Solid State Phenomena Vol. 227

Abstract: Corrosion fatigue small, short and long crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300 ppb Cl^- + 300 ppb SO₄^2- solution and in air at 90 °C. The crack growth rate for short and long cracks was monitored by direct current potential drop (DCPD) and for the small cracks by combining high resolution optical microscopy and DCPD. Comparison of the fatigue growth rate demonstrated that in solution the short crack growth rate was remarkably enhanced in comparison to long cracks, when the crack size is smaller than 250 μm. This enhancement was attributed to the electrochemical crack size effect associated with greater anodic polarisation of the short crack in such low conductivity solution. However, such enhanced growth was not observed for small cracks, which was rationalised on the basis of additional contribution of current from the pit limiting crack-tip polarisation.

Abstract: Studies of stress corrosion in X12Cr13 and X20Cr13 steels used for rotor blades of steam turbines have been conducted for determining the average rate of corrosion cracking development da / dt. Specimens were exposed to the salt mist environment of 1,2 ppm NaCl concentration in temperature of 60° C. The research environment represents the initial condensate of humidity from 3÷4%. Average rate of corrosion cracking development for specimens of the X12Cr13 is by 0,005 µm/h higher from specimens of the X20Cr13 steel. Course of the corrosion cracking initiation depends to a high degree on metal structure, and probably the close value of cracking propagation rate depends on the similar structure of those steels – high-tempered martensite with carbides at grain boundaries.

Abstract: Structural and chemical attributes of amorphous and nanocrystalline metals, which affect their corrosion behaviour are outlined. Effects of the fraction of intercrystalline regions, diffusivity, chemical homogeneity / heterogeneity and local ordering are indicated. These features can lead to higher or lower corrosion resistance as compared to coarse-grained counterparts, depending on the nature of tested metal and corrosive environment. Contributions of these factors are represented by two examples of passive behavior of nanocrystalline metals and amorphous Ni-P alloys.

Abstract: The influence of plastic deformation on the corrosion behavior of as-cast AlMg2 and AlCu4Mg1 aluminum alloys was studied at the microscale using the Electrochemical Microcell Technique and surface microgauges. It was shown that large strain gradients develop locally. The corrosion behavior of both alloys was affected by plastic deformation. The most active sites contain large strain gradients in AlMg2 and cracks with damage in the matrix in AlCu4Mg1.

Abstract: In this paper, the influence of the crystallographic orientation of grains on the corrosion behaviour of pure aluminium is investigated combining the Electrochemical Microcell and the X-Ray Diffraction (XRD) Techniques. Crystallographic orientation has strong influence on the current density in the cathodic branch and the breakdown potential. The influence of plastic deformation (compression) on the corrosion resistance is also investigated. Compression has nearly no influence on the current density in the cathodic domain, but improves the corrosion resistance of aluminium in sodium chloride solution.

Abstract: The work presents a structural and corrosion resistance analysis of Mg-based bulk metallic glasses in “as-cast” state. The studies were performed on bulk glassy samples in the form of plates. The structure analysis of the samples in “as-cast” state was carried out by the XRD and DSC methods. The corrosion behavior of the bulk glassy alloys and pure magnesium samples was studied by electrochemical measurements and immersion tests in 5% NaCl solution at room temperature. For the amorphous alloys the highest corrosion potential was achieved, indicating the formation of protective surface layers with Mg and Cu oxides. The corrosion behavior of the alloys with Zn and Ni addition was found to be better than pure magnesium.

Abstract: Severe Plastic Deformation (SDP) processes including Hydroextrusion (HE) causes the change of the mechanical properties by the introduction of a large number of defects and significant refinement of the microstructure. During the initially stages of HE microstructure becomes more elongated according to the extrusion direction. Only after a certain deformation, grains become more equiaxed. Present studies are intended to determine the influence of the titanium Grade2 microstructure and grain size distribution on its corrosion properties. All corrosion tests were conducted in a 0.9 % NaCl solution at room temperature. Electrochemical Impedance Spectroscopy (EIS) was measured after 2 and 24 hours. Also Potentiodynamic Polarization was conducted after 24 hours. Microstructures were reviled using Scanning Electron Microscopy and Transmission Electron Microscopy. Grain size distribution was determined using the program Micrometer . The results confirmed formation of titanium dioxide and very good corrosion properties of titanium Grade2 at all stages of hydrostatic extrusion.

Abstract: Subject of examinations were ZnAl22Cu3 and ZnAl40Cu3 alloys. The samples after solutionizing 385⁰C/10h and ageing 175⁰C/10h and for comparative purposes, in the initial state (just after casting) have been studied. Corrosion tests were carried out in “acid rain” solution. The scope of the examinations included potentiodynamic and potentiostatic tests , examination of the surface and structure of the samples after corrosion. The research found that corrosion of ZnAl22Cu3 and ZnAl40Cu3 alloys in the "acid rain" environment leads to a depletion of zinc in the upper surface layer and its enrichment in aluminium and copper resulting in decreasing the corrosion rate. Depending on the parameters (temperature, time) solutioning and ageing of ZnAl22Cu3 and ZnAl40Cu3 alloys can lead to an increase or decrease of the corrosion resistance in the "acid rain" environment.

Abstract: The purpose of the study was evaluation of electrochemical corrosion resistance of extruded magnesium alloy AZ31. Corrosion tests were performed in NaCl solutions featuring chloride ions concentration from 0.01 to 2 M NaCl. Potentiodynamic tests enabled to register anodic polarisation curves. Immersion tests were performed in NaCl solution and time 1-5 days. Electrochemical impedance spectroscopy (EIS) was used to evaluate phenomena that take place on the surface of the tested alloy. Impedance spectra of the system were made in the tests, and obtained measurement data was matched to the equivalent system. The results of all performed tests prove explicitly deterioration of corrosion properties of extruded magnesium alloy AZ31 with the increase of molar concentration of NaCl solution. Options of magnesium alloy AZ31 application in aircraft and automotive industry are connected with the need for application of protective layers on elements made of the tested alloy.