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This article presents a concise review of different types of inhibitors for corrosion protection on metal surfaces. Corrosion inhibitors can be of different types, which include organic, inorganic and hybrid (organic/inorganic) materials. They are also classified as cathodic, anodic and/or mixed-type inhibitors, which are based on the active inhibitor molecules that retard the corrosion process. Silicate, nitrites, molybdates, phosphates, zinc salt and cerium salt are widely used as inorganic inhibitors. Many organic compounds have been widely utilized as inhibitors. Corrosion protection will be obtained by various mechanisms such as physisorption, chemisorption, barrier protection, thin-film formation and electrochemical processes. The type of inhibitors, inhibition mechanism and the evaluation methods have been explained in detail.

Globally, researchers have devoted consistent efforts to producing excellent coating properties since coating plays an essential role in enhancing electrochemical performance and surface quality. In this study, TiO2 nanoparticles in varying concentrations of 0.5, 1, 2, and 3 wt.% were added into the acrylic-epoxy polymeric matrix with 90:10 wt.% (90A:10E) ratio incorporated with 1 wt.% graphene, to fabricate graphene/TiO2 -based nanocomposite coating systems. Furthermore, the properties of the graphene/TiO2 composites were investigated by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), ultraviolet-visible (UV-Vis) spectroscopy, water contact angle (WCA) measurements, and cross-hatch test (CHT), respectively. Moreover, the field emission scanning electron microscope (FESEM) and the electrochemical impedance spectroscopy (EIS) tests were conducted to investigate the dispersibility and anticorrosion mechanism of the coatings. The EIS was observed by determining the breakpoint frequencies over a period of 90 days. The results revealed that the TiO2 nanoparticles were successfully decorated on the graphene surface by chemical bonds, which resulted in the graphene/TiO2 nanocomposite coatings exhibiting better dispersibility within the polymeric matrix. The WCA of the graphene/TiO2 coating increased along with the ratio of TiO2 to graphene, achieving the highest CA of 120.85° for 3 wt.% of TiO2. Excellent dispersion and uniform distribution of the TiO2 nanoparticles within the polymer matrix were shown up to 2 wt.% of TiO2 inclusion. Among the coating systems, throughout the immersion time, the graphene/TiO2 (1:1) coating system exhibited the best dispersibility and high impedance modulus values (Z0.01Hz), exceeding 1010 Ωcm2.

In this study, a co-action behavior of a hybrid composed of chitosan and silica was investigated by incorporating it into an epoxy resin to induce an active barrier effect on the metal surface. The chemical structure and porosity of the hybrid chitosan and silica were determined using Fourier-Transform Nuclear Magnetic Resonance (FT-NMR) and Brunauer–Emmett–Teller (BET), respectively. The dispersion of the hybrid chitosan/silica incorporated within the epoxy matrix was observed via Environmental Scanning Electron Microscopy (ESEM) and Fourier Transform Infrared Spectroscopy (FTIR). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were exploited to examine the thermal stability. The wettability and degree of transparency of the hybrid composite coatings were found to be increased after being exposed to the corrosive medium and also were investigated using the contact angle (CA) measurement and ultraviolet-visible (UV–Vis) spectroscopy, respectively. CS0.8 coating has shown an increase in contact angle from 82.4 at normal condition and 89.4 and 83.6 to the exposure to NaCl solution and UV exposure respectively. The active barrier effect of the developed hybrid composite coating on mild steel surface was evaluated by the electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution to which 0.8 wt.% of hybrid chitosan/silica mass fraction exhibited pronounced impedance value up to 1010 Ω cm2 and the breakpoint frequency performed a full capacitive region even after the 60th day of exposure to the accelerated corrosive ions penetration test.