Stainless steel, an iron-carbon alloy containing chromium and other metals, has a wide range of applications, including cookware, building materials, and medical implants. Due to its biocompatibility and resistance to corrosion, it can be used to produce biodevices. To make fine-scale biodevices, it would be helpful to synthesize stainless steel while electroplating it onto the surface of the device. Electrodeposited materials containing iron, nickel and chromium have been made before, but they are too thin or fragile for actual use.
One of the greatest challenges in producing an alloy with iron, nickel, and chromium is getting enough chromium to be electrodeposited. It is difficult to find a chemical that can complex with chromium well enough to stay together until electrodeposition and fall off during the deposition. Another problem involves the presence of chromium itself. Although chromium adds anticorrosive qualities to the alloy by forming a passivated oxide layer at the surface, it increases the likelihood of cracking, limiting the overall thickness of the material. Adding an organic chemical containing sulfur can help reduce the stress on the structure and lower the risk of cracks, but organic compounds can interfere with corrosion protection. Finding the right combination of conditions is essential.
A group of Swiss researchers recently published their method for the electroplating of an iron-nickel-chromium alloy in Chemistry of Materials. They optimized a system by using a copper cathode in a bath of electrolytes that is stirred constantly to maintain homogeneity. A convection flow of 250 rpm agitates the cathode to allow an even deposition of the metals.
The researchers performed systematic tests to find the ideal complexing agent for chromium. The best complexing agent will break off from the chromium as it is simultaneously deposited on the cathode with nickel and iron. They examined glycine, acetic acid, formic acid, and dimethylformamide and, of the four, they found that glycine gave the best deposits with a current density of 10 A/dm2. The alloy can have a maximum thickness of 23 μm, and the composition is 56-58 percent iron, 26 percent nickel, and 14-16 percent chromium.
Next, they tackled the problem of producing stable, crack free microstructures with electrodeposition on a mold. They tested two sulfur-containing organic chemicals: saccharine and sodium dodecyl sulfate. Saccharine additives increased the deposition time and generated a maximum thickness of only 1-2 μm that was subject to cracking. Similarly, using sodium dodecyl sulfate did not allow depositions to be greater than 1 μm in thickness. Instead of trying more organic chemicals, the researchers pushed a strong laminar flow through the mold. They were able to make stainless steel up to 20 μm thick with a rough, but crack-free, surface. In terms of hardness and corrosion resistance, the electrodeposited alloy was comparable to stainless steel 316.
The researchers demonstrated the feasibility of utilizing their electroplating method for depositing stainless steel alloys inside patterned surfaces for applications like microelectromechanical systems. However, more work is necessary to refine the process. Further screening of different organic additives may improve the surface quality and expand the range of deposition thickness. In addition, explorations into using other metals, such as molybdenum, may enhance the properties of the electrodeposited alloys.
The technique used for depositing an alloy inside a mold: UV-LIGA
Chem. Mater., 2008. DOI: 10.1021/cm703591n