Localized Corrosion Morphology Research Articles

Influence of Cu Addition and Autoclave Sterilization on Corrosion Resistance and Biocompatibility of NiTi for orthodontics Applications

Commercial archwires NiTiCu (Ormco) and NiTi (Morelli) and samples of the corresponding raw materials were tested to assess the influence of Cu addition and sterilization on the corrosion resistance and biocompatibility. Raw materials samples and commercial archwires exhibited similar corrosion resistance when compared. The addition of Cu decreased the corrosion resistance of NiTi alloy, whiling the sterilization process had no influence. The biocompatibility tests were carried out in different extracts of the alloys and no cytotoxicity effects were observed for the ionic concentrations used. SEM and Confocal laser analysis permitted to observe the localized corrosion morphology obtained after anodic polarization tests. Although Cu addition on NiTi can be introduced to better adjust the mechanical resistance, this third element induced detrimental effect on the corrosion resistance of raw materials and commercial archwires.

Wet corrosion of Al 1050 alloy in ethyl bromide-containing environment

The natural passivity, or induced by anodizing process, does not always result in an effective corrosion barrier for aluminium when exposed to environments containing specific chemicals that can cause material degradation phenomena with localized corrosion morphology. The present work investigates the effects of ethyl bromide (EtBr), a neutral organic molecule, sometimes used as a fumigant in packaging containers, that has been recently indicated as a possible cause of corrosion in aluminium alloy components. The study was conducted on Al 1050 alloy samples in aqueous solutions of sodium sulphate at increasing EtBr concentrations, ranging from 500 to 8600 ppm. Long-term exposure tests (936 h) and electrochemical experiments (potentiodynamic and potentiostatic polarizations, as well as time-dependent corrosion potential monitoring) were carried out, together with SEM and EDS examinations of the morphology and nature of the corrosion products. The results showed that the alloy undergoes different corrosion behaviours depending on the concentration of EtBr, spanning from passivity at the lowest concentrations to pitting at the highest, passing through the initiation and propagation of crevice at intermediate EtBr contents. A crucial role of the exposure time was detected, tentatively attributed to ethyl bromide hydrolysis in aqueous solution that liberates the bromide anion that is aggressive towards the passive state of aluminium alloys.

Effect of strain on degradation behaviors of WE43, Fe and Zn wires

The biodegradable metallic devices undergo stress/strain-induced corrosion when they are used for load-bearing applications. The stress/strain induced-corrosion behavior causes differences in corrosion rate, corrosion morphology, strain distribution and mechanical performance of the devices. One representative example is the biodegradable stent. Biodegradable stents undergo complex inhomogeneous deformation that can cause dramatic non-uniform stent degradation, resulting in stress concentration and stents failure. The degradation of biodegradable devices requires special attention to the mutual effect between the applied strain and degradation.The quantitative relationship between strain and corrosion of the sample alloys (WE43, Fe and Zn), selected from three typical biodegradable metals, is firstly investigated and compared in this study. The in vitro degradation and the strength retention of WE43, Fe and Zn wires were investigated under different elastic and plastic strain levels ranging from 0.1% to 30%. The results indicated that the applied strain could bring down the corrosion potential, increase corrosion current and accelerate the degradation of three biodegradable metals. Specifically, remarkable enhanced localized corrosion was observed for plastic strained WE43 compared with those with elastic strains. This localized corrosion morphology significantly accelerated the strength decline at first, while the differences diminished with longer immersion period. Fe and Zn exhibited increased degradation with plastic strain applications than those under elastic strains. However, the degradation was not further increased with the increasing magnitude of plastic strains. Moreover, the bended wires were subcutaneously implanted in the dorsal aspect of the rats and the effect of bending deformation on in vitro and in vivo degradation of three metallic wires were also compared. The U-bended WE43 wires suffered more severe in vitro degradation at the stress concentrated region. Surprisingly, the early fracture of the undeformed regions was observed in the in vivo test. In conclusion, the corrosion rate, corrosion morphology and mechanical properties of WE43, Fe and Zn was sensitive to magnitude of the applied strains. The quantification results provided new insights into understanding the strain-dependent corrosion of three biodegradable metals both in vitro and in vivo. Statement of SignificanceBiodegradable implants are subjected to various mechanical environment during the deployment and subsequent physiological activity. It is necessary to have a clear understanding of the effects of the applied stress on degradation. This study addresses the quantitative effects of applied strain/stress on the in vitro and in vivo degradation of three typical biodegradable metals (Mg, Fe and Zn). These quantification results provide new insights into understanding the strain-induced corrosion of three metals.

