Electrochemical and mechanical behavior in polycrystalline CoNi-Ga shape memory alloys

"Electrochemical behavior and microstructure was studied in polycrystalline Co38.3Ni32.1Ga29.6 alloy using the electrochemical technique of polarization curves in a medium of 3.5 wt % NaCl and 0.5M H2SO4. The importance of these alloys is due to their functional behavior, as shape memory alloys with ferromagnetic properties. The alloy was fabricated by induction furnace and the characterizations were performed using differential scanning calorimetry (DSC), nanoindenter tests and X-ray diffraction (XRD) The analysis of the kinetics of corrosion was conducted using cyclic sweep voltammetry curves with potentiostat/galvanostat. The corrosion morphologies were also analyzed by scanning electron microscopy (SEM). The kinetics of corrosion was found to be highest in the acid media and the alloy with thermal treatment in 3.5 wt% NaCl (A-TT-NaCl) shows a icorr lower that the alloy without thermal treatment (A-NaCl) in the same solution. On polarization curves, the current alloys exhibited a general dissolution in the anodic branch until certain potential was reached where a spontaneous passive zone occurred in 3.5 wt% NaCl media, similar behavior was found in acid media where, under high potentials occurs a repassivation zone at 427mV and elements such as Co and Ni were present in a higher percentage for all the corrosion deposits.

Corrosion of Titanium Alloys Anodized Using Electrochemical Techniques

The anodization of titanium has been an excellent option for protecting titanium and its alloys from corrosive environments such as acids and chloride systems, by generating a homogenous oxide layer. The objective of the current investigation was to evaluate the electrochemical corrosion behavior of alloys Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V anodized in 1MH2SO4 andH3PO4 solutions at a current density of 2.5   10–3 A/cm2. The anodization’s electrochemical characterization was achieved in NaCl and H2SO4 at 3.5% wt. electrolytes. Scanning electron microscopy (SEM) was employed to determine the anodized thickness and morphology. Cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS), based on ASTM G61-86 and G106-15 Standards, were the electrochemical techniques mainly employed. The anodized samples presented a change in Ecorr values and a higher passivation zone. The EIS plot showed a higher resistance for samples anodized in H3PO4 and Ti-6Al-2Sn-4Zr-2Mo

Transformation temperatures and electrochemical behavior of polycrystalline Fe-Doped Ni-Mn-Ga and Co-Ni-Ga Alloys

"The effect of Fe addition on martensitic transformation temperatures and electrochemical behavior was studied in polycrystalline Ni51.4Mn24.8-XGa23.8FeX alloys (1<x<2.2) and Co38.3Ni32.1Ga29.6 as alternative to Ni-Mn-Ga alloys which are used as ferromagnetic shape memory alloys. The analysis of corrosion rates was conducted by cyclic polarization curves with potentiostat-galvanostat equipment. The corrosion morphologies were also analyzed by scanning electron microscopy (SEM). The kinetics of corrosion was found to decrease with increasing Fe content in the alloy, while the martensitic transformation temperatures increased with increasing Fe content. The Co38.3Ni32.1Ga29.6 alloy shows i(corr) lower than the Ni-Mn-Ga alloy. From results, the studied alloys exhibited a general dissolution in the anodic branch where a spontaneous passive zone occurred at certain potential and some elements like Co, Mn and also Ni were present in a higher percentage in corrosion deposits.

Electrochemical Noise Response of Cr₂Nb Powders Applying Mechanical Alloying

Cr2Nb alloys are potential candidates for high-temperature structural materials. The influence of different mechanical alloying parameters (milling time) and sintering processes were studied. After mechanical alloying and observation by scanning electron microscope (SEM), nano powders were characterized and then sintered by spark plasma sintering (SPS). Electrochemical noise (EN) tests were also conducted in order to study the electrochemical behavior. From the current experimental results, it was revealed that ball milling times up to 20 h may explain the influence of Nb–Cr alloys and its association to the Laves phase and corrosion behavior. These insights aimed at improving the samples’ predicted behavior before spending time and resources at high-temperature industrial processes

Corrosion Behavior of Passivated CUSTOM450 and AM350 Stainless Steels for Aeronautical Applications

Custom 450 stainless steel and AM 350 stainless steel are both precipitation hardening stainless steels, which are widely used in a variety of aerospace applications. The former steel exhibits very good corrosion resistance with moderate strength, whereas the latter is used for applications requiring high strength along with corrosion resistance. In this study, the corrosion behavior of CUSTOM 450 and AM 350 stainless steels passivated in (a) citric acid and (b) nitric acid solutions for 50 and 75 min at 49 and 70 °C, and subsequently exposed in 5 wt. % NaCl and 1 wt. % H2SO4 solutions are investigated. Two electrochemical techniques were used: electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) according to ASTM G199-09 and ASTM G106-13, respectively. The results indicated that passivation in nitric acid made the surface prone to localized corrosion. Statistical and PSD values showed a tendency toward pitting corrosion. On the whole, passivated CUSTOM 450 stainless steel showed the best corrosion behavior in both, NaCl and H2SO4 test solutions

Electrochemical Noise Analysis of the Corrosion of Titanium Alloys in NaCl and H2SO4 Solutions

