Fundamental Research On Corrosion Properties And Remanufacturing Technology For LPG Spherical Tank

Liquefied petroleum gas (LPG) is commonly held in spherical tanks with large volume and simple structure by decreasing operation temperature or increasing operation pressure. However, due to insufficient desulphurization or H2S run off accidentally from refining units into store system, the content of H2S in LPG may frequently surpass the allowances. Then, the wet H2S corrosion environment with relatively strong corrosivity will be easily formed by H2S dissolving into the thin layer of residual moist deposited on the internal surface of LPG spherical tank. In such conditions, low alloyed steels used to fabricate LPG spherical tanks may have the tendency to suffer the environmental cracking. According to the morphologies of cracks and features of cracking, the problems of environmental cracking related to wet H2S environment are mainly divided into two types, one is hydrogen-induced cracking (HIC) and another is sulfide stress corrosion cracking (SSCC). As a large quantity of crude oil imported from the Middle East is refined in China in recent years, the problems of environmental cracking related to wet H2S corrosion are triggered again, and which has become the major reasons for the premature failure of-LPG spherical tanks.Much research shows that HIC affected rarely by external stress occurs mainly in the base metal of low and medium strength steels. But for SSCC, it primarily appears in heat-affected zone (HAZ) of high-strength and/or high-hardness steel weldments. The relatively high welding residual stress plays an important role in the occurrence of SSCC. Actually, whether HIC or SSCC, it is closely related to the hydrogen atoms produced by electrochemical corrosion in the wet H2S environment. The higher of steel corrosion rate is, the more quantities of hydrogen atoms are produced, which will lead to more hydrogen atoms diffusing into the steel matrix and finally increase the risk of the corrosion cracking for LPG spherical tanks.In the present thesis,16MnR and SPV50Q steel, which are widely used to fabricate LPG spherical tanks in China, are selected as research objects. Based on the facts that some of LPG spherical tanks have been suffered from H2S corrosion cracking in the process of service, the electrochemical corrosion properties, permeation properties and diffusion regularity about hydrogen and the fracture behavior under the interaction between stress and environment for 16MnR and SPV50Q steels as well as their weldments were systematically investigated by electrochemical analysis, hydrogen permeation technique and slow strain rate testing (SSRT) as well as the finite element simulation.The main contents and conclusions in this thesis are as follows:The electrochemical properties of 16MnR and SPV50Q steel as well as their weldments in 5wt.%NaCl-0.5wt.%HAc solution containing H2S were investigated by potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). The effects of H2S content, pH value and testing temperature on corrosion rate as well as the effect of immersion time on corrosion products were considered. The morphologies of corrosion products were observed by scanning electrode microscope and their chemical compositions were analyzed by EDAX. The results show that the presence of H2S accelerates distinctly the corrosion of 16MnR and SPV50Q steel, despite the quantity of H2S content is very small. The corrosion rate of these two steels will increase with increasing H2S content. Similarly, the magnitude of pH value also plays an important role on the corrosion of steels, the corrosion rate of 16MnR and SPV50Q steel will greatly increase with decreasing pH value, which is the result synthesized by the strong solubility of sulfide formed on the electrode surface and the great rate of cathodic reaction. The increasing temperature of solution will also lead to the increase in corrosion of the steels. The sulfide film of corrosion products deposited on the surface of steel has a strong solubility in low-pH value solution, while in high-pH solution, this film has a weak binding force with steel matrix, which indicates that the sulfide film can't provide the effective protection again further corrosion for steels. In the same testing condition, base metal (BM),heat-affected zone (HAZ) and weld metal (WM) with different microstructures, which are cut from the various metals of weldment, show some difference in corrosion properties. In the case of 16MnR steel weldment, the corrosion rate of BM is the maximum, HAZ has the medium corrosion rate, and that of WM is the minimum. While for SPV50Q steel weldment, however, the corrosion rates increase