Abstract:© Versita Sp. z o.o. Keywords: Titanium • Nitriding • Electrochemical corrosion • Depth-profile Research ArticleThick (400 µm) glow-discharge nitrided layers, TiN+Ti 2 N + αTi(N) type, have been produced on the Ti-1Al-1Mn titanium alloy. Using a progressive thinning method, the polarization characteristics at different depths of nitrided layers have been measured. From the plots of obtained potentiodynamic polarization curves the depth profiles of characteristic anodic and cathodic currents (at potentials corresponding to (a) hydride formation, (b) hydrogen evolution, (c) primary passivation, (d) oxygen evolution and (e) secondary passivation) as well as polarization resistance have been determined in 0.5 M Na 2 SO 4 solution acidified to pH = 2. The anomalously high slope of the polarization curves in the cathodic region has been ascribed to the formation of titanium hydride. It has been shown that outer nitrided layers (up to 25 µm) exhibit excellent acid corrosion resistance owing to strong inhibition of the anodic process by TiN phase. Corrosion resistance of deeper situated layers gradually decreases and at depths of 250-370 µm the corrosion process is accelerated by presence of TiO 2 precipitations. Nitrided layers, unlike the alloy core, allow oxygen evolution on the oxy-nitrided surface at potential of +1.6 V and at more positive potentials gradual transformation of the surfacial film into TiO 2 takes place. Secondary passivation on nitrided titanium is less efficient than that in the absence of Ti-N species.

Al‐enriched surface layers containing a Mg17Al12 intermetallic phase and a solid solution of Al in Mg were fabricated by heating Mg specimens in contact with Al powder in a vacuum furnace. The layer formation process proceeded through partial melting at the Mg‐substrate/Al‐powder interface. The test results suggest that a good contact between the Al powder and the Mg substrate is required during heat treatment. In this study, a pressure of 1 MPa was applied to improve the contact of the Al powder with the Mg specimen. When the powder was pressed down during heating, it was possible to reduce the process temperature from 450 °C to 440 °C. The layers produced at 440 °C in a short heating time (40 min) were thick, continuous and uniform. The microhardness of the Al‐enriched layers was much higher than that of the Mg substrate. The results of the electrochemical corrosion tests indicated that the Mg specimens with an Al‐enriched surface layer had better corrosion resistance than the bare Mg. Copyright © 2014 John Wiley & Sons, Ltd.

This paper presents the results of effects of boronizing process of AISI 321 austenitic stainless steel on its electrochemical corrosion resistance. The steel samples were boronized at a temperature of 900 • C for 6 h. In this process, a mixture powder (B4C, SiC, borax), alumina (Al2O3) and potassium fluoroborate (KBF4) was used. The evaluation of the corrosion resistance of boronized stainless steel was carried out by using the so-called progressive thinning method, consisting in determination of polarisation characteristics on increasingly-deeper situated regions of the top layer. This method made it possible to determine changes in particular corrosion parameters read out from potentiodynamic polarisation curves, thus enabling the depth profiles of these parameters. Potentiodynamic polarization tests were carried out in an 0.5 M sulphate solution acidified to pH = 2.0. It is shown that unlike unmodified steel, the boron-rich outer layer does not passivate but undergoes fast dissolution both in active-and transpassive regions in the corrosion solution.

The alloyed surface layers were produced using thin Al or AlSi20 plates, which were first diffusion bonded and then melted into the Mg substrate. The microscopic analysis showed that alloying with Al led to the formation of an Al-enriched surface layer containing a high volume fraction of an Mg 17 Al 12 phase. Applying AlSi20, on the other hand, resulted in the formation of an Al/Sienriched layer containing Mg 2 Si, Mg 2 Al 3 and Mg 17 Al 12 phases. The experimental results indicate that surface alloying of Mg with Al or AlSi20 using the presented method improved the properties of the material. The microhardness of the Al-and Al/Si-enriched layers was much higher than that of the substrate. Higher wear resistance was observed for the Al/Si-enriched layer. The alloyed layers provided a certain level of protection of Mg against corrosion. The specimens alloyed with Al showed better anticorrosive properties than those alloyed with AlSi20.

