Titanium Hydriding: 7 Embrittlement Causes in Acid Service

Last updated: 2026-11-07

Titanium hydriding is the absorption of atomic hydrogen into titanium, where it reacts to form brittle titanium hydride (TiH₂) platelets that strip the metal of its ductility. In hydrometallurgical acid service it shows up as loss of ductility and unexpected cracking — a plate heat exchanger can fail within 14 months — and prevention hinges on keeping the passive oxide film intact and away from conditions that force hydrogen onto the titanium surface. Whether your duty is a leaching circuit, solvent extraction, or an electrowinning feed train, the same seven causes apply.

Key Takeaways

• Titanium hydriding forms brittle TiH₂ inside the metal; it cannot be reversed, only prevented.
• The passive TiO₂ film protects titanium only while oxidizing conditions hold; reducing acids break it down.
• Crevice corrosion, iron contamination, galvanic coupling, and overprotection are the four site-generated causes.
• Hot reducing acid, temperatures above roughly 80 °C, and weld defects account for the remaining three.
• Bulk hydrogen above roughly 100–150 ppm (Grade 2) signals embrittlement risk; metallography confirms it.

What Is Titanium Hydriding and Why Does It Matter in Acid Service?

Four definitions frame the mechanism before the seven causes:

Titanium hydriding is a hydrogen damage mechanism that absorbs atomic hydrogen into the titanium lattice, where it precipitates as brittle titanium hydride (TiH₂) platelets aligned with the metal’s crystal structure. The metal keeps its strength but loses its toughness: bend radii that passed acceptance testing will crack after enough hydrogen has entered. The damage is internal and irreversible — no treatment removes TiH₂ from a finished component.

The passive oxide film is a self-healing TiO₂ layer that forms spontaneously on titanium in the presence of oxygen or oxidizing species and is only a few nanometers thick. It is the basis of titanium’s reputation in acid service: when intact, the metal behaves as a noble material; when the film is breached faster than it can repassivate, bare titanium dissolves and charges hydrogen directly into the surface.

Crevice corrosion is a localized corrosion mode that develops inside tight gaps — under gaskets, deposits, weld backs, and bolt washers — where the stagnant electrolyte depletes oxygen and acidifies locally. In chloride-bearing acid it is the most common way a surface loses its film in service without operator error.

Cathodic (galvanic) hydriding is a hydrogen uptake process that occurs when titanium acts as the cathode of an electrical couple: hydrogen ions are reduced on the titanium surface, and some of that hydrogen enters the metal instead of bubbling away. The oxide film normally suppresses this, but it is not a perfect barrier at cathodic potentials — a fact that surprises engineers who specify titanium precisely because it is “immune.”

Why does this matter in a hydromet plant specifically? The leach-circuit envelope — hot acid, chloride, mixed metals, stagnant shutdowns — attacks every one of the film’s assumptions at once. Titanium does not thin uniformly like carbon steel; it loads with hydrogen quietly until one cold start or pressure spike finds the embrittled metal in heat exchangers, pressure oxidation autoclaves, and pumps.

Why Does the Passive Film Normally Protect Titanium?

Understood correctly, titanium in acid is a ceramic-coated metal whose coating regenerates itself. The TiO₂ film is thermodynamically stable in a wide band of pH and potential, and it repassivates within milliseconds after mechanical damage — provided oxidizing species are present to reform it. That last clause is where plant reality intervenes.

In oxidizing acid systems the film thrives. Sulfuric acid containing dissolved oxygen or ferric ions (Fe³⁺) continuously repassivates the surface; this is why Grade 2 titanium handles naturally aerated dilute to moderate H₂SO₄ at ambient temperature well, and why copper electrowinning electrolytes rich in Cu²⁺ and Fe³⁺ are famously friendly to titanium. A useful heuristic: if the liquor carries a measurable ferric concentration, the film is being maintained every second the equipment runs.

In reducing acid the film starves. Hot dilute hydrochloric acid, deaerated sulfuric acid, and sulfurous species dissolve TiO₂ faster than dissolved oxygen or Fe³⁺ can rebuild it. Once bare metal is exposed, the corrosion reaction couples anodic dissolution to hydrogen evolution on the same surface — the acid literally manufactures the reagent for titanium hydriding at the point of attack. Temperature and acidity shift the boundaries: as general guidance, Grade 2 in naturally aerated sulfuric acid is acceptable at low concentrations near ambient temperature, the risk band widens steeply toward and above 80 °C, and low pH with low oxidant is the worst combination. These thresholds move with chloride content and grade, so treat them as screening guidance and confirm with test work.

Table 1 compares the four workhorse grades against these limits. Composition and mechanical values follow the ASTM B265 plate specification; corrosion service windows are indicative industry practice for acid duties, to be confirmed by corrosion testing on your actual liquor.

