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Reinforced concrete bridge substructures in Florida coastal waters have historically experienced deterioration as a consequence of embedded steel corrosion and resultant concrete cracking and spalling. Ultimately, this deterioration leads to added maintenance costs, reduced service life, and unsafe conditions unless intervention measures are instituted. Galvanic anode cathodic protection (CP), as affected by thermally sprayed zinc for cast-in-place substructure components and zinc mesh jackets for precast ones, is being employed to control this corrosion and extend useful service life. For both types of systems, a submerged bulk zinc anode (SBA) is included to polarize the reinforcement below the waterline and thereby reduce current drain from the lower portion of the thermal spray or zinc mesh. The objective of this research was to determine the extent to which SBAs alone provide protection to the above waterline region of bridge substructures. If adequate or even partial protection is afforded by this means, then considerable cost savings could be realized because of the reduced expense and ease of installation of SBAs compared to zinc thermal spray and CP jackets. To investigate this, two substructure piers on the Bahia Honda Bridge and two on the Niles Channel Bridge, both in the Florida Keys, that were comprised of cast-in-place footers and columns were instrumented with SBAs alone and monitored to determine the level of protection that resulted. For the former bridge, zinc was the SBA type and for the latter magnesium. It was determined that the entire footer reinforcement received protection and that polarization extended up the columns to a certain degree, more so for magnesium than zinc anodes. In addition, the substructures and resultant polarization from the SBAs were modeled using Boundary Element Analysis and two numerical methods. The model results were compared with the field measurements, and the two data sets were determined to be in general agreement. It was concluded that SBAs have utility for extending the useful service life of reinforced concrete bridge substructure elements.
This brief study aims at providing a model to predict the time of service of a cracked bar in corrosive environment, in view of both the fracture mechanics and elastic failure criteria. Dolinskii's assumption on the relationship between stress and the corrosion rate is adopted. It is superimposed with fracture mechanics consideration. A comparison between the time of service of a cracked bar and that of a uniform bar is provided.
No abstract is provided for this article.
Concrete specimens measuring 12 in by 5 in by 3 in were prepared with binary concrete mixtures, in which fly ash was replaced for 20% of the ordinary Portland cement. A water to cementitious ratio of 0.41 was used. The specimens had 0.3 in concrete cover and were reinforced with single rebar. For ponding, different size of solution reservoirs were used, ranging from 2.5 cm to 17.5 cm. NaCl solution (10% by weight) was poured into the solution reservoirs. Accelerated chloride transport technique was employed, and this was continued for a week to a few months depending on the values of the off-rebar potential observed for each embedded specimen. The effect of rebar length under the reservoir for fly ash containing binary blended concrete mixes was examined. In this study, two different types of electrochemical measurements named as EIS and LPR were performed to monitor the values of rebar potential and corrosion current. The corrosion propagation was observed for at least 400 days. Sometimes, corrosion initiated several weeks after the removal of accelerated chloride transport approach. According to the experimental findings, it was observed that the corrosion current value increases with the length of the solution reservoir, except for a rebar embedded in specimens with a solution reservoir of 7.5 cm.
Samples with two different binary blended concrete mixes were prepared, one containing cement replacement of 50% slag (referred here as SL mix) and the other containing cement replacement of 20% fly ash (termed here as FA mix). The water to cementitious ratio used to produce concrete specimens was 0.41. On the top surface of each specimen, various reservoir lengths that ranged from 2.5 cm to 17.5 cm were fitted, and these reservoirs were filled with a 10% NaCl solution. Electromigration was used to accelerate the transport of chlorides, with an applied potential of 9 V at first, and subsequently reduced to 3 V after about a week. The electromigration was applied for a short period (few weeks to a couple of months). For a period of about 1100 days, the corrosion related parameters such as concrete solution resistance, rebar potential, and corrosion current were monitored via the rebar potential measurements, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) measurements, the latter used only to obtain the solution resistance. The corrosion current values determined through experimental observations were then converted to mass loss using Faraday’s law. The readings of corrosion current values (last 7 sets of readings) as well as the calculated mass loss values were found to be larger for the rebars embedded in specimens prepared with SL mix, followed by rebars embedded in specimens prepared with FA mix. Corrosion current and calculated mass loss values in general tended to increase with increasing solution reservoir lengths. No cracks or corrosion products that reached the surface of the concrete were observed on the specimens for the duration of the reported monitored propagation period. This study offers a framework for future studies on accelerated steel corrosion in concrete.
