Galvanic Corrosionby Brian Gatt*

A Guide to Galvanic Corrosion: Protecting your assets

Many boat owners will be familiar with corrosion and the costly nature of attending to the issues associated with metals in water. All metals will corrode when exposed to oxygen and moisture and revert back to the basic natural state of their raw material. Without preventative measures and proper maintenance, corrosion can occur very quickly and be costly. However, there are simple measures which can be taken to significantly reduce the effects of corrosion in terms of time and cost.
Metals are graded in ‘The Galvanic Series of Metals’, reflecting the ease at which they corrode. Softer metals corrode at a higher rate than harder metals. Sea-water is a great electrical conductor and can be a major contributor to the rate of corrosion.
Anode selection should consider the environment, the type of water and the type of vessel. For example, extruded anodes should be used in saltwater to protect non-metallic hull vessels, zinc anodes to protect metallic hull vessels, softer cast sacrificial anodes vessels in fresh water environments, aluminium alloy anodes near a river source and further upriver, and magnesium anodes in environments where there are numerous dissolved minerals, for example a clay water bed or fresh water.
In warm water a harder sacrificial anode is appropriate, in cold saltwater more sacrificial anode surface area is required to achieve adequate protection. The flow rate of the water will also affect the consumption rate of sacrificial anodes. Soft cast sacrificial anodes will be consumed by water erosion and may not achieve the required protection levels for the expected lifetime. Additional anodes or a harder, extruded sacrificial anode will be required in areas of higher water flow rates.
The briefly described scenarios highlight the importance of considering the environment that the vessel resides in to achieve the greatest efficiency and performance of a sacrificial anode. In addition, the outer metal surface area of the precious metal is where the corrosion will occur and where primary protection must be achieved. It is therefore crucial to protect the metallic fittings with a good protective coating and to not solely rely upon the anode to provide complete protection.
Remember that zinc anodes must be calibrated when fitted. Over-protection can result in increased marine growth, paint loss, premature antifoul loss, growth of contaminants on propellers and chloride within heat-exchange heaters. 95% of fibreglass and wooden vessels tested that are shaft-driven are well overprotected using zincs.
Wooden boats using zinc anodes can sometimes see problems with wood rot and alkali attacks. Using a more passive extruded anode will dry wood rot and reduce alkali.
Water type, water pollutants, marina structures, other vessels and the water bed all impact on the expected life-time of protection. Some man-made environments (marinas) can affect vessels in differing ways. Therefore it is necessary to adjust the vessel’s protection method to suit the particular environment.
Shore power is an extremely common cause of corrosion. Imagine that all the vessels connected to shore power are part of one big battery. It is necessary to remove your vessel from what is commonly called the “daisy chain” effect in order to not be part of the battery which is contributing to the anode consumption. There are simple methods, easily employed, most notably the use of an isolation transformer or a galvanic isolator (or electrolysis blocker).
While isolation transformers offer the best protection, they are a lot more expensive than galvanic isolators. Every vessel without at least an electrolysis blocker is at risk of galvanic corrosion, which can lead to excessive anode loss and corrosion beyond the capacity of the boats cathodic protection system. The highest risk vessels within a marina are vessels with sterndrives or saildrives. This is due to the metal being softer compared to other conventional vessels. An electrolysis blocker is essential in ensuring your vessel remains protected from the marina or other vessels with earth leakage problems.
Reducing electrolysis flowing through marinas will reduce individual boat owners’ costs due to anodes dissipating into our waterways, or worse still, expensive maintenance due to electrolysis of expensive parts of the boats. For the marinas too, considerable savings on maintenance of the marina assets can be realized by reducing electrolysis.

* Written on behalf of aquavolt (02) 9417 8455 by specialist contributor Brian Gatt of Electralogic (02) 4381 0790 | www.electralogic.com.au | brian@electralogic.com.au