Cracking up

Cracking up

Bob McCunn explains how sticking to good installation practice can prevent what is the relatively rare, but destructive and costly occurrence of stress corrosion cracking when using copper alloys.
Copper alloy fittings have excellent corrosion resistance, and – if installed correctly – can confidently be expected to provide trouble-free service above and beyond the manufacturer’s guarantee period. Many systems will last the lifetime of the buildings in which they are installed, but designers and installers need to take some basic precautions in order to achieve this. Largely, these precautions have been incorporated into various standards and codes of practice such as the BS6700, and include the need for protection in certain aggressive (or potentially aggressive) environments in order to avoid external contamination.
How does stress corrosion cracking occur?
Stress corrosion cracking occurs in duplex brasses and is commonly referred to as season cracking. In the case of copper alloy fittings, it is manifested by the appearance of inter-granular cracking, usually (but not always) disposed along the longitudinal axis of the component.
Development of stress corrosion cracking depends on the simultaneous occurrence of four essential factors:
• Residual stress in the fitting
• A stress-cracking agent
• Moisture
• An alkaline environment
Residual stress in copper alloy fittings (duplex or yellow brasses)
During most stages of the manufacturing process, copper alloy fittings are cold worked, which leads to the presence of residual stress. Additional stresses are introduced through further cold working etc during installation – such as tightening in threaded connections and compression fittings.
However, stresses that are introduced during manufacture or subsequent cold working operations are normally of no consequence as evidenced by the many millions of copper alloy fittings that have given excellent service throughout the world. Problems only arise when copper alloy fittings are exposed to specific stress cracking agents, whilst under unfavorable conditions.
Stress cracking agents
There are specific stress-cracking agents for brasses, usually ammonia (or its derivatives). Moisture present in an alkaline environment may absorb ammonia, and such solutions – in conjunction with residual stress – can initiate stress corrosion cracking.
Potential sources of ammonia
• Decomposing organic detritus
•    Refrigeration gases
•    Nitrogenous fertilizers
•    Urine
•    Domestic chemical cleaners
•    Floor tile adhesives
•    Concrete additives
•    Insulation materials
Moisture
Moisture can arise from various sources including soils, cement or concrete, condensation etc. However, moisture generally doesn’t pose a problem for copper alloys. It’s only when the moisture absorbs aggressive constituents from component surroundings that problem may arise.
An alkaline environment
Alkalinity itself doesn’t not cause stress corrosion cracking and copper alloys generally have excellent corrosion resistance in alkaline environments. With plaster and concrete being predominantly based on alkaline cement, most construction sites can be considered as alkaline – as can certain insulation materials if they become wet.
Advice for installers
Good installation practice will normally prevent the combination of factors that lead to stress corrosion cracking.
• Use good quality, flat-faced spanners that do not damage the capnut when it’s being tightened. Fittings should not be over-tightened.
• Don’t use of serrated jaw wrenches as they can introduce additional stresses.
• Apply a light oil to the threads on compression fittings above 28mm to reduce the force needed to tighten the fitting, reducing stress.
• Use PTFE tape, or an approved sealant, when tightening male taper threads in female parallel threads so as not to over tighten. Do not use hemp.
• Chilled water applications are one of the highest risk areas that are likely to suffer from stress corrosion cracking due to the presence of moisture, alkalinity and stress in brass.
Installers should make sure that there is no stress-cracking agent (as outlined above) in the vicinity of the installation, including pipe-work insulation materials. If in doubt, alternative jointing systems should be considered.
• Fittings should be protected from aggressive moisture where conditions are likely to be conducive to the development of stress corrosion cracking. Use an appropriate impervious tape spirally wrapped around the fitting. However, bear in mind that insulation materials are primarily designed to reduce heat loss and should not be considered as a protective medium unless stated otherwise by the lagging manufacturer.
In summary, installers should ensure threaded and compression fittings are not over tightened and that the environments into which they are installed are not detrimental as outlined above.