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The oxidation of cold-worked and annealed pure Ni in 1 atm O2 was investigated from 700 ~ to 1270~ in continuous weighing experiments for periods from 1 rain to 20 hr and the oxidized specimens examined by diffraction, electron-optical techniques, and metatlographic cross sections. Cold-worked Ni was found to oxidize faster than annealed Ni and form finer-grained oxide.Plots of the apparent parabolic rate constant, Kp, for cold-worked Ni show an initial high value that decreases rapidly as the oxide coarsens with time. For annealed Ni, Kp is lower, grain size larger, and both change little with time. Accordingly, a range of Kp values is obtained at each temperature and, on an Arrhenius plot of Ni oxidation, there is a corresponding variation in apparent activation energy, EA. These results can be plausibly interpreted on the basis of oxide grain boundaries acting as easy diffusion paths for Ni through the NiO layer: for fine-grained oxide, Kp is higher and EA lower. The oxide is thinnest (and the derived EA highest) on Ni grains which form a single crystal overgrowth. A break in the Arrhenius plot around I000~ is the result of transport below 1000 ~ being largely by leakage paths while at higher temperatures volume transport is more important. The estimated activation energy for growth of the oxide layer is 54 __+ 2 kcal/mol for transport by lattice diffusion and appreciably less for transport by grain boundary diffusion.In previous studies of the oxidation of Fe(1, 2) it had been found that cold-worked Fe oxidized considerably faster than annealed Fe. The explanation proposed was that cold work suppressed the formation of diffusion cavities at the Fe304-Fe interface (cavities which otherwise would block cation transfer) and, in addition, that cation diffusion through Fe304 was faster through Fe304 formed on cold-worked Fe. At temperatures above 600~ i.e., where a layer of FeO developed between the Fe304 and Fe, the effect disappeared: cold-worked and annealed Fe oxidized at the same rate with no diffusion cavities in either case.The purpose of the present work was to obtain further information on the growth mechanism of oxide layers by studying the effect of cold work on the oxidation of Ni. Ni was chosen because only one oxide phase is formed, a factor expected to simplify inter-9 Electrochemical Society Active Member.