A duplex metal pipe is a new type of pipe composed of two metal pipes. The outer pipe is a standard carbon steel pipe, which withstands the working pressure of the pipe system, while the inner pipe is a corrosion-resistant alloy pipe with excellent corrosion resistance. Duplex metal pipes overcome the limitations and single-use nature of traditional metal materials. By using a carbon steel base, the pipes ensure the overall strength and mechanical properties of the pipes, significantly increasing their service life and reducing costs, making duplex metal pipes have broad application prospects.
Process flow for double metal composite pipes
Double metal composite pipes are divided into mechanical composite and metallurgical composite. Mechanical composite is the most common method, which involves inserting a carbon steel pipe and a stainless steel pipe together and producing them using methods such as hydraulic pressure, explosion, and spiral pressing. The process flow is shown in Figure 1.
Figure 1: Process Flow
Common Issues and Causes for Double Metal Composite Pipes
Currently, there are large amounts of H2S or CO2Due to the harsh operating environment, the conveying pipes must have good resistance to H.2S, CO2, Cl-Corrosive, the conveyed medium is a gas-liquid mixture, H2S or CO2Dissolves in water to form a weak acid, which can cause severe corrosion of carbon steel pipes. Specifically, H2S corrosion includes electrochemical corrosion, hydrogen-induced cracking, and sulfide stress corrosion cracking, characterized by localized corrosion; CO2Corrosion mainly manifests as electrochemical corrosion and galvanic corrosion, characterized by localized and uniform corrosion. In addition, Cl-Promotes localized corrosion, allowing corrosion to penetrate deep into the pipe material and form chloride pitting, ultimately leading to pipe perforation.
During inspection of the duplex metal pipe, significant corrosion was observed on a large portion of the inner wall, as well as some instances of internal water corrosion. The corrosion condition of the pipe line is shown in Figure 2.
Figure 2: Corrosion condition of underground pipelines on site
1、The electrochemical corrosion mechanism of duplex metal composite pipes.
(1) The pipe cap seals are not airtight, exhibiting damage. The ends of the double metal composite pipes entering the factory are not sealed with tape, and they are exposed to contaminants such as metal shavings during transport. Additionally, the storage yard has high humidity due to frequent rainfall.
(2) The pipe ends are not properly sealed after sandblasting, and the blasting equipment was unable to completely remove steel sand from the inner walls of the double metal composite pipe; during manual rust removal, steel flakes entered the pipe through the damaged pipe cap.
The 316L stainless steel inner tube of the duplex stainless steel pipe exhibited iron contamination, leading to iron damage and the breakdown of the protective oxide film. This resulted in pitting corrosion at the damaged areas and the protective oxide film region, characterized by a small cathodic area and a large anodic area. The corrosion resistance of 316L stainless steel is due to the formation of a dense, stable, and robust oxide film on the surface, as shown in Figure 3.
Figure 3: Formation of a passivation film on stainless steel surface (Cr)2O3Illustration
The electrochemical corrosion mechanism of duplex metal pipes: When the metal with a lower corrosion potential and the metal with a higher corrosion potential are in contact, a corrosion cell is formed. This results in the metal with the higher corrosion potential undergoing cathodic reaction, which inhibits its corrosion process. Conversely, the metal with the lower corrosion potential undergoes anodic reaction, which accelerates its corrosion process, as shown in Figure 4.
Figure 4 Schematic of electrode corrosion
2、The corrosion of the duplex metal pipe due to the formation of an electrochemical couple.
(1) The surface conditions of the materials vary (scratches, abrasion, welding), as shown in Figure 5.
Figure 5: Formation of an electrochemical corrosion cell due to the presence of two different metals in the composite pipe.
(2) Generally, as the ratio of anode to cathode areas increases, the corrosion rate of the metal acting as the anode also increases.
(3) In media with high conductivity, such as seawater, the current can be distributed over a larger area of the anode surface, resulting in more uniform corrosion. However, in media with lower conductivity, such as soft water or normal air, the corrosion will concentrate on the anode surface closer to the contact point, effectively reducing the effective area of the anode and increasing the local dissolution rate of the anode surface.
3、Measures and Recommendations
(1) Prevent contact between dissimilar metal alloys.
(2) Avoid creating a large cathode-small anode area, which is detrimental to corrosion protection. (3) Implement insulation measures for contact surfaces between metals with different corrosion potentials. (4) If the surface of 316L is damaged due to iron contamination, causing the passivation film to be compromised, the corroded area should be polished, removing loose rust. After polishing, the remaining thickness must meet the minimum thickness requirement. Apply a uniform layer of stainless steel passivation paste to the polished area, ensuring a minimum overlap of 50 mm with the unpolished area. Rinse the passivation area with tap water (with a chloride concentration of no more than 30 mg/L) or wipe the passivation paste with a damp cloth to form a passivation film. After acid passivation of 316L stainless steel, inspect the quality using an endoscope, as shown in Figure 6.
Figure 6: Inspection of the inner wall of the duplex metal pipe using an endoscope.
Conclusion
In highly corrosive environments of sour gas fields, carbon steel is difficult to meet the requirements. While adding corrosion inhibitors can achieve some effect, their stability and cost are not ideal. Dissimilar metal alloy pipes offer significant cost-performance advantages for harsh corrosive environments. To ensure the service life of dissimilar metal alloy pipes and prevent iron contamination and electrochemical corrosion, it is essential to place moisture-absorbing packages at the pipe ends and seal them with good-sealing pipe caps. This ensures the quality of dissimilar metal alloy pipes, enabling them to withstand harsh corrosive environments and extend the service life of dissimilar metal alloy pipes.