Anodic Polarization Scans Research Articles

Laser Surface Melting to Mitigate Intergranular Corrosion of Sensitized AA5083

AA5083 is a solution-strengthened, supersaturated Al-Mg alloy. It has become widely used in corrosive and harsh environments, such as marine settings, due to its exceptional corrosion resistance and impressive strength-to-weight ratio. However, when exposed to moderately elevated temperatures, the alloy undergoes a process called sensitization, resulting in the precipitation of the β phase. This intermetallic precipitate is rich in magnesium and has anodic properties, creating a microgalvanic couple with the more noble aluminum (Al) matrix. Consequently, the sensitized alloy experiences intergranular corrosion due to the anodic dissolution of the grain boundary in a corrosive environment. Various techniques for dissolving intermetallic particles into the matrix have been reported in the literature, but they are often impractical for service components, and traditional solutionizing treatments tend to decrease mechanical properties. This study aimed to investigate the impact of pulsed excimer laser irradiation, as a novel approach, on the surface morphology, chemical composition, and electrochemical behavior of highly sensitized AA5083 samples. To achieve this, various analytical techniques were used, including profilometry, optical microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and localized potentiostat scans. The results of this investigation showed that laser surface melting (LSM) led to a reduction in the open-circuit potential and exchange current density in a 0.6 M NaCl aqueous solution, mainly due to increased surface homogenization. Furthermore, multiple grazing incident x-ray diffraction scans were performed to identify the changes in the metallurgical and crystallographic parameters in the near-surface region. Anodic polarization scans of the LSM surface galvanically coupled with a more cathodic base metal exhibited a lower corrosion current density than the theoretical value suggested by mixed potential theory. The improved performance could potentially be attributed to the surface homogenization and formation of a robust passive layer on the LSM surface.

Optimizing of gas-shielded metal arc welding parameters for 10.5-12.5% chromium steel using 3o8L electrodes

The 10.5-12.5% chromium, titanium-stabilized ferritic stainless steel (FSS) plates used to fabricate coal line jumbo wagons is susceptible to undesirable chromium depletion during welding with 309L electrodes. In this work, non-stabilized FSS plates were welded using 308L welding electrodes. The objective was to weld non-stabilized FSSs using gas-shielded metal arc welding without causing sensitization. The welding process was optimized by lowering the heat input. Sensitization was assessed by rigorous etching techniques and anodic polarization scans. In addition, the welds were subjected to tensile testing, and their fracture surfaces were examined for intergranular cracking. Non-stabilized FSS plates were successfully welded using gas metal arc welding.

Dealloying Behavior in Ni-Cr-Fe Alloys As a Precursor to SCC in Boiling Caustic Environments

