Improved pitting corrosion resistance of S.S 316L by Pulsed Current Gas Tungsten Arc Welding

In this study, S.S 316L was welded using Direct Current Gas Tungsten Arc Welding (DGTAW) and Pulsed Current Gas Tungsten Arc Welding (PGTAW) methods. Optical observations, scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD) were employed to study the effect of continuous and pulse currents on microstructure and phase transformation in weld metal (WM). In addition pits morphology were evaluated by SEM. The corrosion behaviour was analyzed using cyclic polarizaton tests and Mott-schottky measurements. The pulse current resulted in finer grain and more ferrite in WM. This can be due to the decrease in heat input and higher cooling rate encouraged by pulse current. Cyclic potentiodynamic polarization tests showed that the WM of sample produced by pulse current show higher corrosion and pitting resistances than that in sample produced by continuous current. The reason is attributed to lower segregation of solute elements such as chromium and molybdenum into the delta-ferrite and also finer grain size produced in WM due to lower heat input and higher cooling rate. Both of these factors increase the stability of passive layer formed. The results showed that the corrosion behaviour of WM in both conditions (pulse and continuous current) is higher than the base metal (BM). This fact is attributed to the presence of ferrite bands formed in BM due to the segregation of alloy elements. The Mott-schottky plots confirmed that the passive layer formed on welded samples was an n-type semiconductor. The results showed that the samples showed less pitting resistance contained more oxygen vacancies in their passive film structure. It is also concluded that the breakdown of passive layer and pitting formation obey point defect model (PDM). Keywords: S.S 316L, Pulsed Current Gas Tungsten Arc Welding (PGTAW), lacy ferrite, vermicular ferrite, Pitting corrosion, Mott- Schottky

The effect of electrolytic solution composition on the structure, corrosion, and wear resistance of peo coatings on az31 magnesium alloy

Plasma electrolytic oxidation coatings were prepared in aluminate, phosphate, and silicate-based electrolytic solutions using a soft-sparking regime in a multi-frequency stepped process to compare the structure, corrosion, and wear characteristics of the obtained coatings on AZ31 magnesium alloy. The XRD results indicated that all coatings consist of MgO and MgF2, while specific products such as Mg2 SiO4, MgSiO3, Mg2 P2 O7, and MgAl2 O4 were also present in specimens based on the selected solution. Surface morphology of the obtained coatings was strongly affected by the electrolyte composition. Aluminate-containing coating showed volcano-like, nodular particles and craters distributed over the surface. Phosphate-containing coating presented a sintering-crater structure, with non-uniform distributions of micro-pores and micro-cracks. Silicate-containing coating exhibited a scaffold surface involving a network of numerous micro-pores and oxide granules. The aluminate-treated sample offered the highest corrosion resistance and the minimum wear rate (5 × 10−5 mm3 N−1 m−1), owing to its compact structure containing solely 1.75% relative porosity, which is the lowest value in comparison with other samples. The silicate-treated sample was degraded faster in long-term corrosion and wear tests due to its porous structure, and with more delay in the phosphate-containing coating due to its larger thickness (30 µm)

Incorporation mechanism of colloidal TiO2 nanoparticles and their effect on properties of coatings grown on 7075 Al alloy from silicate-based solution using plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was applied using a pulsed unipolar waveform to produce Al2O3-TiO2 composite coatings from sol electrolytic solutions containing colloidal TiO2 nanoparticles. The sol solutions were produced by dissolving 1, 3, and 5 g/L of potassium titanyl oxalate (PTO) in a silicate solution. Scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and Raman spectroscopy were applied to characterizing the coatings. Corrosion behavior of the coatings was investigated using polarization and impedance techniques. The results indicated that TiO2 enters the coating through all types of micro-discharging and is doped into the alumina phase. The higher level of TiO2 incorporation results in the decrease of surface micro-pores, while the lower incorporation shows a reverse effect. It was revealed that the higher TiO2 content makes a more compact outer layer and increases the inner layer thickness of the coating. Electrochemical measurements revealed that the coating obtained from the solution containing 3 g/L PTO exhibits higher corrosion performance than that obtained in the absence of PTO. The coating produced in the absence of PTO consists of gamma-Al2O3, delta-Al2O3 and amorphous phases, while alpha-Al2O3 is promoted by the presence of PTO