in the order of BM,HAZ and WM.For 16MnR and SPV50Q steel as well as their weldments, the properties of hydrogen diffusion were investigated by using electrochemical permeation tests. The effects of H2S content, pH value, additional cathodic current, solution temperature and the thickness of permeation specimens on the hydrogen permeation time (tb), the maximum current density of hydrogen permeation (imax-perm), steady state current density of hydrogen permeation (isteady-perm) and permeation rate (0) etc were considered. The results show, the more of hydrogen atoms produced by corrosion reaction or the additional cathodic current in hydrogen entry surface of specimen is, the larger the gradient of hydrogen concentration is, which will results in the short tb as well as the large imax-perm and isteady-perm.The effect of specimen thickness on the process of hydrogen diffusion and permeation is shown on the numbers of traps suffered, making more hydrogen atoms trapped in the steel matrix. The acceleration of the solution temperature on diffusion is indicated by the increase in the corrosion rate of steels, which will lead to produce much more quantities of hydrogen atoms and make the hydrogen diffusion become much easier. In the same charging hydrogen conditions, due to the strong hydrogen traps in the interface between banded pearlite and matrix as well as the non-metal inclusion and matrix, the diffusion coefficient of BM for 16MnR steel weldment is the minimum. The microstructure of WM for 16MnR steel weldment provides great grain boundary area as a hydrogen diffusion path and results in the maximum diffusion coefficient. The fine-grained microstructure of normalized zone and coarse widmanstatten ferrite of overheat zone in HAZ act as barriers for the hydrogen diffusion, which makes the diffusion coefficient of HAZ locates between those of BM and WM. Similarly, as for SPV50Q steel weldment, the diffusion coefficient of BM is also the minimum, which is correlated with the strong capability to trap hydrogen for the large quantities of dislocation within the lath martensite. The great grain boundary area in the microstructure of WM makes hydrogen easily diffuse, which leads to the maximum diffusion coefficient for WM. The diffusion coefficient of HAZ also lies between those of BM and WM. Using the finite element ABAQUS code, a three-dimensional sequential finite element analysis (FEA) program about the effect of welding residual stress on hydrogen diffusion was performed. The effects of welding residual stress and different microstructures on hydrogen diffusion as well as the distribution regularities about the hydrogen concentration with time were considered. The results show that the hydrogen concentration in WM with the largest diffusion coefficient is the highest in absence of welding residual stress. In the presence of welding residual stress, the hydrogen concentration in HAZ is the highest because the higher welding residual stress exists in HAZ. The hydrogen diffusion in BM is rarely affected by stress because it is far away from welded seam. This proves the facts that diffusion of hydrogen will be accelerated by the action of stress and hydrogen will diffuse toward the higher-stress zones. of the weldment, which is consistent with the results in engineering.In 5wt.%NaCl-0.5wt.%HAc solution, the effect of H2S content or pH value on the environmental fracture behaviors of cross-welded specimens for 16MnR and SPV50Q steel were investigated by SSRT. The feature of fracture and the morphologies of cracks adjacent to the fracture were observed by scanning electrode microscope and optical microscope. The results show that the susceptibility of stress corrosion cracking for the cross-welded specimens of the two steels increases with increasing H2S content or decreasing pH value, and the cracks in the tensile surface accordingly become large deep. Under the interaction of stress and corrosion, all cross-welded specimens for 16MnR steel fracture finally in the zone of BM with relatively large corrosion rate, while for SPV50Q steel, all cross-welded specimens fracture in the zone of HAZ, which indicates that SPV50Q steel weldment has more susceptible to environmental cracking than 16MnR steel weldment.Based on the requirement of the remanufacturing technology for the LPG spherical tanks with cracks, the comprehensive properties of SPV50Q steel weldment experienced with multiple heat treatments were evaluated. The results show that the mechanical and resistance to corrosion properties of SPV50Q steel weldment are rarely affected by multiple heat treatments, and the environmental fracture behavior is not degraded, on the contrary, they are improved to some extent.