oxidation and cracking of copper

[Simon55]

This is sort of a two part question.
At what temperature does copper begin to experience rapid oxidation in atmospheric conditions? Is this temperature related to the type of oxide being formed? (eg CuO vs Cu203).

On a separate issue, has anyone come across branching transgranular type cracking in oxygen free pure copper - if so any idea what could cause that? I have observed this in some rotor bars from an induction motor.

 

[TVP]

ASM HANDBOOK Volume 13 Corrosion is an excellent reference, and is the reference for the following information. You can obtain it from ASM at

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=10488

.

When heated in air, copper develops a Cu₂O film that exhibits a series of interference tints (temper colors) as it increases in thickness. The colors range from dark brown when the layer is 37-38 nm in thickness to red when the layer thickness is 124-126 nm.

Black cupric oxide, CuO, forms over the Cu₂O layer as the film thickness increases above the interference color range. Scaling, or general oxidation, results when copper is exposed to air or oxygen at high temperatures. At temperatures less than 100 C, the oxide film increases in thickness logarithmically with time.

Beyond the interference color range, the growth rate can be approximately determined by the following equation:

W² = kt

where W is weight gain (or increase in equivalent thickness) per unit area, t is time, and k is a constant. k varies with temperature and oxygen concentration, from ~ 4.4 x 10^-8 at 400 C and pure O₂ to 1.78 x 10^-3 for pure O₂ at 1000 C. k in air is obviously less.

As for transgranular cracking, this is likely corrosion fatigue. Corrosion pits start to develop, and then cracks initiate at the base of the pits. Again, ASM HANDBOOK Volume 13 has some good information on this.

 

[kenvlach]

TVP gave an excellent, quantitative description of the oxidation of copper. I just want point out a site with color photos of copper sheet oxidizing in air on an electric kitchen burner. The oxidizing surface goes through the range of colors, and is covered by an opaque black CuO by the time the burner was glowing red. Heating was continued for 1/2 hr to thicken the CuO sufficiently that it formed a brittle layer which detached upon cooling, thereby exposing the pink Cu2O.
[note: there are some nuisance pop-up ads on this site]:

http://www.angelfire.com/ak/egel/solcell.html

 

[kenvlach]

Ran across a useful article:
"The Influence of Water Vapor on the Oxidation of Copper at Intermediate Temperatures"
Reidar Haugsrud
Department of Chemistry, Centre for Material Science, University of Oslo, N-0349 Oslo, Norway

J. Electrochem. Soc., vol. 149, pp. B14-B21 (2002).
[abstract only, copyrighted material]
"The oxidation rate of high-purity Cu has been measured by means of thermogravimetry in oxygen in the pressure range 1×10–4 to 1 atm and water vapor in the pressure range 3×10–5 to 0.022 atm at 500°C, respectively, and as a function of temperature under wet (0.022 atm H2O) and dry (3×10–5 atm H2O) conditions from 400 to 1000°C. The oxidation is parabolic and the rate increases with increasing water vapor pressure below 700°C. The oxide scale consists of two phases, an outer CuO and an inner Cu2O phase. To distinguish between effects of the reaction conditions on the two Cu oxides, the Wagnerian rate constants of the individual Cu oxides have been deconvoluted from the gravimetric rate constant. The functional pressure dependences of the parabolic rate constants are discussed in terms of derived relations between the Wagnerian rate constants and potential prevailing point defects."
©2001 The Electrochemical Society. All rights reserved.

http://ojps.aip.org/getabs/servlet/...00149000001000B14000001&idtype=cvips&gifs=Yes

Re your "branching transgranular type cracking in oxygen free pure copper - if so any idea what could cause that? I have observed this in some rotor bars from an induction motor."
-- I haven't found any reports on this behavior. Are you sure about transgranular? I can hypothesize a theory for intergranular: Guess that it is phosphorus-deoxidized copper, that some phosphate (slag) wasn't completely removed prior to casting and segregated to the grain boundaries, and that the motor was running hot, allowing air permeation and corrosion within the G.B.s. Just speculation on my part, it would take some SEM-EDS work to verify.
Ken