ENGINEERING CONCEPTS: Hydrogen Embrittlement of Copper
Tough-pitch coppers and even oxygen-free coppers are subject to a loss of tensile ductility (embrittlement) when exposed to reducing atmospheres. Bright annealing in a hydrogen-containing furnace atmosphere or brazing using a reducing flame are typical processes that can induce embrittlement of these coppers. Damage can be minimized if the temperature is kept below 750F (400C).
The process of embrittlement involves the diffusion of atomic hydrogen into the copper and subsequent reduction of Cu2O to produce water vapor in equation 1.
Because hydrogen is required for this process, the loss of ductility is referred to as hydrogen embrittlement. An embrittled copper often can be identified by a characteristic surface blistering resulting from expansion of water vapor in voids near the surface. Metallographic examination also will reveal the embrittlement, but it is important to include one of the free surfaces in the mount because this embrittlement is related to a surface diffusion process, and the voids will be more extensive near the surface exposed to the reducing environment. Typically, these voids will be along grain boundaries.
Purchasing oxygen-free copper is no guarantee against the occurrence of hydrogen embrittlement, but the degree of embrittlement will depend on the amount of oxygen present. For example, CDA 101 (oxygen free electronic) allows up to 5 ppm oxygen while CDA 102 (OFHC) permits up to 10 ppm. A hydrogen embrittlement susceptibility test (ASTM B577-93) should be specified when purchasing coppers for use in critical applications where embrittlement is of concern. According to the ASTM specification, heat treatment in a furnace atmosphere containing at least 10% hydrogen for 20 to 40 minutes at 1560F (850C) is sufficient to produce embrittlement. Embrittlement of these coppers is somewhat subjective, since a bend test with a predetermined number of bends must be specified. For example, one might specify a single bend test where any cracks formed on the surface when the specimen is bent into a U shape with the legs of the U closed together is sufficient to fail the test. Some applications that are extremely sensitive could specify that no surface cracks are produced after several fully reversed bends.
Metallographic methods also can be used to determine the amount of Cu2O present prior to heating or brazing. In a polished, unetched specimen, the Cu2O particles usually are located along grain boundaries because these same particles pin the grains during recrystallization. These particles are blue when viewed using white light and ruby red when viewed in cross-polarized light.
Tensile ductility can be estimated using the Brown-Embury equation 2, where the true strain to grow the voids to fracture (_growth) is determined from the volume fraction of voids (Vf) present in the embrittled microstructure.
The fracture strain (_fracture) can then be related to the reduction in area (RA) using equation 3 where Ao and Af are the original cross-sectional area and the final area at fracture, respectively. A reduction in area (RA) of 18% would be predicted for a copper with 10% voids using the Brown-Embury formula whereas a typical soft annealed copper would produce 65 to 70% reduction in area. The ductility is reduced to zero when the volume fraction of voids exceeds 0.159. However, it should be noted that the Brown-Embury analysis assumes a uniform distribution of voids (or particles) in the matrix and that a lower ductility would be expected if void (or particle) clustering were present. Thus, the Brown-Embury analysis is expected to provide only an upper bound to the ductility for these embrittled coppers.
The Brown-Embury equation also can be used for microstructures containing a uniform distribution of second-phase particles. Figure 1 shows the tensile ductility for a variety of copper alloys as a function of the volume fraction of second phase or holes. A similar trend can be shown for resulphurized steel and cast irons. The total strain to fracture for these two-phase microstructures will be a combination of the strain to nucleate a void at the second phase particle and the strain to grow these voids as given by the Brown-Embury equation. At volume fractions of 0.159 and greater, the strain to fracture is entirely related to nucleating voids. In general, a greater strain is required for smaller and more spherical particles, whereas less strain is required as the strength of the second phase is increased.