Titanium alloys have been used in aerospace, aeronautic, automotive, biomedical, structural, and other applications because titanium alloys have less density than materials like steel and support higher stress than Al-alloys. However, components made of titanium alloys are exposed to corrosive environments, the most common being industrial and marine atmospheres. This research shows the corrosion behavior of three titanium alloys, specifically Ti-CP2, Ti-6Al-2Sn-4Zr-2Mo, and Ti-6Al-4V with α, near α, and α + β alloys phases. Alloys were exposed in two electrolytes to a 3.5 wt. % H2SO4 and NaCl solution at room temperature, and their electrochemical behavior was studied by electrochemical noise technique (EN) according to ASTM ASTM-G199 standard. EN signal was filtered by three different methods, and the polynomial method was employed to obtain Rn, kurtosis, skew, and the potential spectral density analysis (PSD). The wavelets method was used, from which energy dispersion plots were obtained. The last method was Hilbert–Huang Transform (HHT), where Hilbert Spectra were analyzed. Results indicated that Rn compared with PSD showed that Ti-6Al-2Sn-4Zr-2Mo presented less dissolution in both electrolytes. Statistical methods showed that the passive layer created on Ti alloys’ surfaces is unstable; this condition is notable for Ti-6Al-2Sn-4Zr-2Mo in NaCl solution

A Comparative Study of Corrosion AA6061 and AlSi10Mg Alloys Produced by Extruded and Additive Manufacturing

The aim of this work was to evaluate the corrosion behavior of the AA6061 and AlSi10Mg alloys produced by extruded and additive manufacturing (selective laser melting, SLM). Alloys were immersed in two electrolytes in H2O and 3.5 wt. % NaCl solutions at room temperature and their corrosion behavior was studied by electrochemical noise technique (EN). Three different methods filtered EN signals, and the statistical analysis was employed to obtain Rn, the localization index (LI), Kurtosis, skew, and the potential spectral density analysis (PSD). The Energy Dispersion Plots (EDP) of wavelets method was employed to determine the type of corrosion and the Hilbert–Huang Transform (HHT), analyzing the Hilbert Spectra. The result indicated that the amplitude of the transients in the time series in potential and current is greater in the AlSi10Mg alloy manufactured by additive manufacturing. The amplitude of the transients decreases in both alloys (AA6061 and AlSi10Mg) as time increases

Electrochemical Corrosion of Titanium and Titanium Alloys Anodized in H2SO4 and H3PO4 Solutions

Titanium and its alloys have superior electrochemical properties compared to other alloy systems due to the formation of a protective TiO2 film on metal surfaces. The ability to generate the protective oxide layer will depend upon the type of alloy to be used. The aim of this work was to characterize the electrochemical corrosion behavior of titanium Ti-CP2 and alloys Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-4V, and Ti Beta-C. Samples were anodized in 1 M H2SO4 and H3PO4 solutions with a current density of 0.025 A/cm2. Electrochemical tests on anodized alloys were carried out using a three-electrode cell and exposed in two electrolytes, i.e., 3.5 wt % NaCl and 3.5 wt % H2SO4 solutions at room temperature. Scanning electron microscopy (SEM) was used to observe the morphology of anodized surfaces. The electrochemical techniques used were cyclic potentiodynamic polarization (CPP) and electrochemical noise (EN), based on the ASTM-G61 and G199 standards. Regarding EN, two methods of data analysis were used: the frequency domain (power spectral density, PSD) and time-frequency domain (discrete wavelet transform). For non-anodized alloys, the results by CCP and EN indicate icorr values of ×10−6 A/cm2. However, under anodizing conditions, the icorr values vary from ×10−7 to ×10−9 A/cm2. The PSD Ψ0 values are higher for non-anodized alloys, while in anodized conditions, the values range from −138/−122 dBi (A2·Hz−1)1/2 to −131/−180 dBi (A2·Hz−1)1/2. Furthermore, the results indicated that the alloys anodized in the H3PO4 bath showed an electrochemical behavior that can be associated with a more homogeneous passive layer when exposed to the 3.5 wt % NaCl electrolyte. Alloys containing more beta-phase stabilizers formed a less homogeneous anodized layer. These alloys are widely used in aeronautical applications; thus, it is essential that these alloys have excellent corrosion performance in chloride and acid rain environment

Aligned TiO2 Scaffolds in the Presence of a Galactopyranose Matrix by Sol-Gel Process

In this work, titanium dioxide scaffolds were synthesized. Titanium isopropoxide (IV) was used as a precursor in its formation, using a polymeric network of galactopyranose as a template. The powder sample obtained was evaluated by scanning tunneling microscopy (STM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, and thermal gravimetric analysis (TGA-DTA). According to the results, it was found that these scaffolds can be successfully synthesized in solution using the sol-gel method. The synthesized scaffolds have diameters from 50 nm with porosity of approximately 0.3&ndash;10 nm. Important parameters, such as pH and the concentration of the metallic precursors, were optimized in this solution. The values of maximum average roughness R(max) and roughness value (Ra) were 0.50 and 1.45, respectively. XRD diffraction analysis shows the formation of crystalline phases in the TiO2 scaffold at 700 &deg;C. The use of biological polymers represents an alternative for the synthesis of new materials at low cost, manipulating the conditions in the production processes and making the proposed system more efficient

Corrosion Behavior of Passivated Martensitic and Semi-Austenitic Precipitation Hardening Stainless Steel

This research aimed to conduct a passive layer state study on martensitic and semi-austenitic precipitation hardening stainless steels (PHSS) passivated in citric acid and nitric acid baths at 49 and 70 °C for 50 and 75 min and subsequently exposed in 5 wt.% NaCl and 1 wt.% H2SO4 solutions. Corrosion behavior of the passivated material was observed by using potentiodynamic polarization (PP) according to the ASTM G5-11 standard. The microstructural analysis was performed by optical microscopy and scanning electron microscopy (SEM), while the passivated layer was characterized by X-ray photoelectron spectroscopy (XPS). The results indicated that the semi-austenitic-NA-50 min-70 °C sample showed the best corrosion resistance behavior in both solutions. The XPS characterization confirmed that the martensitic and semi-austenitic surface film presented a mixture of chemical compounds, such as Cr2O3 and Fe(OH)O, respectively