For a Titanium Heat Rod Used to Reheat a 30% Sodium Thiocyanate Solution at 80°C, How Does the Thiocyanate Anion (SCN⁻) Compete with Chloride for Surface Sites and Raise the Critical Pitting Temperature from 60°C (Cl⁻ only) to 90°C (Cl⁻ + SCN⁻)?

Thiocyanate anion (SCN⁻) is highly adsorbed on the titanium surface in 30% sodium thiocyanate at 80 °C. SCN− competes for surface sites in the presence of chloride. The critical pitting temperature (CPT) increases from 60 °C (Cl− only) to 85-95 °C at a SCN−/Cl− ratio of 2:1. The SCN− produces a passivation film to inhibit chloride adsorption. Pitting is entirely reduced at 80°C for SCN⁻/Cl⁻ ratios higher than 1.5.

The Mechanism of Competitive Adsorption

Thiocyanate has stronger adsorption affinity than chloride. It generates a surface [Ti(SCN)₆]³⁻ complex. This complex is more stable than the chloride complex . SCN− surface coating stops chloride from reaching the metal surface.

Quantitative CPT as a Function of the Ratio of SCN- to Cl-

SCN - /Cl - Molar Ratio Critical Temp. of Pitting (°C) Pitting at 80°C?
0 60 Yes 0.5 65-70 Yes 1.0 75-80 Marginal 1.5 85-90 No 2.0 90-95 No 3.0 95-100 No
Guideline for Thiocyanate Protection

Chloride Concentration (ppm) Required SCN- for CPT >80°C (ppm)SCN⁻/Cl⁻ Ratio 500 750 1.5 1,000 1,500 1.5 2,000 3,000 1.5 5,000 7,500 1.5
Specifying with Assurance

Add sodium thiocyanate to chloride-contaminated solutions at 80°C when SCN⁻/Cl⁻ ratio >1.5. To avoid pitting, the engineer uses competitive adsorption.