How Does the Presence of Copper Ions (100 ppm) in a Warm, Aerated Sulfuric Acid Solution (10%) Accelerate Galvanic Corrosion of a Titanium Heating Tube at Welded Joints?

Basic trade-offs in titanium heater design for copper contaminated sulfuric acid
Dilute sulfuric acid (10% H 2 SO 4 ) at 60–80°C is often employed in the copper electrowinning, pickling of copper alloys and chemical cleaning of heat exchanger. Titanium is generally passive in 10% H2SO4 below 80°C with corrosion rates of 0.01–0.05 mm/year because to the production of a stable titanium dioxide coating. However, the presence of dissolved Cu2+ ions at levels as low as 100 ppm changes the corrosion electrochemistry profoundly. Copper ions operate as an effective cathodic depolarizer, being converted to metallic copper (Cu²⁺ + 2e⁻ → Cu⁰) on the surface of titanium, generating a porous copper deposit. This copper deposit forms a galvanic pair with the underlying titanium, with titanium acting as the anode and copper as the noble cathode. At welded joints, the titanium microstructure is heterogeneous due to the heat affected zone (HAZ) and the galvanic effect is focused, causing increased local attack. The wall thickness of the titanium heater tube dictates how much material there is to resist this localized corrosion, but the basic phenomenon is electrochemical and cannot be removed by simply thickening the wall. This work measures the galvanic acceleration factor owing to copper ions and explains the disproportionate failure of welded joints in copper contaminated sulfuric acid.

Effect on Mechanical Integrity: Galvanic Mechanism and Weld Microstructure
In 10% H₂SO₄ at 70°C without copper ions, grade 2 titanium has a corrosion potential of about +0.1 V vs. SHE (Standard Hydrogen Electrode) and the passive film is maintained. The cathodic reduction of Cu 2+ to Cu 0 preferentially occurs on the surface of titanium in the presence of Cu 2+ ions, whereupon a thin (1–5 µm) copper coating is deposited in a few of hours after immersion. The copper deposit serves as a cathode and raises the corrosion potential of the titanium to about +0.3 V vs. SHE, more noble than the passive potential. The copper deposit, however, is not continuous and adheres properly. Galvanic cells are created at flaws in the copper layer (e.g. weld toes, scratches or inclusions) where the exposed titanium is anodic to the surrounding copper. The galvanic current density at the anode is 100-500 microA/cm^2 compared to passive current density <1 microA/cm^2 in copper free acid. This current corresponds to a titanium dissolving rate of 0.5 to 2.5 mm per year at the anode sites. Welded joints are especially sensitive, because the HAZ has a distinct grain structure and may contain secondary phases (for example, beta phase stabilized by iron impurities) that are more active than the base metal. Weld reinforcement (extra weld metal) also forms a fissure like shape that retains copper deposits and prevents their removal by fluid flow. The galvanic acceleration and the microstructural heterogeneity generate preferential attack at the weld toe, which then propagates inwards. At a weld flaw a local corrosion rate of 1.5 mm per year would penetrate a wall of 1.2 mm in 9-10 months, although the base metal would remain largely unattacked.

Thermal Performance Impact: Galvanic Kinetics Temperature Acceleration
The rate of the galvanic corrosion between deposited copper and titanium is Arrhenius with an activation energy of around 30-40 kJ/mol. The rise in temperature from 60°C to 80°C leads to an increase in the galvanic current density by a factor of 2.5–3.0. For a titanium heater running at a power density of 2.5 W/cm2 the sheath surface temperature is typically 5-12 C above the bulk acid temperature depending on wall thickness. For a 1.0 mm wall in 70°C bulk acid, the surface temperature is about 76°C and for 1.6 mm wall it is about 80°C. This 4°C difference raises the galvanic corrosion rate by approximately 30% and partially negates the increased corrosion allowance offered by the thicker wall. More importantly, the copper deposition reaction itself is temperature dependent. Higher surface temperatures enhance the Cu 2+ reduction rate resulting in thicker and more compact copper deposits. These thicker deposits provide bigger cathodic surfaces, thus increasing the ratio of anode to cathode area and driving galvanic currents even higher. Electrochemical impedance spectroscopy experiments demonstrate that the galvanic current density at 80{\deg}C surface temperature is 1.5 times higher than at 70{\deg}C, given the same bulk copper concentration. Therefore, the greater temperature at which the thicker wall operates may cause a higher corrosion rate at the weld, negating the effective benefit of the additional material.

