032 HPM 0517

HPM May 2017

32 T R A I N I N G & T E C H N I C A L In the last two issues, HPM’s techical expert, John Love, has explained what corrosion is and the fact that it is a very complex process which can occur for a variety of reasons. There are many different causes of corrosion and some, or all, can be present in an untreated This month, the symptoms of corrosion are highlighted heating system. Sometimes, after an initial period of fast corrosion, a stable condition may be reached, with the layer of black iron oxide forming a thin protective layer over the steel, and reducing the rate of further corrosion to a low level. In other cases the corrosion will continue unabated. The effect of corrosion can be evident in many ways, the most obvious being water leakage. With steel panel radiators, corrosion will often occur along a bottom seam due to crevice corrosion, or at the top as a result of general corrosion, which can be accelerated by the presence of any flux residues floating at the water line. Even a fully vented radiator will still have air trapped at the top, above the level of the outlet hole in the vent plug. Corrosion indicators Other indications of corrosion taking place are: • Poor or sluggish water circulation, especially in the lower parts of the system • Bacteriological growth in an open vented system header tank • Frequent venting of radiators required, especially if the vented air has the characteristic 'bad eggs' smell of hydrogen sulphide • Cold areas extending up from an area centrally at the bottom of radiators • Boiler noise due to corrosion debris restricting the water flow through the boiler • Black water coming out of the radiator air vents, when opened • Pump failure. This can be due to bearings being worn as a result of the abrasive effect of the black iron oxide, or as a result of its having seized up due to the oxide sludge, which is magnetic, being attracted to the rotor. “After an initial period of fast corrosion, a stable condition may be reached, with the layer of black iron oxide forming a thin protective layer over the steel, and reducing the rate of further corrosion to a low level” So what actually causes corrosion? The first cause is invariably due to the presence of dissolved air, containing oxygen. In a correctly designed and installed system, most of the air will come out of solution during the initial period of operation and collect in the radiators where it can be vented. During this period oxidic corrosion will occur at an initially fast rate, but gradually reducing to a lower rate as the oxygen in the air Corrosion is common with steel panel radiators that suffer from water leakage is depleted. But if air is continually introduced into the system, the corrosion will continue at a fast rate. This air can get in via a variety of methods. These include: Suction leaks: It’s possible to have a very slight leak at a joint which will not allow water to escape but will allow air to be drawn in. This usually occurs at the union connection onto the pump, where the seal is provided by a composition washer. It is a problem that can be difficult to understand, but I have had jobs where, after exhausting all other possibilities, I have re-made the joint on the inlet to the pump, using a sealant, and the problem disappeared – this can occur with sealed systems. Safety open vent aeration: With an incorrectly designed/installed open vented system, water can be pumped out of the safety vent pipe and drawn back down the cold feed pipe, or air can be pulled down the safety vent pipe when the pump is running. This will result in continuous aeration of the water. It may sometimes only occur briefly when the pump starts (with a low initial flow rate the pump will develop its greatest head) but the result will be the same. Leaks: A system leak will result in fresh aerated water being admitted into the system via the feed and expansion tank (F & E tank), or when a sealed system is “topped up” to rectify a pressure loss. Incorrect water level: With an open vented system, if the floatvalve is adjusted so that the water level is too high in the F & E tank, water will expand back into the cistern when it heats up and raise the water level to the overflow. If water is lost out of the overflow then every time the system cools down, an equal quantity of fresh aerated will enter via the floatvalve. Even without aeration of the system water, there are a number of other causes of corrosion, such as the use of mixed metals (particularly with aluminium), filling with 'corrosive' water and electrical faults resulting in small currents leaking to earth. One additional cause is the use of excessive flux. Next month, I will look at preventing corrosion. John K Love CEng, FCIBSE., FIPHE., FIDHE., MInstR., FConsE What causes corrosion? www.hpmmag.com May 2017


HPM May 2017
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