Vessel corrosion – cathodic protection and sacrificial anodes

To achieve cathodic protection of the vessel with a sacrificial anode system, certain tools and techniques must be used. Vessels are among the structures that are most exposed to corrosion due to the environment in which they operate. Seawater is a very corrosive environment because the salt in it makes it a very good conductor of electricity and leads to the oxidation of iron by releasing many ions. Almost every part of the vessel is subject to corrosion of varying severity.

Parts of the vessel that are underwater or exposed to water, some of which are:

Exterior hull, rudder, impeller shaft, ballast tanks, ballast pipes

There are several ways to protect the hull and other parts of the vessel from corrosion. The method chosen depends on the area to be protected, its shape and environment. There are three major ways to protect against corrosion:

• Coating with anti-corrosion paints
• Cathodic protection system of sacrificial anodes
• Impressed current system

In this article, we will discuss the method of cathodic protection system of sacrificial anodes.

Classification of anodes

Anodes can be classified according to their shape, size, type of raw materials and method of installation at the protected level. Anodes can be used in small or large sizes, and this affects their weight and the overall weight of the protected structure. The size and shape of the area to be protected, availability and access to space, and structural strength considerations are factors that affect the size and weight of the anode.

Anode shape

Anodes can take many forms depending on their application. Sacrifice anodes are produced in various forms such as flat or block shaped, cylindrical or semi-cylindrical, tear anodes, bracelet anodes, disc anodes and tubular anodes.

The choice of anode shape depends on several factors. Some of these factors are:

• Shape of the protected surface,
• Availability of sufficient space to install the anode,
• Availability
• Easy installation.

For example, flat anodes are mostly used for large, flat surfaces such as the hull. In high-speed boats, where flowing water is very important, tear anodes are used because flat anodes increase the boat’s strength. Bracelet anodes are used for pipelines and impeller tentacles, while tubular anodes are used for cables. Of course, in some cases the choice depends on the availability, cost and flexibility in the design. For example, if the cost is high, cylindrical anodes can be used to protect pipelines instead of bracelets.

Anode type

Zinc or aluminum sacrificial anodes are commonly used for marine applications. Two features of anode performance measurement are listed below.

• Closed circuit potential indicates ease of anode corrosion. The more negative the value, the corrosion is easier.
• Electrochemical capacity (Amp-hr / kg) indicates the amount of anode material consumed.

From the table above it can be seen that aluminum has a higher closed circuit potential, so it starts working faster than zinc. It also has a higher electrochemical capacity than zinc and will last longer at the same weight.

In addition, zinc anodes in freshwater tend to form a calcareous coating on the anode surface that prevents them from working effectively. However, zinc anodes are sometimes more reliable in low-oxygen environments, such as marine sediments or areas with high bacterial activity. Therefore, while aluminum is more efficient, the use of zinc may be more effective in some cases.

Aluminum anodes can also ignite if they fall from a height on oxidized steel. Therefore, their use inside cargo tanks is not recommended. Therefore, the choice of materials depends on the type of environment in which the anode is placed and must be done carefully.

Anode installation methods

There are three basic ways to install an anode on a protected structure:

• Welded connection
• Connection by bolts
• Gutter connection

Welded connection ensures the closest and safest contact between the anode and the body, and electrical conduction through the insert is well done. But sometimes welding is not possible due to lack of access or favorable conditions. Therefore, the connection can be used with bolts or studs. In addition, it is easier to replace anodes connected with bolts than to weld.

Tavana Zob Sana’ati Kavir Company, in the field of cathodic protection of vessels with sacrificial anodes and similar tools, is able to produce sacrificial aluminum and zinc anodes in different dimensions and shapes according to the client’s request. You can contact the sales department for more information.