Table 1 — Titanium grades for hydrometallurgical acid service (indicative values; confirm with test work)

Grade 2 is the default because it balances cost, weldability, and corrosion behavior; Grade 7 exists because palladium shifts the hydrogen evolution reaction to higher overpotential in reducing acid, directly attacking the hydriding mechanism; Grade 12 buys most of Grade 7’s crevice performance at a fraction of the cost. Our pressure vessel material selection guide applies the same logic to full vessels under ASME VIII.

What Are the 7 Causes of Titanium Hydriding in Acid Service?

Every titanium hydriding failure traces to one or more of seven causes, presented as phenomenon, then mechanism, then fix. Causes 1 through 4 are conditions the plant creates; causes 5 through 7 are design and fabrication choices made before commissioning.

1. Crevice corrosion under deposits and gaskets in chloride-bearing acid. Phenomenon: pitting and brittle fringes under scale, sand, and gasket lips of chloride-containing duties. Mechanism: the crevice depletes oxygen and traps acid, locally breaching the TiO₂ film; the exposed metal dissolves anodically while the surrounding passive surface acts as the cathode, and hydrogen evolved at the breach enters the metal. Fix: eliminate the crevice — upgrade filtration so solids never settle (a titanium cartridge precision filter keeps scale-forming particles out of exchangers), design for full drainage, avoid gasket geometries that trap liquor, and manage scaling chemistry per our autoclave scaling guidance.

2. Iron contamination and embedded free iron. Phenomenon: rust-colored staining, localized attack, and hydride exactly where a steel brush or carbon-steel sling touched the metal. Mechanism: embedded free iron corrodes preferentially, disrupting the oxide film over each particle and creating micro-crevices and micro-galvanic cells. Fix: titanium-dedicated fabrication and handling tools, no carbon-steel wire brushes, immediate removal of smears by pickling or high-pressure water jetting, and passivation of repaired areas before acid exposure.

3. Galvanic coupling with titanium as the cathode. Phenomenon: hydriding concentrated on titanium connected to graphite heat exchanger sections, platinum-coated sensors, silver brazes, or active mixed-metal-oxide (MMO) anodes. Mechanism: the noble partner drives the titanium potential negative; hydrogen evolution then occurs ON the titanium surface, and a fraction enters the metal. Titanium is almost always the cathode in a couple — it is the more noble metal — so the damage lands on the titanium, not its partner; a laboratory MMO-coated titanium anode plate is safe at its design anodic current but hazardous if left electrically live while idle. Fix: electrically isolate titanium from more noble materials with insulating gaskets and sleeve bolts, and never let an energized anode share a liquor volume with unguarded titanium.

4. Overprotection by impressed-current cathodic protection. Phenomenon: hydriding of titanium in tanks, exchangers, or piping protected by an ICCP system whose potential was set for carbon steel. Mechanism: cathodic protection is deliberately a hydrogen-generating process; pushed beyond roughly −0.8 V versus SCE in acid, the potential that protects steel forces hydrogen onto the titanium cathode. Fix: never apply steel-tuned ICCP potentials to titanium; use coatings on the steel, monitored target potentials, or sacrificial anodes that keep titanium out of the hydrogen-charging window.

5. Reducing acid service beyond grade limits. Phenomenon: loss of luster, thinning on weld beads, and rapid hydride accumulation in hot HCl or high-concentration H₂SO₄ with no oxidant. Mechanism: without Fe³⁺ or dissolved oxygen the film cannot repassivate, so the acid dissolves and charges hydrogen simultaneously — the highest-rate hydriding chemistry on this list. Fix: switch grades (Grade 7 for reducing duties), maintain a deliberate oxidant residual where the process allows, or change materials. Two exclusions: hydrofluoric acid attacks the film directly and is a no-go at any practical concentration, and dry chlorine gas causes rapid attack (titanium is safe in chlorine only if the gas stays wet enough to maintain the film).

6. Elevated temperature in acid. Phenomenon: hydriding rates that roughly double or worse for each temperature step above about 80 °C. Mechanism: hydrogen diffusion and corrosion kinetics at film defects both accelerate strongly with temperature, so a duty safe at 40 °C becomes marginal at 70 °C and dangerous at 90 °C even with unchanged chemistry. Fix: confirm the temperature ceiling per grade with test work, add interlocks or dilution controls, and treat every shutdown drain-and-rinse as mandatory when hot acid would sit stagnant against titanium.

7. Weld and fabrication defects plus missing post-weld cleaning. Phenomenon: hydride platelets and cracking initiating at weld toes, heat-tint zones, and spatter. Mechanism: oxygen and nitrogen pickup from poor shielding embrittles the weld; iron smearing seeds film defects; retained heat-tint colors mark oxide layers that hydrogen uses as entry paths. Fix: qualified weld procedures with trailing shielding, full heat-tint removal followed by pickling, and dye-penetrant plus hardness checks before service. Where welded overlays are in scope, the inspection discipline in our weld overlay failure article applies directly.

No single cause acts alone in real equipment: a crevice (1) inside a hot exchanger (6) running deaerated spent acid (5), on a pack that was iron-smeared during installation (2), fails far sooner than any one factor predicts. Root-cause work should always look for combinations.

