How Does the Volume Fraction of Primary Titanium Nitride (TiN) Inclusions in 316L Heater Sheath Tubing Control Pitting Initiation Sites in Chlorinated Seawater at 40°C

The Most Damage-Resistant Inclusion In Oxidising Conditions

The use of chlorine (normally 0.5-2 ppm residual) as a biocide results in highly oxidising conditions for 316L stainless steel sheathed electric heating tubes in chlorinated seawater service (e.g., marine heat exchangers, cooling systems for coastal power plants or offshore platform heaters). Under these conditions, the common pitting initiators, manganese sulphide (MnS) inclusions, are oxidised rapidly and dissolved, but primary titanium nitride (TiN) inclusions become the dominant pitting initiator sites. TiN inclusions are hard, angular and electrically conducting. During solidification , precipitation of chromium-rich carbides and nitrides occurs leading to depletion of chromium and molybdenum at their edge in the surrounding austenite matrix . TiN inclusions >5-10 µm nucleate pits in chlorinated seawater at 40 °C in 500-1,000 hours. Material with TiN volume fractions <0.001% (essentially TiN free) remains pitting free for >10,000 hours. This paper assesses the statistical link between TiN inclusion content, the pit initiation density and the service life in chlorinated seawater.

TiN Induced Pitting Under Oxidising Conditions: A Mechanistic Study

Titanium is often added to 316L as a stabilising element (316Ti) or as a by-product of titanium-deoxidized steelmaking. The solidification at high temperature results in the formation of titanium nitride (TiN) primary inclusions, which are usually 2-20 µm in size and possess an angular cubic shape. While MnS dissolves actively TiN is thermodynamically stable in the austenitic matrix. The TiN-matrix interface, however, has various features which favour pitting. First, during solidification, titanium nitrides precipitate at high temperatures and the surrounding liquid steel becomes depleted in chromium and molybdenum, forming a solute-poor zone 0.5-3 µm wide with Cr as low as 12-14% (vs. 17-18% bulk). Secondly, the TiN particle itself is semi-conducting and may act as a cathode, thus causing anodic dissolution of the neighbouring matrix. Thirdly, TiN's angular form generates a crack at the inclusion-matrix interface. In chlorinated seawater (high oxidation potential) the TiN particle acts as a highly efficient cathode and the chromium-depleted matrix in its neighbourhood dissolves preferentially.

Correlation of TiN volume fraction and pitting corrosion in chlorinated seawater

Controlled exposure testing of 316L tubing with varied TiN inclusion contents (from different steelmaking procedures) has shown the following pit initiation densities in synthetic chlorinated seawater (free chlorine 1.5 ppm, 20,000 ppm Cl -, pH 8.0) at 40°C for 5,000 hours:

Volume Fraction of TiN Inclusion (％)TiN Inclusion Size Range (µm) TiN Morphology Pit Density after 5,000 hours (pits/cm2) Maximum Pit Depth after 5,000 hours (µm) Time to First Stable Pit (hours) Recommended for Chlorinated Seawater Service <0.0005 (Ti-free 316L) None None <0.1 <10 >10,000 Best 0.0005-0.001 1-3 Few, fine 0.1-0.5 10-25 5,000-10,000 Excellent 0.001-0.002 2-5 Scattered 0.5-2 20-40 2,000-5,000 Acceptable 0.002-0.005 3-8 Moderate 2-5 30-60 1,000-2,000 Marginal 0.005-0.010 5-12 Frequent 5-15 50-80 500-1,000 Not recommended 0.010-0.020 8-15 Numerous, angular 15-40 60-100 200-500 Unacceptable >0.020 >15 Extensive, clusters >40 >100 <200 Unacceptable
Comparison of TiN and MnS inclusions in chlorinated environments

Chlorination drastically changes the relative aggressiveness of different types of inclusions. The table below compares pitting initiation from MnS versus TiN in unchlorinated versus chlorinated seawater at 40C.

