What is its oxidation mechanism in high-temperature steam?
In steam above 400°C, chromium selectively oxidizes to form a protective Cr2O3 film (approximately 2-5μm thick). When temperatures exceed 570°C, the Fe3O4 layer begins to grow epitaxially, accelerating oxidation. After long-term operation, the oxide film may flake off, and the pipe wall thinning rate should be monitored (<0.1mm/year). Adding trace amounts of rare earth elements (such as Ce) can improve oxide film adhesion. ASTM G54 provides specific testing methods.
How to resist hydrogen sulfide stress corrosion?
Hardness should be controlled to ≤22HRC to reduce SCC susceptibility. When the H2S partial pressure in the medium is greater than 0.0003MPa, NACE TM0177 testing should be performed. Heat-treated steel pipes are superior to rolled steel pipes because tempered steel pipes offer greater crack resistance. Welded joints are the weakest link, so thorough PWHT is essential. Consider spraying an aluminum coating (150-200μm) on the interior for enhanced protection.
How does its corrosion resistance compare to 304 stainless steel?
In oxidizing acids (such as nitric acid), 304 stainless steel's corrosion resistance is over 10 times that of 15CrMoG. However, in hot water containing chloride ions (>60°C), 15CrMoG exhibits superior stress corrosion resistance. In high-temperature sulfide environments, 304 is susceptible to intergranular corrosion, while 15CrMoG's chromium-molybdenum combination is more stable. Economically, 15CrMoG's price is only one-third that of 304. Material selection should be based on a comprehensive assessment of the media's characteristics.
Can surface treatment improve corrosion resistance?
Shot peening (0.2-0.3mm coverage) can introduce compressive stress, delaying crack initiation. Aluminizing (900°C x 4h) can increase the surface aluminum content to 30%, increasing high-temperature oxidation resistance by fivefold. Electroplated nickel (50-100μm) is suitable for acidic environments, but the risk of hydrogen embrittlement should be considered. Ceramic coatings (such as Al2O3) can be used in extremely corrosive conditions. All coatings must pass the ISO 4628 adhesion test.
How to assess corrosion damage after long-term service?
Use an ultrasonic thickness gauge to regularly monitor wall thinning (focusing on elbows). Metallographic replication techniques can detect the depth of surface microcracks. Energy dispersive spectroscopy (EDS) can determine the composition of corrosion products and identify the type of corrosion. Remaining life assessment requires the calculation of Larson-Miller parameters. For pipelines with more than 100,000 hours of service, a comprehensive inspection is recommended every five years.
