Silane coatings for mitigation of microbiologically influenced corrosion of mild steel

Abstract Mild steel continues to be the most extensively used construction material in several industries. However, steels suffer from corrosion in aqueous solutions. Coating is one of the common measures to circumvent corrosion. Silane coating is among the recent and promising measures to improve the corrosion resistance of metallic materials in corrosive environments. Silane coatings are becoming increasingly popular particularly over the traditional chromate conversion coatings that have harmful effects on human health and the environment and hence face increasingly regularity restrictions. This study has been conducted to evaluate the influence of silane treatments on corrosion resistance of mild steel in sodium chloride environment and the mitigation of microbial influenced corrosion (MIC) of mild steel due to sulphate reducing bacteria (SRB). The optimum conditions (viz; pHs of hydrolysis and condensation, hydrolysis time, dipping time and application method) of single/two step silane coatings of Bis-[triethoxysilyl]ethane (BTSE), bis-[trimethoxysilylpropyl]amine (bis-amino silane), 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride (quaternary ammonium silane) and octadecyltrimethoxysilane (ODTMS) silanes have been investigated for improving corrosion resistance of mild steel in aggressive chloride solution (similar to sea water). The influence of silane coatings on corrosion performance of mild steel has been explained on the basis of electrochemical and analytical characterizations using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization test, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM). Electrochemical measurements showed that the corrosion resistance of mild steel improved due to the BTSE silane coatings that were deposited at an optimum pH 4 of deposition. The longer the dipping time the better is the corrosion resistance of BTES-coated mild steel in the chloride solution. The time of hydrolysis of 3 h for octadecyltrimethoxysilane (ODTMS) in water alcohol solution at pH 4 was found to be optimum for producing the stable and fully hydrolysed silane. Thus, the silane film developed under this condition was found to provide considerable corrosion resistance up to 24 h in 0.6 M NaCl solution. The significant decrease in corrosion resistance with increase in time of pre-immersion in corrosive solution was attributed to the ingress of corrosive media to the metal surface. As seen from the electrochemical tests, two step ODTMS coating enhanced the inhibition action of mild steel against corrosion in NaCl solution. However; the two step ODTMS coating is still permeable. Similar to what has been reported for the long aliphatic silane films deposited on other metals and alloys, the ODTMS coating on mild steel is suggested to possess defects, such as interconnected pores. Although, quaternary ammonium silane coatings have been used to improve the antimicrobial activity of coated surfaces, the interaction of the positively-charged ammonium group with water causes degradation of silane film when exposed to corrosive environments. The present work addresses the effect of two step silane coatings containing different concentrations of quaternary ammonium silane on corrosion resistance of mild steel in 0.6 M NaCl solution. The two step silane coatings were formed by dipping the mild steel coupons in non-functional silane solution [bis[triethoxysilyl]ethane (BTSE)] followed by dipping either in quaternary ammonium silane alone or in a silane mixture composed of bis-functional and quaternary ammonium silanes. For the second alternative, different silane mixtures were prepared by mixing of bis-[trimethoxysilylpropyl]amine (bis-amino silane) with different concentrations of 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride in different mixing ratios (1:1, 3:1and 5:1 V/V). Based on electrochemical measurement results, the corrosion resistance improved as a result of coatings developed upon two step silane treatments of BTSE followed by the mixture of bis-amino and quaternary ammonium silane. But, the corrosion resistance decreased significantly (and was even lower than that uncoated mild steel) when the quaternary ammonium silane alone was used as the top coating for BTSE-coated mild steel. After determination of the optimum conditions of silane treatments to achieve the best corrosion resistance of mild steel in NaCl solution, the optimum conditions for silane coating to mitigate the microbial influenced corrosion of mild steel in the presence of sulphate reducing bacteria (SRB) have been investigated. Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and cyclic polarization tests were used to evaluate the enhancement in corrosion resistance of mild steel pre-exposed to biotic (media inoculated with SRB) and abiotic environments. To characterize the deposits on mild steel surface before and after MIC process; FTIR analysis were used. The coupons were examined for their surface biofilms and corrosion features, using SEM/EDX technique. Generally, the silane-coated mild steel suffered less acceleration in corrosion in the presence of SRB, as seen from the corrosion potential, which moved less towards cathodic direction compared to that of uncoated specimens. Electrochemical investigations showed that the coating developed upon two step treatment of octadecyltrimethoxysilane had the least corrosion current density and the highest impedance among the investigated coatings. The least tendency to pitting was observed for the coating, developed upon two step silane treatments of BTSE, followed by the mixture of bis-amino and quaternary ammonium silanes in mixing ratios of 5:1 (V/V). BTSE treatment alone did not improve the corrosion resistance of mild steel in the presence of SRB.