Research On The Corrosion Behavior Of Mechanical Clad Pipe Welded Joints During Exposure To CO₂

Clad pipe consists of the outer carbon steel material and the inner corrosion-resistant steel material by the way of mechanical or metallurgical complex. It combines the advantages of external high-strength of carbon steel and internal corrosion-resistant property of stainless steel. It can be applied to transport acidic medium in the high temperature and high pressure environment. Clad pipe manufacturing process can be divided into mechanical and metallurgical clad pipe according to their fabrication technology, and the mechanical clad pipe is widely used because of the advantages of price and manufacture processing.The CO₂ as the associated gas field in the wet environment or dissolved in water is highly corrosive to steel. The pipe walls were soaked in the electrolyte solution, and water and gas are relatively flowing mutually. Hence, the corrosion behavior is complex. The resulting material damage was called as CO₂ corrosionThis thesis mainly investigated the effects of temperature on the CO₂ corrosion behavior of a mechanical clad pipe usually used on gas pipeline through simulating scene corrosion environment of high temperature and pressure. The high temperature autoclave was used to study the corrosion behavior of the inner layer of clad pipe when immersed in simulated sea water, and the relative velocity of the sample is 1m/s. The CO₂ partial pressure is 0.66Ma. Each sample was erroded at 40℃, 60℃and 90℃respectively for 30 days under the continuous erosion corrosion and corroded product film was observed. Lastly, the formation mechanism of corroded product film was analyzed.The macro-analysis, microstructure analysis, hardness test and a series of analysis showed that the welded joint quality is good. The results also show that: stainless steel weld in a medium corrosion, and stainless steel base material in a serious corrosion . Austenitic stainless steel has excellent properties. However, corrosion resistance of the material is not good when immersed in Cl^– excessive condition, and austenitic stainless steel is irresistible to static corrosion and slow flow corrosion (<1.5m/s). Due to the usage of a high chromium high nickel stainless steel welding rod, corrodent product film of welded joints is thin. While, the product film of base metal is very clear. With the increasing temperature, the product film becomes loose and is about to fall off, indicating a more serious corrosion. Inferred by EDS and XRD analysis, the corrodent product film may be composed of Cr₂O₃, FeCO₃, the former by the Cr (OH)₃ derived from dehydration, the latter resulting of the Fe²⁺ and CO₃^2- concentration to the extent. Both of these two can prevent the corrosion to the further.