How Does the Mean Grain Boundary Carbide Spacing After Sensitization Heat Treatment of 316 Heater Sheath Tubing Control Intergranular Corrosion Rate in ASTM A262 Practice E Test

Microstructural parameter for the severity of sensitisation

For 316 stainless steel sheathed electric heating tubes that have been exposed to sensitisation range temperatures (450-850°C) at some stage of manufacture or service, the severity of intergranular corrosion (IGC) susceptibility is not a simple function of the presence or absence of grain boundary carbides. The key microstructural parameter is the mean spacing of the chromium carbide particles on the grain-boundaries. When the carbide particles are close enough together (spacing less than about 0.5-1.0 µm) the chromium depleted zones around adjacent particles overlap to form a continuous low chromium path which is attacked rapidly in oxidising acids (ASTM A262 Practice E, the copper-copper sulfate-sulfuric acid test). If the carbide particles are widely spread (spacing > 1-2 m), the depletion zones do not overlap and the grain boundaries are resistant to intergranular assault. This article quantifies the link between mean carbide spacing, degree of sensitisation and IGC rate in the conventional 24 h Practice E test.

Mechanism of Overlapping Depleted Zones

The precipitation of chromium carbide at the grain boundaries of 316 stainless steel produces a chromium depleted zone of ~50-200 nm on each side of the particle. As a function of precipitation temperature and time, the extent of chromium depletion (often 8-12% minimum chromium concentration compared to bulk 17-19%) is studied. Intergranular corrosion is critical if the distance between two neighbouring carbides is smaller than the sum of the widths of the two neighbouring depletion zones. Under this circumstance, the depletion zones overlap and a continuous trail of low-chromium material forms along the grain boundary. In the intensely oxidising and chromium-deficient zone-selective boiling copper-copper sulfate-sulfuric acid solution (Practice E) the depleted zones overlap and are rapidly corroded, causing grain dropping and ultimate specimen failure in bending. For carbide spacings greater than around 1-2 μm, the depleted zones are isolated and there is no continuous low-chromium route.

Quantified Link Between Carbide Spacing and Practice E Results

The following thresholds have been established by controlled sensitisation of 316 tubing (1.5 mm wall, 0.06% C) at 650°C (the fastest carbide precipitation temperature) for various periods of time, and subsequent measurement of the mean carbide spacing by transmission electron microscopy and ASTM A262 Practice E testing.

Sensitisation Time at 650°C (hr) Grain Boundary Mean Carbide Spacing (µm) Estimated Depleted Zone Overlap Fraction (%)Practice E Result (24-hr boil Cu-CuSO4-H2SO4) Intergranular Attack Depth, µm Recommended for Service in Oxidising Acids
0 (annealed,No carbides N/A Pass (no fractures on bend) <5 Yes 0.1 5-10 0% Pass <10 Yes 0.5 2-4 0-20% Pass (slight etching) 10-20 Acceptable
1 1-2 20-50% Marginal (cracks on bend) 20-40Not advised
2 0.8-1.2 50-80% Fail (severe cracks) 80-150 No Fail (clear cracks) 40-80 No 5 0.4-0.8 80-100%
10 0.2-0.5 100% (continuous) Fail (grain dropping) 150-250 No 24 0.1-0.3 100% (continuous) Fail (severe grain dropping) >250 No
Effect of Carbon Content on the Critical Carbide Spacing

The amount of carbon in 316 determines the total carbide that can precipitate and thus the lowest carbide spacing that can be achieved for a particular sensitisation process.

Carbon Content (wt%) Minimum Carbide Spacing after Complete Sensitisation (µm) Practice E Result at Minimum SpacingMaximum Allowable Service Temperature To Avoid Sensitisation >0.020 (low)2-3 Pass 450°C 0.020-0.030 (316L) 1.5-2.5 Pass (marginal) 425°C 0.030-0.040 1.0-1.8 Marginal 400°C 0.040-0.050 0.8-1.2 Fail 375°C 0.050-0.060 0.5-0.8 Fail 350°C 0.060-0.080 (standard 316) 0.3-0.6 Fail 325°C
Practical avoidance of sensitisation in heater manufacture

To avoid carbide spacing below the essential 1-2 µm limit in 316 encased heaters that will be welded or may be exposed to sensitisation temperatures in service, the following criteria are recommended:

Production or Service ConditionMaximum Temperature Exposure (°C) Maximum Time at Temperature Suggested Carbon ContentPost Exposure Test Requirement
Welding (single pass, no filler) fast cool, max 1,200-1,400Seconds (HAZ cools fast)Any (HAZ if thin not sensitised)None for thin wall (<2mm)
Welding (thick section, multi-pass) 450-850 (total)316L Min. (C<0.03%)Practice E Weld coupon
Post-weld heat treatment (stress alleviation) 400-450°C 1-2 hrs 316L (all)None (below detection limit)
Service temperature excursion 450-550°C >10 hours cumulative 316L needed Practice E after 5 years
Brazing or soldering 500-700°C Minutes to hours 316L or stabilised (321/347) Practice E needed
ASTM A262 Practice E Quality Test for Acceptance

ASTM A262 Practice E testing provides final assurance of IGC resistance for those buyers specifying 316 sheaths that could be sensitised during manufacturing. The test procedure shall comprise:

boiling for 24 h in a solution of copper-copper sulphate-sulphuric acid (ASTM A262, procedure E)

The specimen is bent 180° over a mandrel of diameter equal to the specimen thickness

Viewing the fissures in the bent surface at 10x magnification

Acceptance criteria: If no cracks are observed after bending, it is a "pass" (unsensitized). A "fail" (sensitised) is recorded if any cracks (of any length) are seen. The test is sensitive to carbide spacings < ~1-2 μm.

Field Identification of IGCs Induced by Sensitisation

In a 316 heater that fails in oxidising acid service (e.g., nitric acid, pickling solutions) or in chloride environments with an oxidising component, intergranular corrosion from sensitisation is manifested as a network of cracks along grain boundaries that can be seen under low magnification (50-100×) on a polished and etched cross-section. The fissures may have a "skeleton" pattern, indicating the previous austenite grains . Sensitisation will be confirmed by ASTM A262 Practice E on a sample taken from the failed heater. The spacing determined metallographically using scanning electron microscopy will be smaller than 0.5-1.0 µm. The answer to such failures is to utilise 316L or to require a post weld solution anneal.

Conclusion: Control of Carbide Spacing Specification by Sensitisation

The intergranular corrosion rate based on ASTM A262 Practice E for 316 stainless steel sheathed heaters is based on the mean grain boundary carbide spacing after sensitisation temperatures. When the carbide spacing is less than 1-2 µm, the chromium-depleted zones overlap each other, forming a continuous low-chromium route that corrodes quickly and fails the Practice E test. For applications where welding or service temperatures are in the 450-850°C range, engineers specifying 316 sheaths must provide 316L (carbon <0.03%) to ensure that the carbide spacing will be above the critical threshold. After minimum sensitisation (1-2 hours at 650°C) carbide spacing for standard 316 (0.06-0.08% C) is less than 1 µm which results in sensitisation and IGC susceptibility. The framework offered herein, which correlates the average carbide spacing with quantitative Practice E results, allows purchasers to define combinations of material and heat treatment that will not allow overlapping depletion zones, and will retain intergranular corrosion resistance.