Effects of Cr, Mo and B on the corrosion behavior of Fe3Al intermetallic compounds in H2SO4, H2SO4+KSCN and HCl solutions were investigated using electrochemical tests. The corrosion morphologies and the corrosion products were analyzed by utilizing scanning electron microscopy (SEM) and electron probe micro analysis (EPMA). Addition of Cr and Mo to the Fe3Al intermetallic compound increased corrosion potential, pitting potential and repassivation potential. Active current density, passive current density and reactivation current density decreased as Cr and Mo were added. In the case of Mo addition, the passive current density was slightly higher in the H2SO4 solution than in the solutions containing SCN- and Cl-. When B was added, the corrosion potential and repassivation potential decreased, whereas the active current density, passive current density, reactivation current density and pitting potential increased. Fe3Al intermetallic compounds containing Mo and Cr showed remarkably improved resistance against intergranular and pitting corrosion in SCN- and Cl- solutions. On the other hand, boron-addition accelerated both general and intergranular corrosion by precipitation of borides.
Effect of sulfate and other several oxoacid salts on the crevice corrosion behavior of type 304 stainless steel in chloride solutions has been investigated at 423K. In chloride solutions at 423K, passive films typically broke in pitting mode within crevices. In the existence of oxoacid salts, crevice corrosion potential shifted to nobler values, which was accompanied by the change in corrosion mode within a crevice from pitting to general dissolution. The composition of solution at which crevice corrosion was completely inhibited was expressed by the empirical equation: log [Cl-]=a log [oxoacid]+b. The value of a was almost same as obtained for free surface while b decreased significantly both in case of sulfate and phosphate. Effect of sulfate on crevice corrosion was discussed quantitatively in terms of dissociation equilibrium of hydrogen sulfate ion which buffers a decrease in pH on hydrolysis of chromium (III). Effect of oxoacid salts on the repassivation potential (ER) was also examined. Nitrate and phosphate shifted ER to nobler values, while molybdate worked the other way about. Sulfate did not change ER.
The weight loss tests of the copper pieces immersed in HNO3 solutions added each of four kinds of p-substituted anilines (substituents: NO2, Cl, NH2, OH) show that the substituted anilines have high corrosion inhibition efficiencies of 70-90%. Similar weight loss tests for six kinds of p-substituted anilines (substitiuents: NO2, Cl, NH2, OH, NHCOCH3, CONH2) have done at each of temperatures 30°C, 50°C and 70°C of the solutions and the adsorption energies ΔH (kJ/mol) of each substituted aniline on the copper plane have been obtained in the order for the substituents NO2 (45.37)>CONH2(41.55)>Cl(37.82); NH2(37.82)>OH(28.72)>NHCOCH3 (23.89). The order is the same as that of the strength of substituents, and the strong substituent adsorbes strongly on the copper plane, and the substituent shows strong corrosion inhibition action. ATR IR spectra have been measured the copper pieces immersed in the HNO3 solutions added each of the four p-substituted anilines and the absorptions ν(H-Cu), ν(Cl-Cu), δ(H-Cu) and ν(O-Cu) have been found. The adsorption structures of the p-chloroaniline and p-phenylenediamine on the copper plane are shown and the structures offer a basis of corrosion inhibition mechanism for the p-substituted anilines
Sulfidation behavior of Fe-26at%Cr, -36at%Cr, and -12at%Mn alloys at temperatures of 1073 and 1173K was investigated at a sulfur pressure lower than the dissociation pressure of ferrous sulfide at each temperature. External sulfide scales of (Cr, Fe)3 S4 or (Mn, Fe) S were formed with or without formation of an internal sulfide. The general internal sulfidation appeared for the Fe-Mn alloy at both temperatures and for an Fe-26Cr alloy at 1173K. The former was due to the smaller diffusivity in the alloy and the latter was due to an α to γ phase change. The sulfidation of an Fe-26 at%Cr alloy at 1073 K was confined within grain-boundaries. This was caused by a high sulfur solubility there. A mixture of general and grain-boundary sulfidation appeared when the Fe-36 at%Cr alloy was sulfidized at 1173K. The internal sulfidations of general, grain-boundary and their mixtures were explained by applying the criterion for external or internal oxidation proposed by Wagner.
An in situ infrared reflection absorption spectroscopic technique was developed to investigate metal surfaces during atomospheric corrosion. Sufficient purging of the whole measurement systems with nitrogen gas, strictly constant flow rates of corrosive gases, and reciprocal measurements with p- and s-lights, together with careful optical arrangements, have enabled acquisition of spectra of water and corrosion products from thin surface layers. Pure copper and low carbon steel were tested at room temperature. The corrosive gases containing water vapor with 65% and 80% relative humidities and 10ppm SO2 were introduced into the test cell by nitrogen gas or air synthesized from nitrogen and oxygen. The measurements were carried out under the passage of the gases over the specimens. In using nitrogen as a carrier, only water layer was recognized on copper and the thickness attained a constant value within 30 minutes, whereas in corrosive gases with air as a carrier, sulfite and sulfate together with water bands were observed. On low carbon steel, sulfite and water layers were detected in the nitrogen carrier, and in using air as a carrier, bands attributed to sulfate and probably due to Green Rust II were observed together with sulfite and water bands. These layers grew obeying the parabolic rate law.
High temperature corrosion of the water-wall tubes for the low grades oil firing boilers was investigated during operation under low-oxygen combustion condition. Adjacent to the corroded area near the burner, combution gas contained 0.3-0.6% of oxygen, 0.4-1.0% of hydrogen, hydrogen sulfide and other reducing compounds. The scale deposited on the corroded surface of the water-wall tubes was mainly composed of the metal oxide at outer layer and the metal sulfide at inner layer. Corrosion in these cases was caused by sulfidation reaction of the metal surface with sulfate compounds such as sodium sulfate and unburned carbon in the low oxygen concentration combustion gas. The sulfidation of water-wall tubes in actual boilers can be prevented by Plasma spray coating of 50%Ni-50% Cr alloy.