Tadeusz Hryniewicz

Magnetoelectropolishing for metal surface modification

Abstract This paper aims to present the preliminary results of magnetoelectropolishing, a process derived and named by application of an external magnetic field coupled with the process being controlled and maintained under oxygen evolution to achieve an electropolished surface of a workpiece. The obtained surfaces exhibit reduced microroughness, better surface wetting resulting from increased surface energy, reduced and more uniform corrosion resistance, minimisation of external surface soiling and improved cleanability. The externally applied magnetic force for use with the enhanced electropolishing process may be selected from a group consisting of either permanent magnetic or electromagnetic devices. It has been shown that the enhanced electropolishing process results in better surface performance. In the present paper, for comparison purposes, results of surface roughness, hydrophilicity and corrosion behaviour are presented. Further studies, concerning the corrosion characteristics, the morphology and surface film chemistry, are in progress.

Enhanced oxidation-dissolution theory of electropolishing

Abstract Electropolishing is the electrolytic metal finishing process currently widely used in several high tech applications such as cardiovascular and orthopaedic body implants, pharmaceutical and semiconductor installations, superconductive niobium cavities, among others. The process provides a very clean, smooth, Beilby layer free, corrosion resistant surface. Currently, almost any metal, alloy and intermetallic compound can be electropolished, but in spite of that, we still do not have a single commonly accepted electropolishing theory. To make it even more complicated, the electropolishing process is constantly modified by addition of other physical agents and/or forces such as a magnetic field – magnetoelectropolishing, ultrasounds, or by changing the existing process parameters such as switching from direct to pulse current. The existing electropolishing theories differ in many ways from each other but possess one common ingredient, namely the viscous layer. The aim of this work is to show that the viscous layer is not an indispensible prerequisite to achieve a satisfactory electropolishing finish in every metal–electrolyte system. A supplement to the most broadly accepted electropolishing solid film theory by Hoar, namely the enhanced oxidation–dissolution equilibrium theory, is proposed.

Concept of microsmoothing in the electropolishing process

Abstract Electropolishing of metals and numerous alloys can provide smooth, bright and reflective surfaces that exhibit superior corrosion resistance compared with untreated materials. These features are achieved mainly after electropolishing pure metals and single-phase alloys. Earlier researchers have indicated that, in comparison with other treatments (mechanical polishing, chemical etching), electropolishing produces a much thinner surface layer with changed and specific properties. This implies a difference in the solid surface chemistry which is valuable from the viewpoint of the technical usefulness of such surfaces. Experimental investigations carried out on various metals and alloys have proved that both electrochemical and hydrodynamic conditions are responsible for the positive results obtained in the process. The aim of the present work is to provide a new concept of electropolishing covering transport phenomena such as diffusion and adsorption as well as the hydrodynamic conditions.

Biomaterial Studies on AISI 316L Stainless Steel after Magnetoelectropolishing

The polarisation characteristics of the electropolishing process in a magnetic field (MEP – magnetoelectropolishing), in comparison with those obtained under standard/conventional process (EP) conditions, have been obtained. The occurrence of an EP plateau has been observed in view of the optimization of MEP process. Up-to-date stainless steel surface studies always indicated some amount of free-metal atoms apart from the detected oxides and hydroxides. Such a morphology of the surface film usually affects the thermodynamic stability and corrosion resistance of surface oxide layer and is one of the most important features of stainless steels. With this new MEP process we can improve metal surface properties by making the stainless steel more resistant to halides encountered in a variety of environments. Furthermore, in this paper the stainless steel surface film study results have been presented. The results of the corrosion research carried out by the authors on the behaviour of the most commonly used material − medical grade AISI 316L stainless steel both in Ringer’s body fluid and in aqueous 3% NaCl solution have been investigated and presented earlier elsewhere, though some of these results, concerning the EIS Nyquist plots and polarization curves are also revealed herein. In this paper an attempt to explain this peculiar performance of 316L stainless steel has been undertaken. The SEM studies, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were performed on 316L samples after three treatments: MP – abrasive polishing (800 grit size), EP – conventional electrolytic polishing, and MEP – magnetoelectropolishing. It has been found that the proposed magnetoelectropolishing (MEP) process considerably modifies the morphology and the composition of the surface film, thus leading to improved corrosion resistance of the studied 316L SS.

Nanoindentation and XPS Studies of Titanium TNZ Alloy after Electrochemical Polishing in a Magnetic Field

This work presents the nanoindentation and XPS results of a newly-developed biomaterial of titanium TNZ alloy after different surface treatments. The investigations were performed on the samples AR (as received), EP (after a standard electropolishing) and MEP (after magnetoelectropolishing). The electropolishing processes, both EP and MEP, were conducted in the same proprietary electrolyte based on concentrated sulfuric acid. The mechanical properties of the titanium TNZ alloy biomaterial demonstrated an evident dependence on the surface treatment method, with MEP samples revealing extremely different behavior and mechanical properties. The reason for that different behavior appeared to be influenced by the surface film composition, as revealed by XPS study results displayed in this work. The increase of niobium and zirconium in the surface film of the same titanium TNZ alloy after magnetoelectropolishing MEP treatment is meaningful and especially advantageous considering the application of this alloy as a biomaterial.