Close packed coupled multi-electrodes arrays (MEA) simulating a planar electrode were used to measure the anodic current evolution as a function of position during initiation and propagation of crevice corrosion of AISI 316 stainless steel (UNS S31600) and Ni-Cr-Mo alloy 625 (UNS N06625). Scaling laws derived from polarization data guided the implementation of rescaled crevices providing spatial resolution. Crevice corrosion of AISI 316 stainless steel in 0.6 M NaCl at 50{degree sign}C was found to readily initiate close to the crevice mouth (i.e., xcrit ≈ 0) at modest applied potentials (e.g., 0 VSCE) and to spread inwards and outside the crevice with time. In the case of alloy 625, crevice corrosion initiates further inside the crevice (i.e., xcrit is large) and requires higher applied potential (e.g., 0.05 VSCE). The local crevice current density increased dramatically over a short period to reach a limiting value in both cases.
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A variety of approaches enable deposition of metallic coatings containing rare earth (RE) elements for corrosion protection. Al-TM-RE amorphous and amorphous nanocrystalline alloys are one type of multifunctional material for protection of Al alloys. Ce and Al-Ce coatings have also been deposited. Results for high density, low porosity multifunctional Al-Co-Ce metallic coatings applied to an AA-2024-T351 substrate using a pulsed thermal spray (PTS) are discussed in detail. Three proposed modes of corrosion protection are provided by the coating (i.e. a localized corrosion barrier, a sacrificial anode to supply cathodic protection of any exposed AA 2024-T351, and active inhibitor release) to protect AA 2024-T351. Chemical protection is afforded by either Ce(OH)2 2+ or Ce3+ release.
Most corrosion experiments conducted on reinforced concrete are terminated upon corrosion initiation. This paper presents preliminary results on corrosion initiation and propagation on three corrosion resistant alloys (CRAs). Two types of experiments were conducted: CRAs exposed to simulated pore solution (with chloride concentration gradually increased), CRAs embedded in mortar specimens (most with 0.5 w/c ratio) with a reduced cover (~10 mm) exposed to 15% (m/m%) NaCl (initially permanent ponding and later alternate ponding). Three surface conditions were tested on each CRA type: as received, with mill scale, with mill scale then sandblasted. The open circuit potential was monitored over time. Linear polarization resistance and solution resistance were measured periodically. In this research the experiments were not terminated upon corrosion initiation, rather exposure continued to gain a better understand how corrosion propagates. No cracks have appeared on the mortar specimens with the CRA actively corroding.
For corrosion resistant materials exposed to low-temperature atmospheric environments, the corrosion mode of highest risk is expected to be localized corrosion (pitting, crevice, stress-corrosion cracking) due to accumulation of aggressive species within thin solution layers and/or formation of occluded local geometries. The stability of such a localized corrosion site requires that the corroding site (anode) must dissolve at a sufficient high rate to maintain the critical chemistry, and a robust cathodic area (cathode) must exist that can provide sufficient cathodic current. The characteristics of both the anode and the cathode depend on a large number of physiochemical variables (e.g., temperature, ionic concentration, water layer thickness, etc) and electrochemical parameters (i.e., cathodic and anodic polarization behavior). The effects of all these parameters add significantly to the dimensionality of the problem and a systematic study of these parameters is thus more tractable computationally than experimentally. The objective of this study was to computationally characterize the stability of such a local corrosion site and explore the effects of physiochemical and electrochemical parameters on that stability. The overall goal is to contribute to the establishment of a scientific basis for the prediction of the stabilization of localized attack on wetted, corrosion resistant material surface. A localized corrosion site, illustrated in Figure 1, consists of two parts: (a) the external wetted surface (cathode) and (b) the crevice (anode). This study computationally separated the two and modeled them individually, linking them through the imposition of a common fixed potential at the junction point (i.e., the mouth of the crevice). An objected-oriented computational code, CREVICER, developed at UVa, was extended to study separately both the wet surface (cathode) and the crevice (anode). SS316L was chosen as the material of interest.