Experiments have shown that the susceptibility of Ni-Cr-Fe Alloys to dealloying, and subsequently stress corrosion cracking (SCC), highly depends on alloy composition and electrochemical potential. Dealloying is defined as the selective dissolution of the more active elements in an alloy (e.g., Cr and Fe in a Ni-based alloy). A dealloying process results in a remnant nano-porous layer on the surface that could be a precursor to SCC by a mechanism known as film-induced cleavage (FIC) where cracks propagate through this layer with sufficient velocity to promote energy transmission into the underlying metal resulting in cleavage fracture. Presence of a threshold amount of a more noble element could theoretically suppress this phenomenon by surface diffusion. This was investigated further by previous researchers, and the percolation theory of materials was hypothesized that states that the more noble element in an alloy creates a percolating mass that would limit the ability to retain a connected pathway of the reactive elements from the surface given it reaches a certain threshold of the mass of the material. This percentage was estimated to be between 50-60% for most alloying systems and termed the parting limit. In this study, percolation theory and dealloying is further explored by exposing several Ni-based and Fe-based alloys with varying percentages of Ni to boiling caustic solutions (50% NaOH) and evaluating their dealloying behavior.As mentioned, dealloying highly depends on electrochemical potential and is believed to occur above a threshold potential known as the dealloying critical potential. The dealloying critical potential is defined as the condition at which a transition occurs from a “passivated” alloy surface to the sustained formation of a bi-continuous porous structure. It can be identified by running an anodic polarization scan and initially matching anodic peaks to a pH-potential diagram (Pourbaix diagram) to evaluate the potential dissolution behavior. Bench-top electrochemical measurements were conducted to find the dealloying critical potential and make a correlation between this potential and alloys’ composition. The figure attached shows the anodic polarization scan results for all alloys included in the study in 50% NaOH at 140°C. It was observed that for the samples with high Ni content (i.e., Alloy 600 and Alloy 690) there was only a single active peak, which indicates transitioning to passivation and inhibition of dealloying. All other materials were susceptible to dealloying. Electron microscopy techniques were used to analyze all the samples used in the figure after exposure to 50% NaOH at 140°C at open circuit potential (OCP) and during a potentiostatic hold at the dealloying critical potential. Scanning electron microscopy (SEM) imaging followed by energy dispersive X-ray spectroscopy (EDS) revealed varying degrees of dealloying, with low Ni-containing alloys showing greater severity.The role of dealloying as a precursor to SCC was also evaluated by utilizing U-bend samples for Alloy 690, Alloy 800, and SS 304. The findings were in-line with previous dealloying result; the alloys with Ni content below the parting limit, SS304 and the Alloy 800, both underwent SCC after 48 hours of exposure to 50% NaOH solution at 135 °C, while the Alloy 690 sample (~60Ni) did not show any signs of cracking. A peculiar phenomenon was also noted for alloys containing Mo where it is seen that their dissolution current is approximately two folds higher than their counterparts with approximately the same Ni content. For example, when comparing SS 304 and SS 316 or Alloy C-276 and Alloy G-35. A possible explanation for that is the electrochemical behavior of Mo in caustic conditions where it is shown in its pourbaix diagram that dissolution occurs over a wider range of pH and potential in comparison to the other alloying elements. This earlier dissolution can possibly create more terrace sites on the surface that could accelerate the dissolution of iron and chromium. However, further studies are needed before a conclusion can be reached. Further work has also been conducted on Alloy 800 under the same conditions to evaluate the possibility of controlling the geometry of the dealloyed layer. The results so far indicate that the time of exposure and the electrochemical potential are critical in controlling the geometry of the dealloyed layer. Parallel studies are being conducted on SS 304 to evaluate the effect of Pb on extending the dealloying regime. The results so far have shown that the addition of Pb would shift the OCP more positively towards a region of passivity. It was also noted that dealloying kinetics is hindered in comparison to pure caustic conditions. However, SCC may still be possible with dissolved Pb present. Figure 1

(Invited) Use of ALD by SLRR for the Growth of Layered Bi-Metallic Structures and Alloy Thin Films with Specific Functionality

The ingenious idea of surface limited deposition is among the brightest achievements of Radoslav Adzic. The proposed by his group in the beginning of this century approach of surface limited redox replacement (SLRR) for realization of monolayer thick coatings with catalytically active metals and alloys was used as foundation from others for the development of epitaxial thin film growth methods. Central stage in this direction takes the use of SLRR approach for coating nanoparticles and continuous nanoporous metal layers with ultrathin films of metal, layered bi-metallic structures, and/or alloys with specific functionality. Successful outcome of this coating renders the final product remarkably active and durable catalyst. In this talk, the direct use of SLRR based protocols for the growth of layered bi-metallic films and alloys with targeted functionality. Specific consideration will be given to co- deposition modes in the systems Ag-Cu and Au-Cu. Factors controlling the quality of the deposited films in terms of morphology, orientation and composition will be comparatively considered. Emphasis will be placed on the thermodynamic miscibility of the metals participating in the deposition process as a factor controlling the final growth outcome in both cases; layered structures with sharp interface boundary in the Ag-Cu system and a single phase alloy in the Au-Cu system upon application of identical growth protocols. Results of electrochemical and SEM / STM characterization of the morphology, structure and/or orientation of the deposited films will be presented and critically discussed. Also, the parting behavior of both components upon anodic polarization scans will be considered in detail and studied with emphasis of post-anodization morphological characteristics of the remaining structures. The discussion on the catalyst processing component based on coating of above described bi-metallic structures with ultra thin Pt and/or Pd layers will be the conclusion of this talk. It will be garnered by the demonstration of results of activity and durability testing of accordingly developed catalysts for organic fuel oxidation and/or nitrogen electrochemistry. Finally, a glimpse into the future of this rapidly developing field catalyst design at the atomic scale will be also provided.

