For a Titanium Heating Coil Used to Maintain 60°C in a 5% Sodium Thiosulfate Solution, Which Impurity (Nitrate, Nitrite, or Ammonia) Is Most Aggressive Toward the Passive Film?

Solutions of sodium thiosulfate (Na2S2O3) at 60°C and 5% concentration are employed in photographic fixing, gold leaching and dechlorination. The passive film on titanium is usually stable in pure thiosulfate . However, contaminants from upstream processes like nitrate (NO3−), nitrite (NO2−) or ammonia (NH3) might destabilize the film. Among these, nitrite is the most aggressive contaminant in thiosulfate at 60 °C, towards the titanium passive film. Nitrite functions as a reducing agent in neutral to acidic circumstances and can reduce titanium oxide to lower valence states, potentially leading to active corrosion sites. Nitrate is less aggressive requiring higher temperatures for reduction. Ammonia can complex titanium ions but is benign in thiosulfate at this temperature.

Mechanism of breakdown of the passive film by nitrite

Nitrite ions (NO 2 −) are reduced on the titanium surface in the reaction NO 2 − + 6H + + 5e − = ½N 2 + 3H 2 O in thiosulfate solution at pH 7–8. This uses protons and so increases the local pH. However, more critically, the reduction intermediate (NO) can chemically convert TiO2 to Ti2O3 or Ti. The lower oxide has a better electrical conductivity and a lesser resistance to corrosion. The passive film becomes permeable, losing its protective nature. Pitting initiates at defects in the film. Nitrite is about 10 times more aggressive than nitrate, while ammonia is about 20 times more aggressive than nitrite at the same concentration.

Quantitative Aggressiveness of Various Impurities in Thiosulfate Solution

The corrosion behavior of Grade 2 titanium has been demonstrated by controlled testing with 5% sodium thiosulfate at 60 deg C with pH 7.5 and 100 ppm of each impurity introduced separately over a 1,000 hour period. Corrosion rate is 0.005–0.010 mm per year with no pitting and stable passive film, with no impurity as baseline. The corrosion rate increases slightly to 0.010-0.020 mm/year at 100 ppm nitrate, but no pitting is observed, indicating low aggressiveness. The corrosion rate is 0.008–0.015 mm per year in the presence of 100 ppm ammonia, without pitting, forming a soluble Ti(NH₃)₆³⁺ complex but at a slow rate, representing low to moderate aggressiveness. At 50 ppm nitrite the corrosion rate increases to 0.05–0.1 mm/year with a pitting density of 5–10 pits/cm² after 1000 hours indicating moderate aggressiveness. The corrosion rate is high at 0.15–0.30 mm/year, 20–50 pits/cm² and a pit depth of 0.1–0.3 mm at 100 ppm nitrite. At 500 ppm of nitrite, the corrosion rate is 0.50-1.00 mm per year with severe pitting which is not acceptable.

Aggressiveness Ranking as a Function of Impurity Concentration and pH

The rank of impurity aggressiveness is identical for different concentrations and pH values . At 100 ppm impurity and pH 7.5 nitrate is low, ammonia is low to moderate, and nitrite is significant. At 500 ppm impurity and pH 7.5, nitrate is moderate at a corrosion rate of 0.020–0.040 mm per year and without pitting, ammonia is low to moderate at a corrosion rate of 0.020–0.035 mm per year and with 0–5 pits per cm², and nitrite is severe at a corrosion rate of 0.50–1.00 mm per year and with extensive pitting. At pH 9.0 with 100 ppm nitrite the corrosion rate decreases to 0.080-0.150 mm per year with a pitting density of 5-15 pits per cm2 . The increase of pH reduces the agressiveness of nitrite . At pH 9.0 with 500 ppm nitrite it is 0.20–0.40 mm/year with considerable pitting.

Control of Impurities in Thiosulfate Heaters

The table below shows the maximum permitted levels for impurities in Grade 2 titanium heaters used in 5% sodium thiosulfate at 60°C for a 5-year service life, as well as monitoring frequencies and mitigating techniques.

Impurity Max. Permissible Concentration (ppm) Monitoring Frequency Mitigation if Exceeded Nitrate (NO₃⁻) 500 Monthly Dilute or replace solution
Ammonia (NH3) 200 Monthly Raise pH to >8 (reduces effect of ammonia)
Nitrite (NO2-) 20 Weekly Add sodium azide (10 ppm) to eliminate nitrites
Engineering Beyond Control of Impurities and Grade Selection

Tolerance of nitrite contamination depends of titanium grade. Grade 7 (palladium stabilized) has 2–3 times better nitrite tolerance, allowing a maximum of 50 ppm for a 5 year service life, as nitrite is reduced to nitrogen gas by palladium catalysis without damage to the film. Grade 12 has intermediate progress. Wall thickness provides corrosion allowance. A 2.0 mm wall with pitting at 0.15 mm/year from 100 ppm nitrite lives 13 years. The pH of the thiosulfate is important . Keeping the pH over 8 greatly decreases the aggressiveness of nitrite by converting some NO₂⁻ to the less aggressive NO₃⁻ . Nitrite synthesis from reduction of nitrate is decreased by nitrogen sparging to eliminate oxygen. Add 100 ppm sodium sulfite periodically to decrease nitrite to nitrogen gas.

A specification informed

For a titanium heating coil in 5% sodium thiosulfate at 60°C, the nitrite concentration needs to be kept below 20 ppm for Grade 2 or below 50 ppm for Grade 7. Weekly check solution using ion chromatography or nitrite test strips. Fit a carbon filter in the water supply to remove nitrate and nitrite before adding thiosulfate. If the nitrite level in the system is between 20 and 100 ppm, add 10 ppm of sodium azide or 100 ppm of sodium sulfite to destroy the nitrite or raise the pH to 9 with sodium hydroxide. The engineer can prevent passive film breakdown and pitting in titanium heaters in thiosulfate service by controlling the most aggressive contaminant, nitrite.