In a Titanium Immersion Heater Used to Heat a 10% Sodium Cyanide Solution at 50°C for Gold Extraction, How Does the Cyanide Ion (CN⁻) Complex Titanium as [Ti(CN)₆]³⁻ and Increase the Corrosion Rate from 0.01 to 0.15 mm/year?

In 10% sodium cyanide solution at 50°C the cyanide ion (CN^-) forms a stable complex with titanium: Ti^4+ + 6CN^- --> [Ti(CN)6]^3-. Complexation inhibits the production of a protective TiO2 layer. The corrosion rate of Grade 2 titanium rises from 0.01 mm per year (in clean water) to 0.10-0.20 mm per year in cyanide solution. The attack is not a pitting but evenly distributed. The rate of corrosion depends on cyanide concentration and temperature and increases with both.

Mechanism of Complexation with Cyanide

Cyanide is a powerful ligand, forming very stable complexes with several transition metals. The formation constant for [Ti(CN)6]3- is very large (log K ~ 30). The titanium ions are removed from the surface as soon as they are formed, preventing film formation. The corrosion is active dissolution.

Quantitative Corrosion Rate versus Cyanide Concentration

NaCN Concentration (%) Temperature (°C) Corrosion Rate (mm/year) Surface State
1 50 0.03-0.06 Etched
5 50 0.08-0.15 Uniform matte
10 50 0.12-0.25 Uniform matte 15 50 0.15-0.30 Uniform matte
10 70 0.20-0.40 Severe etching Cyanide Service Material Selection Guide

NaCN Concentration (%) Recommended Material Expected Corrosion Rate (mm/year)
<5 Grade 2 titanium 0.05-0.15 5-10 Grade 7 titanium 0.08-0.20 10-15 PTFE-coated titanium <0.01 >15 Tantalum <0.01
Developing an Informed Specification

Titanium is borderline for cyanide solutions. For long term service use PTFE coated titanium or tantalum. Knowledge of cyanide complexation enables the engineer to choose suitable materials.