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  • [期刊] Investigation of Cathodic Reaction Mechanisms of H2S Corrosion Using a Passive SS304 Rotating Cylinder Electrode
    摘要:The internal corrosion of pipeline steel in the presence of hydrogen sulfide (H2S) represents a significant problem in the oil and gas industry. Its prediction and control pose a challenge for the corrosion engineers. In previously published research by the same authors, an electrochemical model of H2S corrosion was developed in both pure H2S and H2S/CO2 aqueous systems. An additional electrochemical cathodic reaction, direct H2S reduction, was uncovered based upon the carbon steel corrosion experimental results. However, in the carbon steel corrosion experiments, the Tafel's region of cathodic reactions experienced interference by the anodic iron dissolution reaction, making the electrochemical kinetics of cathodic reactions unclear. In the present study, experimentation was conducted to better resolve the direct reduction of H2S while minimizing the effect of the anodic reaction by using a passive stainless steel working electrode. The electrochemical kinetics parameters for H2S reduction (i.e., Tafel slope, exchange current density, and reaction order with H2S concentration) were determined. Moreover, the electrochemical kinetics parameters for H+ reduction were also revisited.
  • [期刊] Mechanism of cathodic reactions in acetic acid corrosion of iron and mild steel
    摘要:The mechanism of the cathodic reaction in acetic acid corrosion of iron and API X65 mild steel was investigated in mildly acidic sodium chloride electrolytes. The behavior of the charge transfer controlled currents in the steady state voltammograms showed that undissociated acetic acid is not significantly electroactive. This behavior was found to be in accordance with the so-called "buffering effect" mechanism, where acetic acid acts solely as an additional source of hydrogen ions. The increased limiting currents in the presence of acetic acid showed a linear correlation with undissociated acetic acid concentration in agreement with the Levich equation, suggesting that the limiting currents are under mass transfer control. Both pure iron and X65 steel exhibited a similar behavior regarding the cathodic currents, indicating that the mechanism of hydrogen ion reduction is not influenced by minor impurities present in the X65 steel.
  • [期刊] Corrosion fatigue tests using strain gauges for measuring load and crack length
    摘要:This paper uses numerical analysis and experimental data to validate the use of strain gauges for load and crack propagation measurements in corrosion fatigue. It was shown that the measurement of crack length and load by strain gauges can be done following the requirements of ASTM E647-15. The results open the possibility for using strain gauges directly positioned on the samples, replacing both the classical load cells to measure load and direct current potential drop to determine fatigue crack growth rate, as neither are easily usable for high-pressure corrosion fatigue tests.
  • [期刊] Influence of carbon sources and concrete on microbiologically influenced corrosion of carbon steel in subterranean groundwater environment
    摘要:Microbiologically influenced corrosion of carbon steel was assessed in a laboratory environment simulating the deep geological repository of radioactive waste. A dense and diverse biofilm was formed on the surfaces of steel in biotic systems without concrete. Addition of nutrients favored biofilm formation and altered the bacterial community; most distinctly, the relative abundance of Alphaproteobacteria decreased, and Deltaproteobacteria or Betaproteobacteria became more abundant, when nutrients were available. Nutrient amendment also increased the corrosion rate and changed the composition and resistance of corrosion products (mostly FeS, Fe2O3, or Fe(OH)2). Presence of concrete inhibited the corrosion of steel and hindered the biofilm formation on steel. Only sparse biofilm consisting of known alkaliphilic bacteria was detected. In the presence of concrete, the corrosion rate was consistently radically decreased, as the properties of the surface deposits (mostly CaCO3) were different from those in the other systems.
  • [期刊] Excimer Laser Processing of Cast Mg-Al-Zn (AZ91D) and Mg-Al (AM60B) Alloys for Improved Corrosion Resistance
    摘要:Poor intrinsic corrosion resistance of Mg alloys is often attributed to recommended heterogeneous microstructures containing more electrochemically noble secondary phases resulting in the preferential dissolution of Mg. The investigation herein demonstrates the laser (excimer laser, λ = 248 nm, 25 ns) processing of cast Mg-Al-Zn (AZ91D) and Mg-Al (AM60B) alloys mitigates the impact β-Mg17Al12 and Al-Mn intermetallic particles have on the corrosion resistance. Control of the laser fluence reduced surface defects commonly formed while improving the corrosion lifetime. Scanning electron microscopy of laser processed specimens revealed significant dissolution of the β phase and Al-Mn intermetallic particles. The corrosion response showed that an order of magnitude reduction in H2 evolution reaction rate was achieved for all times of immersion. Electron impedance spectroscopy showed a 10-fold increase in the estimated polarization resistance with laser processed surfaces, exhibiting a corrosion lifetime of the order of 60 h. The impedance data were verified by 72 h time lapse images of the fully immersed samples exhibiting minimal filiform corrosion coverage for both processed alloys in a quiescent 0.6 M NaCl solution.