A Proposal for Simplifying the Method of Evaluation of Uncertainties in Measurement Results

Abstract The paper deals with the innovative ways of nonstandard, simplifying applications of the valid method for evaluating uncertainties in measurement results and with the definition of conditions of their usability. The evaluation of a substitute criterion for measurement accuracy by means of a relative difference between the measurand and its reference value is proposed. This nonstandard relative uncertainty is comparable with the overall relative standard uncertainty in the measurement result, and thus the evaluation of it enables other simplifications in the calculations of measurement result uncertainties. The use of the simplified evaluation of measurement results is illustrated in two experiments in measurement of the coefficient of thermal conductivity of an insulating material newly developed for the needs of building practice, namely measurement using commercial instruments, and measurement using a newly developed original measuring instrument.

XPS measurements of LDX 2101 duplex steel surface after magnetoelectropolishing

Abstract The work presents the X-ray photoelectron spectroscopy measurement results of LDX 2101 duplex steel surface after magnetoelectropolishing in comparison with the results obtained after a standard electropolishing in still electrolyte and in the electrolyte with moderate mixing. A considerable improvement of the steel surface was found to appear after the magnetoelectropolishing treatment concerning the general composition. The calculated ratio of chromium compounds to iron compounds Cr-X/FeX equals 2 after magnetoelectropolishing and only 0.5 after a standard electropolishing. On the other hand, the ionic Cr6+ content after magnetoelectropolishing was the lowest (2.77 at.%), and after a standard electropolishing it was the highest (30.3 at.%). Such results are expected to influence the corrosion behavior of the steel and increase the corrosion resistance greatly.

Non-contact method for surface roughness measurement after machining

Abstract The paper deals with the measurement and identification of surfaces after machining in a non-contact manner. It presents a new modified measurement method and its implementation, the results of intensity distribution in the defocusing plane, their analysis and interpretation. The scanned intensity distribution at the defocusing plane gives information necessary to assess the second derivatives, and thus, surface functions which can be used to determine groove curvatures of the real surface morphology. The proposed method of measurement has proved to be very sensitive in evaluating the differences between surface finishing methods by which the measured surface standards (etalons) were machined. Two methods of machining were chosen: face grinding and planning. By comparing the roughness standard values Ra, there were obtained relationships between these values and the parameter of the characteristic frequency of vertical inequality being measured according to the presented method. A good correlation between the measured and surface standard values with the correlation coefficient taking a range of values from 0.8 to 1 was achieved.

SEM, EDS and XPS Analysis of the Coatings Obtained on Titanium after Plasma Electrolytic Oxidation in Electrolytes Containing Copper Nitrate

In the paper, the Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) results of the surface layer formed on pure titanium after plasma electrolytic oxidation (micro arc oxidation) at the voltage of 450 V are shown. As an electrolyte, the mixture of copper nitrate Cu(NO3)2 (10–600 g/L) in concentrated phosphoric acid H3PO4 (98 g/mol) was used. The thickness of the obtained porous surface layer equals about 10 μm, and it consists mainly of titanium phosphates and oxygen with embedded copper ions as a bactericidal agent. The maximum percent of copper in the PEO surface layer was equal to 12.2 ± 0.7 wt % (7.6 ± 0.5 at %), which is the best result that the authors obtained. The top surface layer of all obtained plasma electrolytic oxidation (PEO) coatings consisted most likely mainly of Ti3(PO4)4∙nH3PO4 and Cu3(PO4)2∙nH3PO4 with a small addition of CuP2, CuO and Cu2O.

Towards a Better Corrosion Resistance and Biocompatibility Improvement of Nitinol Medical Devices

Haemocompatibility of Nitinol implantable devices and their corrosion resistance as well as resistance to fracture are very important features of advanced medical implants. The authors of the paper present some novel methods capable to improve Nitinol implantable devices to some marked degree beyond currently used electropolishing (EP) processes. Instead, a magnetoelectropolishing process should be advised. The polarization study shows that magnetoelectropolished Nitinol surface is more corrosion resistant than that obtained after a standard EP and has a unique ability to repassivate the surface. Currently used sterilization processes of Nitinol implantable devices can dramatically change physicochemical properties of medical device and by this influence its biocompatibility. The Authors’ experimental results clearly show the way to improve biocompatibility of NiTi alloy surface. The final sodium hypochlorite treatment should replace currently used Nitinol implantable devices sterilization methods which rationale was also given in our previous study.