'Green' Cr(iii)-glycine electrolyte for the production of FeCrNi coatings: electrodeposition mechanisms and role of by-products in terms of coating composition and microstructure

Enrico Bertero, Johann Michler, Stefano Mischler
ROYAL SOC CHEMISTRY, 2019
Journal paper

Abstract

The electrodeposition of stainless steel-like FeCrNi alloys for miniaturised devices is appealing as it would allow combining excellent material properties (e.g. corrosion resistance, hardness, biocompatibility) at low-cost. However, conventional baths often contain hazardous hexavalent chromium. Cr-based alloys electrodeposited from environmentally friendly trivalent chromium electrolytes are crucial for industrial application for facilitating the transition towards sustainable and ecological production and processing. Nevertheless, this process has not been comprehensively studied or understood in depth: especially the role of organic agents (common additives for improving Cr(iii)-based plating; e.g. glycine) in terms of material properties of the electrodeposits. The aim of this work was to investigate the electrodeposition of FeCrNi coatings from a 'green' Cr(iii)-glycine electrolyte. Novel information was attained by analysing films developed under various conditions and characterising them using a combination of advanced techniques. The evolution of microstructure (from amorphous to nanocrystalline) in correlation with film composition (i.e. metals ratio and presence of impurities) and elemental 3D spatial distribution was achieved for coatings produced from different anode materials and thermal post-treatment. The influence of Cr(iii) and glycine in terms of coating atomic contents (i.e. Fe-Cr-Ni-O-C-N-H) was evaluated for films in which both the applied current density and electrolyte composition were varied. These results, together with a thorough analysis on metals speciation/complexation allowed us to propose various Cr(iii)-based electroreduction mechanisms, and to observe, upon annealing, segregation and distribution of impurities, as well as of oxides and metals with respect to microstructure variation, providing an explanation for the amorphisation process.

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Enrico Bertero, Johann Michler, Stefano Mischler
ROYAL SOC CHEMISTRY, 2019
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/270186?ln=en

About this result

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Related publications

'Green' Cr(III)-based electrodeposition of stainless steel-like FeCrNi: synthesis, mechanisms, material properties of films and micro-nanocomponents for bio-medical applications

Enrico Bertero

The electrodeposition (ED) of stainless steel (SS) -like FeCrNi alloys for miniaturised devices is appealing for bio-medicine as it would allow combining excellent material properties (e.g. corrosion resistance, hardness, bio-compatibility) at low-cost. However, conventional baths often contain hazardous Cr(VI). Alloys ED from environmentally friendly Cr(III) electrolytes is crucial for facilitating the transition towards sustainable and ecological production. Still, this process was not comprehensively studied. ED from Cr(III)-based aqueous electrolytes leads to impurities incorporated in films, hydrogen evolution reaction (HER) and side-effects. The role of both electrolyte composition, containing organic additives (e.g. glycine), and deposition parameters on material properties was not clear. Moreover, passing from films to micro-nanocomponents (M-NEMS) via ED was not properly investigated when dealing with more complex Cr(III)-based alloys. The aim of this thesis has been to study the ED of FeCrNi coatings and M-NEMS using a 'green' Cr(III)-glycine electrolyte, understanding better the relations between ED mechanisms, deposition parameters and material properties, as well as their variation due to miniaturisation. Novel information was attained for all-aqueous electrolytes by investigating: films microstructure evolution (amorphous-nanocrystalline) in correlation to their composition and elemental 3D distribution, influence of Cr(III)-glycine in terms of coatings at%, together with a thorough analysis on metals speciation/complexation in the baths. These results allowed to propose various Cr(III)-based ED mechanisms. The material properties of the as-deposited films were evaluated in dependence of morphology and composition variations, then compared to the standard metallurgical ones. FeCrNi electrodeposits showed good passivation and bio-compatibility comparable to AISI SS, and tuneable soft-magnetism. However, tribological tests revealed that the material was hard but brittle. The main issues were correlated to HER leading to low deposition efficiency, material brittleness and porosity. To overcome these issues, a mixed-solvent electrolyte composed of ethylene glycol (EG) has been investigated and compared to the all-aqueous one, showing similar Cr(III)-glycine complexation, but higher deposition efficiency (decrease in HER). EG-based electrolyte was found to be an effective solution for obtaining FeCrNi M-NEMS via template assisted ED. Nanotubes (NTs) and nanowires (NWs) have been achieved via ED into AAO templates: high currents resulted in NTs, whereas low ones in compact NWs. Micro-pillars have been created via ED into UV-LIGA moulds, combining EG-based electrolyte and a CV-like deposition, avoiding moulds delamination caused by HER and under-deposition. A comparison of the material properties was pursued for FeCrNi electrodeposits as-deposited (amorphous) and annealed (nanocrystalline) for both studied electrolytes. This investigation correlated electrolyte type, material composition/microstructure/morphology, vs. other material properties i.e. corrosion resistance, bio-compatibility, magnetic and mechanical properties. In conclusion, this research studied in depth Cr(III)-based ED, achieving improvements and giving useful guidelines for applying this process for creating SS-like coatings and M-NEMS for advanced bio-medical applications.

