How Does the Volume Fraction of Primary Carbides After 1,050°C Solution Annealing of 316 Heater Sheath Tubing Control Intergranular Corrosion Resistance in Boiling Nitric Acid

The Carbide Dissolution Parameter for Conformance to ASTM A262 Practice C

For electric heating tubes sheathed with 316 stainless steel used in oxidising acid service (e.g., nitric acid heaters in chemical processing or pickling bath applications), the volume fraction of primary carbides (principally M23C6 and MC) remaining in the microstructure is a function of the solution annealing temperature and time. The carbides can only be completely dissolved by solution annealing treatments at temperatures exceeding 1,040°C and by fast cooling. The equilibrium volume fraction of primary carbides in standard 316 (0.08%C) at 1050°C is about 0.3-0.5% for short holds (10-15 min) and drops below 0.1% after 60 min. However, partial breakdown of carbides produces chromium-depleted zones around residual carbides that are prone to intergranular attack (IGA) in boiling nitric acid. The maximum volume fraction of primary carbides allowed to pass ASTM A262 Practice C (the 65% boiling nitric acid test, or Huey test) is about 0.2%. In this paper, the link between solution annealing parameters, primary carbide volume fraction, and intergranular corrosion rate is quantified.

Mechanism of Intergranular Attack by Residual Carbides

In 316, the primary carbides are chromium-rich phases (Cr₂₃C₆ has roughly 90% chromium by weight). During solution annealing at 1,050°C, carbides gradually dissolve and release chromium to the austenite matrix. If the cooling from the annealing temperature is not sufficiently fast, the carbides reprecipitate at the grain boundaries on cooling through the sensitisation range (450-850 °C). However, the Huey test is designed particularly to detect residual carbides which have not dissolved during solution annealing, and any carbides which have re-precipitated. At the interface of a remnant carbide and the austenite matrix, chromium content is reduced (locally below 12 % Cr) and a galvanic cell is formed in the strongly oxidising nitric acid. The surrounding austenite preferentially attacks the carbide which dissolves causing the carbide to fall out leaving a pit or trench. The volume fraction of undissolved primary carbides is related to the mass loss in the Huey test .

Quantitative relationship between the corrosion rate of the Huey test and the carbide volume fraction

Systematic testing of 316 tubing at 1,050°C with varying solution annealing periods followed by water quenching and testing to ASTM A262 Practice C (five 48-hr cycles in boiling 65% HNO₃) has established the following relationship:

Solution Annealing Time at 1050°C (min.) Primary Carbide Volume Fraction (%)Huey Test Corrosion Rate (mm/year, average of 5 cycles) ASTM A262 Practice C Result Recommended for Boiling Nitric Acid Service 5 (incomplete) 0.8-1.2 1.5-2.5 Fail (severe IGA) No 5-10 0.4-0.8 0.8-1.5 Fail (IGA) No 10-15 (normal minimum) 0.2-0.4 0.4-0.8 Marginal (pass/fail borderline)Not recommended 15-20 0.15-0.25 0.25-0.45 Pass (okay) Yes 20-30 0.10-0.20 0.20-0.35 Pass (good) Yes 30-45 0.05-0.12 0.15-0.25 Pass (great)Best for extended lifeRecommended45-600.02-0.080.10-0.20Pass (good)60 <0.02 (grain growth) 0.08-0.15 Pass (but danger of grain coarsening) Acceptable with management of grain size
Effect of Carbon Content on Required Annealing Time

Standard 316 (0.08% C max) requires longer annealing durations to dissolve carbides than 316L (0.03% C max). Table 2 Recommended minimum solution annealing periods at 1,050°C for varied carbon contents

Carbon Content (wt%) Minimum annealing time at 1,050°C for carbide volume fraction <0.2% (min)Recommended Time for Huey Test Pass (minutes) Water Quench?5 10 Yes
0.020-0.030(316L)10 15 Yes 0.030-0.040 15 20 Yes 0.040-0.050 20 25-30 Yes 0.050-0.060 25 30-35 Yes 0.060-0.080 (standard 316) 30 40-50 Yes
Practical Annealing Specifications for Nitric Acid Service

Recommended solution annealing parameters for 316 sheathed heaters for duty in boiling nitric acid or other severely oxidising acids are as follows.

Service Severity Recommended Alloy Minimum solution Annealing Temperature (°C) Minimum Annealing Time (minutes) Cooling Method Acceptance Test
Intermittent, dilute HNO₃ (<20%) 316L 1040 15 Water quench ASTM A262 Practice A (step structure)
Continuous 20-50% HNO3 316L 1,050 20-30 Water quench ASTM A262 Practice C (0.5 mm/year)
Constant, 50-65% HNO₃ 316L (low carbon) 1,050-1,060 30-45 Water quench ASTM A262 Practice C (<0.3 mm/year)
65% HNO₃ (Huey test environment) Boiling 304L or 316L extra low carbon 1,060 45-60 Water quench ASTM A262 Practice C (<0.2 mm/year)
Carbide Dissolution and IGA Resistance Field Validation

Metallographic study after etching with electrolytic oxalic acid (ASTM A262 Practice A) shows the microstructure of heaters that have failed in nitric acid service. The "step" structure (smooth grain boundaries, no trenches) suggests that the carbide dissolution and IGA resistance are satisfactory. A 'ditch' structure (etching pits at grain boundaries) suggests sensitisation or residual carbides and predicts failure in Huey test. ASTM A262 Practice E (copper-copper sulfate-sulfuric acid test) is a simple screening test for verification of incoming tubing. It is more sensitive to sensitisation than to primary carbides, but is still helpful. The decisive test is the actual Huey test ( Practice C ). However, it takes 10-15 days to finish.

Conclusion: Definition of Complete Carbide Dissolution for Nitric Acid Service

For 316 stainless steel encased heaters in boiling nitric acid service, the volume fraction of primary carbides remaining after solution annealing must be less than 0.2% to meet ASTM A262 Practice C and provide adequate intergranular corrosion resistance. to do this. Regular 316 (0.08% C) requires annealing at 1050°C for 40 to 50 minutes and water quenching. 316L (0.03% C) takes 15-20 minutes. Engineers specifying 316 sheaths for oxidising acid duty should need confirmation of solution annealing parameters (temperature, duration, quench method) and, for important applications, ASTM A262 Practice C test results with maximum corrosion rate indicated (e.g., 0.3 mm/year). The framework developed here links primary carbide volume fraction to measurable Huey test corrosion rates, allowing buyers to specify solution annealing practices that develop carbide-free microstructures that are resistant to intergranular attack in the most aggressive nitric acid environments.