Stress corrosion cracking (SCC) is a well-documented failure mechanism in brass water fittings and remains a significant concern in plumbing systems. In severe cases, undetected SCC has resulted in sudden failures, leading to extensive water damage and repair costs reaching tens of thousands of pounds.

SCC occurs when a susceptible material is exposed to both a corrosive environment and a tensile stress. In brass components, this typically manifests as cracking that can propagate over time with little external indication of deterioration.

Brass fittings have been shown to suffer SCC in various environments, including ammonia-containing solutions, water systems, and in the presence of ions such as chlorides, sulphates, and fluorides. The interaction between these chemical species and mechanical stress creates ideal conditions for cracking, especially in installations where high residual stresses or over-tightening are present.

Figure 1 – leak of a water fitting which led to flood damage.

Material:
Stress corrosion cracking sensitivity for brass alloys generally increases as zinc contents increases ^[1]. The conditions conducive to this type of mechanism involve exposure to a corrosive environment in combination with stresses within the material related to either forming, manufacture, fit-up, and/or service conditions ^[2][3]. Brass alloys containing a zinc content in excess of 15% are typically susceptible to stress corrosion cracking when in certain environments ^[4].

Stress:
An adequate tensile stress must also be present for stress corrosion cracking to occur. Tensile stresses can be applied externally and/or internally. External stresses are applied by tightening and/or loading during service, while internal stresses are produced either by deformation during cold work or unequal cooling during manufacture ^[5]. The stress applied to brass fitting components is the sum of residual stress from production, the stress due to the water pressure and the stress arising from assembly or installation. The threshold stress below which SCC will not occur can vary from 10 to 70% of the yield strength (depending on the alloy, environment, and temperature) ^[6]. Higher internal stresses are typically associated with higher hardness, though a low hardness does not necessarily correspond to low internal stresses.

Figure 2 – Stress Corrosion Cracks of a bras water fitting.

Corrodent:

The source of ammonia for SCC to occur can be diverse, including the decomposition of organic matter, heat from brazing operations, or even corrosion inhibitors such as Hydrazines or nitrites.^[7][8] Nitrites can be converted to ammoniacal species by breakdown or reduction, which can give rise to the corrosion of copper or its alloys.^[7]

Compounds containing chlorine and sulphur can be found in water being used in the fitting but at levels not considered excessive enough to cause corrosion issues in brass components. However, if these compounds are allowed to concentrate, then the levels can significantly exceed the levels in the bulk environment, increasing the likelihood of stress corrosion cracking. Further, the concentration of aggressive species such as chlorine or sulphur does not need to be high for SCC to occur. Initiation involves a competition between localised corrosion and crack growth. The former is strongly dependent on concentration, but weakly dependent on temperature, whilst the latter is strongly dependent on temperature, but is relatively unaffected by concentration and pH ^[9][10]. Jamal Choucri et al investigated CW602N, along with grades CW617N and CW724R, to assess their corrosion behaviour and susceptibility to SCC in simulated drinking water with different chloride contents (ranging from 100 to 700ppm). At 100ppm of chloride, the SCC susceptibility was found to be greatest where surface dezincification was less intense, while it was visible inside the cracks and the crack tips. The level of chloride in a localised area can be much higher than the bulk environment if allowed to concentrate i.e. areas of stagnant water or crevices. Under the simultaneous presence of stress and aggressive environment, brass dezincification is often reported to accompany and interact with stress corrosion cracking (SCC). In particular, dezincification associated to SCC has been documented in ammonia solutions, drinking water and in solutions of chlorides, fluorides, perchlorates, and molybdates [8]. According to some authors, dealloying and SCC stimulate each other synergistically: strained regions are more prone to selective Zn dissolution, while the obtained brittle Zn-depleted Cu film favours crack initiation and propagation ^[8].

Figure 3 – Stress Corrosion Cracks of a bras water fitting.

Prevention:

1) Review chemical dosing/inhibitor use:

1. Evaluate water treatment protocols to ensure that corrosion inhibitors and chemical dosing agents are used at the correct levels.

2) Correct assembly practices:

1. Tighten fittings to the manufacturer’s recommended torque values. Over-tightening can introduce excessive stress into the fitting, increasing susceptibility to SCC.
2. Avoid the use of tools that may damage threads or introduce stress risers (e.g., pipe wrenches on unsupported fittings).
3. Ensure proper use of thread sealants or lubricants where recommended, to avoid galling and allow uniform stress distribution during tightening.

3) Review of materials selection and whether a more SCC resistant material is required.

4) Minimise residual and external Stresses:

1. Use fittings that have been stress-relief annealed, particularly after cold-forming operations.
2. Design systems to avoid excessive bending or loading on fittings, especially near joints or transitions.
3. Support piping to prevent external mechanical loads from being transferred to fittings.

5) Environmental Controls:

1. Ensure that the plumbing environment is free from sources of ammonia, such as cleaning agents, or decaying organic matter.
2. Maintain good water quality, including control of pH, chloride levels, and biological activity, all of which can influence SCC behaviour.

For more information or expert advice on stress corrosion cracking in water fittings and its prevention, feel free to contact us at info@r-techmaterials.com. Our team of specialists can assist with material integrity challenges and provide support to ensure the safety and longevity of your engineering systems.

References:

• Chen, H., et al., ‘Analysis on Stress Corrosion Cracking of Brass Connecting Nut Utilized in Pneumatic Mechanism for 500kV Tank Circuit Breaker’ in Journal of Physics: Conference Series 1676, UK: IOP Publishing. 2020.
• Fabiszewski, A. S., ‘Failure of Brass Gas Cylinder Valves by Commercial Leak Detector Fluids’ presented at Corrosion 2003, Paper No. 03512, US: NACE International. 2003.
• ASM International, Corrosion in the Petrochemical Industry, US: ASM International. 2011.
• American Petroleum Institute, API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 1^st DC: American Petroleum Institute. 2020.
• LaQue, F. L., Corrosion Resistance of Metals and Alloys, US: Reinhold Publishing.
• Herro, H. M., The Nalco Guide to Cooling Water System Failure Analysis, US:
  McGraw-Hill Inc. 1993.
• Hineman, M. A., Danko, M. J., and Schmidt, F. E., ‘Example of Stress Corrosion Cracking in Copper Piping for Heating and Cooling Systems’ in Microsc. Microanal. 17 (Suppl 2), US: Microscopy Society of America. 2011.
• Copper Development Association Inc., Copper Tube in Buildings: Publication 88, UK: CDA. 1991.
• Rizvi, et. al., ‘Sodium nitrite as a corrosion inhibitor of copper in simulated cooling water’ in Scientific Reports, GER: Springer Nature Ltd. 2021.
• Schroeder, C. D., Solutions to Boiler and Cooling Water Problems, NL: Springer. 1991