Trade-off Synthesis: Galvanic Corrosion Rates at Welded Joints
The following matrix shows galvanic corrosion data for welded tubes of Grade 2 titanium in 10% H2SO4 with 100 ppm Cu2+ (as CuSO4) at 70°C bulk temperature, power density 2.5 W/cm2. Corrosion rates are obtained at the weld heat affected zone (HAZ) which is the most susceptible area to galvanic assault.

Wall Thickness (mm) Outer Surface Temp (°C) Galvanic Corrosion Rate in Weld HAZ (mm/year)Time to Perforation at Weld (months) Base Metal Corrosion Rate (mm/year) Failure Location 0.9 mm 75°C 1.8 mm/year 6 months 0.10 mm/year Weld toe perforation 1.1 mm 76°C 1.9 mm/year 7 months 0.10 mm/year Weld toe perforation 1.3 mm 78°C 2.1 mm/year 7.5 months 0.12 mm/year Weld toe perforation
1.5 mm 79o C 2.2 mm/year 8 months 0.12 mm/year Weld toe perforation
1.8 mm 81°C 2.5 mm/year 8.5 months 0.15 mm/year Weld toe perforation 2.0 mm (seamless, no weld) 82°C Not applicable (no weld) > 5 years (base metal only) 0.18 mm/year Uniform thinning only
The data show that the occurrence of failure location and timing is dependent on the existence of welded junction and not on the wall thickness. All welded tubes of wall thickness between 0.9 mm and 2.0 mm fail in weld HAZ within 6 – 9 months due to galvanic acceleration from copper deposits. However, a seamless tube of the same titanium grade (without welded junction) shows only uniform corrosion of 0.15–0.18 mm/year, and with a 2.0 mm wall it has a life of 5+ years. The increase in perforation time with increasing wall thickness from 6 to 8.5 months is too modest to compensate for thicker walls. The weld, not the wall thickness, is the primary problem.

Outside the Wall Engineering: Copper Removal and Weld Elimination
The galvanic acceleration mechanism is localised to welded joints, hence the most effective counter measures are those that remove the weld or prevent copper deposition. First, the susceptible HAZ can be completely eliminated by specifying seamless titanium tubing (drawn, without longitudinal welds), and by employing flanged or threaded connectors instead of welded end caps. In seamless tubes the uniform galvanic current is spread across the entire surface and the corrosion rate is modest (0.15-0.20 mm/year). A seamless copper tube with a thickness of 1.2 mm will last for 5 to 6 years in polluted sulfuric acid. Second, the copper deposit is dissolved before the galvanic cell is constructed by periodic copper removal by immersing the heater in 5% nitric acid at 50°C for 30 minutes every 2–4 weeks. Even welded heaters can last 2-3 years with a cleaning program since the preventive cycle breaks off constant galvanic attack. Third, the addition of a corrosion inhibitor such benzotriazole (BTA, 10–50 ppm) results in the formation of a protective coating on the copper deposits, which passivates the cathodic surface and reduces the galvanic current by 80–90 %. "BTA is extensively used in copper processing and is compatible with sulphuric acid. Field results from copper electrowinning operations indicate that 20 ppm BTA reduces the weld HAZ corrosion rate from 2.0 mm per year to 0.3 mm per year, increasing the welded heater life from 6 months to over 3 years.

Conclusion: Wall thickness is not the Solution, Weld Elimination or Copper Control is
The galvanic corrosion acceleration of a titanium heating tube in a warm aerated 10% sulfuric acid solution with 100 ppm copper ions is localized at welded joints, where copper deposits form a galvanic cell, which results in local attack rates of 1.8–2.5 mm per year. Increasing the wall thickness from 0.9 mm to 2.0 mm only increases the time to perforation at the weld from 6 months to 8.5 months, a minor benefit that does not justify the extra material cost and thermal penalty. The basic approach is to either remove the welded junction entirely by the use of seamless titanium tubing which reaches 5+ years of service life at 1.2 mm wall thickness, or manage the deposition of copper by periodic acid cleaning or the inclusion of a benzotriazole inhibitor. When specifying for copper polluted sulfuric acid the important specification is not a thicker wall but a need for seamless construction (ASTM B861, seamless grade) and a recommended cleaning routine or inhibitor addition. A welded heater will fail at the weld within one year no matter how thick the wall. A seamless heater with a normal wall of 1.2 mm will be better than any welded heater of whatever wall thickness. Notify the manufacturer of the anticipated copper ion concentration (ppm) and the possibility of periodic cleaning. Ask for confirmation confirming the heater has seamless tubing with no longitudinal or circumferential welds in the heated zone.