April 8

Understanding Marine Electrics: Corrosion Issues with Thru-hulls and Seacocks

A maintenance topic that is vital to your boat’s watertight integrity and which requires special attention is thru-hulls and the associated seacock assemblies. In this post, we'll take a closer look at corrosion issues and how to avoid potential problems.

The Problem with Brass

Brass is widely used in water systems onshore. Unfortunately, brass has a high zinc content, which can result in galvanic corrosion in a saltwater environment. The zinc reacts with copper in the brass, leading to a brittle structure. In a worst case, a moderate kick or knock can cause the seacock assembly to fracture and break off, potentially causing a major water inrush that sinks the boat. 

The Need for Corrosion-resistant Materials

There is a variation of brass that is known as dezincification resistant (DZR) brass that better withstands galvanic corrosion than ‘ordinary’ brass, but even DZR brass is not as good as other widely-available materials.

In Europe, there is a standard for seacocks that requires them to hold up for five years, but this is nowhere near adequate when compared to the average life expectancy of the hull. The DZR fittings meet this standard, and have been widely used, but much better are various bronze alloys, especially silicon bronze.

Plastic Seacock Options

An alternative to metal seacock assemblies is high-density plastic. Brands like Marelon and Trudesign offer reinforced plastic seacock assemblies that are resistant to corrosion and can withstand significant force without damage or breaking. It's important to choose the right product, as some plastic seacocks are bonded together and cannot be disassembled whereas others can be disassembled. In general, plastic fittings require more space than their bronze or DZR brass equivalents. 

In Conclusion

When choosing a seacock assembly, it's essential to consider the materials and the potential for corrosion. Look for options that are made from corrosion-resistant materials like silicon bronze or high-density plastic. Consider the force that the seacock assembly can withstand and ensure that it meets the relevant standards. After installation, inspect seacocks at least annually.

If you want to know more about various metals and corrosion, check out our Advanced Marine Electrics course. The course includes an entire module dedicated to the topic, where we cover all the important issues.

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