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The corrosion inhibition of mild steel in 1 M H₂SO₄ in the presence of polyvinylpyrollidone (PVP) and polyacrylamide (PA) as inhibitors at 30–60°C was studied using gravimetric and gasometric techniques. The inhibition efficiency (1%) increased with increase in concentration of the inhibitors. Increase in temperature increased the corrosion rate in the absence and presence of inhibitors but decreased the inhibition efficiency. Both PVP and PA were found to obey Temkin adsorption isotherm and Kinetic-Thermodynamic Model of El-Awady at all the concentrations and temperatures studied. Phenomenon of physical adsorption is proposed from the activation parameters obtained. Thermodynamic parameters reveal that the adsorption process is spontaneous. PVP was found to be a better inhibitor than PA.

The inhibitive effect of 2,3-diaminonaphthalene (2,3-DAN) for corrosion of aluminum in 1 M HCl was investigated using hydrogen evolution technique at 30 and 40°C. Quantum chemical calculation results showed that 2,3-DAN molecule possessed planar structure with a number of active centers, which aided the adsorption process. The Mulliken charge density, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) were found mainly focused around nitrogen atoms and the cyclic of the benzene as well. The presence of 2,3-DAN molecules in the corrosive medium (1 M HCl solution) inhibits the corrosion process of aluminum and as the concentration of 2,3-DAN increases the inhibition efficiency also increases but decreases with rise in temperature. The corrosion inhibition of 2,3-DAN was discussed in terms of blocking of the Al surface by adsorption of the molecules of the inhibitor at the active centers. It was found that the adsorption of 2,3-DAN onto the Al surface followed the Langmuir adsorption isotherm and 2,3-DAN adsorbed on Al surface probably by physisorption. The proposed physisorption mechanism was supported by the calculated values of E_a, Q_ads, and ΔG_ads.

The inhibitive performance of two water soluble polymers–polyacrylamide (PA) and polyvinylpyrrolidone (PVP) on the corrosion behavior of aluminum alloy 3SR in HCl solution was investigated using weight loss, hydrogen evolution, and thermometric methods at 30–60°C. Results obtained indicate that both polymers inhibited acid-induced corrosion of aluminum at the temperatures studied. PVP was found to be a better corrosion inhibitor than PA. All measurements from the three techniques show that inhibition efficiencies increase with increase in inhibitor concentration and decrease with increase in temperature. This indicates that the inhibitive actions of the polymers were mainly due to adsorption. Adsorption of these inhibitors follows Temkin and El-Awady adsorption isotherm models. Kinetic/thermodynamic parameters (E_a, K_ads, ) of adsorption of the studied inhibitors reveal that the adsorption was physical in nature and spontaneous. Differences in inhibition efficiency of the two polymers could be linked to their differences in molecular structure.

The corrosion inhibition effect of 3-hydroxyflavone was studied on mild steel in 1 M hydrochloric acid (HCl). The anticorrosive effect was evaluated by weight loss and electrochemical methods which include Tafel polarization and AC impedance studies at 300 K. In weight loss method, the inhibition efficiency increased with increase in inhibitor concentration, and decreased with increase in temperature and immersion time, and acid concentration. The inhibitor showed maximum efficiency of 91% at 4 × 10^-4M concentration in 1 M hydrochloric acid. The Tafel polarization study showed that the inhibitor behaves likely as cathodic type. The corrosion inhibition effect measured by weight loss method and electrochemical studies was in good agreement with each other. The surface analysis was done by using scanning electron microscope (SEM). Several adsorption isotherms are assessed to study the adsorption behavior of the inhibitor on the mild steel surface. The negative value of ΔG_ads indicates the spontaneous adsorption of the inhibitor on mild steel surface.

