High temperature corrosion behaviour of nicr-based coatings applied by twin wire arc spray technology

High temperature corrosion is a serious problem related to the combustion of heavy oils with high vanadium contents. Protective overlay coatings are used to allow functioning of engineering components under extreme conditions and provide corrosion resistance to extend the component life. This article is concerned with the high temperature corrosion behaviour of two protective NiCr-based coatings and bare structural steel 1.4959 (W.nr.) in a molten salt environment of 40% Na2SO4 and 60% V2O5 at 750 °C under cyclic condition. NiCr and NiCrMoNbTa coating was deposited on steel 1.4903 (W.nr.) by Twin wire arc spray technology (TWAS). To establish the kinetics of corrosion, the thermogravimetric technique was used. The X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the corrosion products and determine the corrosion mechanism. The hot corrosion resistance of both coatings was better than bare steel samples. The NiCrMoNbTa coating showed better corrosion behaviour than the NiCr coating. However, also the NiCr coating provides sufficient protection to the substrate material

High temperature corrosion studies of HVOF sprayed coatings in molten sulphate salts environment

The generally applied requirements on lowering of emission production in the field of power industry have led, among others, into the application of low-emission combustion process of coal fired power plants. Besides the emissions reduction, this process has led to an increase of boilers parts corrosion degradation due to the reducing atmosphere in low-emission boilers, responsible for sulphide corrosion and molten salt attack. In such environment, the protection of the exposed surfaces gains importance. Among others, the thermal spraying technology can offer a suitable way of surface corrosion protection. In the present work, the high temperature corrosion behaviour of CoCrAlYTaCSi and Cr3C2-25%NiCr High Pressure/High Velocity Oxygen Fuel (HP/HVOF) sprayed coatings is evaluated and compared to the behaviour of bare structural steel (SPT360) substrate. The specimens were exposed to a molten salt environment, i. e., 12% Na2SO4 + 82% Fe2(SO4)3 at 690 °C under cyclic condition, simulating the working environment in low-emission boilers of coal-fired power plants. The thermogravimetric method was used to establish the kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the corrosion products and determine the corrosion mechanism. The thermogravimetric measurement shows high oxidation rate of SPT360 steel. Cr3C2-25%NiCr coating provides slightly better performance compared to CoCrAlYTaCSi coating. Based on the obtained results, both coatings were found to have higher corrosion resistance than the SPT360 steel substrate, offering a sufficient surface protection in a given environment

Restoration of hay meadows on ex-arable land: commercial seed mixtures vs. spontaneous succession.Grass and Forage Science 66

Abstract In many areas of Europe there are policies to restore former arable land to grassland. In practice, this usually involves the use of commercial seed mixtures. The abundance of all vascular plants species in 35 ex-arable fields, sown with a commercial seed mixture or spontaneously revegetated, was studied in one landscape area to compare two methods of grassland restoration. Species abundance was also evaluated in the close surroundings of the fields. Data were processed using multivariate (ordination) and univariate statistics. Period of time since abandonment, size of the field and type of grassland restoration (sown vs. spontaneously revegetated) had a significant influence on vegetation. However, for the target meadow species, the type of restoration did not exhibit any influence. After about 20 years the contribution of meadow species that had established spontaneously in the studied fields was similar to that of their surroundings. We concluded that artificial sowing on ex-arable land is not necessary to develop semi-natural grasslands if (i) there are sources of appropriate diaspores in the immediate surroundings, (ii) the site is not very rich in nutrients and (iii) farmers do not need grass production immediately. In these situations and over this timescale natural regeneration would allow substantial savings of money and labour