Volumen 64 (2020): Edición 1 (March 2020)

The current trend in development of new metallic materials for certain types of implants is turning away from permanent, biologically inert materials to the use of biodegradable materials. Fe–Mn alloys represent high perspective material for development of new generation of temporary and biodegradable implants. The aim of this work was to study mechanical and corrosion characteristics of powder samples containing 25, 30 and 35 wt % of Mn which are fabricated by pressing, sintering, and additional spark plasma sintering. The influence of preparation method (pressing and sintering) to microstructure, phased composition and corrosion behavior of prepared alloys was studied.

This research paper presents an analysis of the corrosion properties of steel-reinforced concrete samples during immersion in 3.5 wt. % NaCl aqueous solution by measuring their response both cathodic and anodic polarization in order to determine the corrosion rates in the function of their calcium nitrate inhibitor content. This cheap inorganic inhibitor was added to the concrete mix in concentrations of 1% and 3% by weight of cement in addition to two different superplasticizers (MapeiDynamon SR 31 and Oxydtron). The compressive strengths of the so prepared samples were also checked according to the relevant European standard and were within the acceptable limits, so this inhibitor does not weaken this important property of the concrete samples.

The test results on steel reinforced samples immersed in 3.5 wt. % NaCl aqueous solutions at room temperature showed promising corrosion mitigating effects just after 6 months testing period. After 6 months the lower corrosion currents (i.e. better corrosion resistance) for both types of superplasticizers were observed with those samples which contained 3% calcium nitrate inhibitor. The best result was observed with sample C4 (in this case 3% calcium nitrate was added to the mixture of cement+Oxydtron superplasticizer). The advantageous inhibition mechanism of nitrate anions is also discussed and interpreted.

Exposure of copper in corrosive environment is possible way, how to obtain artificial patina. Various solutions based on chloride, ammonia or polysulfide are commonly use in this purpose. Furthermore, it appears that the patina is also formed in an environment with an increased concentration of SO₂ in the atmosphere. This procedure was tested in a small (30 l) exposure chamber, where the aggressiveness of the environment was monitored and where the effect of alternating the condensation and drying phases was shown to be positive. Based on this experiment, a 2 m³ pilot chamber was designed for which a water film sensor was developed and tested to ensure drying of the object surface. Monitoring of the aggressiveness of the environment showed that the pH and SO₂ concentrations in the atmosphere are stable after approximately 5 hours and the ideal input SO₂ concentration is 17.7 g m^-3 at which the pH stabilizes at 2.7-3. By recording the voltage variation on the sensor, it was possible to monitor the formation and drying of the water film during the cycling of the condensation and drying phases.

To produce realistic test specimens with realistic flaws, it is necessary to develop appropriate procedure for corrosion flaw production. Tested specimens are made from steels commonly used in power plants, such as carbon steels, stainless steels and their dissimilar weldments. In this study, corrosion damage from NaCl water solution and NaCl water mist are compared. Specimens were tested with and without mechanical bending stress. The corrosion processes produced plane, pitting and galvanic corrosion. On dissimilar weldments galvanic corrosion was observed and resulted to the deepest corrosion damage. Deepest corrosion flaws were formed on welded samples. The corrosion rate was also affected by the solution flow in a contact with the specimens, which results in a corrosion-erosive wear. Produced flaws are suitable as natural crack initiators or as realistic corrosion flaws in test specimens.

AZ31D magnesium alloy is widely used in automotive, aircraft, and aerospace applications because of its high strength to weight ratio. However, the softness of the alloy results in higher wear rate and the high activity results in higher corrosion rate. With an aim of reducing the wear rate and corrosion rate of AZ31 alloy, surface composite of AZ31 alloy is fabricated by reinforcing niobium carbide (NbC) by friction stir processing. The microstructure and dispersion of the reinforcements in AZ31-NbC surface composite is analysed by optical microscopy. In addition, the microhardness and tribological characteristics of the developed AZ31-NbC surface composite are investigated. The results demonstrated an increase in microhardness (23.2 %) and the decrease in wear rate (15.6 % for a normal load of 2 kg) in the developed AZ31-NbC surface composite with respect to the base material. The immersion corrosion test was performed to analyse the corrosion rate of the developed AZ31-NbC surface composite in simulated sea water environment (3.5 wt % NaCl solution). The results indicate that the corrosion rate of the developed AZ31-NbC surface composite is higher than that of base material. A comprehensive analysis on the wear and corrosion mechanism of the developed AZ31-NbC surface composite is presented.