Coffee Husk as Corrosion Inhibitor for Mild Steel in HCl Media

The corrosion inhibition of mild steel by coffee husk aqueous extract (1) and its high molecular weight fraction (2) was investigated in 1 mol L-1 HCl medium. Gravimetric tests showed that for both inhibitors, inhibition efficiency (IE) increased with time and extract concentration, reaching a IE of 89.2% and 90.3% for 800 mg L-1 of inhibitors (1) and (2), respectively, in 24 h of immersion. Gravimetric tests varying temperature showed the activation energy (Ea) barely varied for Inhibitor 1. For Inhibitor 2 a decrease in Ea was noted, a hint it is attached on the metal surface by chemisorption. Polarization curves indicated that both extracts act as mixed type inhibitors. Impedance results showed a decrease in double layer capacitance (Cdl) and an increase in charge transfer resistance (Rct), with a better performance for inhibitor (2). Both extracts presented a good linearity with respect to Langmuir isotherm, an evidence of a monolayer formation. Our results suggest inhibitor 1 acts by purely blocking the metal surface, and in case of inhibitor 2, it screens the metal surface and reduces the activation energy.