How Does the Presence of Oxidizing Ions (Fe³⁺, Cu²⁺) in 5% Sulfuric Acid at 60°C Extend or Reduce the Passive Range of Titanium Grade 2 Heating Tubes?

Grade 2 titanium is close to its active-passive transition in 5 % H2SO4 at 60 °C. Grade 2. It will erode vigorously in deaerated acid at rates from 0.5 to 2.0 mm per year. In the presence of dissolved oxygen or oxidizing ions (Fe³⁺, Cu²⁺, Cr⁶⁺) titanium passivates and the corrosion rate is lower than 0.01 mm/year. But the oxidizing ions can lead to excessive passivation or pitting at high quantities sometimes. The effect of Fe3+ and Cu2+ on the extension or reduction of the passive range of Grade 2 in 5% H2SO4 at 60°C is discussed in this article.

Passivation Mechanism by Oxidizing Ion Source
The reducible oxidizing species are ferric (Fe3+) and cupric (Cu2+) ions. Their reduction at the titanium surface gives a cathodic current that polarizes the titanium into the passive region: Fe3+ + e- → Fe2+ (E0 = +0.77 V vs. SHE) Cu2+ + 2e- → Cu0 (E0 = +0.34 V vs. SHE) The critical potential for titanium passivation in 5% H2SO4 is ~-0.2 to 0 V vs. SCE. Both Fe 3+ and Cu 2+ create enough cathodic current to exceed this barrier.

Effects of Concentration
Grade 2 testing in 5% H2SO4 at 60 °C at different concentrations of Fe3+ and Cu2+:

No oxidizing ions (deaerated): Active corrosion 1.0-2.0 mm/year.

Fe³⁺ 10 ppm Passivation happens. Corrosion rate decreases to 0.02-0.05 mm/year. Passive range is expanded .

50–500 ppm Fe3+ Stable passivation. Corrosion rate 0.01-0.03 mm/year. Best range.

Fe3+ 1,000–5,000 ppm: Still passive, but the risk of pitting increases in the presence of chlorides.

Fe³⁺ 5,000 ppm: Can produce over-passivation at higher temperatures (but not at 60°C).

For Cu 2+ : Same reaction , but Cu 2+ can deposit metallic copper on titanium ( Cu 2+ + 2e − → Cu 0 ) . Copper deposits are galvanic cells, which is good if they are uniform, but bad if nodular.

Synergistic and Antagonistic Actions
Fe³⁺ + Cu²⁺ together: Better passivation stability than each alone. In the presence of Fe3+, the mixed potential is intermediate and the copper deposits are more uniform.

Oxidizing ions + chlorides: In the presence of oxidizing ions, chlorides (even 10-50 ppm) can cause pitting. Passivation plus aggressive anion is the optimum combo for pitting corrosion.

Oxidizing ions + reducing agents If reducing agents (SO₂, H₂S, organic matter) use up Fe³⁺ passivation is lost. Check Fe3+ concentration.

Measured Corrosion Rate
Oxidizing Ion Concentration Corrosion Rate (mm/yr)Passive? Pitting Danger
None (deaerated) 0 ppm 1.0-2.0 No None (active corrosion)
Fe3+10 ppm0.02-0.05YesLowFe3+100 ppm0.01-0.02YesLowFe3+1,000 ppm0.01-0.03YesLow (if Cl- 10 ppm)
Cu²⁺ 10 ppm 0.03 - 0.08Yes (With Cu deposit) Moderate Cu2+ 100 ppm 0.02-0.05 Yes Moderate (Nodular Cu)
Fe³⁺ + Cu²⁺ (1:1) 100 ppm each 0.01-0.02 Yes Low Fe³⁺ + 50 ppm Cl⁻ 100 ppm 0.02-0.05 (pitting) Yes (but pitting) High Application Matrix for Grade 2 in 5% H₂SO₄ at 60°C
Service Condition Level of Oxidizing IonChloride Level Anticipated Grade 2 PerformanceRecommended Action
Deaerated acid None Any Active corrosion (1-2 mm/year)Add Fe3+ or use Grade 7
Air SpargingO₂ (8 ppm) <10 ppm Passive (0.01-0.03 mm/yr)Not Acceptable
Fe3+ from process (eg steel pickling) 50-500 ppm <10 ppmExcellent passive (0.01mm/year) Mint condition
Fe3+ + chlorides e.g. HCl pollution 100 ppm 50 ppm Pitting (0.1-0.3 mm/year pits) Add Fe3+ >500 ppm or lower Cl- Cu2+ from upstream copper processing 100 ppm <10 ppmPassive with Cu depositionCheck copper thickness Clean once a year
Grade 2 heating tubes in 5% H 2 SO 4 at 60 °C with the addition of 10-500 ppm Fe 3+ show an increase in the passive range with corrosion rates <0.03 mm/year compared to >1 mm/year. Fe³⁺ is good and if possible should be maintained. Cu²⁺ also promotes passivation but can deposit metallic copper-uniform deposits are protective, nodular deposits cause galvanic assault. The oxidizing ions combine with chlorides and cause pitting. Keep Fe³⁺ above 500 ppm to avoid pitting caused by chlorides. For new processes, consider adding 50-100 ppm Fe3+ (as Fe2(SO4)3) to assure passivation. If oxidizing ions cannot be maintained, specify Grade 7 titanium. For existing heaters with active corrosion, Fe3+ solution injection moves the heater into passive range; this low-cost intervention can extend the life of the heater from months to years.