Ant-nest corrosion failure of heat exchangers copper pipes

This paper reports on the failure analysis carried out on leaking copper pipes from three different air conditioning systems. Pinhole leaks originating from localized corrosion morphology were reported. Microscopic interconnecting tunnels inside the pipe section were observed; these tunnels were filled with corrosion products. Experimental findings and similarity to previously reported cases pointed out a corrosion mechanism known as “ant-nest corrosion” as failure cause. This type of corrosion could develop in heat exchanger components stored after assembly, during leakage tests or in the initial operational stages. It was shown that an ant-nest corrosion attack could start due to different reasons and propagate in different ways (from the inside or from the outside of the tube). In the first examined case, it was supposed that the hydrolysis of a halocarbon gas circulating in the pipe was the triggering cause of the corrosion event; in the other cases, we ascribed the starting corrosion event to the presence of residual organic contaminants on the external pipe surface.

Study of microbiologically induced corrosion of 5052 aluminum alloy by sulfate-reducing bacteria in seawater

This study evaluated the biocorrosion on 5052 aluminum alloy caused by sulfate-reducing bacteria (SRB) inoculated in sterile seawater. Specimens were immersed in two reactors, one containing only sterile seawater and other containing sterile seawater with inoculum of sulfate reducing bacteria (SRB). The biofilm formation was assessed by periodic analyses of microbial quantification and Scanning Electron Microscopy (SEM) over 48 days of immersion. The Open Circuit Potential (OCP) was monitored for a period of 15 days. To evaluate the corrosion, morphology images were obtained using SEM and optical microscopy. The results demonstrated that the SRB showed high adhesion to the metal substrate. Localized corrosion morphology was identified for the two conditions studied. Better results of mass loss, OCP, polarization and EIS were observed for aluminum alloy exposed to seawater with SRB, evidencing the protective effect of the formed biofilm.

Title Synergistic Effect of Al₂O₃ Inclusion and Pearlite on the Localized Corrosion Evolution Process of Carbon Steel in Marine Environment.

The initiation and evolution of the localized corrosion in carbon steel were investigated in a simulated marine environment of Xisha Island in the South China Sea. In the initial stage, localized corrosion occurred in the form of corrosion spot. The localized corrosion morphology and electrochemical information during corrosion process were tracked by field emission scanning electron microscopy energy dispersive spectrometry (FE-SEM-EDS), scanning vibrating electrode technique (SVET) and scanning Kelvin probe force microscopy (SKPFM). Localized corrosion was induced by the microcrevices around Al2O3 inclusions. The occluded cells and oxygen concentration cell formed in the pits could accelerate the localized corrosion. Pearlite accelerated the dissolution of the inside and surrounding ferrite via the galvanic effect between Fe3C and ferrite. Overall, the localized corrosion was initiated and evaluated under a synergistic effect of crevice corrosion, occluded cells, oxygen concentration cell and the galvanic couple between FeC3 and ferrite.

The effect of tensile and fluid shear stress on the in vitro degradation of magnesium alloy for stent applications

Magnesium alloys have gained great attention as biodegradable materials for stent applications. Cardiovascular stents are continuously exposed to different types of mechanical loadings simultaneously during service, including tensile, compressive and fluid shear stress. In this study, the in vitro degradation of WE43 wires was investigated under combined effect of tensile loading and fluid shear stress and compared with that experienced an individual loading condition. For the individual mechanical loading treatment, the degradation of magnesium wires was more severely affected by tensile loading than fluid shear stress. Under tensile loading, magnesium wires showed faster increment of corrosion rates, loss of mechanical properties and localized corrosion morphology with the increasing tensile loadings. With the combined stress, smaller variation of the corrosion rates as well as the slower strength degeneration was shown with increasing stress levels, in comparison with the individual treatment of tensile loading. This study could help to understand the effect of complex stress condition on the corrosion of magnesium for the optimization of biodegradable magnesium stents.

