How to prevent pipework corrosion


29 July 2019
Insulating pipework and support brackets with the same material improves energy efficiency and protects against bi-metallic corrosion in both cold and hot pipework. Anthony Barnett, technical marketing manager at Armacell, looks at the factors to consider. 

The durability of materials obviously has a bearing on the longevity of all building elements. Factors such as condensation and vibration/structural stress all take their toll on HVAC systems. While it’s the designer’s responsibility to ensure that exposed elements of a building are fit for purpose, it is important that suppliers provide their input, especially in the often overlooked area of bi-metallic material corrosion.
Bi-metallic or galvanic corrosion occurs as a result of the flow of very small electric currents in the presence of moisture and oxygen between two dissimilar metals. This causes the more anodic of the two metals to corrode, whilst the other metal (called the ‘noble’ metal) remains unaffected. For example, brass or copper pipework (ranked higher on the Galvanic series) with steel fittings and hanger brackets (ranked lower) will result in the steel undergoing accelerated corrosion. 

It’s worth bearing in mind that the two metals don’t need to be in contact for galvanic corrosion to occur. When a more ‘noble’ metal corrodes slightly and dissolves in moisture from condensation and subsequently drips over a less noble metal it will result in corrosion of the latter. Galvanic corrosion is therefore an issue that needs to be taken into account at design stage when specifying both pipework, fittings and support brackets.

However, using two or more different metals within an HVAC system does not necessarily mean corrosion will occur if you follow a few basic rules. In most cases insulating the pipe, fittings and hangers with an appropriate material will separate the two metals, eliminate condensation and avoid any galvanic corrosion. 

Insulating copper-zinc alloys (brass) pipework
Specifying an inappropriate insulation can actually aggravate the problem of corrosion on pipework by concentrating sodium chloride particles against the pipe surface, causing stress corrosion cracking (SCC), even on stainless steel. Predominantly, this is caused by the chloride particles always being present in the general construction and building environment, such as on the hands of the installer in the form of sweat.  

The corrosion is triggered by the chloride ions in combination with moisture and operating temperatures above 35°C. As such, the percentage by mass share of water-soluble chloride ions in the insulation material for stainless steel pipes should not exceed 0.05%. Armaflex is especially suitable for use on stainless steel, as it is produced completely without halogens (chlorides). Even then, it is important to avoid contamination of the steel with chlorides from hands or surrounding air.

Copper-zinc alloys (brass) pipework is vulnerable to crystalline stress corrosion cracking at temperatures above 50°C. Ammonia or nitrides (ammonia and nitrogen combinations), in the presence of moisture, cause this type of corrosion. To combat this, we recommend insulation materials such as Armaflex, which has a mass share of ammonia that does not exceed 0.2%.

We also recommend a foil vapour barrier underneath the insulation after a thorough cleaning of pipe surfaces, to avoid issues with both halogen and nitrides.

Support brackets
Pipe support brackets can be a potential trouble spot due to a number of factors. Firstly, they can be susceptible to galvanic corrosion (e.g. steel to copper contact). In addition, direct fixing of cold services pipework to their mounting brackets acts as a thermal bridge from surrounding warmer ambient air. Since these un-insulated supports on chilled water and refrigeration pipework fall below the dew point, moisture is continually drawn to the cold brackets where it condenses into the adjoining insulation material. If the condensation spreads through the insulation, thermal efficiency is lost and corrosion and consequential secondary damage will result. 

A common problem with pipe support brackets is that they compress the insulation. This can place the copper and steel in closer contact (galvanic corrosion) as well as causing a loss of energy efficiency. That’s why pipes in refrigeration and air conditioning applications must always be isolated from their mounting element with an adequate closed-cell insulation thickness, taking into account the line temperature, pipe size, ambient temperature, relative humidity conditions as well as halogen and nitride content of the material. It is important to use pipe support hanger brackets that do not compress the insulation and maintain the same thickness along the length of pipe. 

Within hot water systems there can be significant heat loss as a result of uninsulated pipe brackets or, again, where the material has been compressed, with the consequences being increased energy costs. The extent of this loss of energy can be significant with thermal heat flow calculations for a 60°C hot water pipe in continual use showing a heat loss of 0.06 w/k per bracket (on a 26.9mm diameter copper pipe with a 27mm insulation thickness). This can have a major effect in total energy costs of running a building especially where a large number of supports are used. 

A system-based approach of installing pre-insulated pipe support brackets that prevent the crushing of insulation can result in significant energy savings. For example, when Armafix Ecolight supports are used with the equivalent insulation tube thickness, the required insulation is maintained to provide an energy efficient system. This improves the energy efficiency of both hot and cold pipework, with an annual energy saving of approximately £2.50 to £5 per bracket. This quickly adds up when you consider that a pipework system in a large installation may incorporate hundreds of pipe supports. 

A reliable connection between the clamps and an ArmaFlex insulation can only be achieved by sourcing a compatible pipe support from the same insulation manufacturer, because it ensures matching nominal wall thicknesses and system performance. This requirement has driven advances in pipe support systems and some of these, such as our Armafix Ecolight are able to support and protect the insulation from being crushed or damaged, maintaining the thermal integrity of the system. These pipe supports are now being widely used wherever foam insulated pipes require suspended support. 

Sustainable materials
There has been much work done on improving the sustainability of materials in the building services industry, which is one of the reasons why our pipe supports are now made from 100% recycled PET plastic bottles. This is an extremely environmentally friendly material with a low thermal conductivity and a high compressive strength. They incorporate PET load bearing segments with ArmaFlex insulation on the outer part for connection with the adjoining insulation material.

This approach avoids the pipe coming into direct contact with the bracket and eliminates crushing of the insulation. In terms of mechanical performance it eliminates cold spots and prevents condensation forming on refrigeration and air-conditioning service pipework. By helping to control condensation, the risk of corrosion and secondary damage to surrounding goods and equipment can be prevented.

The PET segments can be completely recycled at the end of their service life, with all production scrap flowing back into new PET blocks. PET has an excellent environmental performance requiring over 60 percent less energy for its production than PUR, and releasing over 80% less CO2.

We can see that pipe brackets represent a potential weak point in both hot and cold pipework systems. If the pipe is not thermally isolated from the pipe bracket, thermal bridges causes condensation or heat loss, as does crushing of the insulation by the bracket hanger. 

Most designers understand that the presence of different metals in any element of a building can result in galvanic corrosion. All metals react with their environment, although some are more reactive than others. Knowing this, and factoring it in at design stage can avoid issues with the HVAC system. 

That’s why insulating both pipework and pipe support brackets plays an important role in maintaining system efficiency and integrity. An integrated approach, where the insulation materials are from a single source offers greater compatibility in terms of condensation control, fire behaviour, energy saving and ease of installation.
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