Crevice corrosion generally occurs on the crevice surface while the exterior or bold surfaces are not damaged. However, for copper and its alloys, the opposite is true; the bold surface is corroded while the crevice remains relatively corrosion-free. This unique type of corrosion is referred to as reverse crevice corrosion (RCC). In this research, commercially pure copper was chosen as the target metal to investigate RCC. Based on electrochemical measurements and surface analysis, reverse crevice corrosion was found to occur at room temperature. At elevated temperature only uniform corrosion was observed while under a deoxygenated environment, as expected, no corrosion was observed.<p> A multiple crevice assembly and a working electrode were designed especially for this research. Exposure test experiments were first performed at room temperature and 50 ºC. Several types of electrochemical tests were conducted including open circuit potential measurement, potentiodynamic measurement and electrochemical impendence spectroscopy (EIS). Atomic Force Microscopy (AFM) and Raman Spectroscopy were used to analyze the surfaces of the copper coupon.<p>The results of the exposure tests showed that RCC occurred at room temperature, but not at elevated temperature. Only uniform corrosion was observed at elevated temperature and no corrosion was occurred under a deoxygenated environment. It was found, based on the open circuit potential measurement, that the RCC process can be divided into three steps, a uniform corrosion phase, a corrosion slow-down step and a reverse crevice corrosion step. The first two steps can be combined into one phase, incubation phase. This hypothesis is supported with the results from Raman spectra and AFM. The EIS measurements revealed that the diffusion process from bulk solution to copper coupon surface is the rate controlling step for incubation phase and this diffusion process combined with the reduction of Cu (I) oxide in the crevice are the rate-controlling step corresponding to the last step.

University of Macau / Faculty of Science and Technology / Department of Electromechanical Engineering

No description available.

Most studies that have examined chloride-induced corrosion of steel in concrete have focused on sound concrete. However, reinforced concrete is seldom uncracked and very few studies have investigated the influence of cracked concrete on rebar corrosion. Furthermore, the studies that have examined the relationship between cracks and corrosion have focused on unloaded or statically loaded cracks. However, in practice, reinforced concrete structures (e.g. bridges) are often dynamically loaded. Hence, the cracks in such structures open and close which could influence the corrosion of the reinforcing steel. Consequently, the objectives of this project were (i) to examine the effect of different types of loading on the corrosion of reinforcing steel, (ii) the influence of concrete mixture design on the corrosion behaviour and (iii) to provide data that can be used in service-life modelling of cracked reinforced concretes. In this project, cracked reinforced concrete beams made with ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) were subjected to no load, static loading and dynamic loading. They were immersed in salt solution to just above the crack level at their mid-point for two weeks out of every four (wet cycle) and, for the remaining two weeks, were left in ambient laboratory conditions to dry (dry cycle). The wet cycle led to three conditions of exposure for each beam: (i) the non-submerged region, (ii) the sound, submerged region and (ii) the cracked mid-section, which was also immersed in the solution. Linear polarization resistance and galvanostatic pulse techniques were used to monitor the corrosion in the three regions. Potentiodynamic polarization, electrochemical current noise and concrete electrical resistance measurements were also performed. These measurements illustrated that (i) rebar corroded faster at cracks than in sound concrete, (ii) HPC was more protective towards the rebar than OPCC even at cracks and (iii) there was a minor effect of the type of loading on rebar corrosion within the period of the project. These measurements also highlighted the problems associated with corrosion measurements, for example, identifying the actual corroding area and the influence of the length of rebar. The numbers of cracks and crack-widths in each beam were measured after the beam’s initial exposure to salt solution and, again, after the final corrosion measurements. HPC beams had more cracks than the OPCC. Also, final measurements illustrated increased crack-widths in dynamically loaded beams, regardless of the concrete type. The cracks in both statically and dynamically loaded OPCC and HPC beams bifurcated at the rebar level and propagated parallel to the rebar. This project also examined the extent of corrosion on the rebars and the distribution of corrosion products in the concrete and on the concrete walls of the cracks. Corrosion occurred only at cracks in the concrete and was spread over a larger area on the rebars in HPC than those in OPCC. The damage due to corrosion was superficial in HPC and crater-like in OPCC. Regardless of the concrete type, there was a larger distribution of corrosion products on the crack walls of the dynamically loaded beams. Corrosion products diffused into the cement paste and the paste-aggregate interface in OPCC but remained in the crack in HPC. The most voluminous corrosion product identified was ferric hydroxide. Elemental analysis of mill-scale on rebar which was not embedded in concrete or exposed to chlorides was compared to that of the bars that had been embedded in uncontaminated concrete and in cracked concrete exposed to chlorides. In uncontaminated concrete, mill-scale absorbed calcium and silicon. At a crack, a layer, composed of a mixture of cement paste and corrosion products, developed between the mill-scale and the substrate steel. Based on the results, it was concluded that (i) corrosion occurred on the rebar only at cracks in the concrete, (ii) corrosion was initiated at the cracks immediately upon exposure to salt solution, (ii) the type of loading had a minor influence on the corrosion rates of reinforcing steel and (iv) the use of polarized area led to a significant underestimation of the current density at the crack. It is recommended that the effect of cover-depth on (i) the time to initiation of corrosion and (ii) the corrosion current density in cracked concrete be investigated.

