3-Hydroxybenzoic acid as AISI 316L stainless steel corrosion inhibitor in a H2SO4–HF–H2O2 pickling solution

3-Hydroxybenzoic acid (3-HBA) was studied for possible use as a AISI 316L stainless steel (SS) corrosion inhibitor in an environmental-friendly aqueous pickling solution of 75 g l−1 sulphuric acid (H2SO4), 25 g l−1 hydrofluoric acid (HF) and 30 g l−1 hydrogen peroxide (H2O2). 3-HBA was tested in concentrations from 5 × 10−5 to 5 × 10−1M at 298 and 313 K temperature. Inhibition efficiency increased with the 3-HBA concentration. The inhibitor mechanism is discussed in terms of the properties of the isotherm equations of Frumkin, Hill-de Boer and Kastening–Holleck mainly. The shape, the trend of the slopes along the curve and the existence of inflection points, were analysed as the characteristics that differentiate one adsorption equation from another. The best fit was obtained using the Frumkin isotherm model. The projected molecular area of 3-HBA was calculated as a structural parameter to elucidate its optimal inhibition mechanism.