Stress- Corrosion cracking of copper single crystals

  • PDF / 3,072,131 Bytes
  • 6 Pages / 594 x 774 pts Page_size
  • 96 Downloads / 223 Views

DOWNLOAD

REPORT


I.

INTRODUCTION

PUREmetals

are usually considered to be very resistant, and perhaps immune, to stress-corrosion cracking (SCC). There have been reports of intergranular SCC in copper,~'2 but it can be argued that this is due to impurity segregation at the grain boundaries. The observation of transgranular SCC in copper was therefore an important development. 3 The environment was a 1 M sodium nitrite solution open to the air at room temperature, and constant extension rate testing was used. The average crack velocity was about 1 nm per second, assuming that cracks were initiated early in the test. The fracture surfaces were too corroded to permit useful microscopic studies of the cracking mechanism. These experiments were repeated elsewhere with applied electrochemical potentials, and it was confirmed that this was a genuine SCC phenomenon. 4 Very recently, similar tests have been carried out on single crystals at the Georgia Institute of Technology;5 evidently the fracture surfaces, though corroded, show cleavage-like features similar to those found in a-brass. The transgranular SCC of brass is now known to proceed by a series of crystallographic cleavage events. 6-9 It is therefore possible that SCC of copper involves micro-cleavage fracture. This would constitute a new development in the mechanical behavior of ductile metals, rivaled only by the embrittlement of aluminum by liquid gallium, l0 In this paper we report a detailed fractographic study of nitrite SCC in copper. The new feature of the experimental procedure is the combined use of single crystals and an applied potential. Acoustic and electrochemical noise are examined for evidence of discontinuous crack growth.

II.

EXPERIMENTAL PROCEDURE

A copper crystal of >99.99 pct purity was obtained from Leico Industries. It was a rod 1.3 cm in diameter, and had an axial orientation about 9 deg from (110). Tensile sheet specimens measuring 30 x 8 x 1 mm were obtained by making longitudinal cuts with a low speed diamond saw. K. SIERADZKI, Metallurgist, and R. L. SABATINI, Metallurgy Associate, are with the Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973. R.C. NEWMAN, formerly Associate Metallurgist, Department of Applied Science, Brookhaven National Laboratory, is now Lecturer in Corrosion Science, Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, United Kingdom. Manuscript submitted February 27, 1984. METALLURGICALTRANSACTIONS A

These were either used smooth (with a reduced gauge section 10 mm long) or notched on one edge to a depth of 2 mm. The specimens were mounted between serrated grips for SCC testing, with a plastic cup to contain the 1 M NaNO2 solution. The notched specimens were coated with lacquer except for the notch root, to improve the sensitivity of the electrochemical measurements. Acoustic instrumentation was used as described previously. 9 A potential of 0 V (SCE) was applied using a potentiostat. The temperature was controlled at various values between 25 and 40 ~ u

Data Loading...