A Closer Look at the Role of Nanometer Scale Solute-Rich Stacking Faults in the Localized Corrosion of a Magnesium Alloy GZ31K

The localized corrosion of as-cast Mg-3Gd-1Zn-0.4Zr (GZ31K) was investigated herein. Assessment of this alloy was carefully selected owing to the presence of chemically distinct stacking faults (SFs) in the α-Mg matrix. The role of a nanometer scale solute-rich SFs on localized corrosion was realized by quasi in-situ scanning transmission electron microscopy (STEM), supplemented by scanning electron microscopy (SEM) and electrochemical methods. It was determined that chemically distinct SFs were highly enriched in Gd and Zn. When the Mg-3Gd-1Zn-0.4Zr alloy was exposed to quiescent 0.1M NaCl, the α-Mg matrix at the periphery of solute-rich SFs was preferentially dissolved. In addition, re-deposition of Zn and Gd was observed upon the α-Mg matrix, accompanying matrix dissolution. Such findings elucidate the relative inertness of solute-rich SFs at this near atomic length scale for the first time, revealing both the nanoscale localized corrosion morphology and unambiguously revealing re-deposition of alloying elements at this scale. Distortion and disintegration of solute-rich SFs at corrosion sites indicate an evolution of stress during the α-Mg matrix dissolution and Mg oxide/hydroxide formation. Localized corrosion morphology on the micrometer scale revealed that the α-Mg matrix surrounding coarser eutectic phase particles was protected as a result of localized alkalinity by the eutectic phase.

Microstructure of localized corrosion front on Mg alloys and the relationship with hydrogen evolution

Actively propagating localized corrosion fronts on pure Mg, Mg-Zn-Ca-Mn, and Mg-Y alloys in NaCl aqueous solutions were investigated by site-specific characterization techniques. Regardless of alloy microstructure and polarization conditions, rising streams of hydrogen bubbles were observed predominantly at the propagating corrosion fronts. Localized corrosion propagated inside the alloys, underneath a surface corrosion film, and exhibited a finger-like morphology at the corrosion region/alloy interface. Hydrogen evolution was found to be related to the formation of corrosion products and followed local anodic dissolution of Mg. Localized corrosion morphology strongly varied with the underlying Mg alloy microstructure and chemistry.

Anodic Hydrogen Evolution and Localized Corrosion during Galvanostatic Polarization of a Peak-Aged Mg-Y-Nd-Zr Alloy

The hydrogen evolution response on a peak-aged Mg-Y-Nd-Zr (WE43) Mg alloy in a 0.1M sodium chloride (NaCl) solution under galvanostatic polarization is investigated by collecting the evolved hydrogen gas and microstructure characterization techniques. The hydrogen evolution rate increases not only with increasing applied cathodic current density, but also with increasing applied anodic current density, indicative of the so-called negative difference effect (NDE). Localized corrosion is induced by anodic polarization and is the site where rising streams of hydrogen gas evolve. Cross sectional characterization of the regions where localized corrosion initiates reveals inward growth and a porous Mg(OH)2 structure with a finger-like propagation front. Neither a film-free region, nor an enrichment of impurity elements was observed at the localized corrosion regions. In addition, no increase in the cathodic activity was observed when comparing samples with and without prior anodic polarization, indicating that the enhanced catalytic activity of the corrosion product may not play a dominant role in the NDE on peak-aged Mg-Y-Nd-Zr. The relationship between the abnormal hydrogen evolution and the localized corrosion morphology is discussed.

