Chromium Electrodeposition from Hydrate Melts: Formation of Metastable δ-Cr Phase in Conjunction with Hydrogen Incorporation

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Abstract
As an alternative to hexavalent chromium plating, chloride-based hydrate melts enable crystalline chromium electrodeposition from trivalent chromium without the use of organic complexing agents. Cr films deposited from these hydrate melt-based baths uniquely contain a metastable δ -Cr phase absent in conventional baths. Achieving hard coatings with adequate mechanical properties requires a systematic understanding of the crystallographic features including phase formation. In this study, the deposition of δ -Cr and its subsequent phase transformation to α -Cr were systematically investigated from the perspectives of crystal growth mode and hydrogen incorporation. The results indicate that δ -Cr preferentially forms during the initial nucleation and coalescence stage, where isotropic fine grains are generated. As electrodeposition proceeds, the crystal growth mode transitions to columnar growth, in which α -Cr becomes the dominant phase. The obtained δ -Cr was confirmed to transform into α -Cr at relatively low temperatures ranging from room temperature to 250 °C. The thermal desorption spectrum of hydrogen and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) experiments using deuterium as a stable isotope label suggest that the formation and stability of the electrodeposited δ -Cr phase may well be attributed to incorporated hydrogen. These findings provide a fundamental basis for microstructural control of electrodeposited Cr from chloride-based hydrate melts.
Funding Information
  • Iketani Science and Technology Foundation (0371053-A)
  • Japan Science and Technology Agency (JPMJAP2309)
  • Japan Society for the Promotion of Science (24KJ1393)
  • the Division of Graduate Studies of Kyoto University (Grant for Overseas Research)

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