In situations where two different metals are immersed in sea water, conditions are created whereby they effectively become a battery. This means that an electrical current flows between the two metals which though completely separate from each other, become linked via the sea water.
The result is that electrons, which make up the current, are supplied from one of the two metals, which in effect loses small particles of itself in the form of metal ions to the sea water. This creates something known as ‘galvanic corrosion’ which if ignored can damage the metals and ultimately cause them to fail.
Marine heat exchangers manufactured using admiralty brass tubes or its variants, are highly susceptible to this problem and in most cases, the only way to prevent the ‘dezincification’ of the brass alloy tubes is to install a zinc anode, (also known as a ‘pencil anode’ or ‘sacrificial anode’).
Zinc anodes work by introducing a third metal into the circuit that gives up, or releases its electrons faster than the other metals in the heat exchanger. So in effect, it ‘sacrifices’ itself to protect the other metals in the heat exchanger.
Whilst zinc anodes are a practical solution in principle, in service they can create problems, as by their very nature they wear out over time and need to be replaced. How soon this is required depends on a number of factors including the amount of zinc required for a given surface area – which varies with the types of metal being protected, and the minerals within the sea water. To monitor this, it is recommended the metals being protected are regularly checked for signs of corrosion, which involves draining, removal and disassembly of the unit to inspect the internal surface areas.
The anode must also be regularly inspected for wear and replaced once around half the zinc has been lost to corrosion. This involves draining the seawater circuit to enable the zinc plug to be removed for inspection and replacement.
So whilst a zinc anode offers protection for heat exchangers using brass alloy tube, wouldn’t life be a whole lot easier, if your heat exchanger didn’t need one?
This is the conclusion Bowman came to when developing their marine heat exchanger and oil cooler range!
Consequently, Bowman heat exchangers are supplied as standard with copper nickel alloy tubes, which eliminates the need for a zinc anode. In fact, were a zinc anode to be fitted to a Bowman heat exchanger, it could actually destroy the copper oxide film built up by the tube as a natural defence, allowing the tube surface to be attacked!
Additionally, Bowman marine specification heat exchangers are manufactured using corrosion-resistant end covers of either composite*, ‘C’ coat* or DZR Brass (dezincification-resistant brass) materials. This means that the possibility of a galvanic reaction between the end covers and the tubes is reduced so significantly as to be virtually eliminated.
There are literally hundreds of thousands of Bowman marine specification heat exchangers and oil coolers in use around the world operating reliably, proving the design and specification Bowman use to be extremely sound for these applications.
And in addition to copper nickel tubes, Bowman also offer titanium tubes as an option, which not only provide the highest levels of corrosion protection, but also have a 10-year guarantee covering all titanium material in contact with sea water!
For more information on Bowman marine heat exchangers and oil coolers, download our ‘Marine Heat Exchangers and Oil Coolers’ brochure below.
*For more information on ‘Composite’ or ‘C Coat’ end covers, download the ‘Product Bulletins’ below.
