News

After the decline in corrosion resistance of duplex steel tee, the repair method needs to be based on the type of corrosion, the degree of damage and the use of the environment comprehensive judgment. The following are common repair strategies and operation points:
First, localized corrosion repair (pitting, crevice corrosion)
1. Sanding and welding repair
Scenario: Corrosion pit depth ≤ 20% of the wall thickness, and not penetrating the local damage.
Operation steps:

Surface treatment: Use sandpaper or power tools to remove the corrosion products and the surrounding oxide skin until the metal luster is revealed, the grinding range should be more than 5~10mm from the edge of the corrosion area.
Welding material selection:
2205 Duplex steel: Use ER2209 welding wire (composition matching austenite + ferrite two-phase, containing 4%~5% molybdenum, 22% chromium).
2507 super duplex steel: ER2594 wire (6% molybdenum, 25% chromium, 0.2%-0.3% nitrogen).
Welding process:
Adopt tungsten argon arc welding (TIG), argon purity ≥ 99.99%, control the welding current 80 ~ 120A, arc voltage 12 ~ 15V, interlayer temperature ≤ 150 ℃, to avoid high temperatures leading to σ phase precipitation.
Pickling passivation of the weld after welding (such as 20% nitric acid + 5% hydrofluoric acid solution soaked for 30 minutes), to restore the surface passivation film.
Inspection and verification: After welding, carry out penetration testing (PT) to detect defects in the weld, and confirm the thickness of the repaired area by ultrasonic thickness measurement if necessary.
2. Surface Coating Coverage
Applicable scenarios: large areas of minor corrosion (such as uniform thinning ≤ 10% wall thickness) or prevent future corrosion.
Coating type and process:

Metal coating:
Thermal spraying stainless steel (e.g. 316L) or nickel-based alloy (e.g. Inconel 625), utilizing supersonic flame spraying (HVOF) technology, with a coating thickness of 0.3~0.5mm, and a bonding strength of ≥50MPa.
Electroplating nickel-phosphorus alloy (Ni-P), coating thickness 20~50μm, need to be activated first (e.g. hydrochloric acid etching) to improve adhesion.
Non-metallic coating:
Epoxy resin coating: suitable for room temperature corrosive environment, need to be sandblasted to Sa2.5 level first, coated with 2~3 layers, total thickness ≥300μm.
Polytetrafluoroethylene (PTFE) lining: suitable for strong corrosive media (such as acid, alkali), using molding or winding method of construction, the thickness of the lining ≥ 2mm, need to ensure that there is no bubble or wrinkle.
Note: No gap between the coating and the substrate, to avoid the formation of new crevice corrosion sources.
Intergranular corrosion repair
Scenario: due to improper heat treatment or high-temperature service leads to grain boundary chromium depletion, corrosion along the grain boundary expansion.
Repair steps:

Solid solution treatment:
Heat the tee to 1050~1100℃ (2205 steel) or 1100~1150℃ (2507 steel), keep warm for 30~60 minutes, so that the carbide and intermetallic phase are fully dissolved.
Rapid water cooling (cooling rate ≥ 50 ℃ / s), inhibit the cooling process of precipitates regeneration, to restore a uniform duplex organization.
Performance verification:
Intergranular corrosion test (e.g. ASTM A262 E method) is performed after repair to detect the corrosion tendency of the specimen in boiling sulfuric acid - copper sulfate solution, and it is required that there is no intergranular cracking after bending.
If solid solution treatment is not feasible (such as large-scale equipment can not be disassembled), can be used for local remelting (such as laser cladding), the use of high-energy density heat source to quickly melt the corrosion area, the formation of a new uniform organization.
Stress Corrosion Cracking (SCC) Repair
Scenario: Cracks are triggered by stress + corrosion synergistically, commonly found in high-temperature and high-pressure environments containing Cl-.
Repair strategy:

Crack removal and patch welding:
Remove cracks by grinding wheel or EDM, and make U-type or V-type bevels with a depth of 2~3mm above the crack tip.
Adopt low hydrogen welding process (such as TIG welding), apply reverse stress to offset the residual stress during welding, and carry out stress relief annealing after welding (such as 150~200℃ for 2 hours).
Stress optimization:
Adjust the pipe support structure to reduce the mechanical stress at the tee (e.g. vibration, installation stress).
If the medium temperature fluctuates greatly, increase the heat insulation layer or adopt flexible connection (such as metal bellows) to reduce thermal stress.
Environmental control:
If the concentration of Cl- in the medium is too high, the Cl- content can be reduced by ion exchange resin or distillation, or add corrosion inhibitors (e.g., thiourea, benzotriazole) to inhibit corrosion.