The role of chromium in the corrosion performance of cobalt- and cobalt-nickel based hardmetal binders: A study centred on X-ray absorption microspectroscopy

DOI: 10.1016/j.ijrmhm.2020.105320

Authors: Benedetto Bozzini (Politecnico di Milano) , Alessandra Gianoncelli (Elettra - Sincrotrone Trieste) , Georgios Kourousias (Elettra - Sincrotrone Trieste) , Marco Boniardi (Politecnico di Milano) , Andrea Casaroli (Politecnico di Milano) , Simone Dal Zilio (IOM-CNR S.S) , Rafaqat Hussain (IOM-CNR S.S) , Majid Kazemian Abyaneh (Diamond Light Source) , Maya Kiskinova (Elettra - Sincrotrone Trieste) , Claudio Mele (Università del Salento) , Sandra Tedeschi (F.I.L.M.S. S.p.A.) , Gian Pietro De Gaudenzi (F.I.L.M.S. S.p.A.)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: International Journal Of Refractory Metals And Hard Materials

State: Published (Approved)
Published: June 2020

Abstract: The corrosion control of WC-Co type hardmetals is steadily gaining momentum for novel crucial applications in the Oil & Gas and food industries. The corrosion rate of the Co-based binder, and the extent to which coupling to WC speeds it up, are strongly influenced by alloying. In this paper, we investigate the impact of Cr on the corrosion-product film formation of Co- and CoNi based hardmetal binders in acidic, neutral and alkaline aqueous solutions. We centred our study on the role of Cr in driving the distribution of oxidized Co and Ni at the micrometre scale, thanks to synchrotron-based soft-X ray microspectroscopy. We have investigated morphochemical distributions for the following grades: Co96Cr4, Co48Ni48Cr4, Co50Cr50. Chemical-state mapping has been complemented by electrochemical measurements and metallographic observations. Amounts of Cr and Ni of ca. 50% notably increase the corrosion resistance in all ambients, with a stronger beneficial effect of the former element. 4% addition of Cr results in slight positive effects, with the exception of the CoNi system in alkaline ambient, that, together with Co50Cr50, outperforms the other grades. In all investigated alloy-ambient combinations, a continuous oxidized metal film grows, onto which micrometric island form of shape and dimensions that depend on the specific grade and aggressive conditions. Quantitative descriptors of chemical-state maps and their theoretical interpretation in terms of electrochemical phase-formation by oxy-hydroxide precipitation, allow to correlate the island patterns with the degree of pseudopassivation.

Journal Keywords: Hardmetal; Binder; Cr; Pseudopassivation; X-ray spectroscopy; X-ray microscopy

Subject Areas: Materials, Chemistry

Facility: TwinMic beamline at Elettra