Steel is the material most used for offshore structures on account of its great strength, low cost and ease of fabrication. However, steel demands corrosion protection and this can be achieved by cathodic protection or by protective coatings. Zinc performs effectively and at low cost as a coating or as a galvanic anode. Reinforced concrete can be made more resistant to degradation by using galvanized steel reinforcement bars.
Zinc anodes operate at over 98% efficiency and need little design skill for successful application. They are low in cost, selfregulating, reliable, need no maintenance and will not cause paint to strip as do other anodes.
As a protective coating, zinc can be applied by hot dipping, or by continuous, low cost, galvanizing of sheet and strip. Additional protection can be readily achieved by painting. On site application can be achieved by zinc spraying, or by using zinc rich paint. Zinc containing paint having over 8% of zinc in the dry film, has proved particularly effective for off-shore structures. Those with inorganic vehicles give outstanding abrasion resistance and such coatings are still giving good protection after twenty years. Those based on organic vehicles require less surface preparation and provide a more flexible coating but have lower abrasion resistance.
Where outstanding resistance to seawater attack is required, zinc can be anodized to a pleasing green or charcoal color. Newly synthesized lead organometallic compounds prove to have outstanding antifouling characteristics and are described.
Many a designer of land-based structures has had reason to regret not having paid sufficient attention to the possible effects of corrosion and the cost of maintenance. Marine structures, on the other hand, are considerably more vulnerable to corrosive attack than are most land-based structures, so that it becomes essential to consult a corrosion engineer at the design stage and to heed his advice at all subsequent stages of off-shore construction. The corrosion engineer must have a broad technical education; he must be part materials engineer, part chemist and electrochemist, and have a knowledge of coating technology. He must have adequate experience in order to be able to discern which of a great many variables will cause corrosion. He must combine the knowledge of a general practitioner with the skill of a specialist.
Fig, 1 - Simplified isometric view, zinc anode installation, centerline tank 9. (Available in full paper)
Zinc Rich Paint. Zinc's corrosion products are voluminous. When zinc is incorporated in a paint formulation these corrosion products tend to seal the coating.
Galvanized and zinc sprayed steel. Oxidation products form a protective coating on the surface of zinc in most environments, thus its inherent corrosion resistance is high.
In sea water, steel corrodes (a) at 2 ½ times the rate of zinc. Atmospheric corrosion of zinc under marine conditions is less than one-ninth of steel (b). Its corrosion products are white, non-staining and in some cases, may act as corrosion inhibitors. Zinc is amphoteric, reacting to both acid and alkali and thus capable of neutralizing a solution which is corrosive on account of its Ph.