INFLUENCE OF A DOPING BY IRON OF NICKEL - MOLYBDENIUM ALLOYS ON PARAMETRES OF THEIR OXIDIZING

Abstract

Influence of alloying by iron on rate of oxidization and heat-resistance of Ni- Mo alloys depending on the concentration of iron has been experimentally investigated.

On the diffractograms of detached oxides, oxides that remained on the surface and surfaces of samples that were cleaned from oxides are observed reflexes of a number of oxides: NiO, Fe2O3, FeMoO3, MoO2. The X-rayed researches showed that with the increase of iron concentration in the alloy the amount of NiO and Fe2O3 oxides changes in favor of the last. Presumably, interrelation of these oxides amounts and presence of other oxides on the surface of investigated alloys determine the anomalies of the alloys oxidization kinetics. 

The presence of areas of a sharp increase in the mass gain rate of samples of Ni-Mo alloys with iron additives in the range of 1073 - 1173 K should be explained by a decrease in the reaction rate of iron with NiMoO4, which leads to the formation of a mixture of NiO and FeMoO3. The accumulation of loose NiMoO4 oxide stipulates periodic destruction of the overhead protective oxide film, as a result of which a temporary acceleration of the oxidation of the studied alloys is observed.

It is taken into account that the oxidation of Ni-Mo alloys with iron additives flows according to the following scheme. Due to the higher affinity of Mo to oxygen than that of other components of the alloys, mainly MoO2 is formed on the alloy surface at the initial stages. As a result, an underlayer of a solid solution of iron in nickel, practically free of molybdenum, is formed. Further interaction with the atmosphere leads to the oxidation of Ni and Fe with the formation of NiO and Fe2O3. MoO2 is also oxidized to MoO3. Then there is the reaction of NiO and MoO3, the product of which is NiMoO4. Most likely NiMoO4 is an intermediate product, which as a result of reaction with iron forms FeMoO3.

When the addition of iron exceeds 15%, the amount of Fe2O3 oxide increases visibly compared with the amount of NiO oxide. As a result, the rate of reactions leading to the formation of FeMoO3 is significantly reduced. Consequently, in order to increase the heat resistance of Ni-Mo double alloys, it is sensible to alloy them with iron in amounts of up to 15%.

Thus, using the methods of thermogravimetry and X-ray diffraction analysis, it was shown that the effect of additives is up to 20 mass % iron on the resistance to oxidation of nickel-molybdenum alloys is not directly dependent on the amount of ligature. It is established that 5% iron additive provides maximum heat resistance of Ni-Mo alloys. It was determined that an effective increase in the resistance to oxidation of nickel-molybdenum alloys is promoted by the addition of iron in amounts up to 15 mass %. Addition of more than 15 mass % leads to a negative effect on the heat resistance of Ni-Mo alloys.