Corrosion and wear resistance of coatings produced on AZ31 Mg alloy by plasma electrolytic oxidation in silicate-based K2TiF6 containing solution: Effect of waveform

In this research, plasma electrolytic oxidation coatings were prepared on AZ31 Mg alloy in a silicate-based solution containing K2TiF6 using bipolar and soft sparking waveforms with 10, 20, and 30% cathodic duty cycles. The coatings displayed a net-like surface morphology consisted of irregular micro-pores, micro-cracks, fused oxide particles, and a sintered structure. Due to the incorporation of TiO2 colloidal particles and the cathodic pulse repair effect, most of the micro-pores were sealed. Long-term corrosion performance of the coatings was investigated using electrochemical impedance spectroscopy during immersion in 3.5 wt.% NaCl solution up to 14 days. The coating grown by the soft sparking waveform with a 20% cathodic duty cycle having the lowest porosity (6.2%) and a sharp layer concentrated in F element at the substrate/coating interface shows the highest corrosion resistance. The friction coefficient of this coating has remained stable during the sliding even under 5 N normal load, showing relatively higher wear resistance than other coatings. The coating produced using the equivalent unipolar waveform, as the reference specimen, showed the highest friction coefficient and the lowest wear resistance despite its highest micro-hardness

Silicate and Hydroxide Concentration Influencing the Properties of Composite Al2 O3-TiO2 PEO Coatings on AA7075 Alloy

This work evaluates the effect of sodium meta-silicate pentahydrate (SMS) and potassium hydroxide concentrations on properties of Al2O3-TiO2 coatings produced through plasma electrolytic oxidation in a solution containing 3 g L−1 potassium titanyl oxalate, (PTO), using a unipolar waveform with constant current density. The surface and cross-section characteristics of PEO coatings including morphology, elemental distribution, and phase composition were evaluated using FESEM, EDS, and XRD techniques. Voltage-time response indicated the concentration of SMS and KOH had a significant effect on the duration of each stage of the PEO process. More cracks and pores were formed at the higher concentrated solutions that resulted in the incorporation of solution components especially Si into the coating inner parts. Ti is distributed throughout the coatings, but it had a dominant distribution in the Si-rich areas. The coating prepared in the electrolyte containing no silicate consisted of non-stoichiometric γ-Al2O3 and/or amorphous Al2O3 phase. Adding silicate into the coating electrolyte resulted in the appearance of α-Al2O3 besides the dominant phase of γ-Al2O3. The corrosion behaviour of the coatings was investigated using the EIS technique. It was found that the coating prepared in the presence of 3 g L−1 SMS and 2 g L−1 KOH, possessed the highest barrier resistance (~10 MΩ cm2), owing to a more compact outer layer, thicker inner layer along with appropriate dielectric property because this layer lacks the Si element. It was discovered that the incorporation of Ti4+ and especially Si4+ in the coating makes the dielectric loss in the coating

Charging Phenomena at the Interface Between High-k Dielectrics and SiOx Interlayers, Journal of Telecommunications and Information Technology, 2010, nr 1

The transition regions of GdSiO/SiOx and HfO2/SiOx interfaces have been studied with the high-k layers deposited on silicon substrates. The existence of transition regions was verified by medium energy ion scattering (MEIS) data and transmission electron microscopy (TEM). From measurements of thermally stimulated current (TSC), electron states were found in the transition region of the HfO2/SiOx structures, exhibiting instability attributed to the flexible structural molecular network expected to surround the trap volumes. The investigations were focused especially on whether the trap states belong to an agglomeration consisting of a single charge polarity or of a dipole constellation. We found that flat-band voltage shifts of MOS structures, that reach constant values for increasing oxide thickness, cannot be taken as unique evidence for the existence of dipole layers

Association between the number of coadministered P-glycoprotein inhibitors and serum digoxin levels in patients on therapeutic drug monitoring

