What are the main types of corrosion of alloy steel pipes in marine environments?
Pitting, crevice corrosion and stress corrosion cracking (SCC) are common in marine environments, and chloride ion penetration is the main cause. Duplex stainless steel 2205 forms a passive film through 35% chromium content, and the pitting resistance equivalent PREN ≥ 35. Deep-sea pipelines also need to consider microbial corrosion (MIC), and 0.1% copper needs to be added to inhibit it. Coating + cathodic protection is a general solution, but the protection potential needs to be tested regularly.
Mechanism and protection of high-temperature sulfidation corrosion?
In the FCC unit of the refinery, sulfur reacts with nickel in steel to form low-melting point Ni₃S₂ (melting point 645℃), which accelerates corrosion. The selection of alloys with an Al content of ≥5% (such as 309Al) can form an Al₂O₃ protective layer. When designing, the metal temperature must be controlled to be more than 30℃ lower than the sulfur dew point, or aluminized steel pipes must be used (life extended by 3 times).
How to design a cathodic protection system?
The sacrificial anode method mostly uses zinc or magnesium alloys, and the output current needs to be ≥1mA/m²; the impressed current method requires a constant potentiostat (-0.85V vs CSE). Deep-sea pipelines use bracelet anodes, with one group arranged every 200 meters. Coke backfill needs to be added to high-resistivity soils, and the effectiveness of the system needs to be verified by the IR drop compensation method.
Common causes of coating failure?
Including mechanical damage (FBE coating impact resistance needs to be ≥10J), cathodic stripping (24-hour test stripping radius <8mm), high temperature degradation (3LPE coating temperature limit 80℃). Arctic pipelines need to pay special attention to low-temperature embrittlement, and polypropylene coating is better than polyethylene. During construction, the surface treatment must reach Sa2.5 level, and the anchor pattern depth is 50-75μm.
How to evaluate the remaining life of alloy steel pipes?
Use ultrasonic thickness measurement (annual corrosion rate = (initial thickness-current thickness)/service life), corrosion coupon weight loss method, electrochemical impedance spectroscopy (EIS), etc. Machine learning models can be combined with historical data for prediction, such as BP's CorrExpert system error <15%. Chemical pipelines usually reserve a 2mm corrosion margin.
