Abstract
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.
Highlights
Steel is the most widely used metallic material thanks to its good mechanical properties and low price
In this paper we extend the synergistic approach by hypothesising that a carefully chosen mixture of non-toxic and environmentally benign inhibitive compounds could modify the rust layer on steel
It has been found that increasing the immersion time of iron and steel in the solution enables thickening of the corrosion product layer resulting in the decrease of the corrosion rate
Summary
Steel is the most widely used metallic material thanks to its good mechanical properties and low price. PEG is used as an alternative solvent for non-ethanolic propolis extraction,[11] and when used together with propolis, could beneficially influence propolis dispersibility in a water solution.[12] it has been shown that a compact layer, with barrier effect for the diffusion of oxygen in 0.1 NaCl solution, has been deposited on mild steel from the solutions containing 1000, 2500 and 5000 g ppm of PEG after 30 min of immersion time, showing that PEG itself can act as a corrosion inhibitor.[13] Formation of the protective ferric ‘‘tannate film’’ deposited from solution of tannin is possible, and it was shown to begin at pH = 3, but relevant efficiencies are attained only at concentrations ~1000 ppm.[14] Recent investigation of low carbon steel inhibition by 1–5% tannic acid solution in 3.5% NaCl shows that efficiencies up to 51% can be obtained.[15] Concentrations of the same order are needed for efficient steel corrosion inhibition in chloride media by starch[16] and sodium benzoate.[17] In the present study, minimal quantities of individual components that would be effective, based on the above literature references, were chosen for the inhibitor mixture composition. High concentration requirement would be counteracted by a long inhibitor free period, following batch application, during which the protective surface film would ensure acceptable corrosion rates
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