| With the development of domestic economy, the import of crude oil in our country has increased year by year, and the amount of the import crude oil has 2.1 million tons by 2007. The degree of dependence upon foreign trade has also climbed up to 57.8%. The price of international crude oil was influenced by many factors and it continued to move up. The output of inferior oil rises gradually in the world, and the oil quality of the domestic crude oil is also deteriorating too. In face of the national strategy and requirements of individual company, processing high-acid, high-sulfur crude oil was inevitable for refining enterprises. Therefore, it is meaningful to deal with the corrosion problems for the refining facilities caused by the poor quality crude oil, investigations on the corrosion behavior, mechanism and the prediction method of steels in the high temperature naphthenic acid has been completed in this thesis. The main works and results are as follows:(1) The corrosion behaviors of six steels in the naphthenic acid were studied, and the influences of acidity, temperature, and time on the rate of the metals corrosion were examined, and the corrosion mechanism of the carbon stell in the naphthenic acid was identified. The experimental results showed that the corrosion rate increased with rising the temperature. Among the six metallic materials, the corrosion rates of carbon steel and low alloy steel increased greatly. The corrosion of metals in the naphthenic acid followed the theory of Arrhenius kinetics, and the apparent activation energy of the six metals calculated. And that of carbon steel was the smallest, 33.8KJ/mol, while that of 316 stainless steel was the biggest,67.9KJ/mol. The corrosion in gas phase was detected and it had low corrosion rate compared to the rate in liquid phase due to the static experiment. The corrosion rate had a linear relationship with the square root of total acid number (TAN), while the influence of time on the corrosion rate of metallic materials is not significant.(2) The corrosion mechanism of carbon steel in naphthenic acid had been studied. The electrochemical measurement was conducted to study carbon steel corrosion in the naphthenic acid media sealed by lab-made stainless steel pitcher, and the results revealed that the open circuit potential was very unstable, the polarization curves was very disturbed and corrosion current was very small in the high temperature naphthenic acid solution, the corrosion current gotton by weight loss method is 126000 times bigger than that by electrochemical measurements. The conductivity of naphthenic acid solution in 0～100℃was among 0.003～0.008μs/cm. The results of the galvanic corrosion experiments showed the corrosion rates of the test metal were very close between in galvanic couple with in single metal, and the similarity was confirmed by the corrosion morphology observed by camera and the scanning electron microscope results. According to these experimental facts, it can be proposed that in naphthenic acid the corrosion mechanism of the metal is followed by a chemical corrosion mechanism basically.(3) According to the multi-adsorption center and strong ability to exclude organic matter in naphthenic acid, three kinds of commercially compounds mercapto benzothiazolyl, benzimidazolyl, and tributyl phosphate (TBP) were selected as single component high temperature corrosion inhibitors, and then the inhibition effect of them were mesured. A phosphate inhibitor, WSH-1, was synthesized and optimized by single factor method and orthogonal test, finally a series of new corrosion inhibitors (WSH-2, WSH-3) were developed by mixture of the above several corrosion inhibitors as effective components and the aniline as solvent. The weight loss test results, scanning electron microscopy, and energy dispersive spectroscopy showed that WSH-1 corrosion inhibitor has strong inhibitive effect on the naphthenic acid corrosion. Higher corrosion inhibition efficiency was gotton when the concentration was above 500 mg/L. Further, the film was non-soluble at solution and could be easily maintained at low concentration (20 mg/L). The corrosion inhibition mechanism could be ascribed to the combined effect of the reaction deposition film and the absorption film. Higher the concentration of corrosion inhibitor was lower the iron ions concentration was more easy formation of the adsorption film is; on the contrary, the reaction deposition film can be easily formed. The WSH-2 and WSH-3 corrosion inhibitor had excellent effects that may be contributed to the combined function of the collaborated deposition and collaborated adsorption of the various corrosion inhibition components. The commercial corrosion inhibitors NS7901, NS7010, tributyl phosphate, benzimidazole (BIM), and mercapto benzothiazole (MBT) were also investigated, respectively. The results showed that at low temperature NS7901 and NS7010 had certain inhibition effect, while at high temperature they almost had no inhibition function; Benzimidazole (BIM) had the lowest inhibition efficiency, which decreased sharply with increasing the temperature. Mercapto benzothiazole (MBT) had higher corrosion inhibition efficiency, but this efficiency would be low as increasing of the time, temperature, and acid number. Thus, it can be concluded that this kind of corrosion inhibitor could absorb quickly and reach the state of equilibrium.(4) The effects of the coating thickness, medium temperature, naphthenic acid number, and corrosion time on the corrosion of electroless plating Ni-P coating in naphthenic acid was investigated through the weight loss method. Meanwhile, the hardness and wear-resistance of the electroless Ni-P coating has been studied. The results revealed that the cnti-corrosion performance of the electroless Ni-P coating in the naphthenic acid corrosion was best when the thickness was 35-40μm. It also showed that the application of the electrolss Ni-P coating was appropriate below 350℃. In addition to these, the corrosion rate of electroless Ni-P coating increased with increasing the acid number and the main reason for this corrosion resistance is that a passive film could be formed on the electroless Ni-P coating in this corrosion medium. Amorphous electroless Ni-P coating with better resistance to naphthenic acid corrosion was obtained on carbon steel via optimizing the process parameters.(5) The effect of sulfide on naphthenic acid corrosion was studied. In the pure sulfide the corrosion rate and weight of the corrosion film increased with increasing the temperature. At a given temperature, the corrosion rates increased with time firstly and then decreased sharply, while the weight of the corrosion film showed a parabolic relationship with the time. The corrosion rate and the weight of the corrosion film increased at first and then decreased with increasing the sulfur content. In the mixed system of sulfide and naphthenic acid, the corrosion behavior was much more complicated than that in the single media respectively. Then, the corrosion classification have been conducted according to the naphthenic acid corrosion index (NACI).(6) Finally, a multi-variables gray forecasting model MGM(1,3) has been established to predict the corrosion of the samples in the vacuum tower in a refinery, showing that the corrosion rate of the carbon steel could be predicted with the model adequately, but it is difficult to predict of the corrosion stainless steel. |
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