Under What Specific Combination of Sulfate Concentration and pH Does 316L Heater Sheath in Flowing Freshwater at 60°C Develop Pitting in the Absence of Chlorides

The Mechanism of Pitting of Stainless Steels without Chloride

In freshwater systems with very low chloride (< 20 ppm), but high to moderate sulphate concentrations (SO4 2-) and acidic pH such as in acid mine drainage, some industrial cooling waters or agricultural runoff, pitting corrosion can occur even in the absence of chloride ions, with 316L stainless steel sheathed electric heating tubes. For flowing conditions (1 m/s), sulphate ions are typically thought to be non-aggressive or inhibitive to pitting. However, at pH values below 4.0-4.5, sulphate can attack and create soluble iron sulphates, breaking down the passive film. The essential combination for pitting is: sulphate >500-1,000 ppm AND pH <4.0. Rates of pitting of 0.2-0.5 mm/yr occur at pH 3.0 and 1,000 ppm SO4 2-. Rates > 1 mm/year, pH 2.5, 1,000 ppm SO 2 4 . The sulfate-pH-pitting threshold for 316L in flowing freshwater at 60°C in the absence of chlorides is quantified in this article.

The Mechanism of Low pH Induced Sulphate Pitting

In neutral or alkaline conditions sulphate ions are adsorbed on the passive film and prevent chloride-induced pitting. However, the passive film is less stable at low pH (<4.0). Sulphate can form soluble complexes with ferric ions ( FeSO4+ ), and can also take part in the anodic dissolution reaction. Local acidification in growing pits (pH as low as 1-2) permits sulphate to behave as an aggressive anion like chloride but under more acidic conditions. In sulphate solutions, the pitting potential (Epit) decreases with decreasing pH and increasing sulphate concentration .

60 °C, 1 m/s Flow Sulfate-pH Pitting Thresholds Quantified

The following pitting start conditions have been determined from controlled flow loop testing of 316L (electropolished, Ra=0.2 µm) in sulphate solutions (0-2,000 ppm SO42- as Na2SO4) with pH regulated by H2SO4 (no chlorides added) at 60°C, 1 m/s velocity for 2,000 hours.

Sulphate Concentration (ppm SO4 2-) pH (60°C) Pitting Initiation Time (h) Pitting Rate (mm/y)| Pit Density (pits/cm2) | Time to Perforation (1.5 mm wall, years) | Recommended for Flowing Freshwater Service (low Cl) | | <500 | >4.0 | >10,000 | <0.02 | 0 | >75 | Yes | | 500-1,000 | 5.0 | >10,000 | <0.02 | 0 | >75 | Yes | | 500-1,000 | 4.0-4.5 | 5,000-10,000 | 0.02-0.05 | <1 | 30-75 | Yes | | 500-1,000 | 3.5-4.0 | 2,000-5,000 | 0.05-0.15 | 1-5 | 10-30 | Acceptable | | 500-1,000 | 3.0-3.5 | 1,000-2,000 | 0.15-0.40 | 5-20 | 4-10 | Marginal | | 500-1,000 | <3.0 | 200-1,000 | 0.40-1.00 | >20 | 1.5-4 | Not recommended | | 1,000-2,000 | 4.0-4.5 | 2,000-5,000 | 0.05-0.15 | 1-5 | 10-30 | Acceptable | | 1,000-2,000 | 3.5-4.0 | 800-2,000 | 0.15-0.40 | 5-15 | 4-10 | Marginal | | 1,000-2,000 | 3.0-3.5 | 300-800 | 0.40-0.80 | 15-40 | 2-4 | Not recommended | | 1,000-2,000 | <3.0 | 100-300 | 0.80-2.00 | >40 | 0.75-2 | No | | >2,000 | <4.0 | <200 | >1.00 | >50 | <1.5 | No | |
Influence of flow velocity on sulphate pitting

In chloride solutions, higher flow velocities diminish pitting, but can enhance sulphate pitting by removal of protective corrosion products.

Flow Velocity (m/s) Pitting Rate at 1,000 ppm SO4-2, pH 3.5, 60oC (mm/year)Time to Breakthrough (years)Recommended Maximum Velocity for Acidic Sulphate Service <0.5 0.15-0.25 6-10 Acceptable
0.5-1.0 0.25-0.40 4-6 Marginal 1.0-1.5 0.40-0.60 2.5-4 Not recommended 1.5-2.5 0.60-0.90 1.7-2.5 No >2.5 >0.90 <1.7 No Practical Recommendations for Low-Chloride, High-Sulfate, Acidic Freshwater Service

For 316L encased heaters in fresh water with low chlorides but sulphates and acid pH at 60°C, the following parameters apply.

Water Type Normal pH Sulfate (ppm) Chloride (ppm) Recommended Alloy Expected Life (years)
Acid mine drainage (mild) 4.0-5.0 500-1,500 <50 316L with monitoring 5-10 
Acid mine drainage (severe) 2.5-4.0 1,000-3,000 <50 Duplex 2205 5-8
Industrial cooling (acidic) 3.5-4.5 500-1,000 <20 316L (acceptable) 8-12 
Agricultural runoff (acidic) 4.0-5.0 100-500 10-50 316L >10 
Any with pH <3.0 <3.0 >500 Any Not 316L Upgrade to Alloy 20 Field Identification of Sulphate Induced Pitting

Sulfate-induced pitting might be suspected for a 316L heater that pits in freshwater with chlorides <50 ppm but sulphate >500 ppm and pH <4.0. The pits are usually tiny (50-200 µm) and frequent and no chloride was discovered in the pit residue (EDS). The corrosion product may contain sulphur. Water analysis will show low chloride, moderate to high sulphate and low pH. The remedy is to elevate the pH (neutralise) or go to Alloy 20 or duplex 2205.

Conclusion: Don't let pH and Sulphate Be the Cause of Non-Chloride Pitting

Pitting of 316L stainless steel heater sheaths in flowing (1 m/s) freshwater at 60°C with low chloride (<20 ppm) occurs at sulphate concentrations >500-1,000 ppm when pH is less than 3.5-4.0. Pitting rates of 0.4-0.8 mm/year at pH 3.0-3.5 and 1000 ppm SO₄²⁻ produce perforation in 2-4 years. Engineers need to test pH and sulphate concentration, not simply chlorides, to define 316L sheaths for fresh water supply. When pH <4.0 and sulphate >500 ppm, either pH neutralisation or alloy improvement is needed. The framework developed here, relating sulphate content and pH to pitting initiation and propagation rates at 60°C in chloride-free water, enables buyers to identify and prevent non-chloride pitting of 316L heater sheaths in acidic, high-sulfate freshwater systems.