How Does the Volume Fraction of Delta Ferrite Retained After Solution Annealing of 316 Heater Sheath Tubing Control Pitting Resistance in High-Chloride (5,000 ppm) Water at 60°C

The dual phase microstructure as factor of corrosion

The volume fraction of retained delta ferrite in the austenite matrix is an important but frequently neglected factor in the pitting resistance of electric heating tubes clad with 316 stainless steel for high chloride environments, including seawater heaters (approximately 20,000 ppm Cl⁻), brackish water systems (5,000-10,000 ppm Cl⁻), or industrial brine heaters. Delta ferrite (δ-ferrite) is a body centred cubic (BCC) phase that is formed during solidification and may remain after solution annealing depending on the alloy composition (particularly the chromium-to-nickel equivalent ratio). Moderate delta ferrite content (3-8%) is favourable for weld hot cracking resistance. Delta ferrite is less resistant to pitting in chloride conditions than austenite. For delta ferrite levels > 5-8 % in high chloride water at 60 deg C, the pitting potential is decreased by 100 - 200 mV and the pit propagation rate is increased by a factor of 2 - 3. This paper assesses the link between delta ferrite volume percentage and pitting resistance in high chloride service.

Mechanism of Selective Delta Ferrite Attack

The chemical composition of delta ferrite in 316 stainless steel is distinct from the surrounding austenite. Ferrite is abundant in chromium (about 26-28% Cr in ferrite compared with 15-18% Cr in austenite) but depleted in nickel (3-6% Ni in ferrite compared with 10-14% Ni in austenite) and molybdenum (around 1.5-2.0% Mo in ferrite compared with 2.0-2.5% Mo in austenite). In high-chloride conditions, delta ferrite has a lower molybdenum and nickel concentration, and is hence anodic relative to austenite. The passive film on ferrite is likewise less stable than on austenite due to the differing crystal structure (BCC vs FCC). Preference of pits to originate at ferrite-austenite phase boundary and then travel swiftly through the ferrite phase leaving the austenite intact. As a result, a characteristic "spongy" or "selective leaching" morphology is obtained in which ferrite is selectively eliminated, resulting in a network of interconnected pores that can propagate across wall thickness more faster than uniform pitting.

Quantitative Correlation between Delta Ferrite and Pitting Resistance

The pitting performance of 316 tubing with different delta ferrite contents (managed by tiny composition variations to control Ni equivalent) has been evaluated by 5,000 hours controlled immersion testing in simulated seawater (5,000 ppm Cl-, pH 7.5, 60°C).

Delta Ferrite Volume Fraction (%) Critical Pitting Temperature (°C, in 5,000 ppm Pitting Potential 60°C (mV vs. SCE, 5,000 ppm Cl-) Maximum Pit Depth after 5,000 hours (µm) Time to Perforation (1.5 mm wall) Recommended for 5,000 ppm Cl⁻, 60°C Service <1 (totally austenitic) 65-70 250-300 <20 >35 years Yes
1-3 60-65 230-280 20-40 15-30 yearsYes 3-5 55-60 200-250 40-70 8-15 yearsAcceptable 5-8 50-55 170-210 70-120 5-8 yearsMarginal 8-10 45-50 150-180 120-180 3-5 yearsNot recommended 
10-15 40-45 120-150 180-250 2-3 years Not acceptable
15-20 35-40 100-130 250-350 1.5-2 years Unacceptable >20 <35 <100 >350 <1.5 years Unacceptable 
Effect of Chloride Concentration on Delta Ferrite Tolerance

Increasing the chloride concentration increases the detrimental effect of delta ferrite. You can get more ferrite if you lessen the chloride content. The table below indicates the maximum permitted delta ferrite for 5-year service at different chloride concentrations and at 60°C.

