Overbased

Overbased refers to a class of chemical additives that contain an excess of base relative to the acidic component they are designed to neutralize. In lubricants, overbased detergents are the most common example, typically based on calcium or magnesium and formed as sulfonates, phenates, or carboxylates dispersed as colloidal metal carbonate in oil. The term highlights the higher-than-necessary base content, which is described by a high base number.

These additives are produced by converting a metal oxide or hydroxide with an acidic surfactant salt and then treating with carbon dioxide to precipitate metal carbonate within the oil phase. The result is a stabilized, micelle-like dispersion in which nano-sized metal carbonate particles are surrounded by surfactant media. This structure provides a reservoir of basicity while maintaining detergency.

The primary purpose of overbased detergents is twofold: neutralization of acidic substances formed during use (such as oxidation products and sulfur-containing compounds) to protect engine and system metals; and dispersion of sludge, varnish, and deposits to keep surfaces clean. The high base number improves corrosion resistance and deposit control, particularly in engines and industrial equipment subjected to harsh operating conditions.

Variants include overbased calcium and magnesium sulfonates, phenates, and related detergents, sometimes referred to as superbased when exceptionally high base content is used. Considerations in their use include compatibility with other additives and seals, potential impacts on low-temperature properties, and cost. They are widely used in automotive engine oils, transmission fluids, and hydraulic fluids to enhance cleanliness and protection.