When Heating Mixed Acid Solutions (H₂SO₄ + HCl) at 80°C, Why Does Grade 7 Titanium Outperform Grade 2 in Preventing Hydrogen-Induced Cracking?

One of the most difficult situations for metallic heating equipment is exposure to mixed acid solutions including both sulfuric and hydrochloric acids . Under elevated temperatures (∼80°C), the combination of chlorides from HCl and the reducing character of H2SO4 provides conditions where commercially pure titanium (Grade 2) becomes vulnerable to hydrogen absorption and subsequent embrittlement, whereas the palladium-stabilized Grade 7 alloy retains its ductility and crack resistance. This discrepancy in performance is attributed to the inherent variance in the maintenance of the passive oxide coating of each alloy in mixed acid electrolytes. Engineers specifying immersion heaters for pickling lines, metal finishing baths, or chemical reactors handling mixed H₂SO₄-HCl solutions must understand the mechanism of hydrogen-induced cracking (HIC) and its dependence on palladium content to specify a heater that will survive past the first maintenance cycle.

Absorption of Hydrogen by Titanium Alloys
Titanium absorbs hydrogen anytime the electrochemical potential at its surface is less than the equilibrium potential for the Ti/H₂O reaction, a situation that is frequently observed in the case of reducing acids where the cathodic half-reaction changes from reduction of oxygen to reduction of protons. Once absorbed, hydrogen atoms migrate interstitially through the titanium lattice and precipitate as brittle titanium hydride (TiH2) platelets when the local hydrogen concentration reaches the solubility limit, which is about 150 ppm at 80°C for Grade 2. Formation of hydrides results in a large lattice expansion (around 15% volume increase) and hence generates internal tensile stresses that cause cracking, especially in areas of triaxial stress, such as weld heat-affected zones or tube sheet rolled joints.

In mixed H₂SO₄-HCl solutions at 80°C, the corrosion potential of Grade 2 titanium is usually in the range -300 to -100 mV versus SCE, which is in the hydrogen evolution region. Laboratory testing (ASTM G5-14 reference) of open-circuit potential measurements shows that Grade 2 immersed in 10% H2SO4 + 5% HCl at 80°C reaches a stable potential of approximately −200 mV vs. SCE, corresponding to a cathodic current density of approximately 0.5 mA/cm2 from proton reduction. Atomic hydrogen produced at this rate diffuses into the titanium at a flux of about 10-8 mol/cm2.s and raises the bulk hydrogen level from an initial 30 ppm (mill-annealed condition) to the critical level of 150 ppm in 400-600 hours of continuous exposure. On the other hand, grade 7 titanium has a potential that is 200 to 350 mV more positive (generally +50 to +150 mV vs. SCE) due to the catalytic effect of palladium on the cathodic reaction. At this potential the major cathodic activity switches from proton reduction to reduction of residual oxygen and the rate of hydrogen absorption reduces by three orders of magnitude.

Effect of Palladium on the Stability of Passive Film
The enhanced performance of Grade 7 (Ti-0.12 to 0.25% Pd) is explained by the ability of the noble metal to induce spontaneous passivation in reducing environments. The palladium islands dispersed in the titanium matrix are excellent cathodic sites for the reduction of any oxidizing species present (dissolved oxygen, ferric ions or even protons at lower overpotential). Such galvanic coupling drives the whole surface of the alloy to the passive region where a continuous TiO2 film exists even in acids that would actively corrode or maintain an active potential on Grade 2. The electrochemical impedance spectroscopy tests showed that a passive film resistance of more than 100 kΩ.cm2 was formed on the surface of Grade 7 after 30 min of immersion in mixed acid at 80°C, while for Grade 2 the film was porous and non-protective with a resistivity below 1 kΩ.cm2 formed under the same conditions.

The essential distinction emerges when trace oxidizing agents are absent. Grade 2 is attacked in 10% H2SO4 + 5% HCl (deaerated) at 80°C at a rate of 0.5 to 1.0 mm per year with a hydrogen uptake of >300 ppm after 1,000 hours. Grade 7 shows a corrosion rate of less than 0.05 mm/year and hydrogen absorption below 50 ppm in the same deaerated atmosphere even after 2,000 hours of continuous testing. That is why we refer to it as a margin of safety . That is why grade 7 is specified for combined acid heaters below boiling point . Grade 2 is restricted to dilute concentrations ( below 3 % each ) or below 50 C .

Consequences of failure and time to failure data
The hydrogen-induced cracking of Grade 2 heating tubes usually occurs as longitudinal cracks along the tube wall, often originating from the inner surface where the hydrogen concentration is maximum and moving outwards. Such failures are unexpected and catastrophic in service-a heater operating at 80°C in a mixed acid tank may exhibit no obvious degradation for 800 hours, and then develop several through-wall cracks in the space of 48 hours. Hydrogen analysis of Grade 2 tubes removed from mixed acid service after failure consistently indicates bulk concentrations between 200 and 500 ppm, with localized hydride layers of 50 to 100 micrometer thickness at crack initiation sites.

