Generic solutions to problems caused by water in heating devices

The correct choice of treatment will depend on the specific problems encountered and will depend on whether the water system involved is direct or indirect. However there are two main categories:

• Chemical Treatment
• Physical Water Conditioning

Below is a brief summary of the solutions identified in general terms. Industry experts in these fields have written factual accounts of the solutions available and how and why these methods work. Where possible links have been included direct to manufacturers product information.

Again falls into two main categories: The first being the most commonly known, the direct internal addition of a chemical inhibitor to an indirect system that inhibits or neutralises the corrosive or hard water condition.

System cleansing

In order to minimise corrosion, treatment of the water with an inhibitor is essential, however, for a corrosion inhibitor to function effectively, the metal surfaces must be clean. The British Standard Code of Practice BS 7593: 1992 details the steps necessary to clean a domestic central heating system. The Code recognises that it is not possible to clean a system without the application of a cleanser. Different products may be used depending on the nature of a system involved.

The most effective corrosion inhibitors act by reacting with the surface of the metal to produce a protective film in the form of a stable complex. The effectiveness of a given corrosion inhibitor will depend on its concentration.

Normally this metal is protected by a film of aluminium oxide which prevents corrosion in water (or in air), but under acid or strongly alkaline conditions the oxide film dissolves exposing the metal. Some waters found in the UK will give rise to sufficiently alkaline conditions in a central heating system to promote corrosion of aluminium and the gassing associated. An increasing number of central heating systems contain aluminium so it is advisable that a neutral (neither acid nor alkaline) corrosion inhibitor product is selected in every case.

John R Lane, EurChem C.Chem M.W.M.Soc MRSC B.Sc ARCS

In aqueous systems, there are two basic ways to inhibit corrosion:-

• Remove the corrosive oxygen from the water, either by mechanical or chemical deaeration.
• Add corrosion inhibitors

Corrosion inhibitors

A corrosion inhibitor is any substance which effectively decreases the corrosion rate when added to an environment.

• Passivating (anodic) inhibitors form a protective oxide film on the metal surface. They are the best inhibitors because they can be used in economical concentrations, and their protective films are tenacious and tend to be rapidly repaired if damaged.
• Precipitating (cathodic) inhibitors are simply chemicals which form insoluble precipitates that can coat and protect the surface
• Adsorption inhibitors have polar properties which cause them to be adsorbed on the surface of the metal. They are usually organic materials.

Passivation inhibitors

Examples of passivators (anodic inhibitors) include chromate, nitrite, molybdate, and orthophosphate. All are oxidizers and promote passivation by increasing the electrical potential of the iron. Chromate and nitrite do not require oxygen and thus can be the most effective. However, due to health and environmental concerns, use of chromate and nitrate has decreased significantly. Both molybdate and orthophosphate are excellent passivators. Molybdate is an expensive material however, Molybdate can be a very effective inhibitor, especially when combined with other chemicals

Preciptating inhibitors

Orthophosphate is a good example of a precipitating inhibitor which exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator.

Copper corrosion inhibitors

The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond directly with cuprous oxide at the metal surface, forming a "chemisorbed" film.

Adsorbtion inhibitors

Adsorption inhibitors must have polar properties in order to be adsorbed, such as amines. Often, these molecules have dual functionality. They contain a hydrophilic group, which adsorbs onto the metal surface and an opposing hydrophobic group, which prevents further wetting of the metal.

Silicates

For many years, silicates have been used to inhibit aqueous corrosion, particularly in potable water systems. Their mechanism of inhibition has not yet been firmly established. They appear to inhibit by an adsorption mechanism. Silicates are slow-acting inhibitors, in some cases, 2 or 3 weeks may be required to establish protection fully.

An alternative chemical dosing system uses high quality food grade polyphosphate crystals which dissolve slowly into the water stream, coating any metal parts which are in contact with the water, with a thin film. This film effectively inhibits the build up of limescale and the corrosion of metal pipes. The automatic process of proportional dosing controls the amount of polyphosphate released into the water. (Liff Industries Ltd see manufacturers section.

The only proven method of external water treatment is the process of base exchange softening. The British Standard Code of Practice BS 7593: 1992 discourages the use of softened water in central heating systems, as softened water tends to be more corrosive, unless a corrosion inhibitor suitable for softened water is added.