Surface resistivity profiles (using a wenner probe) as a function of elevation are being measured on young and mature marine reinforced concrete substructures at and above mean water line on bridges at coastal locations in Florida. The surface resistivity values measured are correlated with chloride diffusion coefficients measured from the same structures. Additionally the wet resistivity values from cored specimens are also correlated to the chloride diffusivity. Previous lab work suggests that a good correlation exists on saturated concrete between these two parameters. The objective of this study is to assess whether a similar correlation can be obtained from field surface resistivity readings, such that surface resistivity could potentially be used as performance based monitoring of new and older structures. Preliminary results suggest that a conditioning method needs to be applied on-site to approximate water saturation conditions at the elevations of interest.
An Al-Co-Ce alloy system has been developed with two important protection abilities when deployed as a metal coating over AA2024-T3. These alloy coatings can both act as a sacrificial anode and supply soluble ions that could function as inhibitors. In this paper, the electrochemical (or electrolytic) throwing power of such an Al-Co-Ce metallic coating under atmospheric conditions is modeled. The criterion for protection is a galvanic couple potential distribution below the pitting potential of AA2024-T3. The effects of scratch size, pH, chloride concentration, metallic coating composition, and electrochemical kinetics of the materials involved were studied. Substantial sacrificial cathodic prevention of AA2024-T3 scratches could be achieved with Al-Co-Ce alloys with performance superior to conventional Alclad coatings in terms of both extent of polarization and maximum scratch size protected. The behavior of the metallic coating can be tailored and provides the best protection (i.e., largest cathodic polarization of the longest scratch lengths) when it contains a low Co content, is exposed to either high or low pH solutions of low chloride concentration, and the AA2024-T3 scratch exhibits slow cathodic kinetics. The Co content should be minimized, in order to maximize the electrochemical throwing power, but must be sufficient for retention of amorphicity.
Once corrosion of the reinforcing steel embedded in concrete has initiated, the corrosion propagation period is typically assumed to last five to ten years for carbon steel reinforcement. However, the duration of the corrosion propagation period could be significantly longer depending on the exposure environment, reinforcement diameter, size of the corroding site, concrete cover, and concrete composition (e.g., w/cm, total cementitious content, presence of supplementary cementitious materials), and resistivity (affected by temperature and moisture content). A better understanding of how corrosion propagates could provide better guidance when conducting the assessment and control of corrosion for structures in which corrosion has initiated. Corrosion propagation was investigated on instrumented reinforced concrete pipe segments, after corrosion of the reinforcement had initiated. The specimens were exposed to lab humidity and temperature for several months, after corrosion initiated. During this time, corrosion continued. It was then decided to apply a current equivalent to 0.5 μA/cm2 via a galvanostat (in some cases this current density was later increased to 1 and 2.5 μA/cm2) in addition to the naturally occurring corrosion current density. The applied current density assumed that half of the steel area under the reservoir was undergoing corrosion. The current was applied: typically current-on for 11 days and then disconnected for 3 days, and the process was then repeated. The specimens during the accelerated corrosion period were stored in high humidity, and after several cycles, selected specimens were covered with saturated sand while others remained under the high humidity exposure. During the disconnected periods, electrochemical measurements such as corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy were performed; the latter two tests at least two days after removal of the applied current. Selected specimens were terminated and gravimetric weight loss measured. These weight loss values were compared to the mass loss calculated by using Faradays law obtained from the measured corrosion current.
