Purpose: To evaluate the inhibition efficiency (IE) of polyvinyl alcohol (PVA) in controlling the corrosion of carbon steel immersed in neutral aqueous solutions containing 60 ppm of Cl⁻, in the absence and presence of Zn²⁺. To investigate the influence of sodium sulphite (Na₂SO₃), sodium dodecyl sulphate (SDS), pH and duration of immersion on the IE of PVA‐Zn²⁺ system. To analyse the protective film formed on the metal surface.Design/methodology/approach: The IE has been evaluated by weight loss method. The protective film was analysed by FTIR and fluorescence spectra.Findings: A formulation consisting of 100 ppm of PVA and 75 ppm of Zn²⁺ offered 81 per cent IE to carbon steel immersed in a solution containing 60 ppm of Cl⁻. A synergistic effect on inhibition of a combination of PVA and Zn²⁺ was observed during the tests. The protective film consisted of the Fe²⁺‐PVA complex and Zn(OH)₂. It was found to be UV‐fluorescent. When SDS was added to the PVA‐Zn²⁺ system, the mixture showed maximum IE at the critical micelle concentration (200 ppm) of SDS (an anionic surfactant). The oxygen‐scavenging effect of Na₂SO₃ increased as the concentration of Na₂SO₃ was increased. At lower concentrations of Na₂SO₃, the transport of the inhibitors played a more major role than did the removal of dissolved oxygen. As the pH value was increased, the IE of the PVA‐Zn²⁺ system decreased. As the duration of immersion was increased, the IE was observed to decrease.Research limitations/implications: Electrochemical studies such as polarization and AC impedance spectra will enlighten more on the mechanistic aspects of corrosion inhibition.Practical implications: If this study is carried out at high temperature under simulated conditions, the findings may find applications in cooling water systems.Originality/value: The role of transport of inhibitors towards the metal surface from the bulk of the solution, formation of micelles by surfactants, removal of dissolved oxygen by oxygen scavenger, competition between formation of insoluble iron‐inhibitor complex on metal surface and formation of soluble iron chloride in influencing the inhibitive property has been investigated. The protective film was analysed by FTIR spectra and fluorescence spectra.

The inhibition efficiency of N‐cetyl‐N,N,N‐trimethylammonium bromide (CTAB), a cationic surfactant, in controlling the corrosion of carbon steel in an aqueous solution containing 60ppm Cl^–, in the absence and presence of Zn²⁺ and also calcium gluconate (CG), had been evaluated by the weight‐loss method. Weight‐loss studies revealed that the behaviour of CTAB changed when its concentration is ≥ 150ppm. Organisation of assembly of CTAB molecules is expected to take place at this critical concentration. The protective film has been analysed using FTIR spectra. In the presence of CTAB, the protective film consists of Fe²⁺‐CTAB complex. In the presence of CTAB and Zn²⁺, the protective film consists of Fe²⁺‐CTAB complex and Zn(OH)₂. In the presence of CTAB and calcium gluconate, the protective film consists of Fe²⁺‐CTAB complex, Fe²⁺‐gluconate complex and Ca(OH)₂.

The inhibition efficiency (IE) of an aqueous extract of white flower, namely, Hibiscus rosa-sinensis Linn., in controlling corrosion of carbon steel immersed in an aqueous solution containing 60 ppm of Cl⁻has been evaluated by the mass loss method. The flower extract (FE) shows good IE. In the presence of Zn²⁺, excellent IE is shown by the flower extract. A synergistic effect exists between the flower extract and Zn²⁺. The mechanistic aspects of corrosion inhibition have been investigated by polarization study and AC impedance spectra. Polarization study reveals that the formulation consisting of flower extract and Zn²⁺ functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. The active principle in the flower extract is quercetin-3-O-glucoside. This has been confirmed by UV-visible absorption spectra. The protective film formed on the metal surface has been analyzed by FT-IR and AFM spectra. It is found that the protective film consists of Fe²⁺-quercetin-3-O-glucoside complex and Zn(OH)₂.