Deterioration of reinforced concrete structures due primarily to chloride induced corrosion of plain carbon-steel reinforcement is a widespread problem, particularly in areas close to marine environments and where de-icing salts are used to keep roadways clear of ice. Replacing plain carbon-steel rebar with highly corrosion resistant stainless steel rebar has been shown to greatly increase the lifespan of concrete structures in harsh environments, and yields favourable life-cycle costs despite high initial costs. In attempt to lower stainless steel rebar’s initial cost of processing, this research compared its corrosion resistance in the pickled (mill scale removed) and as-rolled (mill scale intact) surface conditions. Rebar was embedded in highly-chloride contaminated concrete, and corrosion performance between the two surface types was compared in order to determine if conventional pickling of stainless steel rebar is necessary. A second part of this research addressed possible concern of reduced corrosion resistance of pickled stainless steel rebar in concrete exposed to chlorides when subjected to dynamic loading due to micro-motion at the concrete/crack interface. It was concluded that as-rolled stainless steel rebar in aggressive environments would provide sufficient corrosion resistance for the 75 year lifespan currently specified by the Canadian Bridge Code (CAN/CSA-S6-06, 2006), however it is recommended that monitoring of these specimens be continued to ensure high corrosion rates and/or concrete cracking do not develop. As well, investigation into the effects crevice corrosion cells found in typical concrete structures could have on as-rolled stainless steel rebar’s corrosion resistance should be undertaken. With regard to loading conditions, no significant evidence was found suggesting that pickled stainless steel rebar has reduced corrosion resistance when loaded dynamically versus statically. Therefore pickled stainless steel rebar is recommended for use in dynamically loaded concrete structures if others factors permit. However, the higher electrochemical noise measured during cyclic loading suggests that corrosion behaviour could be influenced largely by frequency of loading, and so further study should be undertaken for applications involving more extreme cyclic loading conditions than those used in this experiment.

