Chemical bother

A great deal of knowledge is brought to bear in ensuring domestic hot and cold water systems meet design specifications for water flow and temperature when they are installed.

Despite this, the likelihood of corrosion affecting the performance and integrity of systems over the long-term is not often considered. This is an oversight because, if a domestic hot water system is not designed correctly, corrosion can cause it to fail.

The factors influencing this type of corrosion are design, installation, commissioning, water quality and operating conditions – each of which is considered later.

The corrosion process

There is always enough dissolved oxygen in drinking water systems to enable corrosion under the right conditions.

The main difference between “once-through” drinking water systems and enclosed recirculating water systems with regard to corrosion is the water’s composition. The water in drinking water systems is highly aerated and, since it is usually regularly renewed, does not change significantly in composition while in the system.

Recirculating water systems, however, contain low, or very low, amounts of dissolved oxygen and the water composition may change radically during its lifetime. Dissolved oxygen in the water acts as the main cathodic reactant (reaction 1, below), which in turn drives anodic metal dissolution reactions (reaction 2).

(1) O2 + 2H2O + 4e- Õ 4OH-

(2) M Õ Mz+ + ze-

eg: Fe Õ Fe2+ + 2e-

The rate of metal and alloy corrosion in drinking water systems is limited by the formation of surface layers. These may be microscopically thin passive layers, as with stainless steels, or substantially thicker corrosion product layers as with copper and galvanised steels, which limit corrosion.

Although it may be possible thermodynamically for corrosion to occur, kinetic factors usually govern whether the reactions take place at a rate sufficient to cause problems. Corrosion failures occur when these normally protective layers become disrupted, or don’t form in the first place.

There are many different forms of general and localised corrosion in domestic water systems. Some are very rare and specific to certain alloy types; for example, stress corrosion cracking of brasses and graphitisation of cast irons.

Others are common to several metals and generally occur more frequently – pitting corrosion, erosion corrosion, microbial corrosion and galvanic corrosion, for instance. Each of these will be discussed in more detail as they relate to the influencing factors described below.

Design as an influencing factor

Most of the corrosion problems relating back to design occur in larger, more complex hot water systems where there is a secondary hot water return.

If the flow rate in a system is too high, it will cause excessively turbulent flow, especially at fittings such as elbows, tee-pieces and changes in cross-section, removing the protective film and leading to erosion corrosion.

Erosion corrosion can occur on all metals, but in drinking water systems it is far more common on copper pipes. An example of this is shown left in Figure 1. Here, the protective basic copper carbonate patina has been removed downstream of the joint, exposing the underlying bright copper surface.

A characteristic of this attack is horse-shoe shaped grooves in the metal, where the 'horse' appears to have walked upstream. If left unchecked, erosion corrosion can result in wall perforation in months, or even weeks.

If flow rates are kept below 2 m/s in hot water systems and 3 m/s in cold water systems, erosion corrosion can be avoided. However, the critical flow velocities may have to be significantly lower than these values if other adverse conditions exist (see examples).

Dead-legs and large diameter pipes, which produce very low water flow conditions, should also be avoided wherever possible.

When flows are less than 0.5 m/s, debris in the system can settle on the bottom of pipes and give rise to under-deposit corrosion. This is caused by a differential aeration effect where, in the anaerobic conditions under the deposit, the metal becomes anodic to the surrounding pipe and corrodes preferentially.

Microbial attack

In addition, anaerobic bacteria may thrive under deposits, which gives rise to the possibility of microbial induced corrosion. This occurs as a result of acids produced by the metabolism of the microbes.

Temperature is important here, since microbial growth is promoted between 25 and 50ºC, so hot water pipes should be insulated, especially if they run next to cold water pipes. An example of microbial induced corrosion, showing mounds of corrosion product above pit sites, which typically have a 'pepper pot' appearance is shown in Figures 2 (a) and 2 (b).

Construction materials

Copper pipe must be specified to BS 1057. Some cheaper imports of copper pipe have carbon film residues left over from the extrusion process which, in certain water conditions, can lead to type one pitting (characterised by a distinctive hemispherical hollow) and perforation of the pipe within a few months. Since the advent of BS 1057, this form of pitting is extremely rare in the UK.

If brass components are to be used, they should be made from dezincification resistant alloys. If not, preferential dissolution of the zinc phase may occur in certain water conditions, leading to a weakened porous structure.

The choice of stainless steel grade depends on the water characteristic, but usually austenitic 300 grades are used in water systems. Galvanised components should not be used in hot water systems, otherwise severe pitting and blistering of the components can occur.

