Corrosion in Commercial Heating and Hot water Systems -3

Part 3 Preventing Corrosion – Glass Versus Stainless Steel

There are several methods for addressing different forms of corrosion that occur within commercial hot water systems.

Adding tin to brass, for example, creates Dezincification Resistant Brass (DZR). Fittings manufactured from this alloy will be marked in the UK with the letters CR (Corrosion Resistant) or DZR (dezincification resistant).

Commercial glass-lined steel water heaters and tanks are also attractive propositions as the glass is, given the right conditions, generally resistant to attack from most chemicals and corrosive materials. The glass is composed of several oxides and silicates blended and heated to the melting point. The first coat of glass is applied to develop a chemical bond with the steel of the tank and has limited corrosion resistance. After the ground coat, layers of chemically resistant glass are then added to create a smooth surface which is more resistant and easier to clean, making them popular in harder water areas.

However, not all glass-lining processes are equal with some being prone to developing microscopic cracks in the lining of the vessels. This means that small areas of the steel cylinder shell may get exposed to water, and there is an opportunity for corrosion to take hold.

Galvanic corrosion in water heaters and tanks is typically managed using cathodic anodes made of magnesium that offer a target for oxidisation in lieu of the steel shell, corroding or being ‘sacrificed’ first. Regular checks and replacement of anodes is critical in delivering ongoing protection from corrosion. Protective epoxy and plastic coatings can also be used to reduce corrosion by preventing conductivity from the water to the metal.

Corrosion in Commercial Heating and Hot Water Systems - Part 3The attack rate is determined by temperature, duration, and the concentration of reagents, for example, the presence of fluorides at any temperature will corrode a glass-lining.

In naturally soft water conditions, despite the use of sacrificial anodes, glass-lined vessels can rapidly succumb to critical corrosive damage. Due to a lack of dissolved metal ions, the purer soft water has low electrical conductivity, so the electrical flow from the anode to the cathode through the water is reduced. This adversely impacts the chemical reaction between sacrificial anode and cylinder shell, reducing the protection. In such cases, when the sacrificial anode is inspected its condition can be extremely good, but this is likely to be a strong indicator that the anode is failing in its role, meaning the water heater itself is being corroded.

The usual alternative to the sacrificial anode is the powered anode. Often made of titanium, the powered anode rather than giving up its own electrons and producing an electrolytic current produces a very low current in the water. This should provide a similar protective effect for a heater’s steel shell but without corroding the anode. However, in soft water areas, a powered anode may still not have a protective effect, as the conductivity required of the water by the anode is too poor.

For this reason, commercial hot water systems in Scotland, south-west and north-west of England and the west of Wales where water is particularly soft will typically need to employ a stainless-steel appliance. Better able to stand up to both water-side and combustion-side assaults, a stainless-steel heater is less susceptible to corrosion, due to the composition of the alloys, which create a protective oxide barrier on the waterside that naturally helps prevents corrosion, even when temperatures increase. Able to withstand higher temperature water (in excess of 80°C) than glass-lined appliances, stainless-steel lends itself to solar thermal and wider commercial applications.

Typically, stainless-steel will be used in indirect DHW heaters, where the internal heat transfer coil is connected to a boiler or a solar thermal collector loop; and in condensing water heaters where the push for higher efficiency condensing units has led to stainless steel being used to construct the heat exchangers. To achieve high efficiencies, flue gases must be cooled below the dew point to release the latent heat of condensation. With very low pH and high acidity, the resultant condensate would have a highly corrosive impact on the surfaces of the heat exchanger which regular steel or copper would struggle to withstand for any length of time.

Stainless steel is therefore preferable due to its versatility for creating intricate forms required by the heat exchanger and its high resistance to corrosion. This does mean stainless steel construction is typically more expensive, due to both higher material and manufacturing costs. Commercially, the investment is worthwhile, especially in soft water areas where, compared with replacement costs as a result of corrosion in glass-lined alternatives, stainless steel can prove far more cost-effective, with its quality reflected in the longer product warranties.

In the commercial world, domestic hot water (DHW) appliances are subjected to extremely hostile conditions, with high temperatures, thermal stress and flue gas condensate on the combustion side and oxygen, minerals and chemical attacks leading to potential corrosion on the waterside. Given this harsh daily treatment, regular servicing and maintenance are key if business-critical service is to be observed. Failure to descale, flush sediment, check anodes or test for corrosion will reduce the operational longevity of any appliance. In soft water areas, poor consideration of prevalent conditions and a lack of regular maintenance can reduce an appliance lifespan from years to a matter of months!

