What Is the Maximum Allowable Copper Ion Concentration (ppm) in a 10% Sulfamic Acid Solution at 80°C Before a Grade 2 Titanium Heater Tube Begins to Plate Copper and Undergo Galvanic Corrosion?

The copper ions (Cu 2+ ) can be plated on the titanium surface as metallic copper in 10% sulfamic acid (H 2 NSO 3 H) at 80°C according to the reaction Cu 2+ + 2e - → Cu 0 . Copper coating presence generates a galvanic pair with titanium, which increases the anodic dissolution of the substrate metal. To avoid plating, the copper content must be less than 50 ppm. Above 50 ppm, copper plates out rapidly and serious galvanic corrosion occurs. The threshold is set by the reduction potential of copper and hydrogen evolution overpotential on titanium.

How does copper plating work and galvanic corrosion on titanium

In sulfamic acid, titanium is passive with a corrosion potential of around -0.1 to +0.1 V versus Ag/AgCl. Copper ions have a reduction potential of +0.34 V . When the copper concentration is higher than a critical concentration, the local concentration overpotential is overcome and copper is deposited on the titanium surface. The coated copper is a cathode with the exposed titanium acting as an anode. The galvanic current density can reach 10–100 µA/cm2, resulting in rapid titanium dissolution at a rate of 0.2–2.0 mm per year.

Quantitative Copper Plating and Galvanic Corrosion against Cu²⁺ Concentration

Grade 2 titanium has been proven in controlled testing in 10% sulfamic at 80°C for 500 hours as follows: At 0 ppm Cu 2+ no copper plating was observed. The galvanic current is 0 µA/cm 2 and the titanium corrosion rate is 0.01-0.03 mm year-1 . Trace plating is possible at 10 ppm, galvanic current is 0 to 1 µA/cm2, and corrosion rate is 0.02 to 0.05 mm per year. Light plating is noticed at 25 ppm, galvanic current 1-5 µA/cm2 and corrosion rate 0.03-0.08 mm/year. At 50 ppm, the moderate level of plating, the galvanic current is 5–15 µA/cm² and the corrosion rate is 0.08–0.20 mm per year, the critical threshold. At 100 ppm, substantial plating is observed, the galvanic current is 15–40 µA/cm2 and the corrosion rate is 0.20–0.50 mm per year. The galvanic current is 40-100 µA/cm 2 and the corrosion rate is 0.50-1.50 mm per year. The surface is completely plated at 250 ppm. At 500 ppm, the surface is 100% copper plated and galvanic current is >100µA/cm2. The corrosion rate is 1.50-3.00 mm/year.

Effect of Sulfamic Acid Concentration and Temperature on Copper Tolerance

The maximum permissible copper concentration falls with increasing acid concentration and temperature. In 5% sulfamic acid at 80°C the critical Cu 2+ level is 100 ppm. At 10% and 80°C the critical level is 50 ppm. The critical level is 30 ppm at 80°C and 15%. The critical threshold at 20% and 80°C is 20 ppm. Critical level 100 ppm at 10% and 60 °C. Critical level 50 ppm at 10% and 80°C. 25 ppm is critical threshold at 100°C and 10% Higher acid concentrations and temperatures enhance the rate of hydrogen evolution and influence the thermodynamics of the plating.

Guide to Copper Contamination in Sulfamic Acid Heaters

The table below recommends the maximum permissible concentration of copper ions and mitigation techniques for Grade 2 titanium heaters in 10% sulfamic acid at 80°C.

Cu2+ Concentration Measured (ppm) Copper Plating Galvanic Current Density (µA/cm2) Titanium Corrosion Rate (mm/year)Recommended Action <25 None 0-5 <0.08None 25–50 Light 5–15 0.05–0.12Monitor weekly 50-75 Moderate 15-30 0.12-0.25Add chelating agent .
75-100 Heavy 30-60 0.25-0.50 Replacement of solution >100 Severe >60 >0.50 Immediate replacement
Engineering Beyond Copper Concentration Control

The grade of titanium affects galvanic corrosion after copper plating. Grade 7 (palladium stabilized) exhibits galvanic corrosion rates 2-3 times lower than Grade 2 with the same copper deposit. Wall thickness gives a corrosion allowance, a 2.0 mm wall with copper induced corrosion at 0.2 mm per year lasts 10 years. Complexing agents e.g. 100 to 200 ppm EDTA combine with copper ions preventing plating. Ion exchange resins can extract copper from sulphamic acid solutions. The plating is inhibited by adsorption on the titanium surface by adding a copper specific inhibitor such as benzotriazole (10 ppm).

An Informed Specification

For a Grade 2 titanium heater in 10% sulfamic acid at 80°C keep the copper ion concentration below 25 ppm to be below the plating threshold of 50 ppm for safety. Check copper levels weekly with atomic absorption spectroscopy or colorimetric test strips. If the concentration is >50 ppm complicate the copper by adding 200 ppm EDTA or change the solution. Existing heaters with copper deposits can be cleaned with 20% nitric acid at 50°C for 30 min to dissolve the copper without harming the titanium. In sulfamic acid service, the engineer controls copper concentration below the critical threshold to avoid copper plating and galvanic corrosion.