Erosion corrosion

After dezincification had been eliminated by the introduction of arsenic-inhibited alloys, the next problem to arise in brass condenser tubes for steam turbines was inlet-end impingement attack associated with higher water speeds. Brasses, like all metals and alloys other than gold, platinum and a few other very expensive "noble" metals, owe their long-term corrosion resistance to the protective effect of thin, adherent, films of corrosion products which form during the early life of the component and form a barrier between the metal surface and its corrosive environment. Water flow conditions which produce high water velocities at the protected metal surface can generate shear forces sufficient to cause local removal of the protective corrosion product film, exposing bare metal to corrosion, and to sweep away the fresh corrosion products resulting from this exposure before they can form a new protective layer. Such conditions are obviously associated with high average water velocities but arise particularly where excessively turbulent flow - as often occurs at the inlet ends of heat-exchanger tubes - gives rise to local water velocity much higher than the average flow rate. The severe local attack that results is commonly termed impingement attack or, more accurately, since it is the result of corrosion of the metal combined with erosion of the corrosion product film, erosion corrosion.

Recognition

Metal that has suffered erosion corrosion exhibits a smooth water-swept surface usually without corrosion products. Localised attack, often associated with local turbulence, immediately downstream of an obstruction, gives individual water-swept pits, undercut on the upstream side and often horseshoe shaped with the open end of the horseshoe pointing downstream. More widespread attach produces a broad smooth surface in which small horseshoe shaped features are often visible.

Apart from its characteristic form, erosion corrosion can often be recognised by its occurrence in regions where local turbulence might be expected. Common situations, apart from the inlet ends of condenser and other heat exchanger tubes, are immediately downstream of elbows, tee pieces and valves - particularly partly-closed valves.

Avoidance

Choice of alloy

The problem of inlet end impingement in sea water cooled condenser tubes was largely cured by the invention of aluminium brass. This alloy, first used for condenser tubes in 1928, remains one of the preferred alloys for this purpose, though in competition with 90/10 and 70./30 copper-nickel and more recently with titanium. Table 24 indicates the relative resistance of admiralty brass, aluminium brass, 90/10 copper nickel and 70/30 copper nickel to erosion corrosion in sea water in terms of recommended maximum design water velocities for tube-and-shell condensers or heat exchangers of conventional design.

Table 28: resistance of copper alloy heat exchanger tubes to erosion corrosion in sea water ()

Slightly different figures are to be found in the literature, with 90/10 copper nickel sometimes shown as marginally superior to aluminium brass. The two alloys are certainly very similar in resistance to erosion corrosion in sea water - small differences in pollution or operating conditions tending to favour one or the other. In polluted conditions (i.e. when the sea water contains sulphide) experience of the relative performance of these two alloys in service is still variable - some users finding aluminium brass superior and others favouring 90/10 copper nickel. It is often stated that for such conditions 70/30 copper nickel CN107 is superior to either, but experience in Japanese coastal power stations shows aluminium brass to be the best of the three alloys under the conditions obtaining there, though still not recommended for badly polluted waters (). While, as the data in Table 24 indicate, the erosion corrosion resistance of admiralty brass in sea water is inferior to that of aluminium brass, the substantially higher water speed required to produce erosion corrosion in fresh water results in admiralty brass being perfectly suitable for fresh water cooled condensers and heat exchangers. It is therefore the alloy most commonly used for fresh water heat exchange service and is, indeed, to be preferred to aluminium brass for this purpose since aluminium brass is liable to pitting corrosion in some fresh waters.

Design features

Having selected the right alloy for service in conditions where there is a possibility of erosion corrosion occurring, it is important also to eliminate design features likely to induce excessive turbulence in the water flow. To this end sharp changes of direction should be avoided by using swept bends rather than elbows, and swept tees or Y-pieces rather than right-angled tees.

Partially open valves not only induce turbulence in the water flow downstream, but may, because of the pressure drop across the valve, cause air bubbles to come out of solution; these can cause erosion corrosion to occur at water velocities below those at which it would occur in their absence. Flow control valves should therefore be sited where there will be least danger of erosion corrosion occurring as a result of air release and downstream turbulence. They should always be on the outlet side of heat exchangers rather than the inlet side and should, if possible, be followed by a straight length of pipe in which the water flow can become smooth again before the next flow-disturbing feature is reached.

Other protective measures

When aluminium brass was first introduced as a condenser tube alloy, it was recognised that it formed the best protective film only if iron compounds were present in the cooling water. Since, however, water boxes and cooling water mains were at that time of unprotected or poorly protected cast iron, there was no shortage of iron corrosion products. Later, with the adoption of coated water boxes and pipes, occasionally unexpected failures of aluminium brass condenser tubes by erosion commenced. It was then found that by providing iron in a suitable form - principally by injection of ferrous sulphate into the cooling water - the optimum performance of the aluminium brass could be ensured.

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