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Electrochemical polarisation characteristics of AISI 405 type stainless steel, carbonitrided in fluid flow, are presented. The evaluation of the corrosion resistance of carbonitrided stainless steel was carried out by using the so called progressive thinning method, consisting in determination of polarisation characteristics on increasingly-deeper situated regions of the top layer. This method made it possible to determine changes in particular corrosion parameters read out from potentiokinetic polarisation curves, thus enabling the depth profiles of these parameters. The resistance of the AISI 405 steel against acid corrosion was determined in acidified 0.5M sulphate solutions having three different pH values, namely 1, 2 and 4. The thickness of the carbonitrided layer has been evaluated on the basis of the spectrophotometric analysis of carbon content and microhardness on the cross section of the surfacial layer.Keywords: corrosion, depth profile, stainless steel, carbonitrided, fluid flow W pracy przedstawiono elektrochemiczne charakterystyki polaryzacyjne stali X6Cr13 (AISI 405) poddanej węgloazo-towaniu w złożu fluidalnym. Ocenę odporności korozyjnej otrzymanej w wyniku obróbki cieplno-chemicznej warstwy wę-gloazotowanej przeprowadzono wykorzystując do tego celu metodę postępującego ścieniania, która polega na wykonywaniu polaryzacyjnych testów korozyjnych na coraz głębiej położonych obszarach warstwy wierzchniej. Metoda ta pozwoliła na określenie zmian poszczególnych charakterystycznych parametrów korozyjnych odczytywanych z potencjokinetycznych krzywych polaryzacji. Badania odporności stali X6Cr13 na korozję ogólną wykonano w 0,5M roztworach siarczanowych o trzech wartościach pH: 1, 2 i 4. Grubość powstałej węgloazotowanej warstwy oceniano na podstawie spektrofotometrycznej analizy zawartości węgla oraz mikrotwardości na przekroju poprzecznym obrobionego cieplnie materiału.

The paper presents results of research under the effect of surface thermo-chemical treatment on the corrosion resistance of X6CrNiTi18-10 steel. The corrosion resistance of the surface layers of stainless steel obtained as a result of thermo-chemical treatment (boronizing process) was assessed using the method of progressive thinning, which consists in performing corrosion tests on deeper and deeper areas of the surface layer. This method allowed for the determination of changes in individual characteristic corrosion parameters read from the potentiokinetic polarization curves and the determination of the depth profiles of these parameters. In the paper, results of tests of X6CrNiTi18-10 steel resistance to general corrosion, performed in acidified sulphate solutions (pH = 1) have been presented. The thickness of the surface layer was assessed on the basis of structural tests and changes in microhardness on the cross-section of the material. It has been found that the extremely high hardness of the boron layer was accompanied by deterioration of the corrosion resistance. The general corrosion rate of the surface layer is 3-4 times higher than the corrosion rate of the material core (substrate). The characteristics of the passive state of steel are particularly worsened, which is manifested by an increase in the value of the critical passivation current, the minimum current in the passive range and by limiting the tendency to secondary passivation.

The paper presents the results of experimental tests of the rolling process of Mg/Al bimetallic bars in two systems of classic passes (horizontal oval-circle-horizontal oval-circle variant I) and modified (multi-radial horizontal oval-multi-radial vertical oval-multi-radial horizontal oval-circle-variant II). The feedstock in the form of round bimetallic bars with a diameter of 22 mm and 30% of the outer aluminum layer was made through explosive welding. The bimetallic bars consisted of an AZ31 magnesium core and a 1050A aluminum outer layer. Bars with a diameter of 17 mm were obtained as a result of rolling in four passes. The rolling process in the passes was conducted at two temperatures of 300 and 400 °C. Based on the analysis of the test results, it was found that the use of modified passes and a lower rolling temperature (300 °C) ensures a more homogenous distribution of the plating layer around the circumference of the core and results in an even grain decreasing, which improves the corrosion resistance of bimetallic bars compared to rolling bars in a classic system of passes and at a higher temperature (400 °C).