Corrosion of carbon (black) steel reinforcing bars (rebar) is the major cause of damage and deterioration of reinforced concrete structures in maritime regions and in climates where de-icing salts are used. The cause of the corrosion is diffusion of chloride ions to the steel surface through the concrete in which it is placed. The bars are naturally passivated by the high pH of concrete interstitial pore fluid, and will not corrode in chloride-free concrete. Chloride ions break down the passive film, allowing dissolution of the steel. Corrosion of reinforcing steel drastically reduces the service lives of concrete structures. Where chlorides can not be avoided, stainless steel is becoming increasingly popular as an alternative reinforcing material. Stainless steel is able to withstand greater concentrations of chlorides, extending the service lives of structure in which they are placed. Due to high initial cost, stainless steel is often avoided in the design of new structures. In order to reduce the cost of stainless rebar, it has been proposed that the standard process of abrasive blasting and pickling of the steels not be performed, as these steps are mainly used to restore a bright and shiny surface, a quality not required for steels embedded in concrete. AISI 304LN, AISI 316LN and 2205 duplex stainless steels were tested with pickled surfaces as well as with mill-scale intact (as-rolled) in order to determine the affect of pickling vs. not pickling on the corrosion behaviour of the steels. Steels were tested in solutions simulating concrete interstitial pore fluid containing from 0 to 16% Cl- by mass of solution, simulating cement paste with 0 to 7.5% Cl- by mass of cement, which is near the solubility limit of Cl- in pore fluid. Steels were also tested in thin mortar shells, with Cl- ions being rapidly diffused to the surface due to an applied potential gradient. The microcell corrosion performance of the as-rolled steels was slightly worse than that of pickled steels; however, the corrosion rates of the as-rolled steels at 16% Cl- in pore fluid are near 3 µm/year, while black steel is normally observed to be actively corroding at 10 µm/year in cement containing as low as 0.1% Cl- by mass of cement, or 0.2% Cl- by mass of solution. No significant difference was observed between different grades of stainless steel in either the as-rolled or pickled conditions. As-rolled stainless steels exhibited poor pitting resistance when an anodic potential is applied, but the corrosion occurs at potentials much higher than experienced in service and at Cl- concentrations far greater than that needed to initiate corrosion on black steel; the time required to reach these higher Cl- levels would allow for maintenance free service long enough to justify the cost of as-rolled stainless steel over black steel. The Canadian Highway Bridge Design Code, CSA S6-06, specifies that reinforced concrete bridges should meet a service life of 75 years. It is concluded that, given the time required for concentrated chlorides to accumulate at the steel, the stainless steel rebar in the as-rolled condition would allow reinforced concrete structures to reach the specified service life, as long as care is taken to avoid contamination of the steel/surface by black steel from handling, or by secondary phases within the steel, Cr23C6 and MnS in particular.

Most studies that have examined chloride-induced corrosion of steel in concrete have focused on sound concrete. However, reinforced concrete is seldom uncracked and very few studies have investigated the influence of cracked concrete on rebar corrosion. Furthermore, the studies that have examined the relationship between cracks and corrosion have focused on unloaded or statically loaded cracks. However, in practice, reinforced concrete structures (e.g. bridges) are often dynamically loaded. Hence, the cracks in such structures open and close which could influence the corrosion of the reinforcing steel. Consequently, the objectives of this project were (i) to examine the effect of different types of loading on the corrosion of reinforcing steel, (ii) the influence of concrete mixture design on the corrosion behaviour and (iii) to provide data that can be used in service-life modelling of cracked reinforced concretes. In this project, cracked reinforced concrete beams made with ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) were subjected to no load, static loading and dynamic loading. They were immersed in salt solution to just above the crack level at their mid-point for two weeks out of every four (wet cycle) and, for the remaining two weeks, were left in ambient laboratory conditions to dry (dry cycle). The wet cycle led to three conditions of exposure for each beam: (i) the non-submerged region, (ii) the sound, submerged region and (ii) the cracked mid-section, which was also immersed in the solution. Linear polarization resistance and galvanostatic pulse techniques were used to monitor the corrosion in the three regions. Potentiodynamic polarization, electrochemical current noise and concrete electrical resistance measurements were also performed. These measurements illustrated that (i) rebar corroded faster at cracks than in sound concrete, (ii) HPC was more protective towards the rebar than OPCC even at cracks and (iii) there was a minor effect of the type of loading on rebar corrosion within the period of the project. These measurements also highlighted the problems associated with corrosion measurements, for example, identifying the actual corroding area and the influence of the length of rebar. The numbers of cracks and crack-widths in each beam were measured after the beam’s initial exposure to salt solution and, again, after the final corrosion measurements. HPC beams had more cracks than the OPCC. Also, final measurements illustrated increased crack-widths in dynamically loaded beams, regardless of the concrete type. The cracks in both statically and dynamically loaded OPCC and HPC beams bifurcated at the rebar level and propagated parallel to the rebar. This project also examined the extent of corrosion on the rebars and the distribution of corrosion products in the concrete and on the concrete walls of the cracks. Corrosion occurred only at cracks in the concrete and was spread over a larger area on the rebars in HPC than those in OPCC. The damage due to corrosion was superficial in HPC and crater-like in OPCC. Regardless of the concrete type, there was a larger distribution of corrosion products on the crack walls of the dynamically loaded beams. Corrosion products diffused into the cement paste and the paste-aggregate interface in OPCC but remained in the crack in HPC. The most voluminous corrosion product identified was ferric hydroxide. Elemental analysis of mill-scale on rebar which was not embedded in concrete or exposed to chlorides was compared to that of the bars that had been embedded in uncontaminated concrete and in cracked concrete exposed to chlorides. In uncontaminated concrete, mill-scale absorbed calcium and silicon. At a crack, a layer, composed of a mixture of cement paste and corrosion products, developed between the mill-scale and the substrate steel. Based on the results, it was concluded that (i) corrosion occurred on the rebar only at cracks in the concrete, (ii) corrosion was initiated at the cracks immediately upon exposure to salt solution, (ii) the type of loading had a minor influence on the corrosion rates of reinforcing steel and (iv) the use of polarized area led to a significant underestimation of the current density at the crack. It is recommended that the effect of cover-depth on (i) the time to initiation of corrosion and (ii) the corrosion current density in cracked concrete be investigated.

