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Electrochemical studies of Na 2 CrO 4 ‐Na 2 So 4 melts at 1200 K
Author(s) -
Shi D. Z.,
Rapp R. A.
Publication year - 1990
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.19900410504
Subject(s) - chromate conversion coating , inorganic chemistry , electrochemistry , chromium , chemistry , cyclic voltammetry , sulfide , oxide , ion , chromite , coulometry , sulfate , electrode , organic chemistry
The electrochemical response of Na 2 SO 4 ‐Na 2 CrO 4 solutions at a Pt working electrode was established by cyclic voltammetry and chrono‐potentiometry measurements at 1200 K. Experiments were made under conditions of controlled and electrochemically measured values of melt basicity for basic, neutral, and acidic melts. Despite the thermodynamic stability of chromite ions under reducing conditions, a direct conversion of chromate to chromite upon cathodic polarization was not observed. Rather, the cathodic reduction of chromate ions proceeds by a reversible one‐electron charge transfer to an electroactive intermediate species formed through an equilibrium between CrO 4 2−and oxide ions in the solution. An irreversible chemical reaction reduces the chromium solute species further to precipitate a solid. As a solute in a corrosive fused salt film, a chromate to chromite transition would not be expected to stabilize the oxidation state and basicity. However, chromate ions to provide a reduction reaction more favorable than sulfate reduction to sulfide; chromate reduction leads to the precipitation of solid NaCrO 2 or Cr 2 O 3 in basic or neutral solutions, respectively. A large shift in melt basicity can only occur for mildly acidic solutions with high chromate solute.