It is well known that the presence of KCl deposited on superheater tubes in biomass- and waste-fired boilers leads to a severe corrosion and premature damage. In order to protect such critical components which are routinely exposed to aggressive environments, thermal sprayings are frequently proposed as a potential solution. By virtue of the techno-commercial benefits that provides as a direct outcome of its ability to cost-effectively deposit coatings virtually free of porosity and in situ formed oxides, the high velocity air-fuel (HVAF) process offers a particularly attractive approach. In the present work, the influence of KCl on the oxidation behavior of four HVAF-sprayed Ni-based coatings (Ni21Cr, Ni5Al, Ni21Cr7Al1Y, and Ni21Cr9Mo) has been investigated. The coatings were deposited onto specimens of 16Mo3 steel, a widely used boiler tube material. High temperature corrosion tests were carried out in ambient air at 600°C, with 0.1 mg/cm2 KCl being sprayed onto the samples prior to the exposure. Uncoated substrates and an identical test environment without KCl were used as reference. SEM/EDS and XRD techniques were utilized to characterize the as-sprayed and exposed samples. The results showed that the small addition of KCl significantly accelerated damage to the coatings. It was further revealed that the alumina-forming NiAl coating was capable of forming a more protective oxide scale compared to other chromia and mixed-oxide scale forming coatings. In general, the oxidation resistance of the coatings based on the kinetic studies had the following ranking (from the best to the worst): NiAl >NiCr> NiCrAlY> NiCrMo.

This study assesses the effect of acid corrosion on the luminescence of YAG:Ce coatings. The feedstock powder is prepared by high-temperature solid phase synthesis and the coatings are deposited by air plasma spraying. Microstructure and phase composition are characterized and the effect of acid immersion duration on luminescent intensity is measured. It is found that the luminescent properties of YAG:Ce 3+ coatings have a tendency to fluctuate with immersion time, which appears to be related to phase composition.

Zirconium (Zr) metal is of interest for chemical corrosion protection and nuclear reactor core applications. Inert chamber plasma spraying has been used to produce thin Zr coatings on stainless steel (SS) substrates. The coatings were deposited while using transferred arc (TA) cleaning/heating at 5 different current levels. In order to better understand thermal diffusion governed processes, the coating porosity, grain size and interdiffusion with the substrate were measured as a function of TA current. Low porosity (3.5% to < 0.5%), recrystallization with fine equiaxed grain size (3-8 µm diameter) and varying elemental diffusion distance (0-50 µm) from the coating substrate interface were observed. In addition, the coatings were low in oxygen content compared to the wrought SS substrates. The Zr coatings sprayed under these conditions look promising for highly demanding applications.

Zinc coatings are widely adopted for cathodic corrosion protection. Mostly the process of choice is hot-dip galvanizing but due to limitations regarding component size and composition of the galvanizing bath it is not always practicable. In the present paper zinc coatings alloyed with Al, Sn, Mg and Cr are applied by twin wire arc spraying to enhance the corrosion protection ability of zinc thermal sprayed coatings. The alloys were characterized and investigated using salt spray test and by means of electrochemical corrosion. Corrosion damage and products were investigated by optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and electron probe microanalysis (EPMA).

Cored wires and high velocity arc spraying technique (HVAS) were used to produce high Mg content Zn-Al-Mg alloy coatings on low carbon steel substrates. The microstructures, mechanical properties and electrochemical corrosion behaviors of the Zn-Al-Mg coatings were investigated comparing with Zn and Zn-Al alloy coatings. And the electrochemical corrosion mechanisms of the coatings were discussed. The coatings show a typical aspect of layered thermal sprayed material structure. Chemical analysis of the coating indicated the composition to be Zn-14.9Al-5.9Mg-3.0O (wt.%). The main phases in the coatings are Zn, Mg 2 Zn 11 , Al 12 Mg 17 and MgAl 2 O 4 , together with a little Al 2 O 3 and ZnO. The Zn-Al-Mg coatings show higher electrochemical corrosion resistance in salt solution than Zn-Al coatings. The corrosion potential of Zn-Al and Zn-Al-Mg coatings decreased a little and then increased towards the noble potential. The analysis of XRD and Electrochemical impedance spectroscopy (EIS) shows that, with addition of Mg, the corrosion products can block off the pores in the Zn-Al-Mg coating, which is so-called self sealing, and thus prevent attack on the underlying steel substrate.

The development of new spraying processes has increased the demand for high quality protecive coatings. Many thermal spraying processes have been developed to obtain coatings for a wide spectrum of materials and substrates. The High Velocity Oxygen Fuel (HVOF) process involves lower temperatures and higher velocities than those required by other techniques to obtain high density coatings. It is desirable to know which are the main factors that affect the corrosion behaviour of coated materials. The corrosion behaviour in chloride solution of a 34CrMo4 steel coated with different kinds of powder have been studied. The electrochemical corrosion of the coating-substrate system was characterised by corrosion potential measurements and potentiodynamic polarisations. Microscopic studies have also been performed by means of SEM. The corrosion tests were performed in synthetic marine water (ASTM D-1l41) in the presence of dissolved air. Polarisation resistances have been obtained from potentiodynamic studies. Measurements were carried out on two different (Ti,W)C+Ni coatings, as well as on the coatings obtained from a TiC+Ni-Ti powder which had been previously Ni coated using an electro less method. The best corrosion results were obtained from these last coatings.