Possible Mechanism of Corrosion Inhibition of a High Strength Low Alloy Steel By a Mussel Adhesive Protein
Authors: Hansen Douglas C , Bennett Brooke N 
Published: February 2020

A proteinaceous biopolymer, mussel adhesive protein #5 (MAP-5), isolated from the common blue mussel (Mytilus edulis L) has been investigated as a model for designing an aqueously soluble candidate corrosion inhibitor system that is non-toxic and environmentally friendly and that could inhibit the flash rusting of exposed high strength low alloy steel surfaces during the paint removal process. It was found that a significant amount of corrosion inhibition is possible (nearly 100% inhibition after 7 days) on HY80 steel in a 100% relative humidity environment at 40 °C. Indeed, these results were shown to surpass the performance of a commercially available flash rust inhibitor (90% inhibition after 7 days) currently in use on maritime vessels. These results indicated that it is possible to utilize the biochemistry of a naturally occurring biopolymer isolated from the marine mussel, Mytilus edulis (L), to develop a non-toxic and environmentally friendly corrosion inhibitor. This protein is unique in containing 27% L-dopa and 20% lysine, two unique amino acids containing catecholic and primary amine functional groups, respectively. To characterize how Mefp-5 interacts with the HY80 steel, a variety of analytical spectroscopy techniques were implemented. Solutions of Mefp-5, L-dopa or lysine dissolved in deionized water, 5% acetic acid, 0.05 M potassium phosphate buffer (pH 5.5), or the same buffer containing mushroom tyrosinase were adsorbed onto HY80 and glass substrates. Energy dispersive spectroscopy (EDS) measurements indicate that the iron content is highest where the MAP-5 biopolymer is adsorbed onto the steel substrate at pH 5.5, twice the content of the iron on the steel surface alone. When the biopolymer is enzymatically cross-linked on the steel surface at pH 5.5, the iron content is decreased by 1/3 of that of the adsorbed biopolymer. Raman infrared spectroscopy suggests that adsorbate, solution composition and pH play a role in the type of iron oxide formed and how the protein orients itself on the HY80 surface. At a pH of 5.5, the isoelectric point (pI) of the Fe2O3 oxide suggests the negatively charged oxide surface attracts lysine’s positively charged ε-amine group. Infrared spectroscopy indicates that L-dopa is also intimately involved in the adsorption of the protein and complexation of iron at the steel surface. 3-dimensional modeling of the MAP-5 protein indicates that the mole percent of L-dopa does not affect the conformation of the polymer in solution; however, results suggest that the amount of L-dopa contained within the polymer may have an effect on the corrosion inhibition efficiency. Overall, the data points toward a synergy existing between L-dopa and lysine, where lysine aids in coulombic attraction to the iron surface, allowing the protein to remain in close proximity and facilitating metal complexation by the L-dopa. The iron at the steel surface can undergo complexation as well as provide possible metal-mediated cross-linking of the protein; subsequent treatment of the adsorbed protein with enzyme results in additional cross-linking of the protein on the steel surface, thus complexing less of the iron. This scenario suggests that it is possible to engineer a corrosion inhibitor that meets the need for low volatile organic content, is aqueously soluble and environmentally benign.