In the hard water areas of the country, many homes now enjoy the luxury of softened water, and as most units are installed on the incoming supply (except for a drinking water tap in the kitchen), the central heating feed and expansion cistern is usually fed with treated water.

A common misconception is that softeners lower the dissolved solids concentration in the water, whereas they actually function by exchanging the hard calcium ions for soft sodium ones. They do this by passing the water through a synthetic resin, which has the appearance of small plastic beads. The resin can combine either with calcium or sodium, but as it prefers to combine with the former, calcium rich water passed through it will be discharged containing sodium in its place. Eventually, the unit will be almost entirely converted to the calcium form. To restore it to the sodium rich state it is regenerated by washing it with strong brine (sodium chloride). The brine has the effect of reversing the normal process, and the calcium can be washed to drain, ready for the cycle to be repeated.

Physical water conditioning

This method includes a number of applications the most common being:

• Permanent Magnet.
• Electro Magnetic.
• Electronic.
• Electrolytic.
• Electrostatic.

These solutions effectively change the way in which water behaves. Using these devices the scaling effects are reduced or delayed and pipes can even be cleaned of existing scale build up. (UKPWCA, lets get physical brochure)

Essentially, physical scale inhibitors whether magnetic, electrolytic or electronic, work in similar ways by changing the behaviour of natural salts in the water so that they remain in suspension rather than sticking to the surfaces of the pipes and associated plant. (quote by Mike Pickavance of the UKPWCA from the June edition of PHPI)

The simplest system uses a series of permanent magnets within a housing, plumbed into the piping in such a way that water passing through the unit is subject to a magnetic field. This magnetic field causes the water to pick up an electrostatic charge, resulting in a temporary alteration in the shape of the salt crystals. Effectively, it changes them from their usual cuboid shape to a needle-like structure, which is more readily washed through the system without adhering to surfaces. (quote by Mike Pickavance of the UKPWCA from the June edition of PHPI

Electro magnetic

Similar characteristics to permanent magnet but have a more powerful magnetic field and longer life. Normally need to be fitted close to the boiler as they work when the water flows through them. Once the flow stops the charge is limited until reactivated. The simplest system uses a series of permanent magnets within a housing, plumbed into the piping in such a way that water passing through the unit is subject to a magnetic field. This magnetic field causes the water to pick up an electrostatic charge, resulting in a temporary alteration in the shape of the salt crystals. Effectively, it changes them from their usual cuboid shape to a needle-like structure, which is more readily washed through the system without adhering to surfaces. (quote by Mike Pickavance of the UKPWCA from the June edition of PHPI)

Electronic

Electronic water conditioners are different in that they are not dependant on the flow of water or the use of chemicals. A high frequency signal is used to impart energy at a molecular level via a device on the outside of the pipe. The signal is transmitted directly into the water 24 hours a day both up and down the system, therefore all the water is treated at the same time.

(Hydroflow (UK) Ltd see manufacturers section) The effect of the radio waves is to change the crystallisation characteristics of the calcium in the water, destabilising them and preventing them from forming into scale. (Liff Industries Ltd see manufacturers section)

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A small electric current passing through the water effectively changes the molecular structure of scale forming crystals, preventing them from forming a hard, insulating layer on boilers, pipework and appliances. This system modifies the physical composition of the ions but no chemical reaction takes place. (Liff Industries Ltd see manufacturers section) In solution calcium, magnesium, and other mineral salts are partly ionised and therefore subject to influence by electromagnetic and electrostatic energy. Increased Ionisation of the soluble water content minimises precipitation or the deposit of mineral salts thus reducing scaling. (Ion water products see manufacturers section.)

Electrostatic

The kinetic energy of a moving stream of water creates a charge, which is simultaneously released into the water. This upsets the stability of particles in the water, which are kept in suspension by the equal charges. By neutralising the charges and upsetting the balance of suspension the particles are encouraged to coagulate and precipitate bringing with them scale producing substances. By stimulating early, unregulated precipitation, low grade irregular crystals are encouraged to form thereby preventing the larger regular crystals, which constitute hard scale. (The Rodin Group see manufacturers section.)