Purpose The purpose of this paper is to study the susceptibility of these three commonly used corrosion resistance fasteners in seawater. For a more practical scenario, a local Atlantic coastal seawater as received was used. Design/methodology/approach Carbon fiber reinforced polymer (CFRP) was fabricated with T700 carbon fiber (Toray Inc.) and VE8084 vinyl ester resin (Ashland) to make a unidirectional composite panel of thickness 1.8 mm. A conductive paint was applied to one of the sample edges that was perpendicular to the fiber direction, providing an electrical contact with carbon fibers to connect a copper wire. This external electric connection was used for potential measurements of both the open circuit potential (OCP) of the CFRP sample, and the mixed potential of the fastened set: consisting of the CFRP and the metallic fastener fastened to it. Three common fastener alloys were selected: 316SS, Monel and Titanium. For this purpose, a high impedance voltmeter was used in conjunction with a saturated calomel reference electrode. Measurements were taken daily. For longer time measurements, a four-channel high impedance analog data logger was used with 30 min sampling rate. Findings For both 316SS and Monel fastened sets, crevice corrosion occurred inside the occluded regions of the set, when immersed in coastal seawater. The attack was more severe for 316 stainless steel set. An isolated island attack of faceted surfaces morphology was seen for 316SS set. While, a circular ring of preferential grain boundary attack appeared for Monel set, indicating an IR (voltage) drop mechanism is more likely operating. Titanium-fastened sets showed high resistance to crevice corrosion when simmered in seawater. However, for long-time exposure, the sets became more susceptible to crevice corrosion attack supported by CFRP attachment (oxygen reduction reaction taking place at the carbon fibers). Originality/value Evidently, titanium, stainless steels and Monel are good candidates for galvanic corrosion resistance. However, their susceptibility to crevice corrosion when coupled with CFRP is a new challenging topic that needs further investigation. This is very important today because the vast application witnessed for CFRP material. This work involves developing an original methodology for this kind of investigation and was done at advanced laboratories of SeaTech at Florida Atlantic University by the Atlantic coastline.
A literature review has been conducted with the goal of identifying alternative low-cost corrosion resistant reinforcement materials compared to epoxycoated reinforcing steel. The new materials are evaluated on the basis of material attributes that are expected to improve initiation and propagation corrosion characteristics including an assessment of the impact of mill scale. Moreover, strength, ductility as well as low cycle fatigue behavior are considered where possible with endurance limit scaled to tensile strength. First, material characteristics that should contribute to improved initiation, propagation resistance, and oxide wedging damage mechanisms will be reviewed. Then, based on the literature survey, the individual materials are assessed and summarized. The most promising alternate reinforcing materials seen to date that are less expensive than 300 series stainless steels include low-Ni austentitic stainless steels, and a variety of ferritic or martensitic 12-15 wt. % Cr steels. Low Ni austentitic stainless steels and other 12-15% Cr steels will likely have inferior corrosion resistance compared to traditional 300 series stainless steels but may be significantly improved over traditional epoxy-coated rebar. It is recommended that a 200 series stainless steel and 12-15% martensitic or ferritic steel be considered for further investigation as reinforcing material in concrete.
In a previous paper (Part I), the utility of Zn-submerged bulk anodes (ZnSBA) for providing cathodic protection to the above-waterline portion of a marine-reinforced concrete bridge substructure was investigated. It was determined that polarizations greater than 100 mV reached to the top of cast-in place footers (about one meter above mean tide) but decayed sharply in the columns above this. As such, it was concluded that these anodes alone did provide some protection in the splash zone, albeit not as much as when combined with either zinc thermal-spray or zincjackets. In the present investigation, similar field trials were performed using Mg-submerged bulk anodes (MgSBA), which were considered to be potentially more effective in providing protection to the above-waterline zone because of greater driving voltage. Results of these trials indicate that 100 mV depolarization was achieved in some cases to as high as 2.5 m above mean water. In addition, the substructure footer and column with MgSBA wa...
An alloy coating has been developed for an AA2024-T3 substrate that can serve as barrier, sacrificial anode, and reservoir to supply soluble inhibitor ions to protect any defect sites. In this paper, the chemical throwing power of such an metallic coating under thin electrolyte films representative of atmospheric conditions is modeled. The geometry is that of an surface with the presence of a scratch simulating exposed AA2024-T3. The model calculates the time necessary to accumulate and inhibitors over the scratch when released from the coating under different conditions. The model factors in the pH-dependent passive dissolution rate of an alloy to define the inhibitor release flux. Transport by both electromigration and diffusion are considered together. The effects of scratch size, initial pH, chloride concentration, and electrochemical kinetics of the material involved were studied. Sufficient accumulation of the released inhibitor (i.e., the concentration surpassed the critical inhibitor concentration over AA2024-T3 scratches) was achieved within a few hours (e.g., for scratches of ) when the initial solution pH was 6 and the coating was adjacent to the AA2024-T3.
