The operational life of a carbon steel heat exchanger has empirically been determined based upon penetration of pitting corrosion in cooling water environment. In the recent years, approach called “extreme value analysis” has been introduced and been proven to be a useful tool for the purpose of life estimation in corrosible environments. However, the growth rate of pitting and its relation with operation conditions have not been made fully clear yet, which gives limitation of the development of the method for accurate estimation of heat exchanger life. The experimental data of a series of the extreme value analysis of heat exchangers were collected and were analyzed in this study, with the data mainly collected from heat exchangers with passing water through tube inside. The time dependence of the location parameter (λ) and of the scale parameter (α) in extreme value analysis were examined at first. As a consequence, it was found that these parameters had similar time dependence and had similar saturated tendency at some years after beginning the operation of the facilities of interest. A multilinear regression analysis was carried out, next, to clear the relations of time dependence, operational conditions and water quality on these parameters. A life estimation equation for heat exchangers was then developed, based on the results of this regression analysis. The effects of operational conditions and water quality on pitting propagation was made clear from these studies and practical application of this approach was emphasized.
Pitting corrosion of carbon steel in a new working fluid for absorption heating and refrigeration was studied. The fluid was a concentrated solution of mixed lithium salt consisting of LiBr, LiCl, LiI and LiNO3, which had been developed aiming to increase the performance of the absorption cycle. The fluid contained not only the oxidizing agent, NO3-, but also, the oxidizing agent, I2 which is the oxidized species of I-. When the solution is kept at an elevated temperature of approximately 150°C, it is found that, regardless of the amount of NO3-, a small quantity of I2 raise the spontaneous electrode potential of platinum, ESP (Pt), to +120mV with respect to the saturated calomel electrode kept at room temperature. While, in an environment with no I2 and only one or greater than one percentage of LiNO3, the ESP (Pt) drops to approximately -120mV. Thus, the fluid forms an oxidizing halide environment that tend to induce localized corrosion in steels. The critical pitting potential, VC, PIT, for the carbon steel was determined to be either -340mV or -310mV for all oxidizing solutions with various concentrations of LiNO3 and I2. By comparing VC, PIT and ESP, it became apparent that pitting corrosion occurred violently in solutions containing I2 for the steel with ESP value more noble than VC, PIT. However, it became also apparent that in solutions containing more than 1% of LiNO3, pitting corrosion ceased when I2 is reduced by NaHSO3 completely. Insusceptibility of the carbon steel to localized corrosion is strengthened with a higher concentration of NaHSO3.
The oxidation behavior of 17Cr-1.2Mo-0.3Ti and 16-19Cr ferritic stainless steels containing up to 2 percent Mo in air at 1000 and 1050°C was studied, and the main results obtained are as follows: (1) The longer the time from polishing of specimens to oxidation tests, the better the oxidation resistance of 17Cr-1.2Mo-0.3Ti steel. (2) Cr2O3 scales were observed in the specimen that was oxidized after a long period of exposure in air at room temperature. On the other hand, the specimen oxidized immediately after polishing developed (Cr, Mn)3O4 scales, (3) Cr concentration in the passive film increased with time elapsed after polishing, (4) Mo addition to ferritic stainless steels promoted the formation of Cr2O3 at the early stage of oxidation, improving their oxidation resistance, (5) The passive film became thinner with increasing Mo content.
In order to evaluate the corrosion fatigue characteristics of the structure materials, the fatigue tests should be carried out in environments which closely approximate the actual corrosive environments. In the present work, the fatigue characteristics of the carbon steel (S45C), coated with thermal sprayed zinc and aluminum layers, were investigated during the accelerated corrosion test in which the specimens were subjected to repeated cycles of salt water spraying, drying in a warm air, and exposure in open air at a room temperature. The results are as follows; 1) The surfaces of the turned and blasted specimens are slightly harder due to the hardening effect. 2) In the thermal sprayed and painted specimens, the fatigue limit under the rotate-bending mode is the same as that of the blasted specimen tested in air. 3) The microstructure near the fatigue limit is the mixture of ferrite and pearlite. 4) Dimples and quasi-striation patterns are obseved in the fatigue surfaces. While, tire-track patterns are observed in the fracture surfaces of the turned specimens.
A method of forming continuous silica and alumina coating films using a sol-gel dip coating and a protectiveness of the coating films are presented. The protectiveness which is related to the defects in the coatings is evaluated by electrochemical methods including polarization curve and open circuit potential measurements in deaerated 0.5M H2SO4 at 303K. The transparent, continuous and adhesive sol-gel silica coating films are obtained when the coatings are applied to single layer alumina-coated Type 316 stainless steel substrate, while the films directly coated onto the mechanically polished substrate tends to produce some defects. The appearance and protectiveness depend on the withdrawal speed and the baking temperature. For instance, the most suitable withdrawal speed and baking temperature are 1.07 and 1.80×10-3m/s, and 673K, respectively. The high protective coating can be obtained by two-layer silica-coatings due to reduction of the defect densities in underlying coatings.
Problems on the corrosion of metal cooking utensils in seasoning solutions has little been studied. The test spesimens were pieces of steel sheet, aluminum sheet and aluminum foil. After these specimens were either soaked at room temperature or boiled in various seasoning solutions, the weight loss and corrosion rate were measured. The surface of specimens was examined by electron microscopy and the surface layer developed on discolored aluminum was examined by electron diffraction method. It has been found that wide or a few corrosion arise depending on kinds of metals and types of cooking liquids. It is well known that iron products rust easily and coating with oil has a preventive effect, and in order to mitigate stickness of pans, it is a customary practice that first iron skilet and wok are heated well, then oil is coated on the surface. These effects were examined by measuring friction coefficient of heating and coating oil mild steel specimens. Some of them formed thin layer. From the infrared spectra of the thin layer forming compound, it has been found that the metal soaps are formed from fatty acid and iron; i.e. chemical adhesion.