What are the differences in corrosion resistance between 316L stainless steel welded pipes and 304

The difference in corrosion resistance between 316L stainless steel welded pipes and 304 stainless steel welded pipes mainly stems from the differences in composition, especially the addition of molybdenum (Mo) and the control of carbon content. The specific comparison is as follows: 

1. Core Component Differences

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II. Corrosion Resistance Comparison

1. Resistance to Chloride Ion Corrosion (Cl⁻ environment)

316L: Molybdenum forms MoO₄²⁻, which, in combination with Cr₂O₃, forms a dense passivation film that inhibits Cl⁻ penetration. In seawater, salt water, or chemical media containing Cl⁻, the corrosion rate is only 1/3 - 1/5 of that of 304. For example, in a 3.5% NaCl solution, the critical crevice corrosion temperature (CCT) of 316L reaches 60 - 80℃, while that of 304 is only 40 - 50℃.

304: Without molybdenum, the passivation film is easily destroyed by Cl⁻, and long-term exposure to high-salt environments (such as seawater) is prone to pitting and local corrosion.

2. Resistance to High-Temperature Oxidation and Corrosion

316L: Molybdenum and Cr form a Cr₂O₃-MoO₃ composite oxide film, which remains stable at 600 - 700℃ and inhibits the volatilization of CrCl₃, reducing oxidation weight loss (annual weight loss of 10 - 15g/m² at 700℃, while 304 reaches 30 - 40g/m²).

304: At high temperatures (>800℃), Cr₂O₃ easily volatilizes, and the oxidation rate increases, and without molybdenum to inhibit the "active oxidation" caused by Cl⁻.

3. Resistance to Pitting and Crevice Corrosion

316L: The equivalent value for resistance to pitting (PREN = 32 - 38) is significantly higher than that of 304 (PREN = 20 - 24), the pitting incubation period is prolonged by 3 - 5 times, and the corrosion depth growth rate is only 1/4 - 1/3 of that of 304.

304: In a weak acid environment containing Cl⁻ (such as pH 3 - 5), pitting is prone to occur at surface defects and rapidly expand.

4. Intergranular Corrosion Resistance

316L: Low-carbon design (C ≤ 0.03%) reduces the precipitation of chromium carbide during welding, avoiding chromium depletion at the grain boundaries, and has better intergranular corrosion resistance than 304.

304: The welding heat-affected zone is prone to the precipitation of chromium carbide, leading to increased intergranular corrosion sensitivity, and requires annealing treatment to alleviate. 

III. Typical Corrosion Scenarios Presentation

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IV. Selection Suggestions

 316L: Suitable for scenarios involving seawater, high salt spray， chemical media containing Cl⁻， high-temperature acidic environments， or those requiring frequent welding (such as marine engineering, pharmaceutical equipment).

Consider 304: Applicable to dry atmosphere， fresh water， weakly acidic and weakly alkaline environments (such as ordinary building decoration， non-corrosive sections of food processing). 

V. Cost and Lifespan Trade-off

Cost: The 316L has a higher price than the 304 due to its higher content of molybdenum and nickel, with a price increase of 30% to 50%.

Lifespan: In an environment with Cl⁻ ions, the lifespan of 316L can reach 3 to 5 times that of 304. Long-term use results in a lower overall cost.

In conclusion, the 316L, with its molybdenum element and low-carbon design, exhibits significantly better corrosion resistance in environments containing chlorine, high temperatures, and complex corrosive conditions compared to the 304. While the 304 is suitable for regular non-corrosive scenarios due to its economic nature.