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An Al-Co-Ce alloy system has been developed with three important protection abilities when deployed as a metal coating over high-strength aerospace alloys such as AA2024-T351 (UNS A92024). These alloy coatings can serve as a local corrosion barrier with tunable anodic properties, act as a sacrificial anode (i.e., its open-circuit potential [OCP] is ca. 20 mV to 300 mV below that of AA2024-T351), and supply soluble ions that function as corrosion inhibitors. These three functions are not mutually exclusive. However, optimization of each may not be achieved in the same way. The Al-Co-Ce alloy system has demonstrated an excellent inherent resistance to corrosion due to its structural amorphicity and chemical composition, with electrochemical tunability based on solid solution Co and Ce composition. For example, the pitting potential (Epit) is more noble than the Epit of AA2024-T351 under acid, neutral, and alkaline conditions with pitting and repassivation potentials optimized by Co content. The gal...
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The ability of a SS316L surface wetted with a thin electrolyte layer to serve as an effective cathode for an active localized corrosion site was studied computationally. The dependence of the total net cathodic current, I{sub net}, supplied at the repassivation potential E{sub rp} (of the anodic crevice) on relevant physical parameters including water layer thickness (WL), chloride concentration ([Cl{sup -}]) and length of cathode (Lc) were investigated using a three-level, full factorial design. The effects of kinetic parameters including the exchange current density (i{sub o,c}) and Tafel slope ({beta}{sub c}) of oxygen reduction, the anodic passive current density (i{sub p}) (on the cathodic surface), and E{sub rp} were studied as well using three-level full factorial designs of [Cl{sup -}] and Lc with a fixed WL of 25 {micro}m. The study found that all the three parameters WL, [Cl{sup -}] and Lc as well as the interactions of Lc x WL and Lc x [Cl{sup -}] had significant impact on I{sub net}. A five-factor regression equation was obtained which fits the computation results reasonably well, but demonstrated that interactions are more complicated than can be explained with a simple linear model. Significant effects on I{sub net} were found upon varying either i{sub o,c}, {beta}{sub c}, or E{sub rp}, whereas i{sub p} in the studied range was found to have little impact. It was observed that I{sub net} asymptotically approached maximum values (I{sub max}) when Lc increased to critical minimum values. I{sub max} can be used to determine the stability of coupled localized corrosion and the critical Lc provides important information for experimental design and corrosion protection.
Cathodic prevention was implemented with commercial Zn bulk anodes on laboratory columns simulating a reinforced concrete marine system. Two computational models of these laboratory columns were implemented. The results obtained from both models were in reasonable agreement with the experimental observations. One of the models was used to predict cathodic prevention throwing power on field scale structures, as a step towards field application. These predictions suggest that with an immersed anode useful levels of cathodic prevention may be reasonably expected, even under conservative assumptions, in the area immediately above high tide where conditions are otherwise very severe.
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Quantitative computational models have been developed to assist in predicting the course of corrosion-induced deterioration and the effect of corrosion protection models in reinforced concrete marine substructures. These models use the concept of a corrosion initiation stage (buildup of chloride ions until reaching a steel corrosion initiation threshold level) followed by a corrosion propagation stage (active corrosion leading to cracking and spalling of the concrete cover). Predictive models were applied to assess the reduction of corrosion rate by submerged and surface sacrificial anodes in marine piles. It was concluded that surface anodes located above water reduced corrosion significantly when the anode extended up to the top of the active steel zone. This predictive model was confirmed by parallel laboratory experiments with partially submerged piles and sacrificial anodes. Additional calculations applied the model to find the extent of cathodic protection feasible for partially submerged bridge footers with only submerged anodes. It was determined that useful protection may be obtained only when the concrete resistivity is very low. Modeling of the initiation stage of propagation resulted in the development of design derating factors to calculate the effect of diffusion geometries other than a flat wall (2- and 3-way corners, circular columns). Chloride ion binding by the concrete did not alter significantly the value of the derating factors, although it increased the time to corrosion initiation compared with a no-binding case. A final task of this work integrated the initiation and propagation phases of corrosion into a damage function model that predicts the amount of spalled area of a pile partially submerged in seawater as a function of service time. The quantitative formulation is suitable for selection of alternative corrosion protection strategies and incorporation in life cycle cost projection models.
