Measurements of membrane potentials arising across chromium hydroxide and chromium-nickel mixed hydroxide precipitate membranes have been carried out to estimate the ion selectivity. The chromium hydroxide precipitate membrane showed a high selectivity for anions in electrolyte solutions of neutral pH. The transport number of anions was evaluated to be 0.96-0.95 in sodium chloride, nitrate, and perchlorate solutions. The fixed ion concentration on the membrane characterizing anion exchanger was determined to be about 0.1g-equiv./l by comparing the experimental membrane potentials with the theoretical values derived from the fixed charge theory of membranes. In sodium hydroxide solution, the transport number of OH^- ions through the membrane was not constant but increased with decreasing solution pH. The order of the selectivity for anions determined with the biionic potentials across the chromium hydroxide precipitate membrane was Cl^->Br^->NO^-₃>I^->ClO₄^-. This order of the selectivity differs significantly from the selectivity for the nickel hydroxide precipitate membrane. The biionic potential across the mixed hydroxide precipitate membranes of chromium and nickel shifted from that of nickel hydroxide to that of chromium hydroxide as the chromium concentration increased. The anion permeability estimated from the membrane potential arising across the biphase hydroxide membrane was very much smaller for the chromium hydroxide than for the nickel hydroxide precipitate membrane. This suggests that chromium is enriched on the surface of hydroxide particles constructing the mixed membranes.

View full abstract

Galvanic interaction between Al-Zn alloys, Al-Zn-In alloys and Al-1100 has been studied in aerated 0.5N NaCl solution at 18°C by measuring the weight loss and by continuously monitoring the galvanic current with a Zero Impedance Ammeter. The surface structure and the composition of corrosion product films formed on the surface were observed by a scanning electron microscope, and an X-ray microanalyzer. Apparent weight loss of Al-Zn and Al-Zn-In alloys coupled to Al-1100 did not correspond to calculated values obtained by graphical integration of galvanic current vs. time curves, which was proved to be due to intergranular corrosion. Dimensional change caused by accumulation of corrosion products at grain boundaries was observed in the anode materials. Al-Zn-In alloys containing 3-4% of Zn and 0.02-0.04% of In showed relatively high performance as sacrificed anodes compared to other binary and ternary alloys.

View full abstract

The effects of carbon content, Ti/C or Nb/C ratio, solution treatment temperature and stabilization treatment on stress corrosion cracking (SCC) of 18-8Ti and 18-8Nb stainless steels have been investigated in polythionic acid solution. The sensitization treatment was made up to 1000 hours at 600°C. The results obtained are as follows: (1) The susceptibility to SCC generally decreases with lowering solution treatment temperature. (2) The stabilization treatment decreases the susceptibility to SCC, its effect being decreased with lowering solution treatment temperature. (3) The critical Ti/C or Nb/C ratio which prevents SCC increases with lowering carbon content in the steel. When the steels were solution treated at 1050°C, the critical Ti/C and Nb/C ratios of the steels containing 0.05-0.08%C are 6-7 and about 10 respectively, while those of the steels containing 0.02-0.03%C are about 10 and 15-20 respectively. (4) In welded joint, the high-temperature heat affected zone of weldment is attacked. This attack can be prevented by the stabilization treatment over 20 minutes at 900°C.

View full abstract

S55C steel roll was used for granulation of ammonium chloride fertilizer produced by the ammonia soda process. Because of highly corrosive and abrasive fertilizer and mixed foreign matters, the roll must be regrinded every month and was discarded every half a year. The life of roll was elongated about 18 times due to prevention of corrosion and wear through powder spraying of Ni-Cr-B alloy on the roll surface.

View full abstract

Recent contributions to the passivity of iron and iron-base alloys were reviewed, and the anodic current-potential and film thickness-potential curves of iron, nickel and cobalt were shown along with the discussions of ionic current in and dissolution current of the passive film. There is a barrier oxide layer on the surface of passivated metals. For iron it is an iron-deficient magnetite layer, for nickel a layer of NiO with excess oxygen, and for cobalt a bi-layer of CoO/Co₃O₄. The ionic current in the barrier oxide layer on iron and in the outer barrier Co₃O₄ layer on cobalt obeys a field-assisted ion migration mechanism. This mechanism, however, does not hold for the barrier NiO layer on nickel and the inner CoO layer on cobalt. The dissolution current of the passive film is controlled by the potential difference across the Helmholtz layer at the film/solution interface, and a Tafel relation is found to hold between the film dissolution current and the overpotential at the Helmholtz layer, leading to the mechanism for iron as Fe³⁺ (oxide)→Fe³⁺ (solution) and for cobalt as Co²⁺ (oxide)→Co²⁺ (solution). A generalized theory was presented in which the metal passivity was attributed to the involvement of a thin oxide film into the structure of electrified interface between the metal and the solution. The passive film formed on a metal changes the electrical double layer at the metal/solution interface into a bi-layer structure consisting of a thin oxide layer and the Helmholtz layer at the film/solution interface, and thus the effective overpotential for metal ion transfer across the Helmholtz layer is reduced resulting in a decrease of metal dissolution rate.

View full abstract