Corrosion Behaviour of Carbon Steel Fasteners in Neutral Chloride Solution

An experimental model for simulating the corrosion of carbon steel fasteners (bolt and nut) composed of a contact carbon steel electrode (CCSE) and an exposed bare carbon steel plate electrode (BCSE) was designed. The effect of coupling on the corrosion process of the galvanically coupled carbon steel electrode was evaluated and compared with the self-corrosion process observed independently at the exposed and contact regions. Results obtained indicated that at an equal area ratio and uncoupled conditions, the corrosion rate is accelerated in the surface directly exposed to bulk solution compared to the bolt surface in contact with the nut. A coupling current was recorded when the exposed surface (BCSE) was electrically connected with the contact surface (CCSE); with the CCSE acting as the anode thereby suppressing the corrosion process in the exposed surface. By implication, the galvanic coupling between CCSE and BCSE increased the corrosion rate of CCSE. The difference in oxygen supply was responsible for the coupling effect observed in the system as there was no decrease in the solution pH. Moreover, varying the cathode-to-anode area (S_c/S_a) ratio significantly influenced the corrosion current density as increased S_c/S_a ratio resulted in an accelerated galvanic corrosion process. The corroded surfaces and interfaces were analysed using stereomicroscopy and scanning electron microscopy. X-ray diffractometry was adopted for corrosion product characterization. The results obtained showed supportive evidence of the corrosion behaviour in carbon steel fasteners.