Welding 317L Stainless Steel Tanks for High-Concentration Sulfuric Acid: Preheating at 100–150°C and Post-Weld Heat Treatment

1. Why 317L Stainless Steel for Sulfuric Acid Storage?

317L is a nitrogen-strengthened, low-carbon austenitic stainless steel with:

High corrosion resistance: PREN (Pitting Resistance Equivalent Number) ≥34. ideal for 98% sulfuric acid at 60°C.

Thermal stability: Retains strength up to 600°C, critical for industrial storage.

Anti-sensitization: Low carbon content (<0.03%) minimizes chromium carbide formation during welding, reducing intergranular corrosion risk.

Case Study: A 2025 chemical plant in Texas replaced 316L tanks with 317L for 98% H₂SO₄ storage, cutting corrosion-related leaks by 80% over 5 years.

2. Preheating: Why 100–150°C is Critical

A. Preventing Cold Cracks

Hydrogen control: Preheating accelerates hydrogen diffusion from the weld zone, reducing hydrogen-induced cracking (HIC) risk.

Thermal stress reduction: Slows cooling rates to avoid martensitic transformation in the heat-affected zone (HAZ).

Example: A 2024 study showed that preheating 317L to 120°C reduced HIC incidence by 65% in 20mm-thick plates.

B. Preheating Guidelines

Temperature range: 100–150°C (adjust based on thickness: 100°C for ≤15mm, 150°C for >25mm).

Heating method: Use induction heating for uniformity; avoid open flames to prevent surface oxidation.

Monitoring: Place thermocouples 50mm from the weld edge to ensure consistent temperature.

3. Welding Process Optimization

A. Filler Material Selection

Match chemistry: Use ER317L or E317L-16 electrodes (≤0.04% C, 12–15% Ni, 3–4% Mo) to align with base metal corrosion resistance.

Low-hydrogen electrodes: Essential for minimizing HIC; store electrodes at 150°C before use.

B. Welding Parameters

Heat input: Limit to 1.5–2.5 kJ/mm to avoid excessive HAZ grain growth.

Interpass temperature: Maintain at 100–150°C (same as preheat) to prevent thermal shock.

Data: A 2023 trial found that controlling interpass temperature within this range reduced HAZ hardness by 30%.

4. Post-Weld Heat Treatment (PWHT)

A. Stress Relief Annealing

Purpose: Eliminate residual stresses from welding, reducing stress corrosion cracking (SCC) risk in sulfuric acid.

Temperature: 620–650°C (below the sensitization range of 317L).

Duration: 2 hours per 25mm of thickness (e.g., 4 hours for 50mm plates).

Case Study: A 2025 Norwegian refinery performed PWHT at 630°C for 3 hours on 317L tanks, extending service life by 10 years.

B. Cooling Rate Control

Furnace cooling: Slow cool to ≤100°C/hour to avoid thermal gradients.

Avoid quenching: Rapid cooling can induce residual stresses and embrittle the weld.

5. Surface Protection for Corrosion Resistance

A. Pickling and Passivation

Pickling: Remove welding oxides with 10–15% nitric acid + 1–3% hydrofluoric acid solution.

Passivation: Apply citric acid-based passivation gel to form a 2–5nm Cr₂O₃ protective layer.

Result: A 2024 study showed passivation increased corrosion resistance by 5x in 98% H₂SO₄ at 60°C.

B. Coating Alternatives

PTFE linings: For tanks handling >98% H₂SO₄, apply a 2mm PTFE layer to isolate the metal from acid.

6. Real-World Success: Saudi Aramco’s 317L Tanks

Challenge: Store 98% sulfuric acid at 50°C in a coastal desert environment.

Solution:

Preheated welds to 130°C.

Used ER317L filler with 1.8 kJ/mm heat input.

PWHT at 640°C for 3 hours.

Outcome: Zero leaks in 8 years of operation.