Q1: What is the corrosion mechanism of A335 steel pipe in high temperature sulfidation environment?
In oil products containing H₂S, A335 steel pipe will undergo high temperature sulfur corrosion (H₂S+Fe→FeS+H₂), forming a loose iron sulfide film, resulting in continuous thinning. P5 steel contains 5% Cr, which is 3~5 times more corrosion resistant than carbon steel, but the wall thickness still needs to be monitored when the temperature is >260°C. The corrosion rate is positively correlated with the H₂S partial pressure, flow rate and Cl⁻ content. Protective measures include injecting corrosion inhibitors and controlling the flow rate to <15m/s. Regular UT thickness measurement is necessary, and the remaining wall thickness must meet the ASME B31G calculation requirements.
Q2: How to improve the corrosion resistance of A335 through coating technology?
The inner coating can be glass flake epoxy (temperature resistance ≤120°C) or zinc silicate (temperature resistance ≤400°C) for water pipelines. FBE (fusion bonded epoxy) or 3LPE (three-layer polyethylene) is commonly used for external corrosion protection, and sandblasting is required to Sa2.5 cleanliness. For high-temperature parts, aluminum or chromium alloy (such as 80Ni20Cr) can be thermally sprayed to form an oxidation protective layer. Pinhole detection (≥5kV DC spark) is required after coating construction. Attention should be paid to the compatibility of the coating with cathodic protection to avoid peeling.
Q3: What media can cause stress corrosion cracking of A335 steel pipes?
Wet H₂S environment (＞50ppm) may cause SCC (must meet NACE MR0175 standard). High-temperature alkali solution (such as NaOH＞5%, temperature＞50°C) will cause alkali embrittlement, especially in the weld area. Polythionic acid (H₂SₓO₆, formed during shutdown) poses a great threat to welds between austenitic stainless steel and A335 dissimilar steel. CO₂-CO-H₂O system may also cause cracks at certain pH. Preventive measures include reducing residual stress, controlling medium purity and adding inhibitors.
Q4: Can austenitizing improve the oxidation resistance of A335?
Austenitizing (such as quenching P91 steel at 1040°C) can refine the grains, increase the solid solubility of chromium, and enhance high-temperature oxidation resistance. The dense Cr₂O₃ film formed after treatment can resist steam oxidation above 600°C. However, tempering (such as 780°C) is required to avoid brittleness. In actual applications, the oxidation rate of P91 steel after normalizing + tempering is 30%~50% lower than that of P22. It should be noted that the peeling of the oxide film may cause erosion of downstream turbines, and regular eddy current testing (ECT) evaluation is required.
Q5: How to detect the wall thickness thinning of A335 steel pipes in service?
Conventional methods include ultrasonic thickness measurement (UT, accuracy ±0.1mm), and fixed-point monitoring of elbows, tees and other corrosion-prone parts every 3 months. Pulsed eddy current (PEC) technology can be used in high-temperature areas without stripping the insulation layer. X-ray DR detection can be combined with wall thickness mapping software to generate a three-dimensional corrosion map. For the inside of the pipeline, the intelligent pipe cleaning device (PIG) equipped with electromagnetic or ultrasonic probes can achieve long-distance continuous detection. The data needs to be recorded and compared with the ASME B31G residual strength formula to evaluate the life.
