A failure of a pump or agitator shaft can be an expensive item because of other consequential damage. Corrosion fatigue is something which can particularly affect rotating components, such as pump or agitator shafts, which are operating in corrosive media. Although it has been known for a long time that sharp steps in shafts can act as stress raisers, this is not always properly accounted for in designs. The local stress intensification factor can be high if the step is very sharp. If a radiused change of diameter is introduced, the local stress can be lowered and the greater the radius of curvature, the lower the local stress during operation.

When specifying cast copper alloys, for instance for pump or valve bodies, there is sometimes excessive concern to prevent dezincification by not allowing any alloys containing zinc. This is because an alloy known as manganese bronze is, in fact, a high tensile brass, and it is very susceptible to dezincification in seawater. Gunmetals are alloys of copper, zinc, tin and lead that have given excellent service in both fresh water and seawater for many years without de-alloying. This is because they all contain less than 10% zinc and this is not sufficient to allow dezincification to occur, even in seawater. Zinc-free alloys like phosphor bronze are difficult to make pressure-tight as castings, while the lead in gunmetals ensures good pressure-tight castings.

Brasses containing 15 to ~37% zinc will dezincify in aggressive waters, but this can be prevented by small additions of arsenic. What is sometimes forgotten is that other alloying elements can have a high zinc equivalence and, if present in significant quantities, can make a notionally single phase brass two phase, and hence susceptible to dezincification. Silicon and aluminium have high zinc equivalence factors and it is important to keep the quantity of these elements low or reduce the zinc content to compensate (as is done with aluminium brass). Where doubt about the composition exists, insist on a corrosion test to BS EN ISO 6509, which will demonstrate resistance to dezincification.

Dealloying is a well known phenomenon, but it is not always appreciated which alloys are susceptible to dezincification and which are not. Brasses containing up to ~15% zinc are single phase and do not dezincify in aggressive waters. Brasses containing 15 to ~37% zinc are single phase and will dezincify in aggressive waters, but this can be prevented by small additions of arsenic (usually 0.03 to 0.1%). Brasses containing more than ~37% zinc are two phase, or duplex brasses and these will dezincify in aggressive waters. Arsenic, or other, additions cannot inhibit the dezincification of two phase brasses, although the addition of ~1% tin, as found in naval brass, will slow it down.

It is frequently thought that the greater the potential difference between two metals, the greater the likelihood of galvanic corrosion. However, it is the magnitude of the current density between them that determines the metal loss. Hence, a large cathode and a small anode will result in a large corrosion rate due to galvanic coupling. Galvanic corrosion can be prevented, or at least minimised, by making the anode much larger than the cathode (100 times greater area or more).