Investigation Of The Mechanism Of Crevice Corrosion And Galvanic Effects In Steel Fasteners Exposed To A Neutral Chloride-containing Environment

Fasteners are essential components in structures and mechanical parts.They typically sustain the framework and aid linkages.Most fasteners are made of steel materials and thus are vulnerable to corrosion degradation when exposed to aggressive chloride environments.Several corrosion-related structural failures have been traced to fasteners.The presence of crevices at the bolt/nut joint makes them susceptible to crevice corrosion.The potential difference established between the exposed and crevice region due to the differential aeration effect drives a galvanic interaction.This galvanic behaviour is similar to what is observed in a battery.Although there is general knowledge on the corrosion of fasteners,few systematic studies have been performed on the mechanisms involved.Some of the corrosion-related research focused on the post-failure analysis of in-service bolted components,thus creating a gap in the knowledge of fastener corrosion.In this study,the mechanism of crevice corrosion and galvanic effect on the degradation of steel fasteners exposed to neutral chloride environments was investigated.The different materials used were Q235 steel(plain),C1045 steel(plain and threaded),A2-70(304SS),galvanized steel and alloy steel(40Cr steel)bolts and nut fasteners.Laboratory testing was performed to collect the electrochemical data and information,while field exposure testing in the marine atmospheric environment was carried out to complement and verify the laboratory findings.Various techniques including non-electrochemical,electrochemical and surface analysis techniques were employed in this study.They include weight loss measurement,open-circuit potential(OCP),electrochemical impedance spectroscopy(EIS),potentiodynamic polarization(PD),zero-resistance ammeter(ZRA),optical microscopy(OM),stereomicroscopy,scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),electron probe micro-analyzer(EPMA),X-ray diffractometry(XRD),X-ray photoelectron spectroscopy(XPS)and Raman spectroscopy.Under the scope of this study,five core aspects which include surface morphology,nature of oxide film,corrosion rate,galvanic behaviour and area ratio effects were assessed.From the study on the corrosion behaviour of C1045 steel fasteners in an aqueous chloride environment,it was deduced that under uncoupled conditions both the crevice and exposed regions of the fasteners behaved as mixed electrodes.The self-corrosion rate was higher for the exposed region due to the increased cathodic oxygen reduction process,and lower for the creviced sample due to the effect of low oxygen linked to the crevice geometry.However,under coupled conditions,the crevice region acted as the anode,while the exposed region was the cathode.This resulted in increased corrosion in the crevice region.The differential aeration/galvanic effect served as the driving force for the corrosion process.However,the galvanic effect was weak.Corrosion product characterization revealed the presence of lepidocrocite(γ-FeOOH)as outer rust and magnetite(Fe3O4)in the inner rust layer after prolonged immersion.However,the crevice region was predominated by akaganeite(β-FeOOH)and magnetite(Fe3O4).No decrease in pH was observed,rather the pH increased slightly due to the formation of Fe3O4.The kinetic data as a function of immersion time showed a deceleration and acceleration in the corrosion process at the exposed and crevice regions,respectively.Increasing the cathode-to-anode area ratio led to accelerated the corrosion process in the fastener.From the analysis,the crevice corrosion in carbon steel fasteners followed the potential(IR)drop mechanism.For 304 stainless steel fasteners exposed to a neutral chloride environment,it was revealed that under uncoupled conditions,the crevice sample was more prone to corrosion degradation when compared with the exposed sample.This was attributed to the decreased passivation effect.Coupling of the exposed and crevice parts demonstrated accelerated crevice corrosion induced by galvanic effect.The driving force for the galvanic-crevice effect under coupled conditions depended on three factors;(i)the potential difference established between the different regions on the fastener surface due to uneven oxygen distribution,(ii)the difference in the solution environment,and(iii)surface properties(active-passive surface).The pH measured for the crevice solution was within the range of 3 to 4.EPMA and XPS depth profile analysis showed evidence of chloride ions in the passive film.The ratio of Cr2O3 to Cr(OH)3 was lower for the crevice sample indicative of an aggressive crevice environment.The crevice corrosion in 304SS fasteners followed the potential(IR)drop and critical crevice solution(CCS)mechanisms.From the corrosion evaluation performed on the dissimilar structural steel bolt fasteners exposed to marine atmospheric environments,it was found that the difference in oxygen,humidity,temperature and salinity levels played key roles in the corrosion of the steel fasteners.The distance from the coastline also impacted the fastener degradation process.The passive steel bolt(304SS)exhibited localized corrosion patterns with no obvious rust build-up.However,the active steel bolts(galvanized steel and 40Cr alloy steel)exhibited uniform corrosion patterns with evidence of rust buildup and cracks of varying degrees.The main constituents of rust found on the corroded carbon and alloy steel fasteners were γ-FeOOH,β-FeOOH,α-FeOOH and the iron oxides Fe3O4/γ-Fe2O3,and Fe2O3.The corrosion resistance at the crevice region was higher in the carbon steel bolts compared to the exposed region,while that of stainless steel was lower at the crevice region and higher in the exposed region.This was attributed to the variation in the corrosion mechanisms.The fastener corrosion process in 304SS followed the critical crevice solution(CCS)and potential(IR)drop mechanism,while that of the carbon steel fastener followed the IR drop mechanism.This confirms the findings from the laboratory analysis.Lastly,the research on the corrosion behaviour of bolted joints consisting of dissimilar metals showed that different variables such as alloying composition,chloride ion content,area ratio,and applied torque influenced the corrosion process,an indication of the synergistic effect of both crevice and galvanic corrosion in the degradation of fastening systems.