In the analysis of corrosion behaviors of metals in oil & gas well and chemical plants containing CO2 and/or H2S gas, detailed knowledge of a stable corrosion product and its solubility is essential. Thus, a computer program has been developed that can automatically construct the potential-pH diagram and calculate the solubility of corrosion product at elevated temperatures for Metal-CO2/H2S-H2O system. The partial pressure of CO2 or H2S gas in this program was used as the input parameter because that can be measured in laboratory tests and actual fields. The thermodynamic data of H2CO3 and H2S (aq) were used in the program, since chemical species such as H2CO3, HCO3- and CO32- are produced when CO2 or H2S gas dissolved are thermodynamically in the equilibrium state. Then, the corrosion of pure iron in CO2 environments was discussed based on the potential-pH diagram for Fe-CO2-H2O system and the solubility of FeCO3. The solubility decreased as the temperature increased, and the pure iron showed the maximum corrosion rate because the FeCO3 film was partially produced on metal surface around 100°C, and then the corrosion rate decreased above 150°C because of the protective FeCO3 film layer. Finally, the solubility of FeS in Fe-H2S-H2O system is very low compared with that of FeCO3, and the corrosion rate of the pure iron was also low at 60 and 100°C, but that increased above 150°C. In this case, the phase transition of FeS from α to β was an important factor.
When higher corrosion resistance than commercially pure titanium is required, Ti-0.15%Pd alloy is used because of insensitivity to crevice corrosion in NaCl solutions and lower corrosion rate in non-oxidizing acid solutions. Since it has been needed to find out substitute alloys having lower price than Ti-0.15%Pd and comparable corrosion resistance, binary titanium alloys were investigated. Among seventeen elements studied, only Ni and Ru were found to increase corrosion resistance in remarkable manner. Crystallographic analyses showed that most of nickel in a Ti-Ni alloys is precipitated as intermetallic Ti2Ni within α-phase matrix, and the intermetallic compound was found to be effective to bring the corrosion potential into the passive range. However, Ti-Ni alloys are not sufficient in terms of corrosion resistance, if compared with Ti-0.15%Pd alloy. As for Ti-Ru alloys, though they exhibit comparable corrosion resistance to Ti-0.15%Pd alloy, there seemed to have no merit in terms of material cost.
Laser heat treatment was applied for desensitization in surface layer of sensitized Type304 stainless steel. Effect of laser heating condition on desensitization was investigated and heat flow by laser heating was calculated by computer simulation. Desensitization by laser heating depends on the maximum temperature of specimen surface in different power densities or beam diameters. The chromium distribution in the vicinity of grain boundary was calculated from the thermal cycles obtained by computer simulation, and the disappearance of chromium depleted zone coincided with desensitization data by laser heating. From these results, it has become clear that chromium depleted zone disappears in short time by laser heating at 1323K and above and laser heat treatment is effective for desensitization of sensitized stainless steel.
Advanced acoustic emission (AE) monitoring and signal processing have been attempted to study the mechanism of the so-called APC-type intergranular SCC of the sensitized AISI 304 steel in 1000ppm NaF solution and 0.1wt% acidified tetrathionic solution at ambient temperature. AE signals (out-plane displacement) from the Mode-I cleavage-like fracture were monitored during both the CERT and the constant load test of a compact tension (CT) and a plate specimen, respectively. Generation of the cleavage-like fracture showed following stress dependancy which was characteristic to the material-environment combination, i.e., AE signals in the fuloride-SCC were mainly observed at the stresses which induced small scale yielding at the tip of the notch or SCC. However, the fracture size was limited to about 20μm. On the contrary, cleavage-like fractures larger than 300μm were observed at higher stresses for tetrathionic-SCC. Generation velocity of the cleavage-like fracture estimated by the AE source inversion processing was found to be almost 107 to 109 times that of SCC propagation rate for the fluoride and the tetrathionic SCC, respectively. Feasible mechanism for such fast fracture; stress sorption grain boundary decohesion fracture for the fluoride SCC and hydrogen assisted fracture of the martensitic phase as well as the stress sorption fracture for the tetrathionic SCC, were discussed.
A review is given of our work on the development of new, novel alloys with high resistance to high temperature corrosion in sulfidizing and oxydizing environments. The corrosion behavior of sputter-deposited amorphous Al-(34-46)Mo and Al-(31-33)Mo-(6-16)Si alloys has been studied as a function of temperature of 973-1273K in sulfur vapor of 103Pa as well as in oxygen of 105Pa and in air. The sulfidation process has been found to follow parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step, determining the overall reaction rate, is the inward diffusion of sulfur through the inner barrier layer of the scale. Over the whole temperature range studied these binary and ternary alloys have shown excellent resistance to sulfide corrosion, their sulfidation rates being comparable to or even lower than the oxidation rates of chromia-forming materials. No influence of silicon on the sulfidation rate of Al-Mo alloys has been observed. The oxidation resistance of binary Al-Mo alloys has been found to be satisfactory, but only at temperatures below 1073K. Above the melting point of MoO3 (1069K), the scale, consisting mainly of Al2O3 with about 5% of MoO3, becomes unprotective because of evaporation of molybdenum oxide. On the other hand, ternary Al-Mo-Si alloys show excellent resistance to oxidizing environments, up to about 1200K, their oxidation rates being comparable to those of alumina formers. No traces of molybdenum have been found in alumina scales on these alloys.
The electrode potential, which plays an important role in corrosion, may be defined by either the electronic or the ionic energy level in the electrode. The electronic electrode potential corresponds to the real potential of electron, i.e. the real free energy for electron transfer from the point at the outer potential of aqueous solution to the point inside the electrode, which is found to be a linear function of the electrode interfacial potential difference. Under equilibrium condition, the electronic potential represents the Fremi level of equilibrium redox electron in the solution for electron transfer electrodes, and the hypothetical Fermi level associated with ion transfer equilibrium for ion transfer electrodes. With localized corrosion, the corrosion potential at the cathode site is more positive than that at the anode site. This corrosion potential difference does not correspond to any local difference in the Fermi level inside the corroding metal, but to the difference in the electrode interfacial potential difference at the two sites.