From extreme value statistical analysis on corrosion rate of bottom plates of oil storage tanks, following results are provided. With heating oil storage tanks installed on sand base and non-heating oil storage tanks installed on asphalt base, corrosion rates of bottom plate are the largest, and the extent of it is reduced in order of non-heating sand base tanks, heating asphalt base tanks. In most non-heating asphalt base tanks, corrosion of bottom plate is restrained, but in some cases, high corrosion rates were almost equal to that in sand base tanks with heating. In such cases, corrosion rate was large and statistics on corrosion rate in sand base tanks and in non-heating asphalt base tanks showed almost the same. It was indicated by direct observation of bottom plates that the presence of airgap between base and bottom plate accelerates corrosion rate. The present counter-measure such as installation of oil storage tanks on asphalt base is not necessarily adequate. The studies on new corrosion prevention method with enough effect which can expect even if gap exists between base and bottom plate, will be the most important subject.
To clarify a complicating behavior of corrosion inhibition in a solution with high concentration of ionic species (HC solution) in cooling water system, the relationship between corrosion rate of mild steel, amount of polymer adsorption and amount of scale deposition was analyzed by using the polymer-polymer complexes (PPCs). The corrosion rate in a HC solution was dependent on the amount of polymer adsorption and the amount of scale deposition. It was corroborated that the corrosion rate was related to the amount of apparent polymer adsorption determined by equation (5) reported previously in ref. 21): In particular, the physical significance of coefficient A in equation (5) was made clear. And then, it suggested that the corrosion rate in this experimental condition was anticipated by polymer adsorption and scale deposition tests in a short term apart from corrosion weight loss test in a long term.
It is known that the stainless steel (SS) like the type 316 SS shows cyclic oscillation of the corrosion potential in the concentrated and the medium temperature sulfuric acid solution environment. With this potential oscillation, the stainless steel suffers the general corrosion. This general corrosion of the type 316 SS is not permitted in some process for the problem of the contamination and the productivity. The authors have studied the prevention method of this general corrosion. It has been found that the anodic protection method, where the noble metals such as gold and platinum come in contact with the type 316 SS, is effective. This anodic protection method has been successfully applied to the actual process environment using the concentrated sulfuric acid. It is considered that the small catholic over voltage of the noble metal compared to the SS is due to the catalytic reducing reaction of the molecular sulfuric acid on the surface of the noble metal.
The high-temperature oxidation behavior of the Fe-X (X=Al, Cr, or Si) alloys in atmospheres containing water vapor and oxygen was investigated using thermo-gravimetry, scanning electron microscopy, electron probe micro-analysis, and X-ray diffraction. Oxidation amounts of all alloys increased remarkably by addition of the water vapor. The oxidation behavior of each alloys showed S-shaped curves that consisted of three stages: slow-incubation, rapid transition, and relatively slow steady state oxidation. There was a little effect of water vapor in the oxidation of Ni-Cr alloy which was investigated for comparison. Without water vapor, the oxide scale was composed of a thin triplex structure of Fe2O3, Fe3O4, and Al, Cr, or Si rich protective oxide with internal oxides of the alloying elements. In atmospheres containing water vapor, the scale structure is similar at the incubation period, in the transition period a thick duplex outer scale of iron oxide and an inner porous scale was formed on all alloys. The inner scale was composed of a duplex phase of FeO and FeAl2O4, (Fe, Cr)3O4, or Fe2SiO4. The results suggested that the change in protective oxide scale to the less protective FeO and complex oxide was caused by increases in oxygen partial pressure at the scale/alloy interface in the atmospheres containing water vapor.
To estimate concentrations of electrolyte layers formed on metal surface in atmospheric environment, relative humidity (RH) in equilibrium with various concentrations of strong electrolyte solutions were calculated with available thermodynamic data. The activity coefficient of water (fH2O(X)) for the solution with molar fraction of water (X) could be given as a function of ionic strength of the solution, which was determined by mean activity coefficients data in literatures for electrolytes in the solution. RH values obtained as RH[%]=100×fH2O(X)⋅X were fitted well with measured values in literatures not only for solutions with individual electrolytes, for example NaCl or MgCl2, but also for solutions with mixed electrolytes, for example sea salt.