Correctly sizing, obtaining and then regularly servicing the right appliances and ancillaries for your application is critical if a hot water system is to operate safely, efficiently and cost-effectively.

Read Part 1 – Recognising the causes of corrosion

Read Part 2 – Testing for corrosion

Corrosion in Commercial Heating and Hot water Systems – 2

Part 2 – Testing for Corrosion

If your premises are located in a soft water area then you need to be aware of the threats of corrosion to your system, and small leaks in your metal plumbing components could well be a sign of a more serious problem.

Testing water for corrosivity factors – pH, calcium concentration, hardness, dissolved solids content and temperature – as well as dissolved lead and copper due to corrosion is relatively straightforward.

Water hardness.The original accepted measurement of water corrosivity, the Langelier Saturation Index (LSI), was developed to determine whether a water source is potentially scale forming or scale dissolving (aggressive).  LSI, however, was not developed with the intention of testing naturally soft water, so relies on the chemical characteristics of the water, such as pH and especially hardness, to estimate the corrosivity. Positive values are indicative of non-corrosive water, negative LSI values indicate water could be corrosive.

The problem with LSI is that it based upon the oft-cited belief that calcium carbonate scale when deposited in a thin, carefully controlled, uniform layer can contribute to a protective barrier against corrosion. There is little scientific evidence to support this assumption

The highly variable physical and chemical conditions within a hot water system mean it is very unlikely that a deposit of scale would ever be truly ‘uniform’. Deposition would typically be prevalent on the hottest elements of the boiler/water heater, and with the formation of crevices actually drive greater localised corrosivity in a fashion similar to pitting which would lead to the potential failure of an appliance. Additionally, as scale accumulates microscopic deposits of metal can be incorporated generating a ‘mini-galvanic’ effect.

Scale formation in a glass-lined indirect tank

Scale formation in a glass-lined indirect tank

Even without the issues of generating corrosion factors, it is never recommended that scale build-up be allowed in a hot water system. An increasing build-up of scale reduces the efficiency of an appliance by restricting the rate of flow and harming efficient transfer of heat. All of which drives up the operational costs.

For a more accurate prediction of calcium carbonate, and levels of corrosivity, the Ryznar Stability Index (RSI) was developed which considers pH, conductivity in TDS, calcium ions (CA²⁺), bicarbonate (HCO₃) and water temperature. An RSI of 6.0 -7.0 indicates water with little scale and the probability of some corrosion, with higher values being increasingly corrosive. Between 7.0 – 7.5, corrosion will be significant, becoming heavy between 7.5 and 9.0. Results above 9.0 are regarded as intolerable.

Both LSI and RSI are predicated on assessing the level of scale build-up, so an alternative approach is to test for metals concentrations, especially copper, which is achieved by drawing a sample to test directly from the hot water system. Detection of iron, manganese or aluminium impurities in the water can also be indicative of corrosivity as localised deposits can also cause pitting in copper.

The only caveat is that the concentrations in open systems, such as where instantaneous water heaters are deployed, will quickly flush by-products of corrosion out of the system. So regular testing is soft water areas can be advantageous when seeking to prevent serious damage to appliances and associated system pipework.

UK water suppliers will typically provide self-assessment help for establishing local water quality, and can often provide details of most recent water tests based on a business’ postcode. Independent testing is also available, though there will be a cost associated. At Adveco our engineers can help and advise with your water assessment as part of the application sizing process.

Read Part 1 – Recognising the causes of corrosion


Corrosion in Commercial Heating and Hot water Systems – 1

Part 1 – Recognising the Causes of Corrosion

Most metals will deteriorate or corrode, sometimes to a more stable chemical state through oxidation or reduction. This occurs over time when metals are in direct contact with any water, rusted iron being the most familiar, but it can also affect copper, lead, aluminium, zinc, and numerous other common metals. This becomes a real issue in water heating and distribution systems where metal appliances and pipes are continuously being attacked to the point of physical failure.

Corrosion in Commercial Heating and Hot water Systems - Hard and soft water areas of the United Kingdom and IrelandCorrosion is a complex phenomenon, and no single dissolved substance is responsible for making water corrosive. There are several factors that can increase the likelihood of corrosion, especially the natural softness of water. When water passes through limestone and chalk in the ground, such as in the South East of the UK, it will pick up calcium and magnesium carbonates, when these minerals are greater than 280ppm the water is classed as hard. However, in Scotland, the North West and South West of England, and Western Wales, where water passes through hard igneous rock it lacks dissolved calcium and magnesium.  This makes the water naturally purer (less than 100 ppm). This soft water exhibits a low pH, low total dissolved solids (TDS) and negligible buffering capacity, all of which makes it more corrosive.

pH measures the hydrogen ion activity in a solution and is used to express the intensity of the acidity of a solution. Typically, the ideal pH for a hot water system is slightly above 7 on the pH scale. Water with a low pH (below 7) is acidic, which is a problem as acids are compounds that release hydrogen ions which oxidize metal, accelerating corrosion. In general, the lower the pH, the more aggressive the corrosion.