Electrochemical Corrosion Study of Weld Joint of Nb and Ti6Al4V in Saline and Water

A Type 1 niobium (Nb) part and a grade 5 titanium (Ti6Al4V) part were joined by laser weld, to form a component to be used in implantable medical devices for electrical stimulation therapies. This component could be exposed to body fluid or particularly water which could penetrate through the medical adhesive seal over the time of implantation. To meet the device reliability, performance, and patient safety goals, electrochemical immersion testing was conducted to study corrosion of the base metals and their galvanic couples and to evaluate corrosion resistance of the weld joints. Samples of as-manufactured Ti6Al4V parts, Nb parts, and weld joints were prepared, cleaned and inspected for testing (Figure 1, Table 1). A wire made of the same material and mostly sealed by medical adhesive, was welded to each sample for electrical connection. The surface areas of samples were calculated. They were divided into groups and tested at 37°C in glass jars containing de-ionized (DI) water or phosphate buffered saline (PBS), respectively. Electrochemical tests including open circuit potential (OCP); cathodic polarization (CP) or anodic polarization (AP); galvanic corrosion or zero resistance ammeter (ZRA) testing; anodic cyclic polarization (ACP) and potentiostatic polarization (PP) were performed at different times during immersion of the samples. The potentiodynamic scan rate for CP, AP and ACP scans was 0.2 mV/s. The tests used three-electrode configuration consisting of a working electrode (WE) which was the testing sample; a reference electrode (Ref.) which was a Ag/AgCl electrode in 3 M NaCl solution; and a counter electrode (CE) which was a piece of platinum sheet, or for galvanic corrosion testing, which was the secondary working electrode of the Nb/Ti6Al4V galvanic couple. Visual and stereomicroscopic examinations were made during and after the testing with a total of immersion time of about 100-130 days. After 3-4 days of immersion, OCP was stabilizing with an average of about 0.08, 0.05 and 0.07 V vs. Ref. in DI water and 0.11, -0.02 and 0.06 V vs. Ref. in PBS, for the Ti6Al4V, Nb and joint samples, respectively. Though the OCP difference was small, it indicated Nb would act as anode when joined or coupled to Ti6Al4V. Galvanic corrosion could occur in Nb of the joint, especially considering the anode (Nb) to cathode (Ti6Al4V) area ratio was as small as 0.01. A galvanic current was estimated to be about 9-13 nA/cm2 in DI water and 24-42 nA/cm2 in PBS based on anodic polarization of Nb and cathodic polarization of Ti6Al4V in potentiodynamic scans (Figure 2a). Further, galvanic corrosion potential and current (Figures 2b and 2c) of the Nb/Ti6Al4V couples was measured by ZRA testing. After ~48 days of testing, the galvanic current was stabilized with an average of about 1 nA/cm2 in both DI water and PBS, indicating a stable passive film formed on Nb surface. The average galvanic corrosion rate of Nb of the couples was about 0.008 μm/year in both DI water and PBS, calculated from the measured galvanic current which was considered more accurate (as obtained in a more steady state) and much lower than the estimation from potentiodynamic scans. ACP curves of the joint and base metal samples showed similar polarization behavior in both DI water and PBS (Figure 2d). All samples displayed stable passivation with low current from OCP to a potential of Ep ≅0.8 V vs. OCP. Beyond Ep, current increased markedly, and small hysteresis formed in ACP scans for the joint and Nb samples. Additional potentiostatic polarization was thus performed on the joint and Nb samples in PBS for ~4 days at potentials of 1.2 V and 1.4 V vs. Ref. which were above Ep and in the hysteresis range. The current would stabilize at the increased levels for both potentials. Neither localized pitting or crevice corrosion attack nor active gas bubbling was observed on the joints and parts. Corrosion rate estimated from the increased current at 1.4 V vs. Ref. for Nb could be as high as 66 μm/year. Depending on the type of device design, an electrochemical potential possibly applied to the joint component was assessed under simulated electrical signals for stimulation, which was measured to be under 0.3 V vs. Ref. and smaller than Ep. Therefore, the joint materials of Nb and Ti6Al4V are stable and suitable for the device application with no concern of corrosion. Figure 1

