The kinetics of the oxidation of chromium and chromium - nickel coated
diamond powders in air at 750 and 850-degrees-C was studied by means
of interrupted weighing. The coatings and metallized powders were subj
ected to x-ray phase analysis. Decrease of the powder mass confirms th
at permeation of the mass with volatile oxides predominates over the i
ncrement due to the weight of the absorbed oxygen in the oxides NiO an
d Cr2O3. The weight loss is caused by the burning away of both the coa
ting (oxidation of chromium and its carbides to CrO with formation of
CO and CO2) and the diamond. Carbon from the diamond is also removed w
ith CO and CO2 as a result of successive reduction of chromium oxide b
y carbon, which diffuses to the oxidized layers, and further reduction
of the carbides by the same mechanism. The higher the oxidation tempe
rature and the thinner the coating, the greater the contribution of th
ese reactions is to the processes responsible for the decrease in weig
ht of the metallized diamonds. It is also encouraged by diffusion of c
arbon to the scale formed and by simultaneous diffusion of oxygen to t
he coating. The reactions of primary oxidation of chromium contained i
n the nickel solid solution, reduction of nickel by carbon and chromiu
m, and dissolution of chromium in the reduced nickel contribute signif
icantly to the oxidation of chromium - nickel coatings.