Adding lignosulphonic acid to organic acids clearly reduces the corrosion compared to 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 will form a physical barrier that prevents the acid from reaching the surface, and thus reduces the corrosion. Hence, adding lignosulphonic acid to organic acids clearly reduces the corrosion compared to pure acids.
The high molecular weight lignosulphonic acid will not penetrate skin. Thus it will also form a barrier on skin.