Internal corrosion of a fire pump can result in costly repair to the pipes, major components, and other pump related accessories on your fire apparatus. This is sometimes described as electrolysis; however, this process is actually galvanic corrosion. Combating galvanic corrosion and its harmful damage to your components can be done by following a few simple steps and recommended procedures.
When dissimilar metals are placed in water and electrically connected, ion migration will occur. One metal will act as an anode and the other as a cathode and these ions will then transfer from the anode to the cathode. In a fire pump, the cast iron body most often becomes the anode and the other parts of the pump become the cathode. The casting will begin to wear away in an attempt to migrate to the other components causing the parts to literally disappear and form pits or holes over time. When a device with a third composition is added intentionally to combat this process it becomes a sacrificial anode. Materials commonly used as sacrificial anodes include, Magnesium, Zinc, and various Aluminum alloys. When these metals are connected physically or electrically, the ion migration will shift from the pump to the weaker sacrificial anode and provide cathodic protection of that fire pump.
One of the most visible and accessible of sacrificial anodes is the suction screen located in the intake piping of the pump. The screen is manufactured not only to prevent large debris from entering the pump but its composition allows it to act as an anode. Overall condition of these parts should be inspected monthly and can be done rather easily by removing the intake valve or cap. The officer’s side intake screen is often missed because it may be capped or unused but it is also an integral part to the pumper’s cathodic protection. Suction screens should be complete with no missing or broken sections and worn or damaged screens should be replaced promptly. A screen and the groove it sits in should be cleaned to remove debris prior to reinstallation to promote a strong physical bond; poor connections can decrease the effectiveness of these anodes.
Most fire pumps also contain additional anodes that are installed into the pump discharge and require routine inspection or replacement. Some installations may be visible by removing an exterior elbow and peering inside the 2 1/2 inch ports on the pump panel, consult your manufacturer’s literature or Department shop to find out if this is an item that can be seen by the operator during routine inspection. The maintenance requirements will be highlighted in the pump manufacturer’s procedures; however, most manufacturers specify inspection every six months. Darley provides an excellent video on their website which walks personnel through the procedures for inspecting the anodes on their fire pumps. Hale also offers an optional tank anode warning system on their fire pumps that will alert the operator with a visual warning when anode replacement is needed.
Departments who have routine service work conducted by qualified personnel will often meet the minimum inspection criteria, but it is important with galvanic corrosion to remember that the ion migration is happening even if the vehicle is not in operation. Lack of use will not necessarily increase the service intervals and poor water quality can further reduce the effective lifespan of these safeguards. Anodes will inevitably wear and disappear, when this occurs the pump will again be the target of corrosion. Adhering to the inspection guidelines is imperative to prolonging pump life and minimizing unnecessary repairs.
Justin Claibourn
Washington Fire Mechanics