Some plumbing fittings are subject to corrosion. When this occurs, they are weakened and eventually leaks develop. There are several ways of preventing this happening in your system. Modem galvanized steel water storage cisterns frequently show signs of rust within a few months of being installed. Older plumbing systems, which were constructed entirely of lead or galvanized steel, could generally be expected to last, without this kind of deterioration, for 50 years or more. In modern systems the use of copper, which itself virtually never corrodes, has greatly increased the risk of corrosion to any galvanized iron or steel fittings incorporated in the system. The process which gives rise to this corrosion is known as electrolytic action.
This is the same principle on which the simple electric battery cell is based; where rods of zinc and copper are in electrical contact with each other and are immersed in a weak acid solution which is able to conduct an electric current (an electrolyte), electricity will pass between the rods, bubbles of oxygen will be produced and the zinc rod will slowly dissolve away. A plumbing system in which copper water supply and distribution pipes are connected to a galvanized steel cold water storage cistern or hot water storage tank, may reproduce these conditions; the copper tubing and the zinc coating of the galvanized steel are in direct contact and the water in the cistern or tank, if very mildly acidic, will act as an electrolyte. This results in rapid failure of the protective galvanized coating allowing aerated water to penetrate to the vulnerable steel underneath; eventually rust will form. A particular form of electrolytic corrosion may result in damage to brass plumbing fittings, such as compression joints and stop valves. Brass is an alloy of copper. and zinc; electrolytic action may result in the zinc in the fittings dissolving away to leave them unchanged in appearance but totally without structural strength. Where these fittings in your plumbing system are showing signs of leakage, it would be worth checking with a local plumber if the type of water in your area is likely to create a situation favourable to electrolytic corrosion. If so, you should replace brass fittings with fittings made of gun-metal (an alloy of copper and tin), which is not susceptible to this kind of damage.
Corrosion as a result of electrolytic action is also likely to occur in pipework if a new length of copper tubing is fitted into an existing galvanized steel hot or cold water system. Always use stainless steel tubing instead – this is not liable to the same risk.
Protecting cisterns and tanks
There are steps you can take to prevent corrosion in galvanized steel cisterns and tanks. For example, when you are installing a new cistern or tank, it is important to make sure you remove every trace of metal dust or shaving resulting from drilling holes for tappings. The least fragment remaining will become a focus for corrosion. One way of protecting a cold water storage cistern is to ensure the metal of the cistern does not come into direct contact with the water it contains. This can be done by painting the internal surfaces with two coats of a taste and odour-free bituminous paint to prevent these surfaces from rusting. Before applying this treatment to a new tank, cut holes for the pipe connections: when you are painting. pay particular attention to the areas in the immediate vicinity of these holes.
Galvanized steel hot water storage tanks, which can still be found in many older homes, cannot be protected by this paint treatment.
Cathodic protection
A sacrificial magnesium anode which dissolves instead of the zinc coating will protect both galvanized steel cold water storage cisterns and hot water tanks. With a cold water storage cistern, clamp the copper wire attached to the anode to the side of the cistern using a G-clamp and suspend the wire over a timber batten into the water in the middle of the cistern. Make sure you replace the anode before it dissolves completely. When protecting a hot water tank, fit the anode to the hand-hole cover of the tank. Turn off the water supply and drain the system from the draincock beside the boiler; unscrew the bolts retaining the hand-hole cover and remove it. Drill a hole in the centre of the cover, use abrasive paper to rub down the area of metal around the hole and screw in the anode before replacing the cover.