Nanocrystalline Stainless Steel Electrodeposition

FeCrNi stainless steel is widely used for industrial applications because of its excellent corrosion resistance and preferable mechanical properties. Electrodeposition of such FeCrNi alloy has attracted attention for years because of its potential application for fabrication of microcomponents by means of LIGA process. However, the technique has not been established so far mainly because of the complexity of bath chemistry, especially related to the instability of trivalent Cr ions as well as the difficulty to control the composition due to anomalous deposition. In order to overcome these technical issues, we have investigated the electrodeposition of FeCrNi stainless steel aiming at achieving the composition and properties close to those of SUS304 (18%Cr and 8%Ni), which is known to exhibit a superior corrosion resistance. In order to stabilize trivalent Cr ions, we have studied the effects of complexing agents for Cr ions on the quality of FeCrNi electrodeposits, and developed the chloride-based FeCrNi plating bath containing glycine as a complexing agent[1]. Further optimization of plating parameters enables us to fill FeCrNi electrodeposit in a UV-LIGA mold with depth of 300µm. Nevertheless, the deposits are brittle and highly stressed, which should be further optimized for practical applications. The crystallographic analysis by X-ray diffraction (XRD) and transmission electron microscope (TEM) showed that the deposit is predominantly amorphous, which may explain brittleness of the deposits. In terms of mechanics, nanocrystalline FeCrNi (nc-FeCrNi) is preferred because of its excellent mechanical strength.

We report here the successful preparation of nanocrystalline FeCrNi electrodeposits by employing a double-compartment electrochemical cell where the anode electrode is separated from the cathode electrode compartment (hereafter referred to as the “double-cells”) instead of a conventional single compartment cell (single-cell), which yields amorphous FeCrNi (a-FeCrNi). Firstly, the microstructure and recrystallization behavior of FeCrNi electrodeposits from the single-cell and the double-cells are characterized and compared. The grazing-incidence XRD (GI-XRD) revealed that as-deposited FeCrNi films prepared in the double-cell is nanocrystalline consisting of the metastable ferrite phase. The analysis of TEM shows that the size of grain is about 7 nm. Further investigation of the microstructure was performed by the in-situ GI- XRD during thermal treatment up to 800°C. The experiments demonstrate that nc-FeCrNi recrystallizes at 550°C along with its phase transformation from the ferrite to the austenite. On the other hand, a-FeCrNi was shown to undergo recrystallization to form primarily the ferrite phase, which is subsequently transformed to the austenite phase. Chemical analysis of deposits revealed that the carbon content of nc-FeCrNi is significantly lower than that of a-FeCrNi, suggesting that the carbon inclusion is responsible for the formation of amorphous phase. The major source of carbon is attributed to reaction compounds at the anode electrode. In order to identify the anodic reactions in our FeCrNi electroplating bath, a series of cyclic voltammetry was carried out. The electrochemical study shows the evidence of the anodic oxidation of glycine as well as the oxidation of ferrous and trivalent chromium ions to ferric and hexavalent chromium ions on a Pt anode. Therefore, the oxidation products of glycine or the compounds generated by their further reactions are thought to be responsible for the inclusion of carbon. In this presentation, we will also discuss the corrosion properties of nc- and a-FeCrNi deposits along with the micromechanical properties of these FeCrNi electrodeposits.

1.         Philippe, L., C. Heiss, and J. Michler, Electroplating of stainless steel. Chemistry of Materials, 2008. 20(10): p. 3377-3384.