ABSTRACT
This study determined the corrosive effect of seawater on as-received and double thermally-aged Al-Si-Mg alloy. It also determined the functional groups present in guava leaf and neem seed extracts and their inhibitive effect on the corrosion of Al-Si-Mg alloy in seawater. This was with a view to enhancing their performance in the aggressive seawater environment. The study involved Fourier transform infrared (FTIR) analysis of the extracts, thermal ageing treatment, hardness measurement, gravimetric based-mass loss test, potentiodynamic polarization test and microstructural examination. The FTIR analysis results showed that the extracts can serve as good corrosion inhibitors due to the presence of aromatic compound and functional group with lone pair of electrons which makes them easily adsorb on surface of the alloy. The thermal ageing treatment employed was Double Thermal Ageing (DTAT) with temper condition at pre-ageing temperature of 105oC at constant time of 2 hrs. The hardness of Al-Si-Mg (SSM-HPDC) alloy was determined using the Rockwell-F scale hardness tester. The hardness value of the alloy increased from 59.1 HRF (as-received sample) to 82.8 HRF (double thermally aged sample) indicating an increase of 40%. Improvement in hardness can be attributed to fine coherent clusters of precipitates which serve as obstacles to dislocation movement. The gravimetric based-mass loss test was carried out at different inhibitor concentrations, time and temperature ranges of 0.1-0.5% v/v, 1-5 hrs and 30-70oC, respectively. From the results obtained the corrosion rate of both the as-received and age-hardened samples decrease with increase in inhibitor concentrations. The maximum inhibition efficiencies of 63.17% and 72.10% were obtained for the as-received and age-hardened samples, respectively, in the presence of guava leaf extract. While in the presence of neem seed extract maximum inhibition efficiencies of 60.49% and 64.26% were obtained for the as-received and age-hardened samples, respectively. The vii mechanism of inhibition was elucidated by kinetic and thermodynamic models. The polarization curves showed that the extracts act as mixed-type inhibitors. Results obtained from the potentiodynamic polarization technique indicate higher potential value for guava (Psidium guajava) leaf extract and lower potential value for neem (Azadirachta indica) seed extract. The Tafel plot showed an increase in polarization resistance (Rp) and lower corrosion current density (jcorr) for inhibited samples as compared with uninhibited samples. From the results, IE of 90.48% and 98.88% was obtained for as-received and age-hardened samples respectively in the presence of guava (Psidium guajava) leaf extract. While in the presence of neem seed extract IE of 96.55% and 80.68% were obtained for the as-received and age-hardened samples respectively. The two methods used for the corrosion evaluation were in agreement and a mixed type corrosion inhibition exist which obeys the Langmuir adsorption isotherms. From OPM result, the microstructure of the age-hardened sample showed finer grains and enhanced grain boundaries than the as-received sample. This was as a result of increase in volume of precipitated intermetallic compounds during ageing process and also refinement of precipitated constituent particles. The surface morphology of as-corroded samples was assessed with scanning electron microscope. Results showed severe damage and pits formation for as-corroded uninhibited condition than as-corroded inhibited condition.