Joanna Michalska

Surface characterisation of Ti–15Mo alloy modified by a PEO process in various suspensions

This paper reports on the surface modification of a Ti-15Mo alloy by plasma electrolytic oxidation (PEO). This process was carried out in solutions of 0.1M Ca(H2PO2)2 with various concentrations of tricalcium phosphate (Ca3(PO4)2), wollastonite (CaSiO3), or silica (SiO2) using voltages of up to 350V. The surface microstructure (SEM, cross-section of coating), roughness and chemical composition (energy-dispersive X-ray spectroscopy, thin layer X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy) of the porous oxide layers were investigated. The concentration of powder added to the solution changed the chemical composition and morphology of PEO coatings on the Ti-15Mo alloy surface. Calcium and phosphorous compounds were detected in the coatings formed on the substrate by the PEO process at 300V.

Modification of niobium surfaces using plasma electrolytic oxidation in silicate solutions

Herein, a study of the plasma electrolytic oxidation (PEO) of niobium in an anodising bath composed of potassium silicate (K2SiO3) and potassium hydroxide (KOH) is reported. The effects of the K2SiO3 concentration in the bath and the process voltage on the characteristics of the obtained oxide layers were assessed. Compact, barrier-type oxide layers were obtained when the process voltage did not exceed the breakdown potential of the oxide layer. When this threshold was breached, the morphology of the oxide layer changed markedly, which is typical of PEO. A significant amount of silicon, in the form of amorphous silica, was incorporated into the oxide coatings under these conditions compared with the amount obtained with conventional anodising. This surface modification technique led to an improvement in the corrosion resistance of niobium in Ringer’s solution, regardless of the imposed process conditions.

Formation of bioactive coatings on Ti–13Nb–13Zr alloy for hard tissue implants

In an attempt to increase the bioactivity of a vanadium-free titanium alloy, Ti–13Nb–13Zr, the plasma electrolytic oxidation (PEO) process for surface modification was utilised. Select samples were subjected to further treatment, either thermal or alkali. The morphology, chemical composition, and phase composition of the treated Ti–13Nb–13Zr alloy were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was observed that during the anodic process under sparking discharge conditions, the simultaneous incorporation of calcium and phosphorus in the forming oxide layer occurs. The resulting layers were porous and exhibited the typical morphology for layers formed during the PEO process. After the alkali treatment of samples oxidised at 150 V, a gel-like titanate layer was formed. The bioactivity investigations in simulated body fluid (SBF) solution indicated that after anodising at 150 V and following alkali treatment the Ti–13Nb–13Zr alloy exhibits osteoinductive properties. The approach presented here may be applied for fabricating Ti–13Nb–13Zr-based implants for hard tissue regeneration.

Influence of electropolishing and anodic oxidation on morphology, chemical composition and corrosion resistance of niobium

The work presents results of the studies performed on electropolishing of pure niobium in a bath that contained: sulphuric acid, hydrofluoric acid, ethylene glycol and acetanilide. After the electropolishing, the specimens were subjected to anodic passivation in a 1moldm(-3) phosphoric acid solution at various voltages. The surface morphology, thickness, roughness and chemical composition of the resulting oxide layers were analysed. Thusly prepared niobium samples were additionally investigated in terms of their corrosion resistance in Ringers solution. The electropolished niobium surface was determined to be smooth and lustrous. The anodisation led to the growth of barrier-like oxide layers, which were enriched in phosphorus species.

Phase analysis in duplex stainless steel: comparison of EBSD and quantitative metallography methods

The purpose of the research was to work out the qualitative and quantitative analysis of phases in DSS in as-received state and after thermal aging. For quantitative purposes, SEM observations, EDS analyses and electron backscattered diffraction (EBSD) methods were employed. Qualitative analysis of phases was performed by two methods: EBSD and classical quantitative metallography. A juxtaposition of different etchants for the revealing of microstructure and brief review of sample preparation methods for EBSD studies were presented. Different ways of sample preparation were tested and based on these results a detailed methodology of DSS phase analysis was developed including: surface finishing, selective etching methods and image acquisition. The advantages and disadvantages of applied methods were pointed out and compared the accuracy of the analysis phase performed by both methods.

Biofilm Formation on NiTi Surface by Different Strains of Sulphate Reducing Bacteria (Desulfovibrio desulfuricans)

Bacteria of Desulfovibrio genus belong to group of widespread sulphate-reducing bacteria (SRB). D. desulfuricans is considered one among many bacterial species involved in microbiologically influenced corrosion (MIC) of metals, mainly of stainless steels and other alloys. SRB can produce gaseous hydrogen sulphide. This gas is released into the environment leading to formation of metal sulphides that significantly influence electrochemical processes and ultimately enhance the corrosion of materials. Biofilms formed by these bacteria are especially harmful for highly alloyed steels and many alloys. The aim of this work was to compare the character of growth and biofilm formation by three strains of D. desulfuricans (standard soil strain DSM and two wild intestinal strains: DV/A and DV/B) on the surface of NiTi alloy.

