GUARDING AGAINST CORROSION

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.

Modern 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, 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, which is not susceptible to this kind of damage. Warning 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 as described earlier in the Course. 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.

Warning 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. Details of installation in a cistern were given earlier in the Course; the procedure for a hot water tank involves fitting the boiler 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.

Protecting central heating systems

A form of electrolytic corrosion can take place in a central heating system where copper tubing is used in conjunction with pressed steel radiators. Some air – a prerequisite of corrosion – will always be present in the system; it dissolves into the surface of the water in the feed and expansion tank and may also enter through minute leaks too small to permit water to escape.

Electrolytic corrosion within a central heating system results in the formation of black iron oxide sludge and hydrogen gas. This leads to impeded water flow and radiators will need continual venting to release airlocks to keep up the required heat level. The iron oxide sludge is drawn towards the magnetic field of the circulating pump and its abrasive qualities contribute towards early pump failure. Also the metal of the radiators, from which the magnetite and hydrogen are produced. becomes thinner until leaks eventually develop in the radiators.

Removing airlocks by venting the radiator is a simple process. A key supplied for this purpose is inserted in the radiator when the water is warm and turned anti-clockwise to open the vent valve. Hold a container underneath the key since some water may escape when the valve is opened. Air will come out of the radiator – when it stops doing so and water begins to flow you should tighten the valve. If a radiator in your heating system needs to be continually vented, it is worth testing for internal corrosion while you are carrying out this operation. Apply a lighted taper to the gas escaping from the radiator; hydrogen gas burns with a blue flame and indicates the presence of corrosion. Protection treatment A chemical corrosion-proofcr can be introduced into the feed and expansion tank to protect the system against corrosion. It is best to do this when the system is first installed, but it can be carried out with an existing system; it will not. however, undo damage already done. Before introducing a corrosion-proofcr into an existing system you should get rid of any magnetite sludge with a special solvent. Like the corrosion-proofer. this is introduced into the feed and expansion tank and you should drain the system first. Disconnect the fuel supply to the boiler and switch off the ignition system several hours before draining to give the water time to cool. Tie up the ball float arm of the feed and expansion tank and fit a hose to the drain-cock near the boiler, running the hose to a drain outside. Undo the draincock. empty the system and, when you have closed the draincock, free the ball float arm in the feed and expansion tank. Allow the system to refill, introducing the solvent at the same time. Follow the manufacturers instructions for the length of time you should allow for the solvent to complete its work before carrying out treatment with the corrosion-proofer.

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