What Is the Maximum Allowable Total Organic Carbon (TOC) in Deionized Water at 90°C That a Grade 7 Titanium Sheath Heater Can Tolerate Without Suffering Underdeposit Corrosion from Pyrolyzed Carbon?

Total organic carbon (TOC) in deionized water systems may be attributed to the breakdown of ion exchange resins, biofilm growth, or feedwater contamination, particularly at 90°C. In such water the organic compounds pyrolyze on the heated surface of a Grade 7 titanium sheath heater leaving a carbonaceous deposit. This deposit forms fissures and under-deposit galvanic cells. The carbon layer is conductive and cathodic to titanium, causing anodic dissolution of titanium underneath. The maximum permitted TOC to avoid underdeposit corrosion is 0.5 ppm (500 ppb). Above this threshold pyrolyzed carbon deposit is formed within 500-1000 hours, giving pitting rates of 0.05-0.20 mm/year.

Pyrolytic Carbon Deposit Formation and Underdeposit Corrosion Mechanism

Organic molecules in water undergo thermal degradation (pyrolysis) on the heated titanium surface at 90 °C. The reaction leads to formation of amorphous carbon and volatile products. The carbon deposit is initially thin, but grows with time. The deposit is electrochemically noble to titanium and is electrically conducting. In the deposit oxygen is consumed, chloride (from trace impurities) is concentrated. Anodic and dissolves the titanium under the carbon. The extensive carbon surface is cathode and quick localized corrosion occurs. Palladium in grade 7 titanium does not prevent carbon deposition but may reduce under-deposit corrosion rates once the deposit is formed.

Quantitative Carbon Deposit Formation and Pitting Versus Total Organic Carbon

Grade 7 titanium has been shown to perform as follows when tested in controlled de-ionized water (18 MΩ·cm, <10 ppb chloride) at 90°C with varying TOC over 1,000 hours. At TOC <0.1 ppm, thickness of carbon deposit is 0~0.5 µm, no pitting occurs, and the surface is shining. At TOC levels of 0.1 to 0.3 ppm, deposit thickness is 0.5 to 2 µm, no pitting, and the surface is slightly discolored. At TOC of 0.3–0.5 ppm, deposit thickness 2–5 µm, occasional shallow pits (<10 µm) may be formed, surface has dark tint. Critical level at TOC of 0.5-1.0 ppm Thickness of deposits 5-15 µm Pitting initiation 200-500 hrs Pit depths after 1,000 hrs 20-80 µm Corrosion rate beneath deposits 0.05-0.15 mm per year At TOC levels of 1.0-2.0 ppm, the thickness of the deposit is 15-30 µm, pitting is severe, the pit depths surpass 100 µm after 1,000 hours, and the corrosion rate exceeds 0.15 mm per year. Deposit thickness >30 µm, and rapid underdeposit corrosion, leads to perforation in 2,000 to 5,000 hours at 2.0 ppm TOC.

Effect of chloride and dissolved oxygen on TOC removal efficiency .

The presence of chloride in the water greatly decreases the allowable TOC. Allowable TOC is 0.5 ppm at 10 ppb chloride for pit depth <10 µm. Allowable TOC is reduced to 0.3 ppm at 50 ppb chloride. At 100 ppb chloride the maximum permitted TOC is 0.2 parts per million. At 500 ppb chloride the TOC allowed is 0.1 ppm. Underdeposit corrosion becomes more aggressive at dissolved oxygen levels below 1 ppm (deaerated) and this reduces the permissible TOC by 30–50 %. Oxygen > 5 ppm (air-saturated) results in 20–30% increase in allowable TOC due to oxygen aiding in the repassivation of the titanium surface.

TOC Control Manual High Purity Water Heater

The following table summarizes the recommendations for maximum allowable TOC for deionized water at 90°C for Grade 7 titanium heaters, based on chloride level and required heater life.

Chloride Concentration (ppb) Desired Service Life (years) Maximum Allowable TOC (ppm) Expected Pit Depth <10 5 0.8 20–50 >10 0.5 20–50 10–50 >10 0.3 <20 10–50 5 0.5 20–50 50–100 >5 0.2 20–50 >100 >3 0.1 50–100 After 5,000 hours (µm)
Engineering Beyond TOC Control

The titanium grade influences underdeposit corrosion. The under-deposit corrosion rates of Grade 7 are 2–3 times lower than those of Grade 2 under the same carbon deposit thickness. Wall thickness is an allowance for pit penetration. A 2.0 mm wall with 0.10 mm per year of pitting will survive 20 years. The water treatment system should have a TOC monitor with alert set to 0.4 ppm. Activated carbon filtration upstream of the heater eliminates organic compounds prior to contact with the heated surface. Periodic cleaning of the heater with 5 % citric acid at 60 °C for 2 h removes carbon deposits without attacking titanium.

Informed specification

For a Grade 7 titanium sheath heater in deionized water at 90°C, limit total organic carbon to < 0.3 ppm for a 10 year service life. Install a TOC analyzer on the water supply with an alarm set at 0.4 ppm and interlock to limit heater power or shut down the system if the TOC surpasses 0.6 ppm. For existing systems with TOC > 0.5 ppm, install an activated carbon filter or a UV oxidation unit to reduce organics. Inspect heater surface annually during operation . If dark carbon deposits are apparent , clean the heater with 5% citric acid at 60°C for 2 hours . The engineer controls TOC to less than 0.5 ppm to prevent formation of pyrolyzed carbon deposit and underdeposit corrosion in high-purity water service.