Experimental Study On Electrochemical Corrosion Of Bottom Plate Of High Sulfur Crude Oil Tank

According to statistics,there are light or heavy corrosion in the high-sulfur crude oil storage tanks in active service in China.Corrosion damage is mostly pitting corrosion.Most of the pits are flaky and the corrosion and perforation of the tank bottom is the most serious.The corrosive environment in the bottom of the high-sulfur crude oil storage tank is mainly controlled by oil and sediment water.Specifically,the direct cause of corrosion of the tank floor is the large amount of soluble salts,soluble gases,pH and microorganisms carried by the two.The external factors are the ambient temperature and the soaking period.The corrosion of a high sulfur crude oil tank in the oil depot of Zhejiang Tianlu Energy Co.,Ltd was investigated on the spot.It was found that the corrosion of the bottom plate of the high sulfur crude oil storage tank was the most serious.The oil properties test in the high sulfur crude oil storage tank shows that the crude oil in the tank belongs to high sulfur and low acid crude oil and has the characteristics of low water content,low salt content and high density.The highest cation content is Na⁺,the highest anion content is Cl^-,HCO₃^-content,the pH value is neutral,the dissolved oxygen content is very low,about0.96mg/L,and the average value of SRB count is 3.0×10³cell/mL,.The conductivity is also as high as 4.15us/cm.According to the above values,the high sulfur crude oil tank can be used to simulate the sedimentation and hydroponics.In this paper,the corrosion laws of Q235B carbon steel in simulated static high-sulfur oil storage tanks were studied in combination with potentiodynamic polarization curves,electrochemical impedance techniques,scanning electron microscopy and energy spectrum analysis.The results show that the i_corr rise of HCO^3-is the fastest in the electrochemical corrosion caused by internal environmental factors,and the degree of corrosion on the bottom of the tank is also the highest.The content of dissolved oxygen and Cl^-reached1.22 mg/L respectively,and after 11700 mg/L,the i_corr tended to calm down,while the icorr of H₂S increased first and then decreased,and reached the maximum at the concentration of 4.18mg/L.Moreover,the content of dissolved oxygen in deposited water of oil tank with high sulfur content is usually less than 2.0 mg/L.Therefore,it can be concluded that the effect of electrochemical corrosion on the bottom plate of high-sulfur crude oil storage tank is HCO^3->Cl^->H₂S>DO.Without SRB,the corrosion is mainly affected by the synergistic action of dissolved CO₂ and chloride.The main corrosion form is pitting corrosion pit,and the sulfide content of corrosion products is very small.When the content of SRB is less than 3.0×10³cell/mL,the corrosion degree of the electrode surface is low,which has a certain protective effect on the electrode.On the contrary,the corrosion degree will be aggravated obviously.When the content of SRB is about 1.2×10⁴cell/mL,the corrosion degree is the most serious,and the corrosion form is ulcerative.Comparing the two external factors,we can find that when the temperature is higher than 25?,the increasing trend of i_corr is increasing.The i_corr of electrode immersed for two days was the largest,and after more than 2 days,the i_corr decreased greatly,and after 15days,it tended to calm down.Therefore,it can be concluded that under the experimental conditions of controlling external factors in this chapter,the corrosion rate of electrode is the highest after 2 days of immersion in 45?simulated solution.Moreover,the corrosion rate of the working electrode soaked at 15?for more than 15 days is the lowest,and the effect of temperature on the electrochemical corrosion of the tank floor is higher than that of the immersion period.