  • [期刊] Technical note: A major loss in tensile strength of friction stir welded aluminum alloy joints resulting from atmospheric corrosion
    摘要:The effect of corrosion on the mechanical properties of aluminum alloy (AA6005-T6) joints produced by friction stir welding is investigated. The tensile strength of the weldments was analyzed after 100 h exposure to 95% relative humidity air at 22°C in the presence of NaCl and CO2. The corrosion exposure caused a significant decrease in the ultimate tensile strength and ductility of the welds. Microstructural studies revealed that the loss of mechanical properties is associated with corrosion in the vicinity of the joint line remnants (JLRs), which are commonly found in friction stir welds. The reasons behind the preferential corrosion of the JLRs and the effect of corrosion on the mechanical properties of the welds are discussed.
  • [期刊] Species-dependent crevice corrosion modeling of Ni-625
    摘要:A coordinated experimental and mathematical modeling effort to develop a three-stage model for determining the spatial and temporal potential, current, ionic species, and damage profiles for nickel 625 crevice corrosion applications in seawater solutions is presented. Stage one is oxygen depletion inside the crevice, stage two is the development of a critical crevice solution (induction stage), and stage three is long-term aggressive dissolution that is consistent with a nearly wellmixed, IR controlled crevice system. In stage one, deoxygenation allows separation of the anodic and cathodic sites. In stage two, local initiation occurs at the crevice tip that diffuses toward the crevice mouth. Minimal dissolution occurs in stage two and complete initiation occurs when the total anodic current is such that the critical IR drop is observed. In stage three, the crevice is saturated at or near the critical crevice solution and metal salt precipitates allow stable crevice corrosion propagation. The key inputs to the three-stage model are concentration-dependent polarization curves, and hydrolysis and hydroxide precipitation reaction data. Model calculations are compared with experimental damage histories and total current measurements. Analytical solutions and 2D modeling efforts are also used to validate the proposed 1D approach for simplified systems leading up to the complex Ni-625 system. All results agree under a thin-film approximation. Further, it is shown that with appropriate experimental input data, and knowledge from solution of the species-dependent system, a damage evolution well-mixed model provides comparable results.
  • [期刊] Investigating the impact of accelerated testing variables on the exfoliation corrosion of AA2060
    摘要:Aluminum-lithium alloys are attractive for aerospace applications because of their improved strength-to-weight and stiffness- to-weight ratios, fracture toughness, and corrosion resistance compared to legacy alloys such as AA2024 and AA7075. Many standardized accelerated tests are used to evaluate the corrosion resistance of high-strength aluminum alloys, but these tests can produce drastically different results for the same alloy. The purpose of this study is to provide a quantitative, technical understanding of the roles of key testing variables in two accelerated tests for exfoliation corrosion, EXCO and ANCIT. Accelerated testing was performed on under-aged and near peak-aged tempers of aluminum-lithium alloy AA2060, and a five-factor design of experiments was used to determine the impact of key testing variables on the corrosion potential and polarization resistance of AA2060. It was found that ANCIT testing produced exfoliation in the susceptible temper (T36) in a much shorter time than EXCO testing did. ANCIT was also more aggressive toward the -T86 temper compared to EXCO. The design of experiments showed that the addition of an oxidizing agent (NO-3) to the testing solution had a statistically significant impact on both corrosion potential and polarization resistance. The solution pH, as well as the interaction between solution pH and added oxidizing agent, had statistically significant effects on polarization resistance.
