How Does the Volume Fraction of Primary Nb(C,N) Carbonitrides from Residual Titanium (0.02-0.08%) in 347 Heater Sheath Control the Pitting Initiation Site Density in Chlorinated Seawater at 35°C with 1.5 ppm Free Chlorine

The Minor Alloying Element Which Becomes a Major Pitting Initiator Under Oxidising Conditions

Typical 347 stainless steel (Nb-stabilized) sheathed electric heating tubes exposed to chlorinated seawater (1.5 ppm free chlorine, 20,000 ppm Cl-) at 35 °C, as used in coastal power plant cooling systems, desalination preheaters or marine heat exchangers with chlorine dosing contain residual titanium (0.02-0.08 wt%) from steelmaking (titanium-deoxidized scrap or intentional addition). Primary titanium-rich niobium carbonitrides (Nb,Ti)(C,N) form. These complicated carbonitrides are more cathodic and more stable than pure NbC, hence they are very efficient pitting initiation sites under high oxidising potential of chlorinated seawater. For titanium levels <0.01% primary carbonitride volume fraction <0.01-0.02 %, pit density <0.5 pits/cm² and service life >10 years. As titanium content increases from 0.03 to 0.05%, the volume fraction of carbonitride increases from 0.04 to 0.08%, pit density from 2 to 5 pits/cm² and longevity from 5 to 7 years. At titanium levels of 0.06-0.08% the volume fraction is 0.10-0.15% and the pit density is over 10-20 pits/cm² and perforation takes place in 2-3 years. This article quantifies the link between residual titanium concentration, primary carbonitride volume percentage, pit initiation density and service life in chlorinated saltwater.

Mechanism of Titanium Enhanced Pittingch Initiation

Titanium has a greater affinity for carbon and nitrogen than niobium. In 347 steelmaking, titanium (up to 0.02-0.08%) preferentially produces titanium-rich carbonitrides (Ti(C,N)) which can also contain niobium (Nb,Ti)(C,N). These particles are formed at high temperature during solidification and are bigger (3-10 µm) and more stable than pure NbC. In chlorinated seawater (high oxidation potential by Cl2) the cathodic process (oxygen reduction + chlorine reduction) is particularly efficient on these carbonitride surfaces, thus forming a strong galvanic couple with the surrounding austenitic matrix. Fast anodic dissolution is induced by high cathodic current density. The high chloride concentration leads to fast pit initiation and propagation.

Quantitative Relation between Titanium Content, Volume Fraction of Carbonitride and Pitting Density

The following pit initiation densities and perforation times have been established for 347 tubing with different residual titanium contents (constant niobium 0.40%, carbon 0.05%, nitrogen 0.04%) in chlorinated synthetic seawater (1.5 ppm free Cl 2 , 20,000 ppm Cl , pH 8.0) at 35°C for 5,000 hours.

Titanium (wt%) Volume Fraction of Primary (Nb,Ti)(C,N) (%)Carbonitride Particle Size (µm) Particle Morphology Pit Density after 5,000h (pits/cm²) Maximum Pit Depth after 5,000h (µm) Time to Perforation (1.5 mm wall, years) Recommended for 35°C Chlorinated Seawater Service <0.005 (Ti-free) <0.01 <1 None/rare <0.1 <10 >15 Best 0.005-0.010 0.01-0.02 1-2 Isolated 0.2-0.5 10-20 10-15 Yes 0.010-0.020 0.02-0.03 1-3 Scattered 0.5-1.0 15-30 8-12 Acceptable 0.020-0.030 0.03-0.05 2-4 Moderate 1-3 25-45 5-8 Marginal 0.030-0.040 0.05-0.07 2-5 Frequent 2-5 35-60 4-6 Not recommended 0.040-0.050 0.07-0.09 3-6 Numerous 4-8 50-80 3-4 No 0.050-0.060 0.09-0.12 3-8 Extensive 8-15 60-100 2-3 No 0.060-0.080 0.12-0.16 4-10 High density 15-30 80-130 1.5-2 No >0.080 >0.16 >5 Very high >30 >130 <1.5 No
On the Interaction Between Ti and Nb on the Formation of Carbonitride

Higher niobium can partially compensate the adverse effect of titanium by preferentially producing NbC but titanium still partitions to carbonitrides.

Titanium (wt%) Niobium (wt%) Carbonitride Composition Pit Density (pits/cm2) Recommended for Chlorinated Seawater 0.02 0.35 Ti-rich (Ti,Nb)(C,N) 2-4 Marginal 0.02 0.45 More Nb-rich 1-2 Acceptable 0.02 0.55 Mostly NbC + trace Ti 0.5-1 Yes 0.05 0.35 Ti+Nb mixed 8-12 No 0.05 0.50 Ti+Nb mixed 4-6 Not recommended 0.05 0.65 More Nb-rich, some Ti 2-4 Marginal
Practical Recommendations for 347 in Sea Water Chlorine

For 347 sheathed heaters in chlorinated saltwater at 35°C, the following limitations of titanium and niobium minimise pitting due to carbonitride formation.

Desired Service Life @ 35C (years) Maximum Titanium (wt%) Minimum Niobium (wt%) Recommended Combined Limit Alternate Alloy >10 <0.01 0.45 Ti+Nb not applicable 347 acceptable 8-10 <0.02 0.50 Upgrade to 316L or duplex
5-8 <0.03 0.55 347 not recommended Duplex 2205 3-5 <0.04Not effective 347 unsuitable Duplex 2205 or titanium <3 >0.04 Not effective 347 unsuitable Titanium
Verification of Titanium Amount and Carbonitride Density

For purchasers needing 347 for chlorinated seawater service there are two verification techniques. The first is a chemical analysis for titanium, either by ICP or OES. Acceptance: Ti <0.01-0.02% for life >8 yr. The second is metallographic (SEM 1,000-2,000×) investigation on a polished cross-section to count primary carbonitride particles (bright, angular, 2-10 µm). Acceptance: <5 particles/mm^2 for 5 year life.

Conclusion: Ultra-Low Titanium Specification for Chlorinated Seawater Pitting Resistance

In 347 stainless steel heater sheaths, residual titanium as low as 0.02-0.03% produces primary (Nb,Ti)(C,N) carbonitride volume fractions of 0.03-0.05% in chlorinated seawater (1.5 ppm Cl₂, 20,000 ppm Cl⁻) at 35°C, and pit density increases from <1 pit/cm² to 2-5 pits/cm² and service life decreases from >8-10 years to 4-6 years. At titanium levels of 0.05-0.08% pit density is >15-30 pits/cm2 and perforation occurs after 2-3 years. Engineers writing specs for 347 sheaths for marine chlorinated service should limit titanium to <0.01-0.02% and for the highest pitting resistance upgrade to 316L or duplex 2205. The framework presented here allows buyers to specify 347 chemistry that minimises titanium induced pitting in the most aggressive oxidising marine environments by correlating residual titanium content with primary carbonitride volume fraction, pit initiation density and perforation time in chlorinated seawater.