There can be a range of reasons for the formation of anodic and cathodic sites, required to produce corrosion. Different materials used in the manufacture of the appliance or pipework, localised stresses, impurities and variances in the production of the metal, its composition and ‘grain size’ can all lead to surface imperfections. If localised variances are relatively small the anodic and cathodic sites will move around on the surface of the metal leading to a more uniform corrosion which is typically seen as surface oxidation or fouling.

Should the anodic sites be more static, localised corrosion can occur. This form of corrosion – which includes pitting, leaching and galvanic corrosion – is a more serious problem which can more rapidly lead to the failure of an appliance or pipework.

Pitting, one of the most destructive types of corrosion, occurs when there are large differences in surface conditions, leading the anodic and cathodic sites to become stationary. The process is exacerbated by low-velocity conditions, leading to the creation of a pit on the surface of the metal, the water inside becomes isolated and, over time, more corrosive as it produces an excess of positively charged metal cations, which attract chloride anions. In addition, hydrolysis produces hydrogen (H+) ions. The subsequent increase in acidity becomes self-sustaining as the concentration within the pit promotes even higher corrosion rates.

Leaching is the selective corrosion of a single element from the alloy. The most common occurrence in a building’s hot water system is the removal of zinc from brass (a copper-zinc alloy), a process also known as dezincification. Though the copper and zinc dissolve out simultaneously, the copper will precipitate back from the solution. The resultant copper alloy will change from a yellow brass to red colour and exhibit poor mechanical property. Common in cheaper valves and fittings where there is likely to be other ‘filler’ metals in the copper alloy, water containing sulphur, carbon dioxide, and oxygen, low pH conditions, low velocity and high free chlorine radicals drive especially aggressive corrosion causing fittings or valves that move to fracture and leak.
The complexity of commercial hot water systems, especially if the project is a refurbishment, can lead to two dissimilar metals (such as copper and stainless steel) coming into contact with each other and water. Under these conditions the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the nobler (cathodic) metal decreases. This is Galvanic corrosion.

The galvanic scale - Recognising corrosion in commercial heating and hot water systemsWhen differing metals are connected in a hot water system, the water in contact with both metals acts as an electrolyte conducting the current. The current flows through the water from the positively charged less noble material to the negatively charged more noble material. Where the current leaves the less noble metal, corrosion will occur. As the current is usually greater close to the contact point of the two metals, this is where corrosion will be a greater issue. The higher the metal is on the Galvanic series, the nobler the metal will be, whilst the greater the distance between the two differing metals in the series, the greater the electrical potential will be and the greater the corrosion rate for the less noble metal.

Another major cause of corrosion found in commercial hot water systems is a direct result of oversizing or the failure to correctly balance water flow. An unfortunately common occurrence, oversizing a system not only raises the capital expenditure and the running costs of a hot water system, but the oversizing of the pumps leads to high-velocity hot water to circulate through the system. If there are any suspended solids in the water, they will be driven against the metal leading to erosional corrosion which is typified by smoothly grooved or rounded holes which mirror the directional or turbulent flow of the water. This erosion is most notable at points where water changes direction or is obstructed, leading to turbulence which further increases velocity and therefore the damage. If the high-velocity flow is not addressed quickly it can result in considerable damage, especially to the circulating pipework.

Certain chemicals (such as chlorine, chloramine and dissolved oxygen) can also make water more corrosive. The presence of oxidizing agents such as dissolved oxygen can cause metals to lose electrons and lead to corrosion. The removal of sulphate, or addition of chloride, the Chloride-to-sulphate mass ratio (CSMR) will accelerate corrosion in the presence of materials that contain lead, leaching it into the water. Sulphates inhibit corrosion by forming passive protective film layers and reducing galvanic currents between dissimilar metals, chlorides prevent the formation of such passive layers and stimulate galvanic current. Should the source water contain natural levels of chloride and treatment be installed to remove sulphate, the expectation is this would push the CSMR up and as a result, accelerate corrosion. The base 60°C requirement for commercial hot water can worsen such cases as high temperatures accelerate almost all chemical reactions. As temperatures hit 70°C, which is not uncommon in commercial systems the rate of corrosion will increase.

Read Part 2 – Testing for corrosion