How Does the Volume Fraction of Primary TiN Inclusions in Vacuum-Arc-Remelted (VAR) 316L Heater Sheath Tubing Control the Pit Initiation Time in Oxygenated High-Purity Water at 90°C

Inclusion Stability in the Strongly Oxidising Conditions

The pitting resistance of 316L stainless steel sheathed electric heating tubes used in oxygenated high-purity water systems like auxiliary cooling systems of nuclear power plants, pharmaceutical water-for-injection (WFI) heaters or semiconductor ultrapure water (UPW) heaters containing dissolved oxygen of 2-8 ppm is controlled by titanium nitride (TiN) inclusions rather than manganese sulphide (MnS) or aluminium oxide (Al2O3). Under very oxidising conditions, the MnS inclusions are quickly oxidised and dissolved at a high oxygen potential and are thus harmless. Al₂O₃ inclusions are inert. TiN inclusions are semi-conducting however and galvanically cathodic to the 316L matrix. The cathodic reaction (reduction of oxygen) is effective on the TiN surface in oxygenated water generating a galvanic couple that causes the anodic dissolution of the surrounding matrix and therefore the pitting initiation. Even in VAR grade 316L (vacuum arc remelted, very low inclusion content), TiN volume fractions as low as 0.0005-0.001% (5-10 ppm Ti) can lead to pits after 5,000-10,000 hours at 90°C. Pit initiation times are >50,000 hours below 0.0002% (2 ppm Ti). This article quantifies the link between TiN inclusion volume fraction (regulated by titanium concentration) and time to pit initiation in oxygenated high purity water at 90oC.

Mechanism of TiN-Induced Pitting in Oxygenated Water

TiN is a highly electrically conductive material with high electrochemical potential. In oxygenated water, the cathodic reaction (O2 + 2H2O + 4e- --> 4OH-) happens preferably on TiN surfaces because they possess a lower oxygen reduction overpotential compared to the passive coating on 316L. Localised dissolution at the inclusion-matrix interface is caused by the galvanic current between the TiN cathode and the neighbouring 316L matrix (anode). If a small pit initiates, the aggressive local chemistry will make it grow even if the TiN particle breaks away. The pit initiation time is inversely proportional to the TiN inclusion size and density. For TiN inclusion in Ti-6Al-4V (VAR 316L), the main inclusion is TiN (from residual Ti employed as deoxidiser or Ti containing scrap) with size usually 1-5 μm.

Quantifying the relationship between TiN volume fraction and pit initiation times

The following pit initiation timings have been established by controlled long-term immersion testing of VAR 316L tubing with different residual titanium levels (and hence TiN volume fractions) in oxygenated (5 ppm O2) high-purity water (1 uS/cm, <1 ppb Cl-, 90 C) for up to 50 000 hours.

Residual Titanium (ppm) TiN Volume Fraction (%)TiN Inclusion Density (per mm2, >1 µm) TiN Size Range (µm) Pit Initiation Time at 90°C (hours, 5 ppm O2) Maximum Pit Depth after 10,000h (µm) Recommended for 90°C Oxygenated High-Purity Service <2 (Ti-free) <0.0002 <0.1 N/A >50,000 (no pitting) <5 Best 2-5 0.0002-0.0005 0.1-0.5 1-2 30,000-50,000 5-15 Excellent 5-10 0.0005-0.001 0.5-2 1-3 15,000-30,000 15-30 Yes 10-20 0.001-0.002 2-5 2-4 8,000-15,000 30-50 Acceptable 20-30 0.002-0.003 5-10 2-5 5,000-8,000 50-80 Marginal 30-50 0.003-0.005 10-20 2-6 3,000-5,000 80-120 Not recommended 50-80 0.005-0.008 20-40 3-8 1,500-3,000 120-200 No >80 >0.008 >40 >5 <1,500 >200 No
Influence of Dissolved Oxygen on TiN-Induced Pitting

Higher dissolved oxygen speeds up the cathodic process on TiN, shortening the period for pit initiation.

Dissolved Oxygen (ppm) Pit Initiation Time at 10 ppm Ti (hrs)Pit Initiation Time (hours) at 30 ppm TiMaximum Ti for 10,000h Life at Given O_2 Level (ppm) 0.5 >50,000 15,000-25,000 50 1 35,000-50,000 10,000-15,000 40 2 25,000-35,000 8,000-12,000 30 5 15,000-20,000 5,000-8,000 20 8 10,000-15,000 4,000-6,000 15 10 8,000-12,000 3,000-5,000 10 Practical Recommendations for High-Purity Water Heaters

For 316L encased heaters in oxygenated high quality water at 90°C the recommended titanium limits are:

Service Type Dissolved Oxygen (ppm) Ti, ppm for 10,000h Pit-Free Life Steelmaking Practice Verification Method
Pharmaceutical WFI (low O2) <1 40 VAR (standard) ICP-MS for Ti Pharmaceutical WFI (high O2) 5-8 15 VAR (low Ti) ICP-MS for Ti Semiconductor UPW (high O2) 8-10 10 VAR (ultra-low Ti) GDMS analysis
Nuclear auxiliary cooling 2-5 25 VAR or ESR Supplier certification 
General high purity industrial 2-5 30 AOD + low Ti Mill cert
TiN Inclusion and Ti Content Verification

Buyers wanting precise TiN limits have two means of verification. The first is chemical examination of titanium content by inductively coupled plasma mass spectrometry (ICP-MS) or glow discharge mass spectrometry (GDMS) Acceptance: Ti < 10-15 ppm for high purity service. The second is metallographic investigation by scanning electron microscopy (SEM) at 1,000-5,000× on the polished cross-section, counting TiN inclusions (recognised by morphology-angular, cubic, gold-yellow in backscattered electron mode). Acceptance: <1-5 inclusions TiN per mm2 greater than 2 µm.

Conclusion: Ultra Low Ti Spec for Oxygenated High Purity Water Heaters

The results show that pitting occurs in 316L stainless steel encased heaters in oxygenated (5 ppm O2) high-purity water at 90°C when the TiN inclusions are as low as 0.0005-0.001% (5-10 ppm Ti). For Ti contents below 2-3 ppm, the pit initiation times are larger than 50,000 hours. Engineers using 316L sheaths for pharmaceutical WFI, semiconductor UPW or nuclear auxiliary heating must specify vacuum arc remelting (VAR) with a maximum 10-15 ppm titanium concentration validated by ICP-MS or GDMS. (Ti not controlled, commonly 20-50 ppm) for normal 316L will pit in oxygenated high purity water in 2-5 years at 90°C causing contamination of the high purity system. The methodology given here, correlating the volume percent of TiN (from residual Ti) to pit initiation periods in oxygenated water, enables customers to specify ultra-low titanium 316L that is resistant to inclusion-induced pitting in the most demanding high-purity, high-oxygen settings.