The effect of tannic acid on the corrosion of mild steel in boiler systems was investigated by physicochemical methods using an autoclave and a test boiler. Tannic acid was found to be a good oxygen scavenger at pH 11 as well as hydrazine and sodium sulphite. Tannic acid inhibited corrosion of steel (i) at room temperature and atmospheric pressure and (ii) at high temperature and pressure. The maximum inhibition efficiency was about 97%. The inhibiting behavior of tannic acid at pH 7 was different from that at pH 11. At pH 7, tannic acid acted as an adsorption-type inhibitor and suppressed catholic reaction of corrosion of steel. At pH 11, tannic acid inhibited corrosion of steel not only as an effective oxygen scavenger but also an effective Schikorr reaction promoter (formation of Fe3O4). Tannic acid is believed to be an effective inhibitor in boiler systems.
Silica films were formed on Ni substrate from sol-solutions of various viscosities. The viscosity was varied by changing the aging time of the prepared solution at 308K from 3.6 to 900ks, with resulting viscosities 2 to 10mPa·s, respectively. The most suitable curing temperature was found to be 673K. The protectiveness was related to the structural defects of the film and was assessed by anodic polarization properties in 0.5kmol·m-3 H2SO4 aqueous solution. The dissolution of the substrate metal was significantly suppressed by the silica coating. However, if the film were associated with defects, local corrosion takes place. The sol-solutions of viscosities around 2.8mPa·s results in the films of very good protectiveness, while those having viscosities larger than 3.7mPa·s lead to exfoliation of the films.
The laser glazing of gas-flame sprayed alumina was attempted to produce a coating resistant to hot corrosion with insulation capability. Superficial laser glazing was achieved by scanning a carbon dioxide laser with a large beam diameter at low velocity over the sprayed alumina coating impregnated with silicone sealant. Successful laser-glazing could be achieved by controlling the temperature of the metallic substrate. The laser-glazed alumina exhibited high insulation capability and good resistance to hot corrosion by Na2SO4-V2O5 at 900°C.
Potentiostatic polarization and slow-strain-rate tests have been used to assess the likelihood of stress corrosion cracking of mild steel immersed in aqueous solutions of di-isopropanolamine (dipa) and in solutions of dipa in tetrahydrothiophene sulfoxide (sulfolane) and water, saturated with CO2 at different potentials and strain rates, at 95°C. Dipa-sulfolane-water solutions show no tendency for stress corrosion cracking either at the free corrosion potential or at potentials of maximum electrochemical reactivity. However, an aqueous solution of 25% dipa saturated with CO2 was found to cause transgranular stress corrosion cracking of carbon steel at the potential of -0.60V (SCE) which is located in the region of maximum anodic electrochemical reactivity.
The aspects of surface analytical techniques by detecting and analysing energy spectrum of outcoming photons from sample surface concerned were summarised. γ-ray, X-ray and light emission spectrometry induced by the ions bombardment offers very unique surface analysis methods. X-ray spectrometry by the irradiation of sample surface with electron beam, X-ray beam or ion beam can serve not only as an elemental analytical techniques but as methods, which give the information on the lateral distribution of elements and the physicochemical state of elements. Fourier transformation of near edge Kossel structure and far edge Kronig structure of X-ray absorption spectrum resulted the radial function or the distance distribution among atoms. X-ray fluorescent spectrometry and diffractometry of the small glancing angle scattering by the use of synchrotron radiation increases the surface sensitivity considerably. Ion, electron and photon induced luminescence are utilized widely for the evaluation of electronical characteristics of semiconductor or phosphor films. Varieties of photothermal and photo-acoustic processes are applied to the surface analyses of both inorganic and organic advanced materials. High sensitivity of Fourier transform infrared spectrometer realized many surface and micro-analytical techniques especially for organic thin films. The newly exploited Fourier transform Raman spectrometer will prove to be very powerful tool of surface analyses even with or without the combination of surface enhanced Raman spectrometry, Raman microprobe analyser. Electron spin resonance absorption and magic angle sample spinning nuclear magnetic resonance spectrometry are proving themselves to be essential methods for the chemical state analyses of compounds adsorbed or modified as functional substrances onto the surface.
Current studies relating to both fabrication processes of plasma sprayed composite coatings and evaluation of some thermal properties of these coatings were reviewed. The reports about the application of mechanically alloyed composite powders to plasma spraying for the fabrication of composite coatings were mainly introduced. Synopses of some researches on both definition of the fracture conditions of thermal shock testing and determination of the evaluation parameters of coating microstructure were also given.