Influence of alloying elements, testing solution and surface roughness on corrosion behaviour of stainless steel reinforcing bars
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The province of Ontario has moved from applying rock salt crystals, predominantly impure sodium chloride, to locally available anti-icing brine solutions with chloride amounts as high as 21%. At the same time, the specified design service life of highway structures has increased from 50 years to 75 years. The exposure to aggressive chloride brines has increased the need for more corrosion resistant reinforcing bar (rebar) than the traditional carbon steel rebar. However, the high cost of many stainless steel rebar alloys made them a last resort when concrete reinforcement options are considered. A major factor for their high cost is the price of their major alloying elements. Therefore, the contributions of these elements towards passive film properties, pitting corrosion resistance, critical chloride threshold (CCRIT) values and overall corrosion performance in the presence of deleterious species in concrete, such as chlorides, must be ascertained. This research aimed to provide a critical evaluation of the various parameters affecting long term corrosion performance of different grades of stainless steel rebar in concrete exposed to anti-icing brines. The first step to achieving this was to determine the variation in pore solution compositions of different concrete mixes so that bars can be tested in a similar environment. To do this, cement pastes of varying admixed chloride content, cementitious materials and water-to-cementitious ratio (w/cm) were cast and their pore solutions were analyzed for ionic composition and pH. The results revealed increasing dissolution of sulphate ions with increasing admixed chloride ions in the pore solution. These actual solutions were used in subsequent assessment procedures involving electrochemical techniques such as Mott-Schottky analysis, potentiostatic linear and non-linear polarization resistance and cyclic potentiodynamic polarization techniques. The goal was to determine the passive film properties, pitting characteristics, critical chloride threshold (CCRIT) and relative corrosion performance of carbon steel and five grades stainless steel rebar. Results showed the addition of sulphates to testing solutions suppressed the damage from chloride ions on steel passive films, by forming iron and nickel sulphides in passive films that provided additional protection. Results also showed that testing in lower pH solution, as done by many researchers, is conservative and underestimates corrosion resistance. The influence of the expensive stainless steel alloying elements (Cr, Ni, Mo, Mn) on the corrosion parameters listed above were then investigated. Chromium significantly improved these properties by decreasing passive film defects and increasing CCRIT values of the rebar. Molybdenum did not improve the corrosion resistance in the austenitic alloys but was beneficial in duplex alloys by concentrating in the ferritic component. Nickel was found to improve the outer layer of passive films properties by forming an Fe-Ni spinel, while manganese improved the inner passive layer. It is necessary for these observations in corrosion behaviour of rebar to be consistent. Consequently, other factors potentially leading to variations in corrosion performance of stainless steel rebar alloys were examined. These included the influence of variability in composition, microstructure and surface roughness between batches of stainless steel alloys from different manufacturers, and results showed surface roughness to be the major and overwhelming factor in corrosion resistance. The most important observation has been that, for the particular concrete mixture used in this research, the critical chloride threshold concentration, found by extrapolation of the experimental data, was greater than the solubility limit of chlorides in cement pores. This implies that chloride induced corrosion of the stainless alloys would not be possible in this concrete in the absence of cracks or major flaws.
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Ibrahim Ogunsanya (2020). Influence of alloying elements, testing solution and surface roughness on corrosion behaviour of stainless steel reinforcing bars. UWSpace. http://hdl.handle.net/10012/15445