Coupling dissimilar metals together may cause galvanic corrosion. This is particularly true if galvanised steel is coupled to copper, due to the large electrochemical potential difference between them. In this case, intense attack would occur on the galvanised component close to the connection to the copper.

Coupling of copper or galvanising to stainless steel is generally not as bad as the potential difference between the metals would suggest, since the cathodic oxygen reduction efficiency of stainless steels is poor. If galvanised steel and copper are used in the same system, the galvanised components must be placed upstream to prevent indirect galvanic corrosion as a result of deposition of copper.

Installation and commissioning

Several factors can create conditions for corrosion when installing systems. If pipe ends are not de-burred after cutting, or if solder deposits are left inside joints, this can cause localised turbulent flow downstream of the imperfections, which may lead to erosion corrosion in copper pipework systems .

Excessive use of soldering flux may also lead to localised corrosion of copper under blobs of dispersed flux. Figure 3 (overleaf) shows a pinhole leak in a greasy flux run from a soldered joint.

Welding stainless steels in air produces oxide films on the surface of the steel close to the weld. If the colour of the scale produced is deeper than straw, this greatly increases the likelihood of pitting corrosion. Welding of stainless steels should therefore be carried out under inert gas shielding or the scales should be removed afterwards by pickling in acid.

Other welding defects, such as filler metal sagging, incomplete root pass and open pores, also increase the likelihood of pitting corrosion.

The main advantage of commissioning water systems is to clean the pipework of any debris and soldering flux, which prevents the corrosion problems above. After installation, both hot and cold systems should be thoroughly flushed according to BS6700.

It is very important that systems are kept full of water after the commissioning stage. If they are drained down, three phase boundaries, air-water-metal, are formed at small residual pockets of water, leading to localised corrosion at the water line. This problem can occur in all metals used in drinking water systems, not just copper.

Water quality

Generally, there is not a lot designers, installers and operators of systems can do about the quality of incoming mains water, apart from fitting filters if there is a risk of particulate matter entering the system. However, the chemical composition of mains water is an important factor in the likelihood of many different forms of corrosion for all metals.

In most cases, soft waters with low bicarbonate hardness are more aggressive for all metals than hard waters, where it is easier to build up protective surface layers.

It is well known that in soft water areas, lead pick-up from old pipes can be quite high and water companies are now dosing phosphate into the water in these areas in an attempt to rectify this. In soft water areas, the critical flow velocities needed to induce erosion corrosion for copper will be reduced.

High chloride levels in water increase the likelihood of pitting and crevice corrosion for most metals, but especially so for austenitic stainless steels. If the chloride content is less than 200 mg/litre in cold water and 60 mg/L in hot water, then 304 grade stainless is usually acceptable. However, at levels of chloride above these values, then the use of molybdenum-bearing 316 grade stainless is recommended.

Copper is unlike other metals, in that increased chloride in the water does not increase the risk of pitting attack. However, as the sulphate levels rise, so does the likelihood of pitting corrosion and general corrosion, leading to an increase in copper ions in the water and causing it to turn blue.

In exceptional circumstances, fitting dolomitic-limestone filters or selective ion membrane filters to the incoming mains supply can increase water hardness.

Operating conditions

It is desirable to bring systems into operation as soon as possible after commissioning. If the water is allowed to stagnate for long in the pipes, this increases the likelihood of microbial activity and, in copper systems, the build-up of non-protective, loosely adherent patinas.

In the latter, this may give rise to the occurrence of blue water after an interval of over a year. Blue water can result in high copper pickup, which may exceed the statutory maximum level of 2 mg/litre, and stain sanitary ware. An example of this type of staining is shown above in Figure 4.

Temperatures in cold water storage tanks should be below 20ºC, and in hot water systems always above 55ºC, to limit the growth of micro organisms including Legionella bacteria (which does not influence corrosion but is a health risk).

As the temperature increases above 60ºC, however, the likelihood of erosion corrosion in copper pipework also increases.

The flow rate in a copper-piped domestic hot water recirculating system, especially in the return pipework, should be limited to 2 m/s maximum (and less in soft water conditions) to prevent erosion corrosion.

This can be achieved by adjusting the speed of the recirculating pump and, if necessary, reducing static pressure in the system. If the static pressure is too high, it may cause high flow rates in parts of the system when taps are opened.

Standards

Each type of corrosion of system components described here can compromise the long-term performance of domestic hot and cold water systems.

However, adhering to British Standard BS EN12502:2004 will assist designers, installers and operators of domestic water systems in minimising the risk of corrosion damage to the system.

Later this year, another guidance standard, BS EN14868, will be published. This will look at factors influencing the likelihood of corrosion in recirculating water systems such as central heating systems, and further reduce the likelihood of corrosion damage.