Intergranular corrosion vs. corrosion resistance comparison: SUS321 stainless steel

What Is Intergranular Corrosion?

In the performance evaluation of stainless steel, intergranular corrosion resistance is a key subdivision of overall corrosion resistance and often decides long-term reliability in high-temperature and corrosive service. Intergranular corrosion attacks the grain boundaries rather than the grain interiors. It is caused by chromium depletion at the boundaries: when austenitic stainless steel is held in the sensitisation temperature range of 450-850 C - for example during welding or long-term high-temperature service - carbon diffuses to the boundaries and precipitates as chromium-rich carbides of the Cr23C6 type. The chromium-depleted zones adjacent to the precipitates lose their passivity, creating electrochemical potential differences with the grain interiors and driving micro-cell corrosion. In severe cases the material loses strength and can embrittle or perforate without visible surface damage. Unstabilised grades such as 304 are particularly vulnerable because they rely only on a carbon limit of 0.08 percent.

Why SUS321 Resists Sensitisation

SUS321 is the JIS G4303/G4304 designation of the titanium-stabilised austenitic grade, equivalent to UNS S32100, EN 1.4541 (X6CrNiTi18-10) and GB 06Cr18Ni11Ti. Its titanium content is at least five times the carbon content (Ti ≥ 5 x C). Because titanium has a stronger affinity for carbon than chromium, titanium carbide (TiC) forms preferentially and the chromium remains in solid solution, so Cr23C6 cannot precipitate at the grain boundaries. This suppresses chromium depletion and eliminates the driving force for intergranular corrosion. In the sensitisation range - for example long-term service at 650 C - SUS321 reduces the risk of intergranular corrosion by more than 90 percent compared with unstabilised 304, which is why it is specified for welded structures and equipment exposed to the sensitisation range.

ASTM A262 Test Comparison

ASTM A262 defines the standard practices for detecting susceptibility to intergranular attack. The table summarises typical results for solution-treated and sensitised material.

Test Method (ASTM A262)	SUS321 (solution-treated)	304 (solution-treated)	304 (sensitised)
Practice A - oxalic acid etch	No grain-boundary attack	No grain-boundary attack	Networked grain-boundary attack
Practice E - sulfuric acid-copper sulfate	No intergranular corrosion tendency	No tendency	Severe intergranular corrosion

Overall Corrosion Resistance of SUS321

Beyond intergranular corrosion, SUS321 offers well-balanced general corrosion resistance. In nitric acid up to about 65 percent concentration at room temperature, in organic acids such as acetic acid, and in neutral salt solutions such as sodium chloride, its uniform corrosion behaviour is similar to 304 and superior to ferritic 400-series grades. In high-temperature water, for example 300 C pressurised water, the stability of the titanium oxide film gives slightly better resistance than 304, which supports its use in steam piping for nuclear power plants. For pitting and crevice corrosion, the pitting potential Eb is about +0.35 V versus SCE - lower than molybdenum-bearing 316L at about +0.5 V - so in environments with chloride concentrations above roughly 300 ppm, cathodic protection or surface treatment should be considered. In oxidising atmospheres at 600-850 C, SUS321 forms a dense chromium oxide film and resists scaling better than 304, which is rated to about 750 C; this suits heating-furnace radiant tubes and heat-treatment fixtures.

Application Scenarios

Scenario	Intergranular Corrosion Risk	Advantage of SUS321	Risk with Unstabilised Alternatives
Chemical welded pipelines	High (weld heat-affected zone)	Titanium stabilisation prevents post-weld intergranular corrosion	304 may crack and perforate after welding
High-temperature components such as furnace tubes	High (long-term sensitisation temperature)	TiC keeps grain boundaries immune	304 may embrittle after long-term service
Medium-chloride vessels	Medium	Balanced corrosion resistance, more economical than 316L	430 ferritic steel may rust easily
Nitric acid production equipment	Low (non-sensitised environment)	Uniform corrosion resistance similar to 304	Higher cost without added benefit

Frequently Asked Questions

Q: What is the difference between intergranular corrosion and general corrosion?

A: General corrosion attacks the whole exposed surface uniformly; intergranular corrosion attacks only the grain boundaries, which are weakened by chromium depletion, and can cause sudden failure with little visible metal loss.

Q: Why is titanium added to SUS321?

A: Titanium, at a minimum of five times the carbon content, forms stable titanium carbides and prevents chromium carbide precipitation at grain boundaries, keeping chromium in solid solution and preserving corrosion resistance.

Q: What is the sensitisation temperature range?

A: Approximately 450-850 C. Exposure in this range, during welding or service, causes chromium carbides to precipitate at grain boundaries in unstabilised or high-carbon austenitic grades.

Q: How is intergranular corrosion susceptibility tested?

A: ASTM A262 standard practices are used, including the oxalic acid etch test (Practice A) and the sulfuric acid-copper sulfate test (Practice E); both distinguish sensitised material from solution-treated material.

Q: Is SUS321 more corrosion resistant than 316L?

A: Not in pitting resistance: SUS321 has no molybdenum, so its pitting potential (about +0.35 V vs SCE) is lower than 316L (about +0.5 V). SUS321 wins in resistance to intergranular corrosion after high-temperature exposure and is usually more economical.

Frequently Asked Questions

Q1. What causes intergranular corrosion?
Chromium depletion at grain boundaries: in the sensitisation range of 450-850 C, carbon precipitates as chromium-rich Cr23C6 carbides, and the depleted zones lose passivity and drive micro-cell corrosion.

Q2. What are the equivalent designations of SUS321?
UNS S32100, EN 1.4541 (X6CrNiTi18-10) and GB 06Cr18Ni11Ti, per JIS G4303/G4304.

Q3. How does SUS321 resist general corrosion?
Similar to 304 in nitric acid up to about 65 percent concentration at room temperature, in organic acids such as acetic acid and in neutral salt solutions, and slightly better than 304 in high-temperature water such as 300 C pressurised water.

Q4. How does SUS321 behave in oxidising atmospheres at high temperature?
It forms a dense chromium oxide film at 600-850 C and resists scaling better than 304, which is rated to about 750 C, suiting heating-furnace radiant tubes and heat-treatment fixtures.

Q5. What is the pitting resistance of SUS321?
The pitting potential Eb is about +0.35 V versus SCE, lower than molybdenum-bearing 316L at about +0.5 V; above roughly 300 ppm chlorides, cathodic protection or surface treatment should be considered.