A thermal barrier coating (TBC) is used as thermal protection of metallic components exposed to hot gas streams in e.g. gas turbine engines. Due to a high thermal expansion coefficient, low thermal conductivity, chemical- and thermal stability, yttria stabilized zirconia (YSZ) is the most widely used material for TBCs today. In the work presented in this master thesis an aqueous nitrate precursor solution was prepared and deposited on stainless steel substrates by spray pyrolysis to produce 8YSZ coatings (8 mol% of Y2O3 in ZrO2). The precursor solution concentration and deposition parameters, including set-point temperature and volume sprayed, were optimized to produce continuous and crack-free green coatings.The deposited green coatings were characterized by scanning electron microscopy, thermogravimetry and Fourier transform infrared spectroscopy to study the influence of substrate temperature on the microstructure of the green coatings. A substantial change in microstructure was observed for the green coatings in a certain temperature range indicating that a minimum deposition temperature was necessary to obtain crack-free green coatings.Heat treatment was necessary to decompose the nitrate species in the deposited film. During heat treatment, vertical cracks were introduced into the coatings due to the nitrate decomposition. The cracking behavior of the coatings was studied for different drying times and conditions, and it was found that the crack propagation can be controlled to obtain the preferred size and geometry of the cracks. Due to built-up stresses in the coating, which can exceed the fracture toughness of the material, it was found that there was a maximum film thickness achievable before spallation of the coating for a given precursor solution. Therefore, the possibility of spraying multi-layered coatings was investigated. The introduction of a second layer showed that it was possible to double the thickness of the coating.

There are currently no standard test methods for testing the cathodic disbonding properties of organic coatings at temperatures above 100 C. There are several subsea oil and gas reservoirs with high temperatures, some as high as 200 C. The main goal of this work was the development of a new apparatus and testing procedure for high temperature cathodic disbonding, hereby referred to as HTCD (High Temperature Cathodic Disbonding).A test method for the cathodic disbonding of organic coatings from submerged steel subjected to high temperatures was studied. The test method requires the use of a specialized HTCD apparatus. In this test method, sample plates were mounted between two channels, one containing a hot (150 C) oil flow and the other containing a pressurized, cold, salt (3.5 % NaCl) water flow. Accelerated conditions made it possible to test the cathodic disbonding properties of several coatings in four weeks. Four weeks is a typical duration for coating prequalification tests. Several commercial coating products of different generic types provided by different manufacturers were tested. The results indicate that adequate coating products for high temperature underwater exposure are available.The required cathodic protection current for the samples tested in the HTCD apparatus was continuously monitored. No correlation between the required cathodic protection current and the extent of cathodic disbonding was observed.A long term test with more field like conditions and a duration of 400 days was performed. Low levels of disbonding for most of the tested products in the 400 day test made comparison to the accelerated tests difficult.An attempt was made to determine the oxygen diffusion coefficient of five coating products. The attempt was unsuccessful. The same method had previously been used to study coatings with a thickness of up to 300 micro m, the coatings studied in this work were between 600 micro-metres and 1200 micro-metres. It remains uncertain whether the chosen method can be used for coatings this thick.Results from electrochemical impedance spectroscopy, performed in a pressurized vessel, showed a large reduction in the ionic resistance of a submerged organic coating upon heating from 30 C to 150 C. This showed that elevated temperatures throughout the coating can reduce the ionic resistance to a level where even an intact coating is incapable of protecting the substrate. Studies of coating samples at ambient temperatures after exposure to higher temperatures showed that exposure to heat causes a lasting reduction in impedance. High impedances correlated with good performance in the HTCD tests.Investigation with a scanning electron microscope (SEM) provided images where the extent of the cathodic disbonding was clearly visible. Electron-dispersive X-ray spectroscopy (EDS) enabled the identification of oxides discovered at the holiday and beneath the disbonded coating. Zinc and calcium oxides were identified at and near the holiday; iron oxide was identified beneath the disbonded coating.

Tap hole refractories constitute critical parts of the refractory lining in submerged arc furnaces. For several hours every day, molten slag and metal flow through the tap hole calling for thorough selection of refractory materials able to withstand the intense thermal, corrosive and erosive conditions present in this area. Carbon-based refractories have shown excellent thermal properties and high strength, as well as low wettability towards process materials, and are therefore utilized in silicomanganese production furnaces both as side lining, in the hearth, and in the tap hole area. The aim of this investigation was to determine the compatibility of five refractory materials utilized in the tap hole area of an industrial silicomanganese furnace with two industrial silicomanganese slags: •Investigate the suitability of the selected refractory materials for confining the process materials during industrial production of silicomanganese alloys. •Identify critical refractory wear mechanisms upon slag-refractory interaction at industrial tapping temperatures.Compatibilities were investigated through 12 static crucible tests and two static plate tests in a vertical tube furnace redesigned during this investigation. Slag-refractory interaction was studied after two and four hours holding time at 1367°C ± 1.8°C, 1464°C ± 2.1°C and 1600°C - 0.6°C /+ 0.2°C. Holding temperatures were verified through the wire-bridge method at the melting points of gold and palladium. Visual inspection, as well as optical microscopy and SEM, were utilized to examine the samples after heat treatment.During compatibility experiments, dissolution of refractory matrix due to solubility of oxide refractory binder phases in silicomanganese slags was observed, as well as disintegration of refractory particles due to gas formation at slag-refractory interface, or expansion as a result of phase transformations in refractory material. Direct reduction of manganese oxide from slags and iron oxide present in refractories by carbon and silicon carbide was also observed. Establishment of partial slag-metal equilibriums between iron oxide and silicon metal originally present in slag was observed, as well as formation of silicon carbide at the slag-refractory interfaces. The latter may serve to protect the refractory from wear caused by slags.Based on observations of extensive interaction between silicomanganese slag sample I and ramming paste at 1600°C, the ramming paste investigated cannot be recommended for usage during tap block repair in an industrial silicomanganese furnace. Incipient electrode paste disintegration by slags and silicon carbide tap block – slag interaction were observed after compatibility tests at 1464°C, calling for further investigations of these refractory materials. Tap hole clay and carbon tap block showed minimal signs of interaction with process materials at 1464°C. Refractory porosity seemed to have a larger effect on refractory wear than refractory ash content. Contrary to industrial observations, silicomanganese slag sample I was more corrosive towards the ramming paste and electrode paste investigated than silicomanganese slag sample II.