Influence of surface condition on corrosion resistance of 304 stainless steel/20 steel bimetallic composite pipe
With the ever-increasing demand for energy, the exploitation of oil and gas fields has gradually extended to deep wells and highly corrosive environments. At present, corrosion-resistant alloys and even nickel-based alloy pipelines are commonly used in highly corrosive oil and gas fields in mining operations and oil and gas transportation.
However, the corrosion-resistant part of ordinary corrosion-resistant pipes only accounts for about 1/3, and the rest is used for structural support, which causes a great waste of resources. In order to reduce costs and prolong the service life of pipelines, domestic and foreign scholars have conducted years of research on the serious corrosion problem of high H2S/CO2/Cl- gas fields.
The results show that the use of corrosion-resistant alloy composite pipes is one of the relatively safe and economical ways to solve the problem of pipeline corrosion. In this study, the most representative 304 stainless steel/20 steel mechanical composite pipe was selected as the research object, and the electrochemical corrosion behavior of 304 stainless steel lined pipe in Cl-containing solution before and after plastic processing and polishing treatment was comparatively studied.
The supplied state (unplasticized) for the test, the 304 stainless steel that has undergone compound deformation, and the 304 stainless steel specimens before and after the polishing treatment are processed into an electrode with a working area of 1cm2. One end of the electrode is welded with a copper wire to conduct the circuit, the other side is used as a working surface, and the rest are sealed with epoxy resin.
During polishing, the working surface of the electrode is polished step by step with 400#, 800#, 1200#, and 2000# sandpaper, and then polished into a mirror surface with a metallographic sample polishing machine, then cleaned with deionized water and placed in the Keep the ionized water in a beaker for later use. The test solution is NaCl.
The test adopts the polarization curve test method. Through the steady-state polarization curve shape, slope, and position of the polarization curve, the electrochemical behavior characteristics of the corrosion process and the control characteristics of the cathode and anode reactions can be studied. Add 400 mL of NaCl solution to the three-electrode system, install the working electrode, and connect it to the computer equipped with the corrosion measurement system software. Set the potential scanning range to -1～+1.6V, the scanning rate to 50mV/min, and the measurement is 0.08 Polarization curve in mol/L Cl-corrosive medium.
Through a comparative study of the corrosion process and corrosion resistance test results of 304 stainless steel/20 steel mechanical composite pipe after plastic processing and polishing treatment in Cl-solution, the following conclusions are obtained:
(1) Unpolished 304 stainless steel is corroded in a neutral Cl-corrosive medium of 0.08mol/L. After plastic processing, the self-corrosion potential shifts negatively, the corrosion current greatly increases, and the corrosion is intensified. This is due to factors such as lattice deformation and residual stress caused by plastic processing, which promotes the corrosion of metals.
(2) Polished 304 stainless steel is corroded in a neutral Cl-corrosive medium of 0.08mol/L. After plastic processing, the self-corrosion potential shifts negatively, and the corrosion current increases, but the relative change is small. It can be seen that the corrosion resistance of 304 stainless steel can be greatly improved by improving the surface processing quality.
(3) Potential polarization curves of samples with different surface roughness in 0.08mol/L neutral Cl-corrosive medium show that the smaller the surface roughness, the better the corrosion resistance of the metal.