Monday, December 16th, 2019

Seasonal Maintenance

Published on February 14, 2012 by   ·   No Comments

It’s all too easy to take an ‘out of sight, out of mind’ attitude to through-hull skin fittings, the valves that shut off the entry/exit points of pipework that terminates underwater. This can include outlets for sinks, basins and holding tanks, as well as engine and generator cooling water intakes. As skin fittings are safety critical items and problems don’t discriminate between boats big or small – they have the potential to sink a shiny superyacht just as easily as a decaying hulk.

There are two potential problems – the first galvanic corrosion, which occurs when different metals are immersed in seawater. With alloys such as brass or bronze this means the most reactive metal will corrode, with the fitting eventually loosing structural strength, unless there is excellent galvanic protection from sacrificial anodes. These are usually made of zinc and will erode faster than the alloys used for underwater hardware.

The second potential problem with skin fittings relates to the valve sticking open or closed, with the former a potential hazard in an emergency situation. It’s easy to check the condition of the fittings while the boat is laid up ashore over the winter, or during a pre-season haul out for antifouling.

Most marine skin fittings are made from manganese bronze or dezincification resistant (DZR) brass. These are much more resilient to galvanic corrosion than most brasses, but the zinc content of the alloy will still tend to erode over time. When this happens the remaining metal has a high copper content, so it takes on a pinkish hue. This is most visible if any surface oxidation is gently sanded away – if the fitting is in good condition the distinctive yellow colour of the alloy will be quickly revealed. However, if a pink tinge remains even after sanding the top surface away the fitting must be regarded as suspect.

Dezincification spells trouble for two reasons – at the best of times copper is very soft, but the loss of the zinc also leaves voids in the material and with these brasses having zinc content of over 30 per cent this can very significantly weaken the fitting.

When buying replacement fittings it’s important to seek out marine grade items: standard plumbing fittings look very similar, but the brass used will decay through galvanic action much more quickly than DZR brass or manganese bronze items, negating the time spent replacing the fittings and more importantly putting the safety of the boat at risk.

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