The inhibitive effect of a seaweed (SM) alcoholic extract on MS corrosion immersed in 1 M HCl, for 30 min, was evaluated by the WL method and PPD technique. The protective film was investigated by the Vickers hardness test. WL revealed that 500 ppm SM offered 84 % IE against MS corrosion in 1 M HCl. The inhibitor molecules adsorption onto the metal surface obeyed Langmuir's adsorption isotherm. The R 2 value was very high (0.998). The PPD study showed that Ecorr slightly shifted to the anodic side. It was inferred that SM functions as a mixed type of corrosion inhibitor, since the shifts were relatively small. In SM presence, LPR value increased and Icorr decreased. Due to the blanket effect (formation of a protective film on the metallic surface, which was found to be a monolayer, blocking anodic and cathodic sites), the electrons transfer from the metal into the solution was blocked. Vickers hardness of the inhibited MS surface was lower than that of the polished one, but higher than that of the corroded one. The outcome of the study may be used in the pickling industry, where HCl is used to remove the rust from the metal surface, simultaneously not attacking it.