The effect of two pyridin-pyrazol derivatives, N-[(3,5-dimethyl-1H-pyrazol)methyl]pyridine-2-amine (TB5) and 5-bromo-N-[(3,5-dimethyl-1H-pyrazol)methyl]pyridine-2-amine (TB6), newly synthesized, on the corrosion of mild steel in 1.0 M HCl was investigated by weight-loss and various electrochemical techniques. Results obtained reveal that these pyridin-pyrazol derivatives perform excellently as corrosion inhibitors for mild steel in hydrochloric solution. Polarization curves showed that both TB5 and TB6 are mixed-type inhibitors in acidic medium. A kinetic study of steel in uninhibited and inhibited acid for TB6 was also achieved to provide suitable investigations on corrosion inhibition mechanism in 1.0 M HCl solution. The effect of concentration and temperature on the inhibiting efficiency has been examined. The thermodynamic parameters for both dissolution and adsorption processes were calculated and discussed for TB6. The adsorption of inhibitor TB6 on the mild steel surface obeys the Langmuir adsorption isotherm equation.

The inhibitive characteristics of aqueous extracts from mangrove (Rhizophora apiculata) bark and leaf on the corrosion of mild steel (MS) coupon in 1 M HCl were examined by means of the gravimetric measurement (weight loss), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The MS surface morphologie were investigated using field emission scanning electron microscopy (FESEM) with Energy Dispersive X-Ray Analysis (EDX) at 120h in the presence and absence of green inhibitors in corrosive medium. Respective inhibition efficiencies of 68.1% and 59.0% were obtained when exposed to 10% v/v Rhizophora apiculata solution for 72h. Examination of the adsorption behavior of the solutions indicated the Langmuir isotherm model as being the most likely relevant adsorption mechanism. In addition, polarization measurements showed that both bark and leaves extract inhibitors act via mixed type inhibition.

The synergistic effect of halide ions such as KCl, KBr and KI on the corrosion inhibition of mild steel in 1 N sulphuric acid by $\gamma$-2,c-6-diphenyl-t-3-methyl piperdin-4-ones with semicarbazone (01SC), $\gamma$-2,c-6-diphenyl-N-methyl-t-3-ethyl piperdin-4-ones with semicarbazone (02SC) and 2,6-diphenyl-t-3-ethyl piperdin-4-one with semicarbazone (03SC) has been examined by weight loss method, potentiodynamic polarization measurements and electrochemical AC impedance spectroscopy. Results show that substituted $\gamma$-2,c-6-diphenyl piperidin-4-ones with semicarbazone act as the perfect corrosion inhibitors and their inhibition efficiency increases with the addition of halide ions. The inhibitor (01SC) shows the inhibition efficiency of 78.28% (0.2mM) by using a weight loss method. The influence of I${}^{-}$, Br${}^{-}$ and Cl${}^{-}$ anions raises the inhibition efficiency of the substituted 2,6-diphenyl piperidin-4-ones with semicarbazone due to the synergistic effect. The synergistic effect of halide ions was formed in the following order: KI $>$ KBr $>$ KCl.

The anti-corrosion activity of a newly synthesized ethylene tetra phosphonic acid (ETPA), namely {Ethylenebis [(2-hydroxy-5,1,3-phenylene) bismethylene]} tetraphosphonic acid, against the corrosion of carbon steel in 0.5-M H₂SO₄ medium and its synergistic effect with Cu${}^{2+}$ ions were studied using potentiodynamic polarizations and electrochemical impedance spectroscopy (EIS). Potentiodynamic polarization studies indicate that ETPA acts as a mixed-type inhibitor and inhibition efficiency increases with increasing ETPA concentration. The adsorption of ETPA at the surface of carbon steel follows Langmuir adsorption isotherm. EIS results demonstrated the adsorption of ETPA onto the carbon steel surface, leading to the formation of ETPA overlaying film. Addition of $5\cdot 10^{-5}$ M Cu${}^{2+}$, at low ETPA concentration ($5\cdot 10^{-5}$ M), results in significant increase in inhibition efficiency (88%), superior to that obtained at high ETPA concentration (81% at $5\cdot 10^{-4}$ M). SEM and EDAX analyses confirmed the existence of a uniform protective film on the electrode surface attributed to ETPA–Cu${}^{2+}$ complex formation.