Characterization of Re-Ni Films after the Initial Stages of Electrodeposition

Abstract

Re-Ni layers produced in the early stages of electrodeposition from citrate electrolytes were studied by high-resolution X-ray photoelectron spectroscopy (HR-XPS) and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). The deposition time was varied in the range of 2 to 300 seconds. The compositional heterogeneity of the thin Re-Ni alloys and the variations of their composition with deposition time, both in the bulk and on the surface, were shown. The results obtained are indicative of the occurrence of chemical reactions at short deposition times and are in agreement with our previous results obtained in studying the early stages of Re-Ni deposition by electrochemical techniques. The mechanism of the deposition process is discussed. Induced electrodeposition of refractory metals with iron-group metals has drawn the attention of many researchers in view of their high corrosion resistance, stability at high temperatures and wear resistance.1 Induced codeposition of rhenium from aqueous solutions has been studied, 2-5 including Re-Ni alloys from citrate-containing solutions. [6][7][8][9] Induced codeposition of Re from an electrolyte containing ReO − 4 can occur readily. Coatings with a thickness of up to 25 μm were obtained within 60 min deposition at a faradaic efficiency of up to 95%. Moreover, the concentration of Re in the resulting alloy can reach up to 90 at.%. This excludes the possibility of having a precursor similar to that found in the case of W, 1,10-12 since it would require the formation of a complex containing nine molecules of the perrhenate ion, which is totally unlikely. Instead, we have suggested a new mechanism, according to which the Ni 2+ ion is first reduced and deposited on the surface of the substrate. The resulting neutral Ni atoms act as strong reducing agents, helping to reduce the 7-valent ReO − 4 probably to the 5-valent ReO − 3 . This is a typical catalytic process, in which the Ni 2+ ion acts as a catalyst, the concentration of which should not change in the overall reaction. Theoretically, this should lead to a deposit of pure Re, but some Ni is found in the resulting alloy due to entrapment of neutral Ni atoms. This represents a rather unique type of process, where the catalyst is formed in situ as part of the redox process.6-9 The deposition of Re-Ni apparently depends on the high catalytic activity of the freshly deposited nickel. We have supported this hypothesized mechanism by different experiments. 7,[13][14][15] The initial stages of Re-Ni deposition, which are critical for understanding the catalytic mechanism, were studied in our recent work. 13There, the electrodeposition of Re-Ni alloys was investigated at deposition times of 0.05-60 s, unlike in earlier publications, 6-8 where the deposition times were on the scale of minutes and hours. The anomalous faradaic efficiency (AFE) of the deposition process, the composition of the deposits and their morphology were considered. The AFE and the Re-content in the deposits were shown to decrease with deposition time. Anom...

Electrodeposition of Re-Ni alloys from aqueous solutions with organic additives

“…Previous work in our laboratory has found that the addition of one of the iron-group metals (Ni, Co, or Fe) to the solution bath leads to high Faradaic efficiency (FE) and high Re-content in the deposits. This catalytic effect of iron-group metals on Re electrodeposition, as well as the mechanism of codeposition has been investigated in detail in our lab in recent years [11][12][13][14][19][20][21]. The Re-Ni alloy system is of particular interest because of the extensive mutual solubility of Re and Ni and the absence of intermetallic compounds in the Re-Ni peritectic phase diagram.…”