BACKGROUND: The ABC transporter P-glycoprotein (P-gp) is recognized as a site for drug-drug interactions and provides a mechanistic explanation for clinically relevant pharmacokinetic interactions with digoxin. The question of whether several P-gp inhibitors may have additive effects has not yet been addressed. METHODS: We evaluated the effects on serum concentrations of digoxin (S-digoxin) in 618 patients undergoing therapeutic drug monitoring. P-gp inhibitors were classified as Class I, with a known effect on digoxin kinetics, or Class II, showing inhibition in vitro but no documented effect on digoxin kinetics in humans. Mean S-digoxin values were compared between groups of patients with different numbers of coadministered P-gp inhibitors by a univariate and a multivariate model, including the potential covariates age, sex, digoxin dose and total number of prescribed drugs. RESULTS: A large proportion (47%) of the digoxin patients undergoing therapeutic drug monitoring had one or more P-gp inhibitor prescribed. In both univariate and multivariate analysis, S-digoxin increased in a stepwise fashion according to the number of coadministered P-gp inhibitors (all P values < 0.01 compared with no P-gp inhibitor). In multivariate analysis, S-digoxin levels were 1.26 ± 0.04, 1.51 ± 0.05, 1.59 ± 0.08 and 2.00 ± 0.25 nmol/L for zero, one, two and three P-gp inhibitors, respectively. The results were even more pronounced when we analyzed only Class I P-gp inhibitors (1.65 ± 0.07 for one and 1.83 ± 0.07 nmol/L for two). CONCLUSIONS: Polypharmacy may lead to multiple drug-drug interactions at the same site, in this case P-gp. The S-digoxin levels increased in a stepwise fashion with an increasing number of coadministered P-gp inhibitors in patients taking P-gp inhibitors and digoxin concomitantly. As coadministration of digoxin and P-gp inhibitors is common, it is important to increase awareness about P-gp interactions among prescribing clinicians

Inhibitive effect of sodium (E)-4-(4-nitrobenzylidenamino) benzoate on the corrosion of some metals in sodium chloride solution

The inhibition performance of a novel anionic carboxylic Schiff base, sodium (E)-4-(4-nitrobenzylideneamino)benzoate (SNBB), was investigated for various metals, namely low carbon steel F111, pure iron and copper, in neutral 10 mM NaCl solution. Potentiodynamic polarization, scanning vibrating electrode technique (SVET), quantum chemical (QC) calculation, and molecular dynamics (MD) simulation were employed. The potentiodynamic polarization data showed that SNBB acts as an effective corrosion inhibitor for both iron and F111 steel, but it is not effective for the copper. In situ spatially-resolved SVET maps evidenced a major change in surface reactivity for Fe and F111 steel immersed in 10 mM aqueous solution in the absence and in the presence of SNBB. Featureless ionic current density distributions were recorded in the presence of SNBB at both their spontaneous open circuit potential (OCP) and under mild anodic polarization conditions, while major ionic flows were monitored above the metals in the absence of SNBB. On the basis of computer simulations, it is proposed that SNBB produces a stable chelate film on iron and steel surfaces that accounts for the good corrosion inhibition efficiency observed. The different inhibition efficiencies of SNBB molecules on the iron and copper was attributed to the special chemical structure of SNBB molecule and its different chelation ability with the released metal ions on the metal surface. The QC calculations also confirmed the high corrosion inhibition efficiency of SNBB. The MD simulation indicated higher binding energy of SNBB on iron surface compared to that of copper surface. The interaction mode of SNBB on iron and F111 steel surfaces corresponds to a mixed chemical and physical adsorption, and it obeys the Langmuir isother

Effect of pulse current mode on microstructure, composition and corrosion performance of the coatings produced by plasma electrolytic oxidation on AZ31 Mg alloy

Plasma electrolytic oxidation (PEO) coatings were grown on AZ31 Mg alloy in a silicate-based electrolyte containing KF using unipolar and bipolar (usual and soft-sparking) waveforms. The coatings were dual-layered consisting of MgO, MgF2 and Mg2SiO4 phases. Surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks and granules of oxide compounds. Deep pores were observed in the coating produced by unipolar and usual bipolar waveforms. The soft-sparking eliminated the deep pores and produced the lowest porosity in the coatings. It was found that the corrosion performance of the coatings evaluated using EIS in 3.5 wt. % NaCl solution is mostly determined by the inner layer resistance, because of its higher compactness. After 4 days of immersion, the inner layer resistances were almost the same for all coatings. However, the coatings produced by unipolar and usual bipolar waveforms showed sharp decays in inner layer resistances after 1 week and even the barrier eect of outer layer was lost for the unipolar-produced coating after 3 weeks. The low-frequency inductive loops appeared after a 3-week immersion for all coatings indicated that the substrate was under local corrosion attack. However, both coatings produced by soft-sparking waveforms provided the highest corrosion performance