Marine buildings are mainly steel structures, which makes them extremely vulnerable to corrosion. Seawater, as an electrolyte, transfers electrons from the steel structure through oxidation, resulting in the destruction of Marine structures. If left untreated, this corrosion process can reduce the integrity of the structure to the point of failure. In addition to coating, the standard method of protection is the use of sacrificial anodes. They are made from a more reactive or less precious metal (usually zinc or aluminum). The sacrificial anodes are attached to the steel structure, and since they are more easily oxidized, they will be turned into cathodes by the protective structure itself. Electrons leave the protected body through the anode, and the anode slowly dissolves. The application of this principle can protect steel structures from corrosion.
When the coating of the ballast tank is damaged or eventually ruptured, the only way to prevent corrosion is to use a sacrificial anode. Design considerations for cathodic protection of sacrificial anodes in ballast tanks typically include the size, shape and area to be protected in ballast tanks, coating systems and ballast procedures. A feasible approach is to distribute the sacrificial anodes evenly over all uncoated tank structures and focus on horizontal surfaces.
Chemical composition of aluminum anode
| Types | Zn | In | Cd | Sn | Mg | Si | Ti | Impurities, not more than | Al |
| Si | Fe | Cu |
| Al-Zn-In-Cd | 2.5-4.5 | 0.018-0.050 | 0.005-0.02 | — | — | — | — | 0.1 | 0.15 | 0.01 | Balance |
| Al-Zn-In-Sn | 2.2-5.2 | 0.020-0.045 | — | 0.018-0.035 | — | — | — | 0.1 | 0.15 | 0.01 | Balance |
| Al-Zn-In-Si | 5.5-7.0 | 0.025-0.035 | — | — | — | 0.10-0.15 | — | 0.1 | 0.15 | 0.01 | Balance |
| Al-Zn-In-Sn-Mg | 2.5-4.0 | 0.020-0.050 | — | 0.025-0.075 | 0.50-1.00 | — | — | 0.1 | 0.15 | 0.01 | Balance |
| Al-Zn-In-Mg-Ti | 4.0-7.0 | 0.020-0.050 | — | — | 0.50-1.50 | — | 0.01-0.08 | 0.1 | 0.15 | 0.01 | Balance |
Electrochemical performance of aluminum anode
| Performance, types, indicators | Open circuit potential
-V(SCE) | Working potential
-V(SCE) | Actual capacitance A · h/kg | Current efficiency
% | condition |
| Ordinary aluminum alloy anode | 1.10-1.18 | 1.05-1.12 | ≥2400 | ≥85 | Corrosion products are easy to fall off and dissolve evenly on the surface. |
| High-efficiency aluminum alloy anode | 1.10-1.18 | 1.05-1.12 | ≥2600 | ≥90 |
| Highly activated aluminum alloy anode | 1.45-1.50 | 1.40-1.45 | ≥2080 | ≥70 |
