Detergents are cleaning agents that adhere to dirt particles, preventing them from attaching to critical surfaces.  Detergents also adhere to the metal surface itself to keep it clean and prevent corrosion from occurring.

To provide additional protection, over-based detergents neutralize acidic combustion and oxidation products, helping to control rust, corrosion, and resinous build-up in the engine. Some detergents can also act as oxidation inhibitors.

The rate of oxidation doubles with every 10º C rise in temperature.  If not controlled, the lubricant decomposition will lead to oil thickening and the formation of sludge, varnish, resin, and corrosive acids.

This vicious cycle leads to an increase in rust and corrosion, resulting in metal damage from oxygen and acid attack.  Water and polar impurities increase the speed of attack, and internal combustion engines contain plenty of these elements.

Rust and corrosion inhibitors provide a barrier between the metal surface and harmful elements. Some inhibitors neutralize acids, others form protective films. Basic detergents are excellent rust and corrosion inhibitors, because they protect in both ways.