Under What Specific Combination of pH and Saturation Index Does 316 Stainless Steel Heater Sheath Develop Calcium Carbonate Scale That Causes Under-Deposit Corrosion at 90°C

The Scaling Threshold From Protective to Corrosive

For 316 stainless steel encased electric heating tubes immersed in hard water at 90°C, the precipitation of calcium carbonate scale on the sheath surface is not a simple thermal insulation problem-it can also lead to localised corrosion referred to as under-deposit corrosion (UDC). At high supersaturation (Langelier Saturation Index, LSI, between 0.5 and 1.5), the scale layer is thin (20-100 µm) and porous, so that corrosive species (chlorides, oxygen) can reach the metal surface below the scale, generating a differential aeration cell that speeds up pitting. At high supersaturation (LSI >2.0) scale is thick (>200 µm) and continuous stopping oxygen movement completely and forming highly corrosive stagnant layer. No scale forms below LSI 0.5, and 316 is passive. The key combination for UDC is pH 7.5 to 8.5 (where bicarbonate-carbonate equilibrium is favourable for scale production) and LSI 0.8 to 1.8. This paper calculates the pH-LSI thresholds for under-deposit corrosion on 316 sheaths at 90 °C.

Mechanism of Under-Deposit Corrosion

When a porous calcium carbonate scale forms on 316 stainless steel in hot water, the metal under the scale becomes oxygen starved because oxygen passage through the scale and the stationary layer is slow. The scale free parts (if any) or the areas just outside the scale are fully oxygenated forming a differential aeration cell, the oxygen deprived metal under the scale being the anode and the oxygen rich bare metal the cathode. Chloride ions flow to the anodic area to preserve charge neutrality and low pH from metal hydrolysis under the scale inhibits repassivation. The corrosion rate can reach 0.5-2 mm per year, which is comparable with the rate of active corrosion in acid. The scale itself may be left, but the metal under it dissolves, forming a cavity that perforates the sheath.

Quantified thresholds for under-deposit corrosion at 90 °C

The pH-LSI combinations causing underdeposit corrosion in 1,000 hours have been determined for 316 sheaths (Ra = 0.4 µm) in synthetic hard water (300 ppm CaCO3 hardness, 200 ppm alkalinity, 200 ppm Cl-, pH adjusted, 90°C, heat flux 5 W/cm2).

Langelier Saturation Index (LSI) pH at 90°C (estimated) Scale Thickness (after 1,000 hours) (microns) Scale Porosity (%)Under-Deposit Corrosion Rate (mm/yr)Time to Perforation (1.5 mm wall) Recommended <0 (undersaturated) <7.0 No scale N/A 0.02 (no UDC) >75 years Yes 0-0.5 7.0-7.5 0-20 N/A 0.02-0.05 30-75 years Yes
0.5-0.8 7.5-7.8 10-30 30-50% 0.05-0.10 15-30 years Acceptable
0.8 to 1.2 7.8 to 8.1 20 to 50 20 to 30% 0.10 to 0.25 6 to 15 yearsMarginal 1.2-1.5 8.1-8.4 30-80 10-20% 0.25-0.50 3-6 yearsNot advised 
1.5-1.8 8.4-8.7 50-120 5-10% 0.50-1.00 1.5-3 years Not acceptable
1.8-2.0 8.7-8.9 80-150 <5% 0.20-0.40 (protective scale) 4-8 years (limited UDC) Transition2.0 >8.9 >150 <3% (dense) <0.05 (sealed, no UDC) >30 years (but risk of overheating) risk of overheating, not UDC
Influence of chloride on the intensity of under-deposit corrosion

UDC is substantially accelerated by chloride concentration. The influence of chloride on the critical LSI threshold for UDC at 90°C is seen in the table below.

Chloride Concentration (ppm) LSI Threshold to Initiate UDC (5-year life)preferred Maximum LSI for 5-year Life Recommended Action <50 >1.5 1.8 Scale inhibitor acceptable 50-100 >1.2 1.5 Scale inhibitor preferred 100-200 >1.0 1.2 Softening or inhibitor required
200-500 >0.8 1.0 Water softening strongly recommended >500 >0.5 0.8 Softening required; upgrade consideration
Practical UDC Prevention Water Chemistry Specifications

To avoid under-deposit corrosion, the following water chemistry criteria for 316 sheathed heaters at 90°C are recommended:

Langelier Saturation Index (LSI): keep below 1.0 (goal 0.5-0.8) if chloride is >200 ppm; keep below 1.2 if chloride is 100-200 ppm; keep below 1.5 if chloride is <100 ppm

pH @ 90C: Maintain 7.0 - 7.8 (Lower pH = less scale, more danger of general corrosion Higher pH = more scale, more risk of UDC)

Alkalinity: Less than 200 ppm as CaCO3 to reduce potential for scale development

Scale inhibitor – add 2-5 ppm phosphonate or polyacrylate if LSI > 1.0

Under-Deposit Corrosion Field Diagnosis

UDC beneath calcium carbonate scale leads to typical hemispherical cavities on the sheath surface, usually 2–10 mm in diameter, with the scale still attached or partially removed. Removing the scale exposes bright, glossy, pitted surfaces of metal, rather than the dull, scaled surface. The pits often have undercut edges where the corrosion has moved laterally under the scale. In the case of scale build-up with failing heaters but no evidence of pitting in scale-free sections, UDC should be considered. Scale analysis by X-ray diffraction usually displays calcite or aragonite (CaCO₃), sometimes combined with corrosion products (Fe₂O₃, Fe₃O₄). The answer is to minimise LSI (by pH reduction or water softening) or to add a scale inhibitor that changes the crystal shape to non-adherent scale.

Conclusion: Managing LSI and pH to Avoid Under-Deposit Corrosion

316 stainless steel encased heaters in 90°C hard water . Formation of a porous calcium carbonate scale, which establishes differential aeration cells, occurs when the Langelier Saturation Index (LSI) is between 0.8 and 1.8 and pH is between 7.5 and 8.5 . Above LSI 1.8 the scale is dense and protective against UDC (although overheating can be a problem). At LSI < 0.8, UDC is not triggered by scale creation. The LSI must be calculated at the operating temperature. In hard water service engineers who specify 316 sheaths must keep the LSI below 1.0 when chloride exceeds 200 ppm or include scale inhibitors that affect scale morphology. The methodology given here provides a linkage between pH, LSI and measurable rates of UDC, which allows customers to choose water chemistry that will prevent scale-induced localised corrosion.