Accelerated life testing with U-bend specimens (ASTM G30) can provide comparative data. Grade 2 U-bends exposed to 15% H 2 SO 4 + 3% HCl at 80°C develop visible cracks after 300 to 500 hours and total fracture after 700 hours. Metallographic studies of identical Grade 7 specimens demonstrate no production of hydride phase and no cracking after 3,000 hours. The time-to-failure of immersed heating tubes under residual tensile stress from manufacture or mounting is inversely proportional to the hydrogen flux. A 25 mm diameter tube of Grade 2 with thickness of 1.65 mm and residual stresses of 150 MPa in the as-welded condition failed after about 1200 hours in mixed acid at 80°C while a Grade 7 tube of the same dimensions was still functional after 8000 hours.

Selection Guide Mixed Acid Heaters Application
The following decision matrix translates the electrochemical and mechanical analysis into practical specifications for engineers who purchase or design heating equipment for mixed H 2 SO 4 -HCl service.

Application Environment Acid Composition Temperature Recommended Alloy & Wall Thickness Rationale & Key Trade-Offs
Steel pickling line (carbon steel scale removal) 5-10% H 2 SO 4 + 1-2% HCl 75-85°C Grade 7, 1.2-1.5 mmGrade 2: Palladium stabilization necessary. Chloride promotes hydrogen absorption. 1.2 mm is adequate corrosion allowance.
Aluminum anodizing bath (mixed acid)2-4% H2SO4 + 0.5-1% HCl 30-40°C Grade 2, 1.0-1.2 mmGrade 2 Economical Monitor Bath Chloride Low Temperature Reduced Rate of Hydrogen Diffusion Replenish Every 24 Months
Titanium etch bath (PCB production)3-5% HCl + 1-2% HF (w/o H2SO4) 45-55°C Grade 7, at least 1.5 mmFluoride presence speeds up attack; Grade 7 heavy gauge necessary; avoid Grade 2 entirely because of quick hydrogen uptake.
Ferric chloride regeneration reactor 8% HCl + 2% H₂SO₄ + Fe³⁺ 85-90°C Grade 7, 1.65 mm Palladium alloy requires oxidation of Fe³⁺ ions for passivation, chlorides and temperature; inspect annually for hydride.
Waste acid concentration system 15-20% mixed acids, ratio varies 80-95°C Grade 7, 2.0 mm or Grade 12 (Ti-0.3Mo-0.8Ni)High temperature and concentration requires greatest corrosion resistance. Grade 12 may be a cheaper cost alternative to Grade 7.
Hydrogen Control Complementary Design Considerations
Increasing wall thickness alone does not prevent hydrogen-induced cracking, but it does increase the diffusion channel and the bulk volume available for diluting absorbed hydrogen. For mixed acid service, the alloy selection is independent of wall thickness consideration. Grade 7 allows a thinner wall (1.2 mm) to be safely employed, while even a 2.5 mm Grade 2 tube may eventually embrittle because to the strong hydrogen flux. Welds must also be of good quality: welds on Grade 7 must be made with suitable filler metal (Grade 7 matching filler) and back-purged with argon to avoid contamination. The alpha-case layer produced by welding or heat treatment is brittle and susceptible to hydrogen attack and must be removed. Operational controls can alter the potential of Grade 2 into the passive range, such as maintaining dissolved oxygen above 1 ppm (by air sparging) or adding small amounts of ferric sulfate (Fe2(SO4)3) as an oxidizing agent. However, these methods are not reliable for unattended heating equipment.

SUMMARY OF SPECIFICATION FOR THE MIXED ACID HEATER
Thus, using Grade 7 titanium instead of Grade 2 is not only a recommended but a must for reliable operation in mixed H₂SO₄-HCl solutions at 80°C for longer than 2,000 hours. The palladium content has a profound effect on the electrochemical behavior. It maintains a passive oxide film that suppresses hydrogen evolution and prevents hydrogen absorption which leads to brittle hydride formation and crack propagation. Grade 2 is economically desirable in many corrosive situations, but in hot mixed reducing acids it cannot maintain its passive layer and will absorb hydrogen at a rate that ensures embrittlement within conventional maintenance intervals. For any immersion heater application in which the combined H2SO4 concentration is greater than 5% and the HCl concentration is greater than 1% at temperatures above 60°C, engineers should use Grade 7 titanium. For borderline conditions, monitoring electrochemical potential (targeting sustained potential above 0 mV vs. SCE) can verify whether Grade 2 may survive. But the conservative way-and the one that avoids catastrophic heater failure and production downtime-is to select Grade 7 with wall thickness appropriate to the mechanical load and desired service life.