document: A surfactant is a surface active agent. In this work a surfactant term will be used for compounds which improve the dispersability of the CI in the acid (as emulsifiers providing dispersed emulsion – not separated) while wetting the surface of the metallic material [14,20,24]. However, surfactants can offer corrosion protection themselves. Some examples when the same compound was used as a surfactant or active corrosion inhibitor ingredient are given below. Typical surfactants in the oilfield services industry are alkylphenol ethoxylates, e.g. nonylphenol ethoxylate (NPE) [14,15,30,106,107]. However, NPEs have been banned from use in the North Sea because of their toxicity. On the other hand, ethoxylated linear alcohols are more acceptable [20]. The quaternary ammonium salts and amines (when protonated) are the most used compounds of the cationic surfactants class, where the cation is the surface active specie. As the amines only function as a surfactant in the protonated state, they cannot be used at high pH. On the other hand, quaternary ammonium compounds, frequently abbreviated as “quats”, are not pH sensitive. Long-chain quaternary ammonium bromides were also reported to work as efficient CIs for steel materials [106]. A frequently employed surfactant was N-dodecylpyridinium bromide (DDPB) [9,60,61,108,109]. Anionic sulphates, anionic sulphonates, alkoxylated alkylphenol resins, and polyoxyethylene sorbitan oleates are also useful surfactants. Ali reported that a particularly useful surfactant is a blend of polyethylene glycol esters of fatty acids and ethoxylated alkylphenols [15]. Several examples of the surfactants used are given below in Section 5.6.