There is a chemical reaction that takes place between ferrous steel and aluminum that will occur if you don't use some agent to keep them from coming in direct contact. Salt will displace grease as [MENTION=159729]hairnet[/MENTION] said, and anti-seize compound is a better choice. Also, cheap stamped cogs are often a poor fit and will ruin your hub threads over time. It sounds like it's time to spring for a new hub or wheel.