Anode and Sacrificial Anode

Anode and Sacrificial Anode

The anode is the negative electrode in a battery or galvanic cell. It loses electrons to the cathode and attracts positively charged ions from the electrolyte solution. The anode thereby takes on the role of the sacrificial element, protecting the more valuable metal system to which it is attached.

Sacrificial anodes protect a variety of metal structures such as iron and steel ship hulls, underground and above ground pipelines and boilers or any other steel structure immersed in water or soil conditions where corrosion can occur. By sacrificing themselves, sacrificial anodes extend the lifespan of the more valuable metal structures to which they are attached.

Anode materials come in a range of forms and price points, but all function in the same basic way. They are typically made of carbon-based material such as graphite, silicon or a combination of the two. Graphite anodes are found in household (alkaline) batteries and lithium-ion batteries, while silicon anodes are used for larger applications such as power stations.

To function as a sacrificial anode, the anode must have low-resistance contact with the metal to which it is attached. This contact can be achieved by mounting the anode directly to the metal or, as is more common with zinc anodes, by attaching them with a wire. In either case, the metal to which the anode is attached must be bare and bright prior to installation, as this is essential for proper functioning.

The current produced by sacrificial anodes is measured in amperes per second and can be very high. To reduce the potential for stray currents on other metal structures, it is important to mount the anodes in a strategic fashion and to plan the total number of anodes required for a given system, taking into consideration the surface area of the anode and the system to which it will be attached.

There are a number of anode shapes available for different applications, such as flat, slender and cylindrical. The choice of anode shape is based on the availability of space and the desired installation method; for example, a flat anode can be bolted directly to the structure being protected, while slender anodes are commonly used for cylindrical structures such as pipelines.

When selecting an anode, it is also important to consider the reactivity of the material. The reactivity of anode materials can be categorized in a series that includes magnesium (Mg), aluminium (Al), zinc (Zn) and chromium (Cr). Metals at the highest end of this reactivity series can be used to protect iron or steel structures such as ship hulls, while the lower-reactivity anodes are often made from magnesium or aluminium alloys.

Aluminum anodes are starting to replace zinc anodes in some marine applications, as they can be used for longer in freshwater environments and work better than zinc in brackish or salt water. They are also lighter in weight and more flexible than zinc, making them easier to install and remove as necessary.