The Effect of Pre-Hydrogen Charging on Corrosion Behaviour of Superduplex Stainless Steel Welded Joints

The paper presents results of research undertaken to determine the influence of hydrogen on passivity and corrosion resistance of 2507 super duplex stainless steel welded joints. Butt welded joints were made with low heat input using flux corded arc welding (FCAW) method. Coupons of 5.0 x 5.0 mm were cut from the welded joints containing weld metal (WM), heat affected zone (HAZ) and parent metal (PM). Hydrogen was introduced into the samples by cathodic current method under galvanostatic condition at room temperature. Corrosion resistance was qualified with the polarization curves registered in synthetic sea water. Electrochemical hydrogen charging affected the native passive layer. The internal hydrogen shifted corrosion potential to the more active direction and cause an increase in corrosion current density. The corrosion sensitivity increases for higher current density applied during hydrogen charging. Weld metal area has been revealed as the most sensitive to corrosion attack.

The Corrosion Resistance of the Mg-Al-Ca-Sr Sand Casting Magnesium Alloys

In this paper, the corrosion resistance of two sand-casting creep resistant magnesium alloys Mg-9Al-1.5Ca-0.3Sr and Mg-9Al-2.2Ca-0.8Sr in the salt environment has been investigated. Specimens of each alloy has been immersed in 3.5% NaCl solution at room temperature and successively taken out after 1, 2, 4, 5 and 9 days. After immersion test, the microstructure and the appearances of the corroded structure were examined. The corrosion rates of both investigated alloys increased lineally with increasing the exposure time in both solutions. Mg-9Al-1.5Ca-0.3Sr alloy exhibits the higher corrosion rate during the immersion test than Mg-9Al-2.2Ca-0.8Sr. The corrosion layer of both alloys consists of MgO, MgOH and phases containing Cl, Na, Al and Ca. The increase of Ca content in the Mg-9Al-2.2Ca-0.8Sr alloy improved the corrosion resistance due to the formation of the reticular (Mg,Al)2Ca phase, which acted as an effective barrier against corrosion.

Microbial Aspects in Corrosion Studies of Stainless Steels

Microbially influenced corrosion (MIC) has been focusing increasing attention from different research areas in the last years, as an answer to the demand of wide variety of industries and degradation cases. The difficulty in reaching an adequate understanding of corrosion processes induced by microorganisms has result in cross-fertilization of ideas between researchers from different disciplines like microbiology, electrochemistry, metallurgy and materials engineering. Different MIC mechanisms can be simultaneously or alternately occuring on various materials, where a complex boundary layer of corrosion products, bacterial cells and other environmental factors are present on the surface. The paper discusses new aspects of MIC mechanisms induced by sulphate reducing bacteria species, taking into consideration modern techniques and new approaches in the study. Practical cases concerning microbial risks, failures illustrate the complexity of this phenomena: from local corrosion problems through stress corrosion cracking and hydrogen damage. The role of microstructure and chemical composition of materials is also highlited as a key factor in microbial damaging. In addition to this, the principal morphologies of attack and cracking are described.

Removal and transformation of benzotriazole in manganese-oxide biofilters with Mn(II) feeding

The aim of this study was to evaluate the treatment of organic-carbon-deficient wastewater containing benzotriazole (BTA) in lab-scale aerated biofilters filled with natural manganese oxide ore, sand coated with synthetic manganese oxides and sand (as a control material) in terms of BTA removal efficiency, its transformation products and ecotoxicological impact of the treated wastewater. Additionally, the effect of Mn(II) feeding was tested. The removal of BTA in all the biofilters was ≥97%. The contribution of the biotic removal of this compound was 15%, 50%, and 75% in the systems filled with sand, synthetic and natural manganese oxides, respectively. Only the columns filled with natural manganese oxides provided significant removal of DOC and decrease of UV254 and SUVA254, with even more pronounced effect with Mn(II) feeding. The presence of Mn(II) was also found to enhance the removal of NNH4 in the systems filled with either form of manganese oxides, otherwise the removal of NNH4 was negligible or negative. The transformation reactions of BTA were methylation, hydroxylation, and triazole ring cleavage. Based on the number of compounds and their relative abundance, the methylated transformation products were predominant in the effluent. The reduction of the ecotoxicity (Microtox bioassay) of the effluents was positively correlated with the decrease of UV254, SUVA and DOC and only moderately with the removal of BTA. This study has shown that the natural manganese oxide ore provides the broadest set of services as a filtering material for aerated biofilters treating carbon-deficient wastewater containing BTA.