High-concentration sulfuric acid (93–98% concentration) is a workhorse chemical in industries like petrochemicals, pharmaceuticals, and mining—but it’s also highly corrosive. Storage tanks for this acid need a material that can stand up to long-term exposure without rusting, leaking, or failing. 317L stainless steel is the top choice here: its high molybdenum content (3–4%) and low carbon content (<0.03%) make it resistant to both general corrosion and intergranular corrosion—two major risks for sulfuric acid storage.
But even the best material fails if welded poorly. A single weld defect—like a tiny crack, pore, or un 融合 (lack of fusion)—can become a “corrosion channel” for sulfuric acid, leading to leaks, tank damage, or even safety hazards. That’s why a strict welding procedure specification (WPS) isn’t optional—it’s the backbone of a reliable, long-lasting tank. We’re breaking down the key steps of this WPS, from choosing the right 焊丝 to post-weld passivation, so you can weld 317L tanks that hold up to high-concentration sulfuric acid for 15–20 years.
1. Pre-Welding Preparation: Get the Materials and Surfaces Right
Welding success starts before you strike an arc. For 317L stainless steel tanks, two things matter most: using matching consumables and ensuring spotless surfaces.
a. Consumable Selection
317L’s corrosion resistance comes from its precise alloy mix—so your welding wire, 焊条 (electrode), or flux must match this composition to avoid creating weak, corrosion-prone welds. The industry standard consumables are:
Welding Wire: ER317L (for TIG/MIG welding). It has the same molybdenum and low carbon content as 317L base metal, ensuring the weld joint is just as corrosion-resistant as the tank itself.
Welding Electrode: E317L-16 (for SMAW/handheld arc welding). Choose low-hydrogen electrodes (H4 or lower) to reduce the risk of hydrogen-induced cracks—critical for thick tank walls.
Protection Gas: For TIG/MIG welding, use high-purity argon (99.99% minimum). Impurities like oxygen or nitrogen will create oxide inclusions in the weld, which act as corrosion starting points.
Always check the consumable’s mill test report (MTR) to confirm it meets ASTM standards (e.g., ER317L meets ASTM A5.9). Using off-brand or mismatched consumables is a shortcut that leads to early tank failure.
b. Workpiece Cleaning
Sulfuric acid attacks even tiny impurities—oil, rust, paint, or dirt—trapped in welds. Before welding:
Use a stainless steel wire brush to remove surface rust or scale. Never use carbon steel brushes—they leave iron particles that cause “rust spots” in the weld.
Wipe the welding area (20mm on either side of the joint) with acetone or isopropyl alcohol to remove oil or grease. Let it dry completely before welding.
For thick plates (over 12mm), use a grinder with a stainless steel grinding wheel to bevel the joint. A 30–35° bevel ensures full weld penetration—partial penetration leaves gaps where acid can collect.
2. Welding Method Selection: TIG for Precision, MIG for Efficiency
High-concentration sulfuric acid tanks have varying wall thicknesses (usually 6–25mm, depending on tank size). The best welding method depends on the thickness and joint type:
a. TIG Welding (GTAW) for Root Passes
The “root pass” (the first weld that joins two plates) is the most critical—it must be fully fused and free of defects to prevent acid leaks. TIG welding is ideal here because:
It uses a non-consumable tungsten electrode, so there’s no risk of wire contamination.
It provides precise heat control, reducing the chance of overheating (which weakens 317L’s corrosion resistance).
For root passes on 6–12mm thick 317L plates:
Tungsten electrode: 2.4mm diameter, pure tungsten (or thoriated tungsten for better arc stability).
Current: 80–100A (DCEN, direct current electrode negative).
Voltage: 10–12V.
Travel speed: 50–70mm/min.
Backing gas: Argon (flow rate 5–8 L/min) to protect the back of the weld from oxidation—this is non-negotiable for acid tanks, as backside oxidation leads to rapid corrosion.
b. MIG Welding (GMAW) for Filler and Cover Passes
Once the root pass is complete, MIG welding is used for filling and covering the joint—it’s faster than TIG and works well for thicker walls (12–25mm). For these passes:
Wire diameter: 1.2mm ER317L.
Current: 180–220A.
Voltage: 22–24V.
Wire feed speed: 4–6 m/min.
Argon flow rate: 15–20 L/min.
For very thick walls (over 25mm), or for hard-to-reach joints (like tank corners), SMAW (handheld arc welding) with E317L-16 electrodes works—just keep the arc short to minimize spatter (spatter left on the surface can trap acid).
