When a Grade 7 Titanium Sheath Heater Is Used to Maintain 95°C in a 40% Ammonium Nitrate Solution for Fertilizer Production, How Does the Nitrate Ion Reduction at the Cathodic Potential of -0.8 V vs. Ag/AgCl Change the Surface pH from 7 to 10 and Promote the Formation of a Protective Titanate Layer?

Reduction of nitrate ions NO₃⁻ + H₂O + 2e⁻ → NO₂⁻ + 2OH⁻ At 95 °C in 40 % ammonium nitrate under cathodic protection at −0.8 V vs. Ag/AgCl The local pH at the titanium surface increases from 7 to 10-11 . At such high pH titanium creates a protective coating of sodium titanate (Na₂TiO₃) or ammonium titanate. The corrosion rate is reduced from 0.10 mm per year (unprotected) to 0.01-0.03 mm per year (protected). The titanate layer is stable and very adherent.

Mechanism of Passivation Through Cathodic Protection

Cathodic current converts nitrate to nitrite and hydroxide. The hydroxide ions raise the ph. Titanium is passive at pH >9. The titanate layer is formed by reacting TiO2 with alkali. It is a coating 1-5 µm thick which prevents further rusting.

Quantitative corrosion rate versus cathodic potential

Cathodic Potential (V vs. Ag/AgCl) Surface pH Corrosion Rate (mm/year) Titanate Layer Thickness (microns)
0 (unprotected) 7 0.08-0.15 0 -0.6 8-9 0.04-0.08 0.5-1
-0.7 9-10 0.02-0.05 1-2
-0.8 10-11 0.01-0.03 2-4
-0.9 11-12 0.005-0.015 3-5
Cathodic Protection (CP) In Detail

Desired Corrosion Rate (mm/year) Minimum Cathodic Potential (V vs. Ag/AgCl) Expected Life (1.5 mm wall) <0.08 -0.6 15-30 years <0.05 -0.7 30-50 years <0.03 -0.8 50-100 years
Informed Specification

For titanium heater in ammonium nitrate at 95°C, apply cathodic protection at -0.8 V vs. Ag/AgCl to raise the surface pH and form a protective titanate layer. The engineer controls the potential to reduce corrosion.