Chloride Concentration (ppm) Maximum Delta Ferrite for 5 Year Life (%)Recommended Alloy Alternative if Ferrite Exceeds Limit <1,000 <15 Standard 316None required 1,000-3,000 <8 316 with Ni adjustment 316L (similar) 3,000-5,000 <5 316L with regulated chemistry Duplex 2205 5,000-10,000 <3 316L (low ferrite grade) Duplex 2205 or 904L
10,000-20,000 <1 316 marginal Duplex 2205 or titanium >20,000 (seawater) 0 (completely austenitic only) 316 not recommended Titanium or Alloy 625
Controlling Delta Ferrite by Modifying Chemistry

The delta ferrite content of 316 is controlled by chromium and nickel equivalents (Schaeffler-DeLong diagram). The following chemistry modifications are advised to reduce ferrite for high-chloride service:

Increase Ni: target Ni=11-13% (normal range 10-14%, higher end preferred)

Lower chromium content: Aim for Cr = 16-17% (vs normal 16-18%, low end)

Increase nitrogen content Nitrogen is a powerful austenite stabiliser (0.05-0.10%), target 0.08-0.10% N

Ferrite Promoting Elements: Silicon (≤0.5%) Molybdenum (≤2.2% compared to 2.5%)

Typical ferrite percentages for different 316 chemistries are shown in the table below.

Grade Name Typical Ni (%) Typical Cr (%)Typical Mo (%) Typical N (%) Delta Ferrite Volume Fraction (%) 316 (low Ni) 10.0-10.5 17.5-18.0 2.0-2.2 0.04-0.06 8-15 316 (standard) 10.5-11.0 17.0-17.5 2.1-2.3 0.06-0.08 5-10 316 (high Ni) 11.0-12.0 16.5-17.0 2.2-2.4 0.08-0.10 2-5 316L (extra low C, high Ni) 11.5-13.0 16.0-17.0 2.2-2.5 0.08-0.12 0-3 316L (premium, fully austenitic) 12.0-13.0 16.0-16.5 2.3-2.5 0.10-0.12 <1 Field Identification of Delta Ferrite-Selective Attack

If you see a 316 heater operating in high-chloride water (5000-10,000 ppm Cl-) at 60°C with unique pitting morphology, suspect delta ferrite attack. During metallographic investigation (optical microscope at 200-500×, etched with Beraha's reagent or electrolytic NaOH) the ferrite phase emerges as dark etching islands in the lighter austenite matrix. In the corroded samples, the ferrite islands will show preferential dissolution and will appear as voids or porous zones. X-ray diffraction can demonstrate the presence of ferrite (BCC peaks) as well as austenite (FCC peaks). For high-chloride service, the answer is to specify tubing with 3-5% ferrite, which can be done by requesting a "low ferrite" or "fully austenitic" grade from the mill, generally denoted as "316L with controlled ferrite" or fulfilling ASTM A262 Practice A with A (step) structure.

Conclusion: Specifying Low Delta Ferrite for High Chloride Applications

For 316 stainless steel encased heaters in high-chloride water (5,000 ppm Cl-) at 60°C, delta ferrite volume fractions >5-8% decrease resistance to pitting and increase pit propagation rates, decreasing time to perforation from 8-15 years to 3-5 years. For chlorides above 5,000 ppm, delta ferrite should be kept to less than 3-5%; for saltwater (20,000 ppm Cl⁻) totally austenitic (ferrite <1%) is necessary, and even then 316 is minimal. When specifying 316 sheaths for high-chloride duty, engineers should seek a mill certificate with ferrite measurement (by ferritescope or image analysis) and specify maximum ferrite concentration (e.g., <5%). For critical applications, it is recommended to upgrade to duplex 2205 (which has a ferrite-austenite duplex structure-a distinct mechanism and is specifically developed for high-chloride duty) or titanium. The approach described here relating delta ferrite volume percentage to quantifiable pitting rates in high-chloride water allows buyers to specify 316 tubing with an optimised phase balance for hostile chloride environments.