Deterioration of reinforced concrete structures due primarily to chloride induced corrosion of plain carbon-steel reinforcement is a widespread problem, particularly in areas close to marine environments and where de-icing salts are used to keep roadways clear of ice. Replacing plain carbon-steel rebar with highly corrosion resistant stainless steel rebar has been shown to greatly increase the lifespan of concrete structures in harsh environments, and yields favourable life-cycle costs despite high initial costs. In attempt to lower stainless steel rebar’s initial cost of processing, this research compared its corrosion resistance in the pickled (mill scale removed) and as-rolled (mill scale intact) surface conditions. Rebar was embedded in highly-chloride contaminated concrete, and corrosion performance between the two surface types was compared in order to determine if conventional pickling of stainless steel rebar is necessary. A second part of this research addressed possible concern of reduced corrosion resistance of pickled stainless steel rebar in concrete exposed to chlorides when subjected to dynamic loading due to micro-motion at the concrete/crack interface. It was concluded that as-rolled stainless steel rebar in aggressive environments would provide sufficient corrosion resistance for the 75 year lifespan currently specified by the Canadian Bridge Code (CAN/CSA-S6-06, 2006), however it is recommended that monitoring of these specimens be continued to ensure high corrosion rates and/or concrete cracking do not develop. As well, investigation into the effects crevice corrosion cells found in typical concrete structures could have on as-rolled stainless steel rebar’s corrosion resistance should be undertaken. With regard to loading conditions, no significant evidence was found suggesting that pickled stainless steel rebar has reduced corrosion resistance when loaded dynamically versus statically. Therefore pickled stainless steel rebar is recommended for use in dynamically loaded concrete structures if others factors permit. However, the higher electrochemical noise measured during cyclic loading suggests that corrosion behaviour could be influenced largely by frequency of loading, and so further study should be undertaken for applications involving more extreme cyclic loading conditions than those used in this experiment.

