For a Titanium Heat Rod Used to Reheat a 30% Sodium Thiocyanate Solution at 80°C, Why Does the Thiocyanate Anion (SCN⁻) Competitively Inhibit Chloride-Induced Pitting?

Thiocyanate anions (SCN⁻) present in 30% sodium thiocyanate (NaSCN) at 80°C strongly adsorb to the titanium surface and generate a protective barrier. SCN- competes for surface sites in the presence of chloride ions. SCN− has a higher adsorption affinity than Cl− and hence it displaces chloride at the surface and thus prevents the chloride produced pitting. The inhibitory efficiency increases with the concentration of SCN- ions. For the SCN⁻/Cl⁻ molar ratio above 2, the pitting is fully suppressed.

Quantitative Inhibition of Pitting by SCN–

For a solution of 1000 ppm Cl- at 80 C:

SCN- Concentration (ppm) SCN-/Cl- Molar Ratio Pitting Density (pits/cm2) Maximum Pit Depth (µm) 0 0 50-150 200-500 500 0.5 10-30 50-100 1,000 1.0 2-10 10-30 2,000 2.0 0 0 5,000 5.0 0 0 Inhibition efficiency with temperature

Temperature (°C) Minimum SCN- / Cl- Ratio for Complete Protection
Engineering Recommendation 40 0.8 60 1.2 80 2.0 100 3.0

To avoid pitting in chloride containing solutions, the SCN-/Cl- molar ratio should be kept above 2.0 at 80°C. The engineer prevents pitting of titanium heaters caused by chlorides by employing thiocyanate as a competitive adsorbate.