Zinc as a Self-Regulating Galvanic Anode For Ship Hulls

Abstract

Abstract This report presents results of service trials of up to three years duration for high purity and alloyed zinc hull anodes. The trials demonstrated that zinc anodes of suitable composition are capable of providing complete cathodic protection for ship hulls for extended periods in sea water. The current output of alloyed anodes (0.3% aluminum or 0.1% aluminum + 0.05% cadmium) is reliable and predictable because these materials do not polarize with time. Zinc hull anodes of suitable design can have useful current outputs for periods of eight to ten years in sea water. Bolting is a satisfactory method of fastening zinc anodes to a ship's hull. Zinc anodes cause little or no accelerated break-down of the adjacent hull paint. The two outstanding advantages of the zinc anode systems namely, self-regulation and improved current distribution, are available at a competitive price. Compared to the cheapest of the controlled type systems, whether of the galvanic or impressed voltage type, the zinc anode system at 13 cents per sq. ft. per year costs: (a) slightly more for large wetted hull areas of around 50,000 sq. ft., (b) slightly less for hull areas of around 15,000 sq. ft. and (c) markedly less for smaller hull areas. Recommendations are made for: (1) The more widespread use of zinc anodes for cathodic protection systems both on and in ships. (2) The use of high purity (ASTM Special High Grade) zinc alloyed with either aluminum, or aluminum plus cadmium, as the standard zinc anode material for all sea water applications. 5.2.2