Potentiometric determination of the concentration of aggressive anions that can be tolerated by a certain passivator.
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概要
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The effect of various additives of Cl<SUP>−</SUP>, Br<SUP>−</SUP>, and I<SUP>−</SUP> on the potential of a passivated steel electrode was established. In solutions of Na<SUB>2</SUB>CrO<SUB>4</SUB>, Na<SUB>2</SUB>HPO<SUB>4</SUB>, and Na<SUB>2</SUB>WO<SUB>4</SUB>, S-shaped relations were obtained between the steady-state potential and the quantity of the aggressive salt added. The resulting active potentials were established after an induction period which decreases with increase in the concentration of the attacking ions and/or decrease in that of the inhibiting anions. The concentration, <I>C</I><SUB>agg</SUB>, that can be tolerated by a certain concentration of the inhibiting anion, <I>C</I><SUB>inh</SUB>, is given by log<I>C</I><SUB>inh</SUB>=<I>K</I>+<I>n</I>log<I>C</I><SUB>agg</SUB>, where <I>K</I> and <I>n</I> are constants. The equation was derived theoretically on the basis of the changes produced in the structure of the double layer caused by specific adsorption of both aggressive and passivating ions. In solutions of KMnO<SUB>4</SUB>, NaNO<SUB>2</SUB>, and K<SUB>3</SUB>[(Fe(CN)<SUB>6</SUB>)], the passivating film could not withstand the corroding action of the halide ions.
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