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Corrosion Basics: Protecting Fixed Structures in Seawater

Structures such as steel bulkheads, steel piles supporting piers or wharfs, offshore drilling platforms, and other similar structures may be cathodically protected with either sacrificial galvanic anode systems or impressed current systems.

Galvanic anode systems in seawater, for the most part, use much heavier anodes than those used in soil. The low-resistivity seawater environment tends to require greater protective current densities and also permits greater current outputs from the anodes. Consequently, the greater corrodible mass is needed to provide reasonably long life.

For structures that can be polarized, a low-potential galvanic anode material such as zinc or aluminum is generally preferable to a high-potential material such as magnesium. Magnesium will work perfectly well, but may discharge more current than needed. This results in reduced efficiency and shorter service life for the anodes. In some chloride environments, magnesium anodes have a greater tendency than other galvanic materials to self-corrode, which further reduces their service life.

Zinc or suitable aluminum alloy anodes can polarize a steel structure in seawater to within a few millivolts of the characteristic potential of the anode itself. Assume that the stabilized driving voltage between the anode and the polarized structure is 0.050 V, although it can be less. This ability to maintain polarization at a relatively modest current will consume the anodes at an efficient rate.

Compare the typical behavior of magnesium anodes: the structure does not tend to polarize to a potential more negative than ~–1.1 V vs. a silver/silver chloride (Ag/AgCl) electrode (SCE) because the hydrogen overvoltage potential is reached, resulting in the evolution of free hydrogen rather than additional polarization. This means that with a standard magnesium alloy working voltage of ~–1.4 V vs. an SCE electrode, there will be a driving voltage of ~0.3 V. Thus, on a comparable basis, the magnesium will discharge about six times as much current as is actually required to achieve the desired polarization.

Because less electronegative anodes can provide an efficient cathodic protection system, the surplus current from a more powerful anode is, in effect, wasted. However, there is one advantage offered by magnesium anodes in seawater: the greater driving voltage tends to force the more rapid formation of thicker protective calcareous deposits on the structure surface than would be obtained with less powerful anodes.

Special chemical backfills are not needed in the uniform seawater environment because galvanic anodes work satisfactorily without them.

Impressed current systems for fixed seawater structures may use suitable anode materials, also without backfill, suspended from the structure being protected or placed on the ocean floor. In the past, various materials such as treated graphite, high-silicon cast iron, platinized titanium, or lead/silver have been used as anodes. However, the introduction of highly efficient, dimensionally stable anodes (DSAs), which are basically mixed metal oxide coatings on titanium, has rendered these other anodes almost obsolete.

Particular attention must be given to the design of the rectifier positioning, header cable distribution system, and anode suspension or placement details. Above-water components are subject to severe marine atmospheric attack, whereas submerged portions must be protected from, or designed to withstand, the mechanical forces exerted by moving seawater as well as by water-carried debris or shipping traffic.

This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 514-516.

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