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The corrosion resistance of dual-phase steel tee usually has good stability under normal use conditions, but in the process of long-term use, its corrosion resistance may decline to a certain extent due to environmental factors, use conditions or improper maintenance. The following is a specific analysis:
First, the main factors affecting the corrosion resistance
1. The stability of the material itself
Microstructure changes: duplex steel (such as 2205, 2507) corrosion resistance and its austenite and ferrite two-phase ratio (usually close to 1:1) is closely related.
Long-term service at high temperatures (e.g. over 300°C): This may lead to the precipitation of intermetallic phases (e.g. σ-phase, χ-phase), which are brittle and hard phases that not only reduce the toughness of the material, but also consume corrosion-resistant elements such as chromium and molybdenum in the matrix, forming chromium-poor zones and thus accelerating corrosion (e.g. intergranular corrosion).
Welding heat-affected zone: If the welding process heat input is too high or improper cooling rate, it may lead to local organizational abnormalities (such as excessive ferrite or coarse austenite grain), which may become the starting point of corrosion after long-term use.
Elemental stability: the nitrogen (N) element in duplex steel can be solidly dissolved in the austenite to improve pitting resistance. However, if the material in processing or use due to high temperature denitrogenation, may reduce corrosion resistance.
2. Changes in the corrosive environment
Medium properties:
If the medium corrosive ions (such as Cl-, S ² -) concentration increases, the pH value decreases or the temperature increases, may break through the corrosion resistance limit of the material, resulting in accelerated corrosion rate.
The presence of solid particles in the medium (such as sand, slag), may be due to scouring corrosion (wear + corrosion synergistic effect) to accelerate the surface damage, exposure of fresh metal, reduce corrosion resistance.
Stress: Although duplex steel is better than austenitic stainless steel in stress corrosion resistance, but in the long-term alternating stress or residual stress (such as pipeline vibration, installation stress), may lead to stress corrosion cracking (SCC), especially in the Cl-containing environments with a higher risk.
3. Surface damage and maintenance
Mechanical damage: surface scratches, pits, etc. caused during transportation, installation or maintenance may become a source of corrosion (e.g., pitting starting point), and corrosion at the damage may expand after long-term use.
Oxide film damage: the passivation film (Cr₂O₃, etc.) on the surface of duplex steel is the key barrier to corrosion resistance. If the surface is oil, welding slag, corrosive media attached for a long time, may damage the passivation film, resulting in localized corrosion (such as crevice corrosion).
Inadequate maintenance: not regularly clean up the deposits in the pipeline, not repair the damaged parts, or not according to the requirements of the maintenance of anti-corrosion coatings (such as coated), may accelerate the corrosion process.
Second, the specific performance of the decline in corrosion resistance
1. Localized corrosion
Pitting / crevice corrosion: in the Cl-containing environment, after long-term use, if the surface passivation film is locally broken, pitting pits may be formed, and then to the depth of the expansion.
Intergranular corrosion: If the material precipitates carbides (e.g. Cr₂₃C₆) or intermetallic phases at grain boundaries as a result of high temperature or welding, and chromium content near the grain boundaries decreases, intergranular corrosion may occur in a specific medium, destroying the material structure.
2. uniform corrosion
In strong acids, alkalis and other strong corrosive media, if the material corrosion resistance is close to the critical value, long-term service may lead to uniform thinning of the wall thickness, although the corrosion rate is slow, but the cumulative effect will reduce the strength, and ultimately affect the safety.
3. Stress Corrosion Cracking (SCC)
Prolonged exposure to high stress + corrosive media (e.g. high temperature and high pressure environment containing Cl-) may lead to micro-cracks, cracks gradually expand over time, eventually leading to leakage or fracture.
How to slow down the decline of corrosion resistance?
1. Reasonable material selection and design
According to the characteristics of the medium to choose the appropriate grade of duplex steel:
Low corrosive environment: 2205 duplex steel (Cl-concentration ≤ 2000ppm);
High corrosive environment: 2507 super duplex steel (Cl-concentration ≤ 10000ppm).
Avoid stress concentration: optimize piping design, reduce turbulence and impact at the tee, and reduce mechanical stress and thermal stress.
2. Control processing and welding quality
Avoid surface scratches during processing, and carry out surface polishing or pickling passivation after molding to restore the passivation film.
When welding, adopt low heat input process (such as TIG welding), control the interlayer temperature ≤ 150 ℃, and carry out non-destructive testing (such as penetration flaw detection) after welding to ensure that there are no welding defects (such as porosity, not fused).