Improvement of corrosion resistance of alloy seamless pipes: Effect of Cr and MO element content on service life

I. Chromium is a key element in the formation of passivation film in alloy seamless pipes. Its content must reach a critical value (usually ≥12%) to form a dense Cr₂O₃ oxide film that effectively isolates the corrosive medium. Chromium improves the material life in the following ways:

(1) Passivation film stability: In acidic or chlorine-containing environments, the Cr₂O₃ film can significantly slow down the uniform corrosion rate. For example, the corrosion rate of 304 stainless steel in 5% NaCl solution can be reduced by more than 80%.

(2) Electrode potential enhancement: High chromium content (such as 20-30%) causes the metal electrode potential to increase suddenly, inhibiting electrochemical corrosion, especially in high-temperature oxidizing acids (such as nitric acid).

(3) High-temperature oxidation resistance: When the chromium content exceeds 15%, the alloy can still maintain the integrity of the oxide film above 600°C, extending the high-temperature service life.

II. Synergistic optimization of corrosion resistance by molybdenum (Mo) element

(1) Molybdenum forms a synergistic effect with chromium by refining the passive film structure and inhibiting local corrosion:

(2) Resistance to pitting and crevice corrosion: Mo can enhance the repair ability of the passive film. For example, the corrosion resistance of 316 stainless steel (containing 2-3% Mo) in marine environment is significantly better than that of 304 stainless steel, and its service life is extended by 2-3 times.

(3) High-temperature corrosion resistance: When molybdenum and chromium work together, a stable chloride passivation film is formed in chlorine-containing high-temperature flue gas, and the corrosion rate can be reduced by 85%.

(4) Content threshold effect: When the molybdenum content reaches 5%, the pitting resistance is significantly improved. When the content is above 15% (such as Hastelloy C276 alloy), an ultra-long service life can be achieved in strong reducing acid.

III. Cr-Mo synergistic effect and life prediction model

Composition design principles:

Conventional corrosion environment: Cr ≥ 12%, Mo 2-3% (such as 316 stainless steel) can meet the service requirements of more than 10 years.

Extreme environment: Cr 20-30%, Mo 10-15% (such as NS333 alloy) in chlorine-containing flue gas at 550℃, the corrosion loss is only 0.03mm in 3 years.

Life degradation mechanism: Low Cr-Mo alloys (such as 15CrMo) are prone to film rupture below 500℃ and need to be combined with passivation treatment; high Cr-Mo alloys achieve long-term protection through self-repairing film.

IV. Application scenarios and material selection recommendations

(1) Petrochemical pipelines: 12Cr5Mo (Cr 12%, Mo 0.5%) is recommended, taking into account both cost and sulfide corrosion resistance.

(2) Nuclear power equipment: NS333 (Cr 20%, Mo 10%) is used to resist high-temperature radiation corrosion.

(3) Offshore platforms: Duplex stainless steel containing Mo ≥ 4% is preferred to inhibit chloride ion penetration.