To Prevent Corrosion at Sea Think Aluminum Anodes!
Authors: Sansonetti S.J. 
Published: October 2008

This paper has been prepared as a ready reference for the design and corrosion engineers who have the responsibility of protecting offshore structures and marine equipment. It discusses how metals corrode and enumerates the many ways in which steel can be destroyed by seawater. The latest advances in the development of aluminum galvanic anodes by the aluminum industry are presented. The chemistry of the aluminum alloys available, their electrical characteristics, and their performance in seawater with varying degrees of salinity, temperature, and motion are discussed. The corrosion control of exposed structures and equipment, splash zone areas, and areas submerged in mud are recommended. Design criteria for cathodic protection as they relate to optimizing anode selection, i.e., shape, size, weight, current density, and methods of attachment are presented. The cost effectiveness of aluminum anodes is compared with zinc and magnesium anodes. A general guide to design of cathodic protection systems for offshore structures is included, together with a more detailed outline of step-by-step procedure in the appendix. A comprehensive bibliography of papers relating to the field of aluminum galvanic anodes for cathodic protection is included.

INTRODUCTION

Over 100 countries have frontages on the oceans. Several have taken steps in the past decades to explore and develop the tremendous wealth that exists in and under the seas.

The oceans and lakes cover 69% of the total surface of planet earth. The average depth of the ocean is 13,000 feet with approximately 2% being greater than 20,000 feet deep (Figure 1). The maximum depth of 35,888 feet was located by Jacques Piccard in the Trieste bathysphere in the Pacific off the coast of Guam.

Importance of the Oceans to Man

The nations of the world are rapidly realizing that in order to support the ever growing population of the earth, attention must be turned to tapping the mineral wealth and harvesting food from the sea. Man has gathered food from the sea for ages, but he has not yet begun to fully tap the potential available to him. No major attempt has been made to cultivate the oceans for animal and plant foodstuffs in comparison to man's exploitation of land above sea level. Long before man focuses his attention on fully harvesting the oceans, he must attempt to meet his most pressing need-- fresh water--by turning to desalination of seawater.

Over the past four decades, industry has extracted bromine and magnesium metal as well as sodium and potassium from seawater. While there are tremendous quantities of all elements present--$6 billions worth per cubic mile (l)--they are so dilute that they are not economical to recover by today's technology (Table 1). Exploration of the ocean bottom indicates that major mineral concentrates, in the form of large nodules, are available to be mined. These nodules contain rich concentrations of phosphorus, manganese, and may contain molybdenmu, nickel, cobalt, titanium, uranium, and zirconium.

The tempo of exploration and development of the mineral wealth is accelerating. Many large companies are going it alone while others are combining their efforts