Potential Dependence of Intergranular Corrosion Propagation in Sensitized Al-Mg Alloys

Applied potential has been found to influence localized corrosion morphology in a wide range of aluminum alloys.[1-4] The concept of a potential window for intergranular corrosion (IGC) is often utilized to define the zone of IGC in IGC-susceptible materials.[2-5] In general, the IGC zone is usually the potential range between the breakdown potentials of the more anodic intermetallic phase and that of the matrix phase. In the case of Al-Mg alloys (AA5XXX), particularly AA5083, it was established that the range of potentials between the breakdown potentials of a sensitized AA5083 alloy and that of the more anodic β-phase precipitate (Al3Mg2) describes the potential window for IGC spreading.[5] Limited studies involving IGC propagation into the depth from the surface for sensitized Al-Mg alloys also demonstrated potential dependence. Mizuno and Kelly[ 6] have shown that the maximum IGC damage depth in sensitized AA5083-H131 after 100 hours of galvanic coupling with 4340 stainless steel is linearly related to the galvanic couple potential. The same trend of linearly increasing IGC damage depths with measured applied potentials were observed in a 100-h galvanostatic tests conducted by Lim, et al.[7] on sensitized AA5083-H131. Both studies showed that this linear potential dependence changed with increasing degrees of sensitization (DoS). Additionally, there exists a threshold potential, the minimum potential at which IGC occurred, which decreased with increasing DoS, within the narrow range of -0.85 to -0.80 VSCE. This threshold potential is more negative than the IGC spreading potential for AA5083,[5] but more positive than the β-phase breakdown potential,[8] suggesting an IGC propagation zone within the suggested IGC spreading potential window. The aforementioned studies were limited to AA5083-H131; other alloy composition, strain-hardening tempers, as well as plate gauge thickness were not explored. The influence of these factors on the potential dependence and IGC propagation kinetics are particularly important as preliminary research (Figure 1) revealed that metallurgical and physical factors influence DoS, and possibly subsequent IGC behavior, when exposed to the same sensitization heat treatment. In is then the aim of this study to investigate the effects of alloy Mg content and plate gauge thickness on the potential dependence, specifically the threshold potential, of IGC in these sensitized Al-Mg alloys. Following predetermined appropriate current densities from Lim, et al.,[7] galvanostatic tests at anodic current densities, ranging from 1.5x10-4 to 3x10-3 A/cm2, for 100 h were carried out on sensitized AA5456-H116 with ST surfaces exposed to 0.6 M NaCl solution at pH 8.3, and IGC propagation in the L direction. Various sensitization levels, from different heats, and varying plate gauge thicknesses were considered. IGC propagation depths were characterized through appropriate metallographic techniques, microscopy, and image analyses. Initial results for AA5456-H116 (0.25” gauge) in Figure 2 show the same linear potential dependence of maximum IGC damage depth as that of AA5083-H131,[7] but higher threshold potentials (-0.80 to -0.77 VSCE). The results of this study provide insights into the critical gaps regarding alloy, temper, and gauge thickness effects on the IGC propagation behavior of sensitized Al-Mg alloys applied in marine environments. Acknowledgements Office of Naval Research under the contract ONR: N00014-08-10315.

Inhibition of the Cu65/Zn35 brass corrosion by natural extract of Camellia sinensis

• Camellia sinensis plant extract is non-toxic and can be use for brass corrosion inhibition. • An inhibitive effect is observed in both acid and neutral media. • The extract decreases the cathodic reaction rate more strongly than the anodic reaction rate. • The inhibition efficiency increase with immersion time. • The inhibitors easily adsorb on the brass surface by forming an inhibitive layer which prevent the surface from corrosion. In this work, the corrosion inhibition of brass was studied using natural plant extract, Camellia sinensis , in 0.1 M Na2SO4 solutions with pH 7 and pH 4. Electrochemical techniques (potentiodynamic polarization, electrochemical impedance spectroscopy) and scanning electron microscopy (SEM) were applied to study the brass corrosion behavior in presence and absence of the extract. The results indicated that the extract is a very effective corrosion inhibitor for brass corrosion process in both the acidic and neutral media by virtue of adsorption. The inhibition effect increases by time as demonstrated by the EIS monitoring for 120 h. In the blank solution the corrosion process leads to the formation of a dark oxide patina at pH 7 and induces localized corrosion morphology at pH 4. The extract presence can avoid both the dark patina and the pits formation.

