Adding a few per cent of lignosulphonic acid to strong acids clearly reduces the corrosion of the mixture relative to the pure acids. Lignosulphonic acid is a surface active compound that will adsorb onto many surfaces. The negatively charged lignosulphonic acid will readily adsorb onto positively charged surfaces. In the presence of divalent or trivalent cations lignosulphonic acid will also adsorb onto negatively charged surfaces. The cations can be supplied from the oxide layer covering many metal surfaces. (B. O. Myrvold, Journal of Power Sources, 117 (2003) 187-202). Lignosulphonic acid adsorbed to the surface might form a physical barrier that prevents the acid from reaching the surface, and thus reduces the corrosion. The high molecular weight lignosulphonic acid will not penetrate skin. Also on skin it might thus form a barrier.

 

It is absolutely not a buffer. Corrosivity of the pure organic acids is reduced by adding another acid, lignosulphonic acid. Hence, SoftAcid has a very low pH and thus is very effective