3. Critical Welding Parameters: Avoid Overheating and Defects
317L stainless steel is sensitive to heat—too much heat causes grain growth, which reduces corrosion resistance. Follow these parameter rules:
Heat Input Control: Keep heat input below 1.5 kJ/mm. For example, a 10mm thick plate welded with MIG (200A, 23V, 60mm/min travel speed) has a heat input of ~1.15 kJ/mm—safe for 317L.
Interpass Temperature: Don’t let the weld cool below 60°C or above 150°C between passes. If it cools too much, you risk cold cracks; if it’s too hot, grain growth occurs. Use a temperature gun to check between passes.
Joint Alignment: Keep the gap between plates at 2–3mm (for root passes). A too-small gap leads to incomplete penetration; a too-large gap requires more filler metal, increasing heat input.
4. Preheating and Post-Weld Heat Treatment
Unlike carbon steel, 317L doesn’t need high-temperature preheating—but extreme conditions demand small adjustments:
Preheating: If the ambient temperature is below 0°C, or the wall thickness is over 20mm, preheat the joint to 50–100°C (using a propane torch or induction heater). This prevents cold cracking from rapid cooling.
Post-Weld Heat Treatment (PWHT): Focus on hydrogen relief (not full stress relief, which can reduce corrosion resistance). Heat the weld area to 250–300°C, hold for 1–2 hours, then cool slowly (50°C/hour) to room temperature. This removes trapped hydrogen, which causes brittle cracks that acid can exploit.
5. Post-Weld Treatment: Restore Corrosion Resistance
Welding destroys 317L’s protective oxide layer (passive layer) and leaves behind heat-affected zones (HAZs) that are prone to corrosion. Fix this with two key steps:
a. Acid Pickling
Use a pickling solution (10–15% nitric acid + 2–3% hydrofluoric acid) to remove weld scale, spatter, and HAZ discoloration. Apply the solution with a brush, let it sit for 5–10 minutes (until the surface is bright), then rinse thoroughly with deionized water. Neutralize any remaining acid with a 5% sodium bicarbonate solution—residue acid will corrode the tank from the inside.
b. Passivation
After pickling, apply a 20–25% nitric acid solution to the weld area and let it sit for 20 minutes. This rebuilds the passive layer, boosting corrosion resistance by 40–50% compared to unpickled welds. Rinse again with deionized water and dry with a clean, lint-free cloth.
6. Welding Quality Inspection: No Defects Allowed
Even small defects can lead to big leaks. Inspect every weld using these steps:
Visual Inspection: Check for surface defects—no pores, slag inclusions, cracks, or undercut (a groove along the weld edge). Use a magnifying glass (10x) to spot tiny cracks.
Non-Destructive Testing (NDT): For all critical joints (tank bottoms, seams, and nozzles):
RT (Radiographic Testing): Detects internal defects like voids or incomplete penetration.
UT (Ultrasonic Testing): Finds subsurface cracks or inclusions in thick walls.
Pressure Testing: After welding, fill the tank with water (or a non-corrosive test fluid) and apply 1.5 times the operating pressure (e.g., 1.5 bar for a tank that operates at 1 bar). Hold for 24 hours—no leaks or pressure drops mean the welds are sound.
7. Real-World Example: A Petrochemical Plant’s Success
A petrochemical plant in Texas built a 50m³ high-concentration sulfuric acid tank using 317L stainless steel, following this WPS:
Root passes: TIG with ER317L wire, argon backing gas.
Filler/cover passes: MIG with ER317L wire.
Post-weld: Pickling, passivation, and RT/UT testing.
After 3 years of use, the tank shows no signs of corrosion or leaks. The plant’s maintenance manager noted: “We used to have issues with thinner stainless steel tanks leaking after 1–2 years, but the 317L tank—welded to this spec—has been trouble-free.”
Conclusion
Welding 317L stainless steel for high-concentration sulfuric acid tanks isn’t about speed—it’s about precision. Every step, from choosing ER317L consumables to post-weld passivation, is designed to eliminate defects and restore corrosion resistance. For plant operators, this WPS isn’t just a set of rules—it’s a way to avoid costly downtime, safety risks, and environmental damage.
As industries rely more on high-concentration sulfuric acid, mastering this welding specification will keep tanks running reliably for decades. It’s proof that with the right process, even the most corrosive chemicals can be stored safely—one sound weld at a time.