Corrosion resistance of surface modified nickel titanium archwires

To compare the corrosion behavior of commercially available surface modified nickel titanium (NiTi) arch wires with respect to a conventional NiTi and to evaluate its association with surface characteristics. Five types of surface modified arch wires and a conventional NiTi arch wire, all from different manufacturers, were evaluated for their corrosion resistance from breakdown potential in an anodic polarization scan in Ringer's solution. Surface characteristics were determined from scanning electron microscopy, atomic force microscopy, and energy dispersive analysis. One-way analysis of variance and post hoc Duncan's multiple range tests were used to evaluate statistical significance. Surface modified NiTi wires showed significant improvement in corrosion resistance and reduction in surface roughness values. Breakdown potentials increased in the order of group 6 (conventional; 204 mV) < group 1 (nitride; 333 mV) < group 5 (epoxy resin; 346mV) < group 3 (oxide; 523 mV) < group 2 (gold; 872 mV) < group 4 (Teflon; 1181 mV), but root mean square (RMS) roughness values, which indicated surface roughness, followed a different pattern: group 3 (oxide; 74.12 nm) < group 1 (nitride; 221.651 nm) < group 4 (Teflon; 278.523 nm) < group 2 (gold; 317.894 nm) < group 5 (epoxy resin; 344.236 nm) < group 6 (conventional; 578.555 nm). Surface modification of NiTi wires proved to be effective in improving its corrosion resistance and decreasing surface roughness. However, neither factor could maintain a direct, one-to-one relationship. It meant that the type and nature of coating material can effectively influence the anticorrosive features of NiTi wires, compared with its surface roughness values.

Corrosion of WC-VC-Co Hardmetal in Neutral Chloride Containing Media

The effect of varying VC content on the corrosion behavior of WC-10 wt% Co hardmetals in sodium chloride (NaCl) and synthetic mine water (SMW) solutions has been investigated using anodic polarization scans and surface analytical methods. It is shown that the polarization behavior is active-pseudopassive in NaCl and active in SMW regardless of the VC content, while the corrosion resistance is poorer and independent of VC content in NaCl but better at high VC contents in SMW. The corrosion behavior of samples is explained using the effect of VC on the chemical composition of the binder.

Corrosion-wear evaluation of a UHMWPE/Co–Cr couple in sliding contact under relatively low contact stress in physiological saline solution

Abstract Several tribocorrosion studies have been carried out using metal on metal (MOM) and ceramic on metal (COM) materials combinations in order to understand the behavior of these materials when applied in human joint replacement. However, there are few studies in this area focused on processes of tribocorrosion occurring at the interface between a metal and a polymer. Thus, the aim of this study was to evaluate the tribocorrosion behavior between cobalt chromium and ultra-high molecular weight polyethylene (UHMWPE) under relatively low contact stress. Reciprocating corrosion–wear tests were performed using plates of cobalt-chromium alloy and UHMWPE pins under a load of 1.84 N (giving a contact stress of 39 kPa) in a 0.9% NaCl solution, during 1 h. The tribocorrosion tests were performed at open circuit potential (OCP), during anodic polarization scans and at potentiostatic conditions of −150, −100, 200 and −800 mV vs. SCE. The electrochemical wear rates were estimated using the Faraday equation. The OCP results showed a dramatically decrease of potential during the mechanical contact, while the anodic polarization scans revealed an increase of current during mechanical contact. Finally, potentiostatic tests carried out at −150, −100 and 200 mV vs. SCE displayed an increase of anodic current during mechanical contact, while the tests carried out under −800 mV vs. SCE showed an increase of the cathodic current during mechanical contact. Thus, this set of results provides evidences of tribocorrosion when the mechanical contact was applied to the system, even though the applied stress was relatively low.

Influence of cold work and sigma phase on the pitting corrosion behavior of 25 chromium super duplex stainless steel in 3.5% sodium chloride solution.

