The grit in your plaster pits the surface of your tools, especially if the plaster still contains moisture while you're carving it. And then when you clean your tools with water the water gets into the pits and if left to sit there it will eat way at the metal.
Try to get the water off your tools as fast as possible.
After cleaning your tools with water, dry throughly with a dry cloth and then blow dry the tools to get rid of all the moisture (moisture hidden below the surface residing in the pitted surface).
There are anti-rust sprays. But in your case coating the tools with a rust inhibitor would not be a good idea as the coating material could stain your plaster.
Clean and dry quickly is my recommendation.
If you store your tools in a container with a lid thrown into the container a bag of silica gel desiccant. The desiccant will help absorb any remaining moisture.