The ubiquitous use of plain and low-alloy steels in neutral, chloride bearing environments presents an everyday challenge for corrosion protection professionals. This paper explores the possibility of developing a non-toxic and environmentally friendly synergistic inhibitor mixture made of propolis, tannin, sodium benzoate, PEG400 and starch that could be applied into the solution to induce the formation of a persistent protective layer on steel. Components of the mixture were chosen based on the references giving their characteristics relevant to their possible action in the solution and/or at the surface of the steel. The efficiency of the protective layer formed under the influence of the inhibitor mixture, and then measured by the LPR probe in the inhibitor-free solution, was the lowest under quiescent conditions (75%) and the highest (95%) under flow conditions. Both, LPR and EIS data indicate that the inhibitor modified layer presents a barrier for diffusion of oxygen that acts as a primary corrosion reaction depolarizer in the investigated neutral chloride solution. The demonstrated persistence of the inhibitor modified layer is of primary interest as it gives possibility batch application.
Concomitant rust growth on a rust-free surface and conversion by a solution of tannin, propolis and benzoate has been investigated in the present study with the aim of producing a modified rust layer, more resilient and protective than a pure rust layer. Under such circumstances, balancing simultaneous but contradictory processes of corrosion acceleration by iron chelation and corrosion inhibition by ferric tannate precipitation is crucial for layer formation. Benzoate served as an iron corrosion inhibitor in the layer formation stage while the propolis added to the tannin solution improved the modified rust layer protective efficiency. Corrosion rates lower than 5 mpy that are acceptable from the practical viewpoint are attained with the inhibitor-modified rust layer, both, in quiescent solution and up to a linear velocity of the solution approximately equal to 1.5 m s −1 .
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