Research On Corrosion Mechanisms Of Liquefied Petroleum Gas Spherical Tank

Liquefied petroleum gas (LPG) is an important chemical raw material for industrial and civil use. The demand for LPG is increasing with the rapid development of industry. In recent years, as the domestic production of crude oil can hardly meet the demand of Chinese plants, a large quantity of crude oil has been imported from Middle East. Unfortunately, the higher content of sulphur in the crude oil and design deviation of the installations lead to insufficient desulphurization and dehydration. The content of HaS and H2O in LPG may frequently surpass the allowances, forming the wet fyS corrosion environment in the LPG spherical tank of low-alloyed steel. Final failure may thus appear as sulfide stress corrosion cracking, hydrogen-induced cracking and hydrogen blistering, significantly threaten the safety of the spherical tank.To clarify the corrosion mechanisms involved in the LPG tanks, the present thesishas performed a comprehensive study of two kinds of materials-high-strength lowalloyed steel and middle- strength low alloyed steel which are usually used to fabricate LPG spherical tanks. The material corrosion characteristics, cracking patterns and corrosion process in the wet H2S are analyzed and summarized in the course of research based on open literature and the field investigation. Necessary references to the effective and economical prevention of corrosion failure of the tanks are provided.Sulfide stress corrosion cracking frequently occurs in weld-joint of LPG spherical tank made of SPV50Q high-strength steel in wet H2S environment. Accordingly, slow strain rate tensile test (SSRT) is performed on a weld-joint of SPV50Q in solutions of 100ppm H2S and 5()0ppm H2S respectively in laboratory condition. Susceptibility of stress corrosion cracking and fracture feature of the joint are studied and evaluated by analyzing the stress-strain curve, stress-time curve and fractography with scanning electronic microscope.Moreover, some severely corroded plates of spherical shell removed from one LPG spherical tank of 16MnR material and served for about 16 years, are taken as an example. Based on macro-observation, metallographic micro-observation and fractographic observation by scanning electronic microscope, together with the records of working conditions and inspection reports given by the plants, the failure mechanisms are analyzed. At the same lime, mechanical properties of the material arc measured in order to evaluate the over-all properties of material served for long time under the environmentof wet HiS. The evaluation may provide a reference for engineering material selection. The comparisons of resistance against uniform corrosion and hydrogen-induced cracking between the aged 16MnR and new 16MnR in the severe corrosion environment of 5%NaCl + 0.5%CH3COOH + 7000()ppml 12S are carried out in laboratory.It is concluded that in the environment of wet H2S hydrogen induced cracking and hydrogen blistering may appear which results in severe lamination of the 16MnR parent metal and a reduced wall thickness. Operation risk is thus increased though catastrophic failure would not occur immediately. Strip MnS along with pearlite band in material is found to be the most important metallurgical factor to cause severe corrosion. The formation causes of strip MnS are further summarized and the related prevention mensurcs arc nlso provided in the thesis.