Corrosion of carbon (black) steel reinforcing bars (rebar) is the major cause of damage and deterioration of reinforced concrete structures in maritime regions and in climates where de-icing salts are used. The cause of the corrosion is diffusion of chloride ions to the steel surface through the concrete in which it is placed. The bars are naturally passivated by the high pH of concrete interstitial pore fluid, and will not corrode in chloride-free concrete. Chloride ions break down the passive film, allowing dissolution of the steel. Corrosion of reinforcing steel drastically reduces the service lives of concrete structures. Where chlorides can not be avoided, stainless steel is becoming increasingly popular as an alternative reinforcing material. Stainless steel is able to withstand greater concentrations of chlorides, extending the service lives of structure in which they are placed. Due to high initial cost, stainless steel is often avoided in the design of new structures. In order to reduce the cost of stainless rebar, it has been proposed that the standard process of abrasive blasting and pickling of the steels not be performed, as these steps are mainly used to restore a bright and shiny surface, a quality not required for steels embedded in concrete. AISI 304LN, AISI 316LN and 2205 duplex stainless steels were tested with pickled surfaces as well as with mill-scale intact (as-rolled) in order to determine the affect of pickling vs. not pickling on the corrosion behaviour of the steels. Steels were tested in solutions simulating concrete interstitial pore fluid containing from 0 to 16% Cl- by mass of solution, simulating cement paste with 0 to 7.5% Cl- by mass of cement, which is near the solubility limit of Cl- in pore fluid. Steels were also tested in thin mortar shells, with Cl- ions being rapidly diffused to the surface due to an applied potential gradient. The microcell corrosion performance of the as-rolled steels was slightly worse than that of pickled steels; however, the corrosion rates of the as-rolled steels at 16% Cl- in pore fluid are near 3 µm/year, while black steel is normally observed to be actively corroding at 10 µm/year in cement containing as low as 0.1% Cl- by mass of cement, or 0.2% Cl- by mass of solution. No significant difference was observed between different grades of stainless steel in either the as-rolled or pickled conditions. As-rolled stainless steels exhibited poor pitting resistance when an anodic potential is applied, but the corrosion occurs at potentials much higher than experienced in service and at Cl- concentrations far greater than that needed to initiate corrosion on black steel; the time required to reach these higher Cl- levels would allow for maintenance free service long enough to justify the cost of as-rolled stainless steel over black steel. The Canadian Highway Bridge Design Code, CSA S6-06, specifies that reinforced concrete bridges should meet a service life of 75 years. It is concluded that, given the time required for concentrated chlorides to accumulate at the steel, the stainless steel rebar in the as-rolled condition would allow reinforced concrete structures to reach the specified service life, as long as care is taken to avoid contamination of the steel/surface by black steel from handling, or by secondary phases within the steel, Cr23C6 and MnS in particular.

The aim of this study was to use the finite element method to model crack, corrosion, and Crack-in-Corrosion defects in a pipeline. The pipe material under investigation for this study was API 5L X60, 508 mm diameter with a wall thickness of 5.7 mm. The pipe material was evaluated using Tensile, Charpy, and J testing in order to model the defects and to establish the numerical failure criteria. Corrosion defects were modeled as flat-bottomed grooves. The collapse pressure was predicted when the deepest point in the bottom of the defect reached a critical stress. Based on this criterion, the FE corrosion failure pressure predictions were conservative compared to the experimental failure pressures, conducted by Hosseini [9], with an average error of 10.13%. For crack modeling, the failure criteria were established considering the plastic collapse limit and the fracture limit. Both the Von Mises stress in the crack ligament and the J-integral values around the crack were monitored to predict the failure pressure of the model. The crack modeling was done based on two approaches, the uniform depth profile and the semi-elliptical profile. The crack with uniform depth profile was done because the uniform shape is the logical equivalent shape for a colony of cracks. The crack with the semi-elliptical profile was done to have a less conservative results and because the experiments were done with semi-elliptical cracks. The FE crack modeling results were conservative compared to the experimental collapse pressure with an average error of 19.64% for the uniform depth profile and 5.35% for the semi-elliptical profile. In crack-in-corrosion (CIC) defect modeling, the crack was modeled with uniform depth because it was very difficult to model the semi-elliptical crack profile when the crack defect is coincident with a corrosion defect. The results were conservative compared to the experimental results with an average error of 22.18%. In general, the FE modeling provides the least conservative failure pressure prediction over the existing analytical solutions for pipe with longitudinal corrosion, crack, and CIC defects.