Preliminary Research On Mechanism And Protection Methods Of Carbon Steel In Oilfield-produced Water

Due to the complexity of the transmission medium(oil,water,gas mixed transmission)of oil and gas pipelines,there is a universal risk of under deposit corrosion(UDC),resulting in pipeline perforations and leaks,and even huge economic losses and environmental disasters.In this paper,the UDC of carbon steel pipelines in oilfield-produced water is taken as the research object to systematically study the in-situ growth of Ca CO₃ on the surface of carbon steel,the galvanic corrosion between the Ca CO₃-covered electrode and the bare electrode at different temperatures,and the inhibitory process of under composite deposit corrosion(composed of sludge,corrosion products and sand)in the presence of different corrosion inhibitors.By establishing a basic UDC finite element model,the effectiveness of cathodic protection for UDC was discussed.The following results have been achieved:(1)The potentiostatic method can generate a uniform and dense Ca CO₃ deposit in situ on the wire beam electrodes(WBE)surface.Subsequent UDC tests show that during the initiation of corrosion,the Ca CO₃ deposit-covered electrodes act as the anode,and the bare electrodes act as the cathode due to the oxygen concentration difference between the inside and the outside of the deposit,resulting in accelerated corrosion under the deposit.The polarity reversal may be the result of the sealing effect of the FeCO₃ corrosion products in deposit,which makes the deposit denser and becomes a physical barrier that prevents the penetration of aggressive ions.So it is inferred that the Ca CO₃ deposit has protective ability for lower steel.It was found that the LP parameter can well characterize the corrosion inhomogeneity of the WBE surface through experiments,which increases with increasing temperature,and the local corrosion effect is enhanced.(2)Corrosion experiments under composite deposit show that deposit-covered steel is corroded as an anode during galvanic corrosion without inhibitors,and the bare steel will also be corroded in the late stage,therefore the protections of both in the transport pipeline are equally important.Although the added inhibitors have their limitations,they still have reference value for corrosion protection of the pipeline in engineering.Among them,zinc gluconate can only form a protective film on the bare electrode due to its low permeability to the composite deposit,which strengthens the galvanic effect between the inside and outside of the deposit,causing more serious corrosion under the deposit.And the protection to the bare electrode cannot long-term maintenance.On the other hand,NO₂^-can penetrate into the deposit freely,but due to the uneven diffusion in it,the under-deposit metal appears severe pitting.However,it has good long-term protection for the rest of the metal surface.In addition,the constant frequency impedance,surface potential and current scanning of WBE show that the corrosion rate of carbon steel is in good agreement with the above electrochemical parameters,which can accurately reflect the degree of local corrosion under the deposit.(3)COMSOL was used to simulate the development of UDC and cathodic protection efficiency.It was found that the under-deposit steel was dissolved as an anode,and the cathodic protection could provide certain protection for the carbon steel under the deposit.When the applied cathode potential rises to-1.0 V vs.SCE,the under-deposit carbon steel is fully protected,but at this time there is a risk of hydrogen evolution on the surface of the bare metal outside the deposit.