Lisa Pavelka writes "In some of my collage work, I feature clay gears and keys. People always ask me where I "bought" these, especially the ones that are rusted. They are shocked when they find out I made them and now you can too." This PDF tutorial is free to you. Read on for supplies listed.
Lisa Pavelka's Steampunk Innie Texture Stamp or Wings and Things Texture Stamp
Swellegant - Iron Metal Coating and Tiffany Green Rust Patina
You could also use the Victorian Keys or the Key to My Heart mold by Best Flexible Molds