Carbon Steel Nuts/Stainless Steel Flange Nuts Manufacturers

Carbon Steel Nuts/Stainless Steel Flange Nuts Industry Knowledge Extension

Nut selection guide under high temperature and high pressure conditions: corrosion resistance boundary of carbon steel and stainless steel flange nuts

Under high temperature and high pressure conditions, nuts are key fasteners, and their material selection directly affects the safety and service life of the equipment. Carbon steel nuts and stainless steel flange nuts have different corrosion resistance due to differences in material properties. The following systematically analyzes the corrosion resistance boundary and selection points of the two from four dimensions: corrosion mechanism, working condition adaptation, material properties and protection strategy.

1. Corrosion mechanism comparison: failure mode of carbon steel and stainless steel
Corrosion risk of carbon steel nuts
Oxidative corrosion: Iron oxide (Fe₂O₃/Fe₃O₄) is easily formed on the surface of carbon steel at high temperature. If the oxide layer is dense, corrosion can be slowed down, but high pressure scouring or medium penetration will destroy the oxide film.
Stress corrosion cracking (SCC): In media containing Cl⁻, H₂S, etc., carbon steel is prone to hydrogen-induced cracking, especially at high temperatures above 300°C. The risk increases significantly.
Case: In a petrochemical plant, carbon steel nuts were corroded by H₂S, causing thread breakage, and the frequency of leakage accidents increased by 30%.
Corrosion characteristics of stainless steel flange nuts
Intergranular corrosion: Austenitic stainless steel (such as 304, 316) is prone to precipitate chromium carbide (Cr₂₃C₆) in the sensitization range of 500-800℃, resulting in chromium depletion at the grain boundary and decreased corrosion resistance.
Pitting and crevice corrosion: When the Cl⁻ concentration is greater than 100ppm, the local passivation film on the stainless steel surface breaks, forming pitting pits, which accelerates the corrosion rate.
Advantages: Duplex stainless steel (such as 2205) has better pitting and stress corrosion resistance than the 300 series through the balance of ferrite and austenite.

2. Material properties and selection strategy
Optimization direction of carbon steel nuts
Material upgrade: Alloy steel (such as 42CrMo) is used to improve high-temperature strength, and Cr and Ni elements are added to enhance corrosion resistance.
Surface treatment: galvanizing (sacrificial anode protection) or aluminizing (forming Al₂O₃ oxide film) can increase corrosion resistance by 2-3 times.
Cost consideration: The cost of carbon steel nuts is only 1/3-1/2 of that of stainless steel, and is suitable for medium and low corrosive conditions.
Key points for selecting stainless steel flange nuts
Grade selection:
316L: Suitable for wet H₂S environment containing Cl⁻≤500ppm.
254SMO (super austenite): Cl⁻ concentration＞1000ppm or high temperature strong acid environment.
Hardness matching: The hardness of the nut needs to be 15-20HB lower than the flange surface to avoid biting or thread damage.
Low temperature toughness: Austenitic stainless steel still maintains ductility at -196℃, which is suitable for LNG low temperature conditions.

3. Protection strategy and life extension plan
Environmental control
Injection of corrosion inhibitors (such as imidazolines) reduces the corrosiveness of the medium and slows down the corrosion rate by 50%-70%.
Cathodic protection (sacrificial anode or impressed current) is especially suitable for buried pipelines or seawater environments.
Monitoring and maintenance
Regular ultrasonic detection of cracks at the root of the thread, recommended cycle: once every 6 months under high temperature and high pressure conditions.
Torque re-inspection: High temperature will cause the thread grease to fail, and it needs to be re-tightened at 80%-90% of the initial torque value.
Design optimization
Increasing the nut thickness (such as from the standard 1.5D to 2D) improves fatigue resistance.
Using conical flange nuts (such as DIN 6923) to disperse stress concentration and reduce the risk of corrosion cracking.