Copper and its alloys have been widely used in plumbing for over 50 years. This has been due to relatively low cost, ease of use and non-toxic nature. In the vast majority of cases, copper and its alloys have given excellent service over the expected life of the system.

Failures are usually due to a combination of adverse circumstances, which can be grouped under design and installation, operating condi-tions, water quality and chemical composition and surface condition of the metal.

Copper tube can pinhole due to a number of different mechanisms of pitting or erosion corrosion. General corrosion stains sanitary equipment, and copper ions in water can cause attack on other metals downstream.

General corrosion

The generally excellent corrosion behaviour of copper pipework is due to the formation of a deposit or scale on the surface. In soft, acidic waters with relatively high sulphate levels, protective scales form slowly or not at all. This water is termed cupro-solvent. Under normal use and flows, there is seldom a problem. However, if very low flow or stagnant conditions exist, e.g. in dead-legs, the films may be non-adherent and disrupted, giving rise to blue-water problems.

Erosion corrosion

Probably the most common cause of pin-holing is due to erosion corrosion. This occurs where protective scale is disrupted or removed due to local high, turbulent-flow velocities. The attack manifests itself in steep-sided grooves or horseshoes within the region cleaned of green scale (Fig. 1). It usually occurs at elbows, or tee-pieces or just downstream of valves or fittings. In cold-water pipes, erosion corrosion can occur at flows in excess of 3 m/s, but in hot-water pipes it can occur at less than 1.5 m/s. The critical velocity is influenced by water quality, with soft water inducing attack at lower values. Localised turbulence from burrs on pipe ends may cause erosion corrosion, even if the mean flow rate is too low to cause such attack.

Pitting corrosion

Pitting is due to separate and distinct mechanisms. The classic type I pitting due to continuous carbon films inside pipes only occurs with organically pure, moderately hard waters from deep wells or bore-holes. Since copper pipes to standards such as BS EN1057 are free of such residues, the problem only occurs with cheap imported pipe.

Hot, soft-water pitting, sometimes referred to as type II pitting, is also very rare. These irregular shaped pits are usually covered with blue copper hydroxy-sulphate.

Perforation due to type II pitting takes several years and only occurs in waters with a relatively low bicarbonate : sulphate ratio and at over 60oC.

Localised attack may also occur under deposits, usually at the 6 o’clock position. This is due to a differential aeration cell being set-up, with the area under the deposit becoming anodic compared to the surrounding copper surface. The time to perforation is usually in excess of seven years. However, if anaerobic bacteria, especially sulphate-reducing bacteria, grow under the deposit, microbially influenced corrosion (MIC) may occur as a number of tiny pits under a nodule of corrosion product, and is thus sometimes called pepper-pot corrosion. A classic appearance of MIC on a copper pipe showing green nodules and the pitted surface under a nodule can be seen in Fig. 2.

Microbial growth and, hence, MIC is most likely at temperatures of 25-55OC. Both deposit attack and microbially influenced pitting attack are more likely in stagnant or very slow flowing water. Low flow rates allow debris and corrosion products to settle, and stagnant conditions allow the stabilisation of corrosion cells. Under-deposit attack can occur under lagging if it is damaged and moisture can enter.

Excess flux may cause perforation of copper pipes, sometimes some distance downstream of the joint because fluxes are aggressive to copper. Self-cleaning fluxes are more aggressive. Modern fluxes are water dispersible and, unless a large excess is used, seldom give problems.

Other corrosion forms

Components made from copper alloys, particularly brasses, may suffer dezincifi-cation. Zinc is selectively removed, leaving white corrosion products and porous copper, with reduced mechanical strength. The alloy may change colour from brass yellow to red. Brasses may be prone to attack in soft waters of low pH. However, the likelihood of dezincifi-cation decreases with increasing copper content, and brasses with more than 85% copper are highly resistant. Copper-tin-zinc-lead alloys (gunmetal) are considered immune.

Brasses may also be subject to stress-corrosion cracking. The material must be subject to tensile stresses, either residual from manufacture or the presence of specific chemical agents. For brasses, this can only occur in waters with much higher levels of nitrite or ammonia than are found in drinking waters. However, reduction of nitrates in the water may occur in critical areas, e.g. in crevices or under deposits, to give sufficient concentration of nitrite/ammonia to cause stress-corrosion cracking (SCC).

A particular localised attack known as Rosette corrosion has been found in domestic copper hot-water cylinders, but only in cylinders with aluminium protector rods and in certain waters. The attack occurs under the aluminium hydroxide corrosion deposits at the base of the cylinder. Work by the Water Research Council indicated that the water had to contain a relatively high level of nitrate. As described for SCC above, the mechanism is believed to involve the reduction of nitrate to ammonia under the deposit.

Preventing corrosion

Almost all these problems can be prevented by good practice in design, installation and operation, even with adverse water quality. Design and installation should be executed to BS 6700 and CIBSE guidelines.

Type I pitting can be avoided by using copper pipe to BS EN1057 with a BS kite-mark. Dezincification can be avoided using components of dezincifi-cation-resistant brass or gunmetal. The system should be designed for flows between 0.5 and 1.0 m/s for hot water and 0.5 and 2 m/s for cold water. Higher flow rates may cause erosion corrosion at elbows and bends, and lower flows allow debris to settle in the pipes. The design must avoid dead-legs.

Cut pipe-ends should be deburred and, if soldering is used, excess flux avoided. If the system is hydraulically tested, it must not be drained prior to use or 3-phase boundaries (air/water /metal) will be set-up at pockets of water, producing localised corrosion cells, which may lead to perforation. The system should be flushed with clean water to thoroughly remove debris and flux residues and put into operation as soon as possible after commissioning.

In large buildings like hotels, hot-water systems are often unvented and have recirculating pumps. Pressure settings and pump speeds should be sufficient to give a ready supply of hot water to all rooms but not excessive, or erosion corrosion may occur. Temperatures at calorifier heaters should be set to about 60-65OC, with returns about 55OC. To avoid the growth of micro-organisms, the temperature in hot- and cold pipes should not remain between 25 and 50OC for long periods. Hot-water pipes should thus be lagged and inspected periodically. If torn lagging becomes wet due to leaks or condensation, copper pipe-work can be attacked from the outside.

Larger systems, such as those in hotels and hospitals should be disinfected before entering use and periodically thereafter according to ACOP 2000 and BS6700 to prevent the risk of legionella. Should signs of MIC be found, either disinfection with chlorine or pasteur-isation with water at about 90oC for an hour should be carried out.

If private water supplies are used, where the pH is low or where the bicarbonate: sulphate ratio is relatively low, consideration may given to using a dolomitic limescale filter. If dissolved iron and/or manganese are more than a few mg/l, a manganese filter on the supply pipe may be necessary.

Summary

Tubes and components made of copper or its alloys generally give excellent performance in domestic hot- and cold-water and heating systems. Where corrosion does occur, it is almost always due to non-adherence to good practice in design, installation, operation or maintenance. Even then, failures may not be apparent, unless other adverse circumstances, such as particular water chemistry, are also present.

Phillip Munn is with Corrosion & Environmental Services Ltd, Midway Business Centre, Bridge Street Industrial Estate, Clay Cross, Derbyshire S45 9NU.
[pmunn@cesl.biz]

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