest Marine Iron Works, who is vice chairman for the Columbia River Area. Mr. Mitchell announced that John Cauduro, general manager of Progress Electronics Company, Portland, had been appointed Technical and Research Committee chairman for the section.

For the technical session, Joseph A. Lehmann, P.E., vice president and general manager for Norton Corrosion Ltd., Woodinville, Wash., presented a paper titled "Marine Cathodic Protection." The presentation was augmented with slides depicting a variety of cathodic protection system installations and some component failures.

In his paper, Mr. Lehmann stated that cathodic protection is the one method by which virtually complete corrosion control can be achieved on new or existing subsurface steel structures without prior treatment of the metal surface of the adjoining environment.

Essentially, the cathodic protection process is simply "corrosion in reverse." On marine structures, there are multiple conditions resulting in the establishment of numerous and intense galvanic cells. For example, seawater near the surface contains a large amount of oxygen as compared to water at greater depths. The steel surfaces which are oxygen starved become anodic to those areas where oxygen is plentiful. Hot worked welded seams are usually anodic to adjacent cold rolled plate. Submerged or buried steel encased in concrete interconnected to bare steel in mud or water results in corrosion attack on the bare steel at the junction with the encased steel. Variations in the environment give rise to local concentration cells. These cells can be set up by differences in oxygen, temperature, agitation, water velocity and, in fact, by almost any heterogeneous exposure condition. Once a potential difference has been established, the corrosion cell will begin to function.

Realizing that no corrosion occurs at the cathode, a structure can be protected if its entire surface can be made to be cathodic with relation to the auxiliary anode which is expendable. Essentially, there are two types of cathodic protection—galvanic anode and impressed current systems. The two systems are similar in that they both deliver a d-c current to the structure being protected, forcing it to be cathodic or "cathodically protected." The paper then continues on to explain the criteria for protection and design considerations. The test includes information on how to calculate galvanic anode requirements.

Copies of the paper are available through the Section librarian, Mrs. L.S. Bracken, Todd Shipyards, P.O. Box 3806, Seattle, Wash. 98124.

Other stories from June 15, 1977 issue


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