AbstractThe effect of cold work (up to 16% strain) and sigma phase precipitation (at 850 °C for 10 and 60 min) on the pitting resistance of 25 chromium super duplex stainless steel were investigated in 3.5% sodium chloride solution at 70 and 90 °C. Anodic polarization scans for cold worked samples revealed immunity to pitting attack at 70 °C even with 16% strain. At 90 °C, the alloy still showed high pitting resistance, pitting occurring at about 600 mV (SCE) for the 16% plastic strain samples. A serious deterioration of the pitting corrosion resistance was found after heating the alloy at 850 °C for 10 min resulting in a clear drop in the pitting potential at 90 °C. After heating for 60 min, the material showed rapid deterioration of pitting corrosion resistance at 70 °C.

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N 2–H 2 (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.

Mechanical properties, bioactivity and corrosion resistance of oxygen and sodium plasma treated nickel titanium shape memory alloy

Nickel titanium (NiTi) shape memory alloy is a unique material displaying the shape memory effect and superelastic property making it attractive to the orthopedic field. However, with its high nickel content of ∼ 50%, there is concern on health and safety when this material is implanted inside the human body for a prolonged period of time as toxic nickel ions may leach into the body due to corrosion under physiological environment. Previous work demonstrated that the corrosion resistance of this material could be enhanced by implanting a packed oxide layer on the substrate surface using oxygen plasma immersion ion implantation (PIII). The present study aims at improving its bioactivity by further implanting sodium ions into the modified surface using PIII. The chemical composition of the modified surface is characterized by X-ray photoelectron spectroscopy (XPS). Simulated body fluid (SBF) immersion tests for 21 days indicate that implantation of sodium successfully enhances the accumulation of calcium/phosphorus-rich deposits on the modified surface. Anodic polarization scans suggest that sodium PIII does not affect the corrosion resistance of the oxygen plasma implanted surface. Three-point bending test reveals changes in the bulk mechanical property after PIII. However, superelasticity can be retained in the implanted NiTi materials. Differential scanning calorimetry (DSC) suggests a change in the transition temperature of the substrate which likely causes the change in mechanical property. In conclusion, oxygen and sodium PIII can enhance the bioactivity and corrosion resistance of NiTi, but care must be exercised because this treatment may change the bulk mechanical property.

NITRIC ACID RESISTANCE OF NEW TYPE Fe-Mn-Al STAINLESS STEELS

Several high-Mn (25-30 wt%) Fe-Mn-Al alloys containing various Al and Cr levels with fully austenitic or duplex microstructures have been studied. The effect of the alloying elements such as Al and Al-Cr on the passivation mechanism in 5-50% HNO3 solutions was investigated by using electrochemical measurements, X-ray diffraction and scanning electron microscopy. The results of anodic polarization scans showed that the acid corrosion resistance of Fe-30Mn-4Al-4Cr and Fe-30Mn- 8Al-6Cr alloys is superior to that of alloys without Cr. The X-ray analysis confirmed the presence of Al2O3 and Cr2O3 in the passive film. Scanning electron microscopy showed that the formation of the passive film takes place by the direct nucleation and growth mechanism. It is apparent that the corrosion resistance of Fe-Mn-Al steels is not dependent on their phase structure but rather on the chemical composition. Comparing with AISI 304 stainless steel, the passive film that formed on Fe-Mn-Al alloys bearing high Al and Cr, was found to be more stable in 5-50% HNO3 solutions.On a étudié plusieurs alliages de Fe-Mn-Al à haute teneur en Mn (25-30% en poids) contenant des niveaux variés d’Al et de Cr, à microstructure complètement austénitique ou duplex. On a étudié l’effet des éléments d’alliage comme l’Al et Al-Cr sur le mécanisme de passivation dans des solutions de 5-50% HNO3 en utilisant des mesures électrochimiques, la diffraction des rayons x et la microscopie électronique à balayage. Les résultats de balayages de polarisation anodique ont montré que la résistance à la corrosion acide des alliages Fe-30Mn-4Al-4Cr et Fe-30Mn-8Al-6Cr était supérieure à celle d’alliages sans Cr. L’analyse de rayons x a confirmé la présence d’Al2O3 et de Cr2O3 dans la couche passivante. La microscopie électronique à balayage a montré que la formation de la couche passivante se faisait par le mécanisme de nucléation directe et de croissance. Il est apparent que la résistance à la corrosion des aciers Fe-Mn-Al ne dépend pas de la structure de phase mais plutôt de la composition chimique. Comparant à l’acier inoxydable AISI 304, on a trouvé que la couche passivante qui s’est formée sur les alliages Fe-Mn-Al à teneur élevée en Al et en Cr était plus stable dans des solutions de 5-50% HNO3.