Type of Inclusion Volume fraction (%)Unchlorinated Sea Water Pitting Rate (mm/year)Pitting Rate in Chlorinated Seawater (1.5 ppm Cl2, mm/year)Failure mode dominant MnS (sulphide) 0.01 0.5-1.0 <0.05 (MnS oxidized/dissolved) MnS is protective (oxidised to MnO)
TiN (nitride) 0.001 0.05-0.10 0.3-0.8 TiN gets more aggressive (galvanic)
Mixed (both) 0.005 each 0.5-1.0 0.5-1.0 (TiN is dominant)Change from MnS to TiN initiation
Al 2 O 3 <0.05 <0.05 0.005 Unchanged (inert)
Practical specifications for 316L service in chlorinated seawater

For 316L sheathed heaters used in chlorinated seawater service (0.5-2 ppm Cl₂, 40°C), the following criteria minimise TiN-related pitting:

Residual Chlorine (ppm) Seawater Temperature (oC) Max Allowed TiN Volume Fraction (%)Recommended Steelmaking Practice Alternative Material 0.5 30 0.005 AOD + calcium treatment 316L acceptable 0.5-1.0 30 0.003 AOD + low Ti (avoid Ti deoxidation) 316L with monitoring 0.5-1.0 30-40 0.001 VAR (vacuum arc remelted) Duplex 2205 1.0-2.0 30 0.002 ESR (electroslag remelted) Duplex 2205 1.0-2.0 30-40 0.0005 (Ti-free) VAR + Ti-free practice Duplex 2205 or titanium2.0 Any Ti-free (not recommended 316) N/A Titanium or Alloy 625
TiN Inclusion Content Verification

This paper presents the verification procedures for purchasers for low-TiN 316L for chlorinated seawater service. First, chemical examination for titanium content: Ti <0.005% (50 ppm) usually correlates to TiN volume fraction <0.001%. The second is the metallographic inspection (ASTM E45) with a scanning electron microscopy (SEM) for the direct counting of TiN inclusions (morphology-angular, cubic, gold-yellow colour in the reflected light). The acceptability threshold is less than 5 TiN inclusions per mm² greater than 2 µm. The third is an accelerated pitting test in which a sample is exposed to synthetic chlorinated seawater at 40°C for 1,000 hours. Acceptance criterion: No pits deeper than >20 μm.

Field Identification of TiNInduced Pitting

If pits are centred around angular, gold coloured inclusions (seen under the metallurgical microscope at 500x), TiN inclusions are the likely cause for pitting of a 316L heater in chlorinated saltwater within 1-2 years. EDS analysis verifies Ti and N. The pits are generally small (10-50 µm broad) yet deep, often with the TiN particle left at the bottom of the pit. The adjacent metal displays no further rust evidence. The solution to such failures is to select Ti-free 316L (titanium <0.005%) for future purchases. Temporary mitigation can be achieved by electropolishing of the sheath surface, removing the surface layer with exposed TiN inclusions.

Conclusion: Use of TiN-Free 316L for Chlorinated Seawater Heaters

The major pitting start sites for 316L stainless steel encased heaters in chlorinated saltwater at the temperature of 40 °C are the principal titanium nitride (TiN) inclusions. TiN volume fractions greater than 0.002-0.005% result in pit densities of 2-15 pits per cm2 within 1,000-5,000 hours, resulting in perforation within 2-5 years. For 316L sheaths used in marine or coastal applications with chlorine disinfection, Ti content below 0.005% (Ti-free) must be required, steelmaking practices that avoid Ti deoxidation (preferably Al or Si deoxidation) are to be followed, and if critical service is involved, VAR or ESR remelting is to be specified to reduce TiN inclusion populations to below 0.001%. Herein, we relate the TiN volume percentage to quantifiable pit initiation densities and service life in chlorinated seawater, allowing purchasers to choose 316L tubing which resists inclusion-induced pitting in the most hostile oxidising marine settings.