  • [期刊] Environmentally assisted cracking measurements in structural aluminum alloys under accelerated test conditions
    摘要:Environmentally assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is an important maintenance and safety issue for U.S. Department of Defense assets. EAC initiation and propagation of cracks is influenced by the complex interactions of load, environment, and alloy properties. Traditional environmental fracture testing conducted under immersion or constant humidity conditions may produce results that are different than measurements collected under thin electrolyte layers or droplets formed during atmospheric exposure. In addition, most standard methods do not provide instantaneous measures of crack velocity that can be used to identify specific environmental conditions that promote cracking. Improved assessment of EAC susceptibility and the conditions that promote cracking of aluminum alloys has been accomplished with an autonomous, in situ measurement system that can be used in accelerated corrosion test chambers and outdoor exposure sites. Continuous measurements of crack length throughout a corrosion study can be obtained using a tensile loaded notched specimen, compact load frame, and a force sensor to track load shedding with crack propagation. These measurements can be used to compare alloy performance, determine environmental conditions that promote EAC, and evaluate the effectiveness of corrosion control coatings and methods. Aluminum alloy testing with varying environmental and mechanical parameters (e.g., relative humidity, salt composition, degree of sensitization, and stress intensity) has demonstrated a strong dependence of crack velocity on cyclic relative humidity (RH). Specifically, in a number of tests, crack velocity increased to a maximum during drying (decreasing RH) at intermediate humidity. This result may be important to understanding the processes that promote EAC and indicates that high humidity and salt loading may not always be the most aggressive conditions for evaluating EAC susceptibility. Results of AA5083 alloy testing in cyclic accelerated corrosion tests, including ASTM B117, ASTM G85 A5, and GM9540P are reported. A subset of results for AA7075- T651 is also presented to demonstrate applicability of the test method for a different alloy that was not as highly sensitized to EAC.
  • [期刊] Performance of a magnesium-rich primer on pretreated AA2024-T351 in selected laboratory and field environments: Conversion coating pretreatments
    摘要:The effect of conversion coatings on the corrosion protection of AA2024-T351 by magnesium-rich primer (MgRP) was evaluated in topcoated and non-topcoated, scribed conditions. Protection of remote scratches and global protection by the coating after exposure in selected laboratory and field environments was investigated. Exposure studies focused on chromate conversion coating, trivalent chromium pretreatment (TCP), and non-chromium pretreatment, and compared to non-film forming (NFF) surface pretreatment. Exposures were conducted in the field under two different environments: at a coastal marine site and at an inland rural site. ASTM B117 with 5 wt% NaCl, modified ASTM B117 with acidified ASTM substitute ocean water and UV light, as well as full immersion in ambiently aerated 5 wt% NaCl solution were compared to field environments. Mg pigment depletion rate, global galvanic protection potential, coating barrier properties, and scribe protection were investigated. In systems without a topcoat, full immersion studies resulted in significant depletion of Mg, and all other environments led to depletion of Mg at different rates. In contrast, a polyurethane topcoat limited the Mg metallic pigment depletion, resulting in only partial Mg depletion in all chosen environments. In NFF pretreated AA2024-T351 with MgRP, magnesium was galvanically coupled to AA2024-T351 immediately and was available for cathodic protection from the beginning of exposure. This is indicated by a shift in global galvanic protection potential from -1.4 VSCE to more positive potentials with increasing exposure time. In the case of conversion coating pretreated AA2024-T351, there was limited galvanic coupling with the MgRP. Upon prolonged exposure in full immersion, the global galvanic protection potential decreased to more negative potentials below the open-circuit potential of AA2024-T351, indicative of galvanic coupling. In the case of systems with topcoat, the global galvanic protection potential was heavily regulated by the polyurethane topcoat and there was no significant global galvanic coupling between AA2024-T351 and Mg in the timeframe over which experiments were conducted. Mg was preserved and available for any future sacrificial anode based cathodic protection and local protection. The barrier properties of the MgRP pigmented coating also degraded with time at a higher rate in systems in the absence of topcoat. This was attributed to UV degradation of the pigmented coating resin and could be reduced with the polyurethane topcoat. Scanning electron microscopy/energy dispersive spectroscopy characterization of the scribe after different B117/field exposure times indicated that the protective throwing power increased as a function of exposure time in both AA2024-T351/NFF/MgRP and AA2024-T351/TCP/ MgRP systems. Moreover, a secondary protection mode was identified.

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