Investigation and Discussion of Characteristics for Intermetallic Phases Common to Aluminum Alloys as a Function of Solution pH

This paper presents results for corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high strength aluminum-based alloys, for tests conducted in a 0.1 M NaCl solution of varying pH via the use of a microcapillary electrochemical cell. The intermetallics investigated were , , , , , and . Elaboration of the results reveals that the electrochemical behavior of such compounds varies markedly with pH, with attendant ramifications for localized corrosion and protection in Al alloys. Examples of this are shown for AA7075-T651, where it is shown that the localized corrosion morphology is drastically different upon the bulk alloy depending on the pH of the test environment. A stochastic pitting is observed at an acid pH, near-neutral conditions result in a deterministic-type pitting, and a general corrosion is observed at an alkaline pH.

Transitions between pitting and intergranular corrosion in AA2024

The pitting and intergranular corrosion (IGC) behavior of various tempers of AA2024 was investigated in 1 M NaCl. The breakdown potentials associated with pitting or IGC were determined. The breakdown potentials were found to be almost independent of sample orientation for any given temper. Artificial aging had a strong effect on polarization behavior and localized corrosion morphology. The anodic polarization curves of AA2024 in the solution heat treated and water-quenched condition, T3, and T3+ tempers exhibited two breakdown potentials, whereas overaged AA2024-T8, T8+, and solutionized and furnace cooled AA2024 exhibited only one breakdown potential. When two breakdown potentials were observed, the more active one was found to be related to the transient dissolution of S phase Al 2CuMg particles leading to pitting while the noble one was thought to result primarily from initiation and growth of IGC. The breakdown potentials decreased with increasing aging time at 190 °C, and only one breakdown potential was measured for T8 and T8+ tempers. Unlike the T3 temper, no sharp IGC was found for these tempers. Selected granular attack from breakdown of the copper-depleted matrix was believed to be the cause for localized corrosion in the T8 and T8+ tempers. The effect of nitrate and sulfate ions on the localized corrosion behavior was also studied.

Correlation Between Localized Corrosion Morphology And TheState Of Interface Of Coated TiN Thin Film.

Ceramic thin film was widely used due to its superior mechanical and chemical characteristics. However, defects and micro cracks were always existed from the initial stage. In corrosive environment. these defects caused the problem of localized corrosion. Moreover, the evaluations of changes of structure and the state of interface between thin film and substrate which was brought about through employing different coating methods become extremely important. Therefore, TiN thin film coating used in this study was made by two different types of coating methods, that is, plasma CVD and Dynamic Ion Mixing (DIM), which have different interfacial strength. Then, defect morphology and nm or pm orders localized corrosion generated in 3% NaCl aqueous solution was investigated by Atomic Force Microscopy (AFM). As a result, in TiN film coating made by the plasma CVD method, exfoliation of film itself was recognized from the initial stage. The crevice corrosion has been developed from this exfoliation. On the other hand, exfoliation of film itself was not confirmed in thin film coated by the DIM method. Therefore, differences in the initial state of film structure and the generated corrosion morphologies from those defects were observed. And also, improvements in corrosion and exfoliation resistances were recognized in case when superior adhesive state of interface betw.een TiN thin film and substrate were realized. Transactions on Engineering Sciences vol 33, © 2001 WIT Press, www.witpress.com, ISSN 1743-3533

Electrochemical and corrosion behaviour of ion-implanted and pulsed-laser-irradiated metals

Generalized corrosion of metals under active corrosion conditions is hindered by directed-energy-beam radiation. A trend towards a more uniform corrosion (the suppression of galvanic microcells by amorphizing) is observed. Non-spontaneously passivating metals show an improved localized corrosion resistance in a passivating environment. Passivating alloys, in contrast, exhibit a more localized corrosion morphology under pitting attack, after irradiation. Also stress corrosion cracking is accelerated, by the action of galvanic cells, between the cathodic amorphous near-surface layers and the anodic inner unmodified metal.