Corrosion of amalgams under sliding wear

Objectives. During mastication, dental amalgams are simultaneously subjected to corrosion by the oral environment and to a sliding-wear process by biting forces. In the present study, the effect of sliding wear on the corrosion behavior of two high-copper dental amalgams was investigated.Methods. An experimental apparatus was utilized that allows electrochemical testing under sliding-wear conditions. Corrosion potential measurements and anodic polarization scans were conducted in 0.1 M NaCl solution under sliding wear to characterize the behavior of two commercial, high-copper, single composition dental amalgams. In addition, long duration tests were conducted to assess possible corrosion and wear synergistic effects.Results. The results showed that sliding wear caused a sharp reduction in the corrosion potential, a significant increase in the corrosion rate and a decrease in the repassivation rate of both amalgams. These effects are due to the mechanical removal by the wear process of the surface protective film formed on dental amalgams. The simultaneous action of sliding wear and corrosion can also induce embrittlement that leads to cracking.Significance. The present evidence suggests that this cracking may be one of the major contributors to marginal failures of dental amalgam restorations.

The effect of σ-phase precipitation at 800°C on the corrosion resistance in sea-water of a high alloyed duplex stainless steel

Super-duplex stainless steels are recently developed high alloyed stainless steels that combine good mechanical properties with excellent corrosion resistance. Because of a high content of chromium and molybdenum, these alloys are susceptible to σ-phase precipitation during short exposure to temperatures between 650 and 950°C. The effect of 800°C aging on σ-phase formation and on the mechanical properties of a super-duplex stainless steel have been reported previously by the authors. 1 This investigation concerns the effect of σ-phase precipitation at 800°C on the corrosion behaviour in seawater and comprises anodic polarization scans and potentiostatic crevice corrosion tests. A serious deterioration of the corrosion resistance is found after aging times longer than 7 min, resulting in a drop of both the critical crevice corrosion temperature (CCT) and the breakdown potential ( E bd). The initiation of localized corrosion takes place next to the σ-phase, in the newly formed secondary austenite ( γ 2).

Use of potentiodynamic polarization technique for corrosion testing of dental alloys

The number and in particular the diversity in types and composition of alloys used in dentistry are increasing, thus enhancing the need for predictive corrosion testing. No corrosion test is generally accepted as being both applicable to and relevant for all dental alloys. However, some methods have gained a certain recognition, and among these are the potentiodynamic polarization techniques. The purpose of the present work was to study the application, the reproducibility and the influence of some methodological variables on the results of potentiodynamic polarization corrosion testing. Embedded specimens of 18 different dental alloys, representing all major types, were subjected to anodic polarization scan within the potential range of -700 mV to 1000 mV (SCE). Artificial saliva, with and without organic compounds, and sodium sulfide solution were used as electrolytes. The results were processed on a microcomputer using locally developed software. The curves of current density versus potential showed fair reproducibility, with great differences between some of the alloys, showing a clear relationship to nobility or passivity of the alloy. There were marked differences between the results obtained in artificial saliva and in sodium sulfide solution, while addition of organic components to the artificial saliva only had a minor effect. The potentiodynamic polarization corrosion test supplies detailed information such as anodic charge, and open circuit, rupture, and passivation potential. Furthermore, it indicates the passive range and sensitivity to pitting corrosion. These are all useful when evaluating the corrosion properties of a dental alloy. An analysis of variance of the results in this study showed that possible differences between the alloys were clearly detectable with this method. However, for alloys releasing non-ionic products during corrosion/degradation, the method may be inappropriate. It is thus concluded that the method appears applicable as an in vitro corrosion test for most dental alloys, with the possible exception of amalgams.

