Intergranular corrosion vs. corrosion resistance comparison: SUS321 stainless steel

Intergranular Corrosion vs. Corrosion Resistance: Core Advantages and Applications of SUS321 Stainless Steel

In the performance evaluation system for stainless steel, intergranular corrosion resistance is a key subdivision indicator of corrosion resistance, directly determining the long-term reliability of materials in high-temperature and corrosive environments. SUS321 stainless steel (06Cr18Ni11Ti), through the precise addition of titanium (Ti), forms unique advantages in controlling intergranular corrosion and enhancing overall corrosion resistance, particularly in scenarios that require strict resistance to corrosion in welded structures. The following is a comparative analysis from the perspectives of mechanism, performance, influencing factors, and application scenarios:

1. What is Intergranular Corrosion and Corrosion Resistance of SUS321 Stainless Steel?

Intergranular Corrosion: This type of corrosion occurs at the grain boundaries, caused by chromium depletion at the grain boundaries (forming Cr₂₃C₆ carbides), which leads to electrochemical potential differences and the formation of micro-cell corrosion. Austenitic stainless steels (such as 304) are prone to intergranular corrosion when subjected to long-term service or welding in the "sensitization temperature range" (450-850°C). The precipitation of carbides can lead to the sudden drop in material strength and structural embrittlement.

Corrosion Resistance: This refers to the material's ability to resist corrosion in chemical media, covering various forms such as uniform corrosion, pitting corrosion, crevice corrosion, and stress corrosion. SUS321's corrosion resistance is a "comprehensive ability," while intergranular corrosion resistance is a specific optimization for the "weak points" at the grain boundaries.

Intergranular Corrosion Resistance of SUS321:

SUS321 has a titanium content (Ti≥5×C%) that forms titanium carbide (TiC), as titanium has a stronger affinity for carbon than chromium, preventing the formation of Cr₂₃C₆ at the grain boundaries. This effectively suppresses chromium depletion at the grain boundaries.

In comparison to unstabilized 304 stainless steel (which relies on controlling carbon content ≤0.08% to manage intergranular corrosion), SUS321 reduces the risk of intergranular corrosion in the sensitization temperature range (e.g., long-term service at 650°C) by over 90%.

Performance Data Comparison (ASTM A262 Standard Testing):

Test Method	SUS321 (Solution-treated)	304 (Solution-treated)	304 (Sensitized)
Oxalic Acid Electrolytic Corrosion	No corrosion at grain boundaries	No corrosion at grain boundaries	Networked corrosion at grain boundaries
Sulfuric Acid - Copper Sulfate Method	No intergranular corrosion tendency	No tendency	Severe intergranular corrosion

 

2. Comprehensive Corrosion Resistance of SUS321

Uniform Corrosion Resistance: In environments with nitric acid (≤65% concentration, at room temperature), organic acids (such as acetic acid), and neutral salt solutions (such as NaCl), SUS321's corrosion resistance is similar to 304 and superior to ferritic stainless steel (400 series).

High-Temperature Water Resistance: In high-temperature water (e.g., 300°C high-pressure water), due to the stability of the titanium oxide film, SUS321's corrosion resistance is slightly better than 304, making it suitable for steam pipes in nuclear power plants.

Pitting and Crevice Corrosion Resistance: The pitting potential (E_b) is about +0.35V (vs SCE), slightly lower than that of 316L (which contains Mo and has E_b ≈ +0.5V). Therefore, in environments with high Cl⁻ concentration (e.g., >300ppm), cathodic protection or surface treatments should be applied.

High-Temperature Oxidation Resistance: SUS321 forms a dense Cr₂O₃ oxide film when subjected to long-term service at 600-850°C, providing better oxidation resistance than 304 (which has a maximum temperature tolerance of 750°C). It is suitable for use in heating furnace radiation tubes and heat treatment fixtures.

4. Application Scenarios Comparison: Intergranular Corrosion Resistance Leading to Material Selection in Specific Fields

Scenario	Intergranular Corrosion Risk	Advantages of SUS321	Risks of Alternative Materials
Chemical Welding Pipelines	High (Weld Heat-Affected Zone)	Titanium-stabilized design prevents post-weld intergranular corrosion	304 may crack and perforate after welding
High-Temperature Corrosion-Resistant Components (e.g., furnace tubes)	High (Long-Term Sensitization Temperature)	Titanium suppresses TiC precipitation, maintaining grain boundary corrosion resistance	304 may embrittle after long-term service
Medium Cl⁻ Environment Vessels	Medium	Balanced overall corrosion resistance, more cost-effective than 316L	430 ferritic steel may rust easily
Nitric Acid Production Equipment	Low (Non-Sensitized Environment)	Uniform corrosion resistance similar to 304, with a better cost-performance ratio