EPFL2020

The chemical-mechanical polishing process (CMP) is an essential part of the production of integrated circuits. Metal to be polished in the CMP reacts with the oxidants from the aqueous suspension (slurry) and the passive film is formed on the metal surface. Suspended abrasive nano particles from the slurry cause detachment of the formed friable passive films. The CMP represents the tribocorrosion process where material deterioration results from the interaction of wear and corrosion which takes place in tribological contact exposed to an aggressive environment. Due to increasing number of the materials to be polished in the modern integrated circuits it is necessary to gather fundamental understanding of the removal mechanism of each material. The aim of this work is to elucidate the removal mechanisms of tungsten (W), as a typical microelectronic metal (chosen as a model system), under different oxidising conditions (represented by electrochemically imposed potential). Also, we want to evaluate the effect of the slurry composition (chelating agents lactic and phosphoric acid) on material removal. To achieve this electrochemical techniques (chrono amperometry, cyclic voltametry, passivation kinetics measurements under mass transport control), and tribocorrosion techniques (tests on tribometer with electrochemical cell with reciprocating sliding motion of alumina ball) were used. Surface analysis techniques (SEM, XPS, AES) were used to assess worn and unworn surfaces' morphology and chemistry. In addition, electrochemical tests and polishing tests on the CMP machine were performed with technical CMP slurries. The sliding action of the alumina ball on tungsten in the 0.01 M H2SO4 solution was found to cause three distinct wear responses depending on imposed electrode potential. Below the first threshold potential the wear is low due to negligible oxide film formation. In the following wear region, up to the second threshold potential, materials deterioration in the rubbed area proceeds by cyclic mechanical removal of the WO3 passive film followed by repassivation of tungsten. The amount of anodic oxidation corresponds to the overall removed metal volume. Beyond the second threshold potential a compact tribolayer of WO3 (more than 400 nm thick), probably formed by agglomeration of oxide particles detached from the metal surface, forms and covers the wear track. Similar tribocorrosion behavior of tungsten was also observed in the presence of 0.1 M H3PO4. The presence of chelating agents, lactic acid and phosphoric acid, in the 0.01 M H2SO4 solution does not change the volume of material wastage at potentials below the second threshold potential. Beyond this electrode potential, lactic acid promotes WO3 dissolution and thus impedes the third body formation and enhances wear. Electrochemical factors have a strong influence on the tungsten CMP removal rate. It was found that CMP removal rate increases with the open circuit potential (electrode potential) and with passivation charge density. A good correlation of tribocorrosion results and CMP practice was observed. Tribocorrosion models can be successfully applied to the tribocorrosion of tungsten and as well can be fairly used for modelling the material removal in CMP.

EPFL2009

Analysis of High Temperature Degradation of Alloys in Solid Oxide Fuel Cell

Manuel Bianco

Solid oxide fuel cells (SOFCs) are approaching commercialization to improve power production efficiency. Currently, cost and lifetime reliability limit their spread in the market. The SOFC is ceramic-based, but refractory metal alloys are in fact the majority materials present in SOFC systems. The lifetime behaviour of SOFC stacks, and of the metallic components in particular, remain in need of further investigation. The degradation processes behind the possible failure of selected metal interconnect (MIC) and balance of plant (BoP) alloys were investigated. Samples were extracted from stacks operated for periods up to 18000 hours and analysed with scanning electron microscopy coupled with X-ray energy dispersion spectroscopy (SEM/EDS). Scanning transmission electron microscopy (STEM) coupled with EDS and electron diffraction was applied to identify corrosion products. Electrochemical impedance spectroscopy (EIS) analysis completed the characterization of the stacks. Degradation processes of steam generator BoP alloy were investigated with SEM/EDS as well. Various protective coating solutions to limit interconnect and related cathode degradation were tested as small coupon samples. Area specific resistance (ASR) monitoring and evaluation of Cr evaporation blocking verified the effectiveness of these solutions. A special set-up able to test 8 small MIC samples simultaneously in different gas atmospheres was built and used to correlate operating variables to material degradation through a design of experiment (DoE) approach. The results indicate that in commercial ferritic stainless steel, corrosion kinetics are fast in the first few thousand hours of operation and then tend to stabilize. The main influencing parameter is the initial density of the protective coating, independently of the steel and coating compositions. However, even porous coatings demonstrated to be viable for periods at least up to 2 years. The strong chemical interaction with the cell materials is their main drawback, but they present a competitive advantage in manufacturing cost. The steam generator alloy used in the BoP suffered stress corrosion cracking induced by the strong thermal gradient between hot and cold gas and the aggressive hot outlet gas atmosphere. Aluminizing treatment of the alloy seems to solve the issue.

EPFL2019