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?
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In 10% sulfamic acid (H₂NSO₃H) at 80°C, copper ions (Cu²⁺) can plate onto the titanium surface as metallic copper. The plated copper forms a galvanic couple with titanium, accelerating anodic dissolution of the underlying metal. The maximum allowable copper concentration to prevent plating is 50 ppm. Above 50 ppm, copper plates rapidly; above 100 ppm, severe galvanic corrosion occurs.
Quantitative Copper Plating and Galvanic Corrosion
In 10% sulfamic acid at 80°C for 500 hours:
| Cu²⁺ (ppm) | Copper Plating Observed? | Galvanic Current (µA/cm²) | Titanium Corrosion Rate (mm/year) |
|---|---|---|---|
| 0 | No | 0 | 0.01–0.03 |
| 10 | No | 0–1 | 0.02–0.05 |
| 25 | Trace | 1–5 | 0.03–0.08 |
| 50 | Light | 5–15 | 0.08–0.20 |
| 100 | Moderate | 15–40 | 0.20–0.50 |
| 250 | Heavy | 40–100 | 0.50–1.50 |
| 500 | Continuous | 100–200 | 1.50–3.00 |
Effect of Acid Concentration and Temperature
| Sulfamic Acid (%) | Temperature (°C) | Critical Cu²⁺ for Plating (ppm) | Recommended Max Cu²⁺ (ppm) |
|---|---|---|---|
| 5 | 80 | 100 | 75 |
| 10 | 60 | 100 | 75 |
| 10 | 80 | 50 | 30 |
| 15 | 80 | 30 | 20 |
| 20 | 80 | 20 | 10 |
Copper Contamination Management Guide
| Measured Cu²⁺ (ppm) | Recommended Action | Expected Ti Corrosion Rate (mm/year) |
|---|---|---|
| <25 | None | <0.05 |
| 25–50 | Monitor weekly; add chelating agent | 0.05–0.15 |
| 50–100 | Replace solution or use ion exchange | 0.15–0.40 |
| >100 | Immediate solution replacement | >0.40 |
Engineering Recommendation
For sulfamic acid service at 80°C, maintain copper concentration below 30 ppm to provide a safety margin below the 50 ppm plating threshold. Use ion exchange or chelating resins to remove copper. By controlling copper ion concentration, the engineer prevents galvanic corrosion from copper plating.
