When connecting stainless steel hoses and accessories to ordinary carbon steel fittings, three key issues must be addressed: electrochemical corrosion, welding process compatibility, and mechanical stress concentration. Due to the significant difference in electrode potential between stainless steel and carbon steel, direct contact can create a galvanic effect. This can accelerate corrosion on the carbon steel side in humid or salt-fog environments, even leading to pitting and perforation. For example, in coastal buildings, if isolation measures are not implemented, carbon steel pipe fittings can fail within a few years due to galvanic corrosion. Therefore, physical isolation or material modification are necessary to block corrosion pathways. For example, installing a polytetrafluoroethylene insulating gasket on the connection surface or using a bimetallic composite transition joint with a nickel-based alloy overlay on the carbon steel end to prevent carbon diffusion.
Welding process compatibility directly impacts joint strength and durability. Stainless steel and carbon steel have significant differences in melting point, thermal expansion coefficient, and thermal conductivity. Direct welding can easily produce a brittle martensitic structure, leading to weld cracking. For example, when welding 304 stainless steel to Q235 carbon steel, if high-alloy filler materials such as 309L are not used, carbon migration will form a chromium-depleted layer in the weld area, significantly reducing corrosion resistance. The specification requires the use of a TIG primer followed by a CO₂ gas shielded welding process. The argon purity must be ≥99.99%, and the interpass temperature must be controlled below 150°C. After welding, the stainless steel side must be pickled and passivated, and the carbon steel side must be coated with an epoxy zinc-rich primer to isolate the corrosive media.
Mechanical stress concentration is another cause of connection failure. Stainless steel hoses and accessories have a lower elastic modulus than carbon steel, making them susceptible to localized deformation due to vibration or thermal expansion and contraction. For example, when connecting a stainless steel bellows to a carbon steel flange in an automotive drive shaft, if sufficient compensation is not provided, repeated bending can cause fatigue cracks on the stainless steel side. During design, the appropriate hose length should be selected based on the pipe's displacement, and a bellows compensator or ball joint should be used to absorb axial and lateral displacement.
Material surface treatment is crucial to connection quality. Before welding, carbon steel pipe fittings must be sandblasted to remove scale. Stainless steel pipes must be degreased with acetone to prevent oil-induced carbon contamination. For example, if zinc coating remains on the carbon steel surface, zinc vapor can penetrate the weld during welding, causing "liquid metal embrittlement" and cracking along the penetration line. Groove design should be tailored to the material's characteristics. A 60° V-groove (1mm blunt edge) should be used on the stainless steel side to reduce dilution, while the gap between the components should be controlled on the carbon steel side to prevent lack of fusion.
Environmental compatibility is a key consideration in selecting a connection solution. If welding is necessary in dry indoor environments, the "Petrochemical Dissimilar Steel Welding Specification" SH/T3526-2015 must be clearly referenced in the drawings, and third-party penetrant testing (PT) and potential difference testing (ASTM G71) must be performed. For coastal structures frequently exposed to moisture and salt spray, a mechanical connection method using bolts and insulating washers is preferred. Non-metallic washers block the current path, while bolt torque is controlled to prevent stress corrosion cracking.
Long-term stability maintenance requires a full lifecycle management system. Regularly inspect the coating integrity of joints and promptly repair damaged primer on the carbon steel side. Perform ultrasonic testing on welded joints to monitor crack growth. Install vibration-damping brackets in vibration-prone environments to reduce the impact of dynamic loads on the joint structure. For example, a chemical company has managed to control the corrosion rate of dissimilar steel joints to less than 0.02mm/year by testing the potential difference every six months.
Future technological development focuses on standardizing and intelligentizing joining processes. Bimetallic composite transition joint processes are being promoted, with automated overlay equipment controlling the thickness uniformity of the nickel-based alloy layer. Intelligent welding robots are being developed to monitor the weld pool temperature and composition in real time and dynamically adjust welding parameters. Digital twin technology is being applied to simulate stress distribution in joints under complex working conditions to optimize design parameters. These innovations will significantly improve the reliability and cost-effectiveness of joining stainless steel hoses and accessories to carbon steel fittings.
