In the corrosion protection system of ships, aluminum sacrificial anodes play a crucial role as silent guardians. A complete Aluminum sacrificial anode application ship solution begins with precise calculations, usually in accordance with classification society specifications such as DNV-GL or ABS, The amount of anode used is determined based on the immersion area of the ship (for example, a 200-meter-long container ship can reach 10,000 square meters), the coating damage rate (the design value is usually 1% to 3%), and the required protection current density (for steel hulls, it is generally 10 to 20 milliamperes per square meter in a seawater environment). A 50,000-ton bulk carrier may need to install 150 to 200 standard-sized aluminum alloy anodes of 100mm×200mm×1000mm, each weighing approximately 25 kilograms, with a total weight exceeding 4 tons. The design objective is to ensure that the protective potential at each point on the hull surface remains stable within the effective range of -0.80V to -1.05V (relative to the Ag/AgCl reference electrode), thereby suppressing the average annual corrosion rate from over 0.1 mm to below 0.02 mm.
The arrangement of these anodes is by no means random; rather, it is optimized through computer-aided modeling to eliminate protection blind spots. Key areas include the areas with intense turbulence near the propellers, the bow area subjected to wave impact, and the more corrosive gas-liquid interfaces in the ballast tanks. The current density requirements at these locations may be 50% higher than the average level. For instance, in ballast tanks, the distribution density of anodes may reach one per 15 square meters, while on flat ship bottom shells, the density can be reduced to one per 30 square meters. The inspection report of an oil tanker that had been in service for 15 years in 2021 showed that its aluminum anode system, which was strictly arranged in accordance with the specifications, successfully controlled the average thickness loss of the hull steel plates within 5% of the original design thickness, far lower than the usual 15% loss rate of underprotected ships. This enabled the vessel to pass the special inspection smoothly and extend its operational life by at least 10 years.
From an economic benefit analysis, aluminum-based anodes demonstrate significant advantages. Compared with traditional zinc anodes, the capacitance of high-performance aluminum-zinc-indium alloy anodes can reach up to 2800 ampere-hours per kilogram, which is nearly 260% higher than that of zinc anodes at 780 ampere-hours per kilogram. This means that under the premise of providing the same total protective charge, the weight and volume of aluminum anodes can be reduced by approximately 40%, directly lowering the load burden and fuel consumption of ships. A set of aluminum anode system equipped for a newly-built 200,000-ton oil tanker has a total procurement and installation cost of approximately 150,000 US dollars, accounting for only 0.1% of the vessel’s construction cost. However, it can extend the cycle of special inspection and maintenance in dry docks from the traditional 2.5 years to 5 years. Each dry dock entry can save about 2 million US dollars in downtime losses and maintenance costs, with an investment return rate exceeding 300%.
Practical cases have confirmed its reliability. In 2019, on a container ship sailing on the high-temperature and high-salt Southeast Asian route, due to the selection of high-purity aluminum anodes that met the MIL-A-24779 standard and the consideration of the influence of seawater flow rate on anode polarization at a speed of 28 knots during the design, the performance degradation rate of its protection system was less than 5% within a four-year service cycle. A negative example is that a small cargo ship used non-standard aluminum anodes to save 5% of the initial budget, which led to the protection potential not meeting the standard. After 18 months, pitting corrosion with a depth of more than 1.5 millimeters occurred in some areas of the hull, and the repair cost was as high as 20 times the cost of the anode. Therefore, the scientific and standardized application of aluminum sacrificial anodes is a strategic investment to ensure the safe operation of ship assets worth hundreds of millions of dollars in harsh Marine environments for more than 25 years.