Two distinct binary blended concrete mixes were prepared for the study. The first mix involved a cement replacement of 50% slag, denoted as SL. The second mix incorporated a cement replacement of 20% fly ash, referred to as FA. No chlorides were added during the preparation of these concrete specimens. To accelerate chloride transport, electromigration was employed by placing specimens with varying reservoir lengths (ranging from 2.5 cm to 17.5 cm) on their top surfaces. These reservoirs were subsequently filled with a 10% NaCl solution. In this paper, corrosion propagation was monitored over a period of approximately 650 days using electrochemical measurements such as open circuit potential, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). The evolution of rebar potential, polarization resistance, solution resistance, and corrosion current were analyzed to understand the corrosion behavior. This paper focuses on how the length of the solution reservoirs influences the corrosion-related parameters such as polarization resistance, solution resistance, rebar potential, and corrosion current. During the monitored propagation period, the corrosion current values (last 7 sets of readings) exhibited higher magnitudes for the embedded rebars in specimens made with SL mix in comparison to those made with FA mix. Corrosion current measurements likewise showed an increasing trend as the reservoir lengths increased. None of the specimens had any visible cracks or corroded products that could reach the concrete surface throughout the monitored period. The experimental results provide insights into the corrosion mechanisms and the effectiveness of accelerated corrosion techniques in simulating real-life conditions.
We describe recent computational and experimental studies on the corrosion properties of metallic coatings that can be tailored (tuned) to deliver up to three corrosion-inhibiting functions to an underlying substrate. Attributes are tuned by a selection of alloy compositions and nanostructures, ideally in alloy systems that offer flexibility of choice to optimize the corrosion-resisting properties. An amorphous Al-based coating is tuned for corrosion protection by on-demand release of ionic inhibitors to protect defects in the coating, by formation of an optimized barrier to local corrosion in Cl− containing environments, as well as by sacrificial cathodic prevention. Further progress in this field could lead to the design of the next generation of adaptive or tunable coatings that inhibit corrosion of underlying substrates.
The time-to-corrosion initiation (T i ) of reinforcement and the chloride threshold content (C th ) of a series of G109 concrete slabs were experimentally investigated. Results indicate that T i is increased by decreasing water-to-cement ratio or increasing cement alkalinity. The effects of a superplasticizer on initiation of corrosion of reinforcing steel bars (rebar) depend on concrete mix. Measurements reveal that chloride contents at both active (anodic) and passive (cathodic) sites increase linearly with the logarithm of T i regardless of concrete mix, whereas chloride contents measured at anodic sites are always greater than at cathodic sites. Defects at or nearby corrosion anodic spots could facilitate chloride accumulation: rebar corrosion would occur earlier than if no defect were present. Further analysis reveals that the weakest-link theory is applicable to predict the probability of initiation of rebar corrosion, as indicated by the Weibull distributions of T i and C th .
A computational code that was originally designed to model crevice corrosion was extended to multifunctional coatings on Al alloys exposed to thin layers of electrolytes. The model is able to calculate the transient distributions of potential, current density, and all chemical species concentration, enabling the dynamic simulation of inhibitor release, inhibitor transport, and sacrificial cathodic protection. The model has been applied to both inhibitor release from and aggressive anion capture by hydrotalcites (HTs) pigments in epoxy primer coatings applied to AA2024-T3. Computational studies were carried out to investigate the effects of HT/vanadate (HT/V) epoxy coating system parameters including scratch size, inhibitor release rate, Cl− gettering rate (GR), cathodic kinetics on the bare AA2024-T3, and solution layer thickness on system performance. The analyses of the computational results have quantified the important factors controlling successful corrosion inhibition by inhibitor release from coatings. The pH-dependence of the steady state inhibitor release rate was found to be the most important parameter controlling system performance. Cl− gettering can also reduce the aggressiveness of solution at long times, especially when considered in conjunction with inhibitor release. However, the ion exchange capacity required poses stiff design challenges involving the loading of the ion exchanger into the resin and the service conditions. The effectiveness of inhibition decreased significantly for the larger scratch sizes. The cathodic kinetics within the scratch play an important role in determining the ability of a given inhibitor to function effectively. When the scratch is the cathode in the galvanic couple with the substrate under the coating, inhibition was more effective. For the conditions simulated here, the net effect of a decreased solution layer thickness is to increase the protection ability of the system. The increase in the inhibitor concentration overcomes the decrease in the pH at the anode.
