This invention relates to the use of water-soluble 3-mercaptopropionic acid disulphide salts as extreme pressure additives for aqueous functional fluids. These salts may be prepared by the oxidation of 3-mercaptopropionic acid followed by bringing the disulphide obtained into contact with an organic or inorganic base. Functional fluids are used during industrial operations such as, for example, the machining of metals.
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