Corrosion Studies of Rapidly Solidified Magnesium Alloys

Effects of alloying elements on the corrosion resistance of mostly binary rapidly solidified (RS) magnesium alloys were examined using electrochemical techniques. Uniform corrosion rates were measured at room temperature in a pH 9.2 sodium borate buffer using electrochemical impedance spectroscopy (EIS). Comparisons of electrochemically and gravimetrically determined corrosion rates of bulk magnesium and magnesium alloys showed that the charge transfer resistance could be used to accurately measure the corrosion rates of these materials provided the attack was indeed uniform. The correlation broke down for pure magnesium, which suffered a nonuniform attack due to the undermining of small particles. Corrosion rate measurements of binary alloys showed that aluminum was the only element which caused a decrease in the corrosion rate of magnesium; the corrosion rate decreased with increasing aluminum content. Low concentrations of zinc and lithium resulted in alloys with corrosion rates slightly higher than that of the pure magnesium. Anodic polarization scans were used to compare as‐cast and rapidly solidified commercial ternary alloy AZ61 in borate (pH 9.2) and carbonate/bicarbonate (pH 10.0) buffer solutions containing 100 ppm, 1000 ppm, and 3.5 weight percent sodium chloride. The RS AZ61 formed a more protective film than the cast material under these conditions.

Crevice Corrosion Test Cell: A New Design

Abstract A new cell for studying crevice corrosion has been developed. The crevice is made by pressing a glass lens against a flat polished specimen and this ensures a very reproducible geometry. The susceptibility to crevice corrosion of weld metal samples has been studied with this cell by anodic polarization scans and it is shown that the presence of a high-molybdenum content, greater than 7%, completely eliminates this form of attack.

Corrosion of Galvanized and Galvannealed Steel in Solutions of pH 9.0 to 14.0

Abstract Well-characterized zinc coatings on steel have been prepared so that either the eta or zeta phase was present as the outermost layer. Their electrochemical behavior was studied in a range of solutions by galvanostatic anodic polarization and potentiodynamic anodic polarization scans. The corrosion products were identified using scanning electron microscopy (SEM) and x-ray analysis. Results show that the solution pH has the most marked effect on electrochemical behavior and the nature of the corrosion product formed. At a given solution pH, the behavior and products formed are modified according to the concentration of chloride in the solution and the type of iron-zinc alloy layer investigated. The results are discussed in relation to the practical use of galvanized reinforcement in concrete.

Electrochemical behaviour and corrosion of 18-8 austenitic stainless steel in sulphate solutions

The electrochemical behaviour and corrosion of a commercial Egyptian 18-8 austenitic stainless steel was investigated in sulphuric acid and in sodium sulphate solutions using the potentiodynamic technique. In acid solution, the anodic polarization scans are characterized by the appearance of active, passive and transpassive transitions before oxygen evolution. The current densities corresponding to these three regions are increased by increasing either the acid concentration or the voltage scanning rate. The passive electrode does not regain its activity when subjected to cathodic treatment. Sensitization of the electrode enhances its susceptibility to intergranular corrosion. In sodium sulphate solution, the polarization scans are characterized by the disappearance of the active region. On sensitization, a second transpassive region appears before oxygen evolution.

The Effect of Prior Cold Work on the Degree of Sensitization in Type 304 Stainless Steel

Abstract The effect of prior cold work of 0 to 60% on subsequent sensitization of Type 304 stainless steel at 660 C was studied from anodic polarization scans in 1N HClO4 + 0.2N NaCl and 0.5M H2SO4 + 0.01 M KSCN solutions and from SCC behavior in room temperature 1N H2SO4 at 150 mVSHE at a strain rate of 10−6 sec−1. Small amounts (5 to 20%) of prior cold work sharply enhanced the susceptibility to subsequent sensitization. With increasing amounts of prior cold work, heating at 660° for 1 or 16 hours resulted in progressively more widespread precipitation of carbides within and across the grains. This presumably shortened diffusion paths for chromium and, along with the expectedly faster chromium diffusion rate in cold worked metal, resulted in partial to total absence of sensitization in specimens that were highly cold worked (>40%) prior to the sensitization treatment.