How do 316L stainless steel welded pipes and 304 stainless steel welded pipes perform in terms of

The difference in corrosion resistance of 316L stainless steel welded pipes and 304 stainless steel welded pipes under high temperatures mainly stems from the composition design (molybdenum content, carbon content) and the stability of the passivation film. The specific comparison is as follows: 

I. Core Differences in High Temperature Corrosion Resistance

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II. Performance of High Temperature Corrosion Scenarios

1. Chlorine-containing medium under high-temperature conditions (such as seawater desalination, chemical reactors)

316L:

Resistance to pitting corrosion: Mo forms MoO₄²⁻ to fill the defects in the passivation film and inhibit Cl⁻ adsorption. The critical pitting corrosion temperature (CPT) reaches 110℃ (304 is 80℃).

Resistance to crevice corrosion: In 600℃ Cl⁻-containing flue gas condensate (Cl⁻ 500ppm), the annual corrosion rate is only 0.3-0.8mm (304 is 1.5-3mm).

Resistance to Cl⁻ activation oxidation: MoO₃ reacts with CrCl₃ to form stable Cr₂(MoO₄)₃, inhibiting Cr loss, and the oxidation rate at 700℃ is 50% lower than that of 304.

304:

At high temperatures (>500℃), Cr₂O₃ is prone to volatilization, and Cl⁻ penetrates the passivation film to cause pitting corrosion. At 600℃, the CPT drops below 50℃.

Long-term exposure to high-temperature environments containing Cl⁻ (such as flue gas desulfurization systems) is prone to intergranular corrosion, and annealing treatment is required to alleviate the problem.

2. High-temperature acidic environment (such as sulfuric acid, nitric acid reactors)

316L:

In 10% H₂SO₄ (600℃), the corrosion rate is less than 0.1mm/year, which is better than 304's 0.3mm/year.

Stronger resistance to HNO₃ oxidation, with a year-on-year weight loss of only 5g/m² at 800℃ (304 is 15g/m²).

304:

At high temperatures (>70%) concentrated sulfuric acid, it has poor stability and is prone to sulfur corrosion; in dilute sulfuric acid, the temperature needs to be controlled below 500℃.

3. High-temperature oxidation and sulfur corrosion (such as petroleum cracking furnaces)

316L:

Short-term resistance to 870℃ high temperature, Mo inhibits the reaction of sulfur oxides (SO₂/SO₃) with Cr to form volatile CrS, reducing oxidation weight loss.

In high-temperature environments with H₂S (such as hydrogenation reactors), its ability to resist sulfur stress corrosion cracking (SSCC) is significantly better than 304.

304:

At temperatures above 800℃, the stability of the Cr₂O₃ film decreases, and the sulfur corrosion rate accelerates. The usage temperature needs to be limited to <700℃. 

III. Comparison of High-Temperature Mechanical Properties

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IV. Selection Suggestions

1. Preferred scenarios for 316L:

Temperature > 500℃ and containing Cl⁻ (such as seawater desalination, chemical reactors)

High-temperature acidic environment (H₂SO₄/HNO₃ treatment system)

Requires long-term antioxidant and anti-sulfur corrosion (petroleum cracking furnaces, gas turbine blades)

2. Consider 304 for scenarios:

Temperature ≤ 600℃ and without Cl⁻ (such as ordinary boiler feed water pipes)

Dry gas transportation (such as high-temperature steam pipes)

Cost-sensitive and less corrosive environment 

V. Cost vs. Lifespan Trade-off

Cost: Due to the Mo and low-carbon design, the price of 316L is 30% - 50% higher than that of 304.

Lifespan: In an environment with high temperature and Cl⁻ ions, the lifespan of 316L can reach 3 - 5 times that of 304, with a lower overall cost. 

Summary

316L, through the incorporation of Mo elements and the use of low-carbon design, forms a more stable composite passivation film at high temperatures, significantly outperforming 304 in terms of resistance to Cl⁻ corrosion, oxidation resistance, and creep resistance. It is suitable for high-temperature and highly corrosive core scenarios. While 304, due to its economic nature, is applicable to conventional high-temperature environments with lower temperatures or weaker corrosiveness.