Although there are many metals that can corrode (weaken through oxidation), only iron and iron-based metals can form rust. It is water interacting with air that causes rust to form on iron nails. As rust forms on the surface of iron, it oxidizes (interacts with oxygen in the air) and forms a protective coating (rust). This protects the underlying metal from rusting away. However, when salt is added to the water, the choride ions in the salt will cause this protective oxidized rust to separate from the metal, thereby exposing new metal. This process continues until all of the metal has rusted away or mechanically fails. So the salt, with it’s ability to accelerate rusting and attract more water, causes rust to destroy iron and iron alloys. Unfortunately, I don’t know how sugar affects rusting. I researched it and could find nothing.
Air is not always necessary to cause rust, only the presence of oxygen, and oxygen can be found in water as well as air. In fact, water is usually considered the third part of the oxidation process, since it allows the oxygen and the iron to meet. Salt water shows even more adverse effects on the process of rusting. The reason is that the process of rusting involves electrons moving around, and electrons move more easily in salt-water than they do in clean water. Sugar water may cause less corrosion of metal than that caused by plain water. Best way to prevent stainless steel corrosion is by keeping it away from forms of water.