- Design an accelerated test for Cu-Ag-P brazed sections to determine susceptibility to corrosion.
- Obtain empirical data on corrosion rates for standard brazed sections at different temperatures.
- Determine the mechanism of sulphide-induced selective attack on copper brazes.
The concept of this CRP is to develop an accelerated testing procedure for brazed assemblies which can be used to evaluate whether their products/pipwork are at risk. The key outputs from this work will be:
- An accelerated testing procedure for qualifying Cu-Ag-P brazed samples.
- Data on the corrosion rates of Cu-Ag-P brazed standard test specimens at several different temperatures.
- Knowledge summary on the mechanism of sulphide-induced selective attack on copper brazes.
- Provide guidance on appropriate sulphur corrosion testing and characterisation procedures for Cu-Ag-P brazed copper tubing.
- Provide a unique source of quantitative information on the corrosion rates of Cu-Ag-P brazed copper tubing.
Relevant Industry Sectors
Copper-silver-phosphorus (Cu-Ag-P) brazes are currently widely used for joining copper-to-copper pipe sections and in heat exchangers for air conditioning, plumbing and refrigeration.
The method is cheap and avoids the use of fluxes and any associated cleaning procedures. However, if the fluids being conveyed by the pipework contain sulphur compounds, a corrosion phenomenon known as sulphide-induced selective attack may occur. This process can occur in a range of different environments and is characterised by dissolution of copper in braze metal and often in the adjacent parent metal, typically leading to the formation of black corrosion products.
Sulphur can originate from a range of sources, including bacteria in water, lubricant oil, and vulcanised rubber hoses. The brazing quality can also influence the corrosion rate, as porosity traps corrosive fluids and amplifies the problem. Sulphur corrosion of Cu-Ag-P brazes is not widely reported in the technical literature, although recognised as a problem within the industry. TWI has recently performed an arbitration regarding this subject on behalf of two non-Member companies (a hose manufacturer and a global supplier of air conditioning units). There have also been several Member enquiries on the issue of sulphur corrosion, asking for advice on the maximum “safe” level of sulphur for a given temperature and time of operation.
There is a need to understand in more detail the conditions which result in sulphur corrosion, and what level of corrosion should be expected for a given temperature and time. An accelerated test is required to allow end-users to check susceptible pipework quickly.