For a Titanium Heat Rod Used to Reheat a 30% Sodium Thiocyanate Solution at 80°C, How Does the Thiocyanate Anion Competitively Inhibit Chloride-Induced Pitting?
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It is found that the thiocyanate anions (SCN-) strongly adsorb on the titanium surface in 30% sodium thiocyanate (NaSCN) at 80°C to form a protective barrier. Cl- ions compete with SCN- for adsorption sites on the surface. SCN- has a higher adsorption affinity than Cl- and, therefore, displaces chloride from the surface and prevents chloride-induced pitting. The inhibition efficiency is increased by the concentration of SCN- . Pitting is totally suppressed when the molar ratio SCN- / Cl- is higher than 2. The protective coating is a titanium-thiocyanate complex which prevents the entry of chloride to the metal surface.
Mechanism of Competitive Adsorption and Pitting Inhibition
Thiocyanate is a soft base, which bonds covalently and strongly to surface atoms of titanium. Chloride is a hard base and forms weaker electrostatic connections. In mixed solutions SCN⁻ preferentially adsorbs and blocks surface sites that would be available to chloride. The deposited SCN layer also modifies the surface charge and repels chloride ions. The inhibition obeys the Langmuir isotherm θ K[SCN ] 1 K[SCN ] (1) where θ is the surface coverage and K is the adsorption equilibrium constant.
Quantitative pitting inhibition as a function of SCN- / Cl- ratio
The following has been established for Grade 2 titanium by controlled testing in 1,000 ppm Cl- solutions at 80°C and various SCN- concentrations for 500 hours. SCN - /Cl - molar ratio 0 (0 ppm SCN - ) Pitting density 50-150 pits/cm 2 Maximum pit depth 200-500 microns The pitting density at a ratio of 0.5 (500 ppm SCN-) is 10-30 pits/cm2 and the pit depth is 50-100 µm. Pitting density is 2-10 pits/cm 2 and pit depth is 10-30 µm at a ratio of 1.0 (1,000 ppm SCN -) 2.0 ratio (2,000 ppm SCN-) has a pitting density of 0 pit/cm2 and pit depth of 0µm (complete protection). At the ratio of 5.0 (5000 ppm SCN -), the pitting density is 0 and the pit depth is 0.
Critical Ratio as a Function of Temperature and Chloride Concentration
As the temperature increases the minimal ratio of SCN-/Cl- for full protection increases. At 40 °C the ratio is 0.8. At 60°C the ratio is 1.2. The ratio is 2.0 at 80 °C. The ratio at 100°C is 3.0. The required ratio also increases with chloride concentration. At 500 ppm Cl⁻ the ratio is 1.5, at 1,000 ppm Cl⁻ the ratio is 2.0 and at 5,000 ppm Cl⁻ the ratio is 3.0.
Thiocyanate: The Secret to Chloride Pitting Prevention
The following table gives guidance on SCN⁻ concentration to avoid chloride-induced pitting on Grade 2 titanium heaters operating at 80°C.
Concentration of chloride, ppm Minimum SCN− for complete protection, ppmSCN⁻ (ppm) suggestedSCN-/Cl- Molar Ratio 100 200 300 2.0 250 500 750 2.0 500 1,000 1,500 2.0 1,000 2,000 3,000 2.0 2,000 4,000 6,000 2.0 5,000 10,000 15,000 2.0 Engineering beyond thiocyanate concentration control
The grade of titanium dictates the SCN⁻ concentration necessary. Grade 7 (palladium stabilized) has a lower need (1.0-1.5 SCN-/Cl- ratio at 80 C) due to its more noble surface. Wall thickness provides a pit penetration allowance if inhibition is not complete. The stability of SCN- is pH dependent. At pH < 4, SCN- hydrolyzes to HCN and is lost. SCN⁻ is stable at pH > 10. Oxidizing substances (e.g., Fe³⁺) can oxidize SCN⁻ to sulfate, thus decreasing inhibition.
An Informed Specification
For a titanium heat rod in 30% sodium thiocyanate at 80°C with a chloride contamination, the molar ratio SCN-/Cl- should be maintained above 2.0 to avoid pitting. If chloride content is uncertain, use SCN- at 5,000 ppm (0.5%) as general inhibitor. Weekly titration to check the SCN- concentration. If the ratio falls below 2.0, add sodium thiocyanate. For existing heaters with pitting, increase the SCN⁻ concentration up to 10,000 ppm for a short time to repassivate the surface. The engineer uses thiocyanate as a competitive adsorbate to protect titanium heaters against chloride pitting.
