ISSN:
1572-8838
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Electrical Engineering, Measurement and Control Technology
Notes:
Abstract The corrosion behaviour of Ni in molten Na2SO4, NaCl, and in mixtures of these two salts, at 900° C, in laboratory air and under O2+SO2/SO3 atmospheres has been examined by electrochemical curves and topochemical analysis of corrosion products. Ni passivity in pure Na2SO4 was observed under potentiodynamic and potentiometric conditions, the passive film corresponding to NiO. Passivity was not so easy to achieve in chloride melts as in sulphate alone, but once a thick oxide film forms on the specimen, the Cl− addition is accompanied by an increase in the film stability. The inhibiting role of NaCl on Ni in the passive-transpassive area was also evidenced. In opposition, halide additions (especially those up to 25%) increased the dissolution current densities of Ni in the active region. These higher dissolution rates are represented by the equation Ni3S2+4NaCl+1/2 O2 = 2NiCl2+2Na2S + NiO which is also suggested as a critical factor in the Ni passivation. The passive capability found for Ni in Na2SO4/NaCl melts, in air, is destroyed by SO3 atmospheres. This corrosion-stimulation is due to the SO3 role in promoting reactions such as NiS + 3O2− = Ni2++SO3+8e which would be potential-determining at the Ni surface until Ni2+ precipitates or the conjugate oxygen cathodic reduction process takes place. Microprobe analysis also evidenced S penetration which might be the reason for the Ni embrittlement. The polarization curves for Ni in pure NaCl showed the lack of a passive region; occurrence of extensive intergranular attack was also indicated by metallographic observation. The observed dissolution must occur at the expense of the Ni interactions with the species which intervene in the reaction equilibrium between O2 and molten NaCl (O2, Cl−, Cl2 and O2−) as well as with the Na+ cations, as has been discussed elsewhere. Its self-sustaining nature is enhanced by the continuous reduction of the nickel ion content of the melt by NiCl2 evaporation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00617672
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