Crude Oil Storage Tank Floor Corrosion Mechanism And Protective Measures

The corrosion in the bottom of crude oil storage tank is one of the major problems confronting the designer and operator in the petroleum industry. It is closely associated with the presence of water and contaminates which accumulates in the bottom of tanks, and contains a number of complex components, including chlorides, sulphates, H2S and microorganisms as well, although the petroleum itself may not corrode tanks. In other hand, the effects of vibration, collision of floating roof tank and the turbulence of oil could aggravate the destruction, such as crevice corrosion for the storage tanks. In this paper, the mechanisms of corrosion for such storage tanks are studied and discussed extensively, as follows:Effects of Cl", S2", temperature, pH value on corrosion behavior of carbon steel, and the influence of different kinds of carbon steel are investigated by electrochemical tests. Experiments show that carbon steel has great tendency to be dissolved in the water of bottom, Cl" will influence both anodic and cathodic polarization behavior, S2" serves a dual purpose in terms of media and structure of iron sulfide flfm, sometimes it corrodes mildly in the media of neutral and base, however corrosion will -be accelerated in the condition of acid media and high temperature."Field test" was designed and established in order to observe the effect ofvibration under different stresses. The experiment results show that the vibration may accelerate abrasion and aggravate the localized corrosion due to the fact of non-homogeneous distribution of corrosives and debris created in above process.According to corrosion rate of carbon steel, no obvious change was found with the increase of sulfur-reducing bacteria(SRB) numbers. With alternating current(AC) impedance, the effect of ferrous ion on corrosion of carbon steel in SRB media was studied, and it has shown that the pitting was emerged on thesurface of sample as the Fe existed in the SRB media from 2~20mg 玊 , while in media without Fe2+ or containing 120 mg ?I"1 Fe2+, average corrosion was the main corrosion form instead. In addition, the potentiodynamic curves has also proved precipitate of ferrous sulphide corrosion production played an important role in pits initiation and propagation, and the surface is inavoidable in the state of pitting under potentials >-0.17V.In summary, corrosion and perforation in the bottom of crude oil tank ismainly caused by abrasion, and then the crevice or SRB corrosion may aggravate it.Based on above mechanics study, several protection methods available were compared, ^"coating + cathodic protection" is the best, and further more research approve that conductive coating may accelerate anode dissolve, reduce protection efficiency. Due to the fact that the bottom is always attacked by water in special, the insulated coating could be considered to put in use, inwhich reduce the anode consumption and get perfect purpose.