Engine heat exchangers have a common problem of corrosion due to the
difference in metal between casing and tube stack. Employing
zinc pencil anodes reduces the corrosion on the casing prolonging
the life of both casing and tube stack whilst in seawater.
Zinc is the only material that has an official specification, the
American MIL-A-18001 K spec. Zinc is a well-proven anode
material and its capacity is virtually unaffected by the operating
current density, although the driving voltage diminishes with
increasing temperature therefore it has limited application.
If zinc is not to this specification i.e. high purity zinc 99.99%,
it will have a limited use as an anode material because of iron
present as an impurity.