How Do You Diagnose Titanium Hydriding Before It Cracks Equipment?

Titanium hydriding gives you two diagnostic windows: the metal tells you how much hydrogen is in, and the operating record tells you how fast it is going in.

Hydrogen analysis is the primary measurement. A sample milled from the hottest or most crevice-prone location is analyzed for bulk hydrogen by hot extraction. For Grade 2, embrittlement concern typically begins once bulk hydrogen exceeds roughly 100–150 ppm; the ASTM B265 acceptance limit for new plate sits at 150 ppm maximum, so a component trending toward that number is consuming its manufactured margin. Baseline every major component at commissioning and trend the numbers — a slope means far more than a single reading.

Bend and impact testing reads the mechanical consequence. A guided bend test on a coupon cut from service equipment — or from a companion coupon rack hung in the same liquor — fails early if TiH₂ has accumulated: embrittled titanium cracks on the outer fiber where sound metal would simply bend. Impact energy falls on the same trend. These tests are destructive, which is why coupon racks belong in the original design.

Metallography is the confirmation step. A sectioned, polished, etched sample shows titanium hydride platelets — acicular needles aligned with specific crystallographic planes, denser toward the surface where hydrogen entered. Metallography distinguishes superficial uptake (platelets in a thin skin, arrestable) from deep saturation (platelets through the wall, terminal), and separates hydriding from look-alikes such as corrosion fatigue and stress corrosion cracking.

Hardness mapping supports the other three. TiH₂ raises local hardness, so a grid of microhardness indents across a cross-section reveals a gradient mirroring the hydrogen gradient — highest at the entry surface, falling inward. It is ideal for screening many locations before full hydrogen analysis on the worst ones.

As for cadence: inspect titanium in continuous acid service annually with visual plus deposit sampling, run companion coupons every 6–12 months, and pull metallographic samples at every major shutdown after the second year. Shorten the interval when two or more causes coexist, and immediately after any excursion — one stagnant week of depleted ferric can load more hydrogen than six months of normal running.

How Do You Design Against Titanium Hydriding?

Prevention is a package of material, chemistry, and electrical decisions; any one of them alone can be defeated by the others.

Grade choice comes first. Grade 2 remains the default for oxidizing and mildly reducing acid duties because it is economical and forgiving. Grade 7, with its palladium addition, is the correct call for reducing acid and for chloride duties where crevices cannot be designed out; its higher cost is trivial against replacement cost. Grade 12 is the sensible compromise where strength matters and the liquor stays mostly oxidizing. Do not upgrade grades to compensate for a fixable site condition — a Grade 7 exchanger crevice-corroded by settled solids is an expensive way to learn cause 1.

Maintain the oxidant. Where process chemistry allows, hold a ferric residual so the film repassivates continuously. This is usually free: in copper and many SX circuits the oxidant is already there, and the design task is only to avoid depleting it — manage shutdowns, avoid stagnant holds of hot deaerated acid, and drain and rinse during extended outages. A shutdown procedure is a corrosion procedure.

Kill the galvanic paths. Isolate titanium from graphite, platinum, silver, and any active anode system with insulating kits at every metallic junction. Specify non-metallic or isolation-gasketed joints where titanium meets more noble metals — graphite gaskets against titanium are a known bad pairing — and never allow steel-tuned impressed-current protection to reach a titanium surface.

Eliminate crevices and deposits. Design for full drainage, weld rather than fasten where practical, select gasket materials and geometries that do not trap liquor, and filter the liquor hard enough that solids never settle. Filtration is the cheapest corrosion insurance in the plant.

Control fabrication quality. Dedicated tools, correct shielding gas practice, heat-tint removal, pickling and passivation after every weld repair, and hardness verification of weld zones. The cheapest moment to fix hydriding risk is before the equipment ever sees acid.

Treat titanium-clad steel with extra caution. Clad construction is economical for large vessels, but every discontinuity in the titanium layer — weld seams, nozzle transitions, repair patches — is a place where the steel substrate can polarize the titanium locally and hydriding can start. Design clad vessels with generous steel corrosion allowance at discontinuities, verify bond integrity, and monitor the liner. For pressure leaching duties, a laboratory pressure leaching autoclave is the right place to prove liquor compatibility before full-scale commitment.

What Does Prevention Cost Compared With Failure?

All figures below are indicative planning ranges in US dollars, to be confirmed by vendor quotations.

Prevention side. A Grade 2 plate-and-frame heat exchanger for spent electrolyte service, roughly 20–40 m² of area, typically lands in the $25,000–60,000 range as equipment. Stepping up to Grade 7 for a reducing-acid duty multiplies the titanium material cost by roughly three to four, moving the exchanger toward $50,000–110,000. A coupon-rack and inspection program — companion coupons, annual hydrogen analysis on two to four samples, and shutdown metallography — costs on the order of $8,000–20,000 per year including laboratory charges and labor. Insulating kits, filtration upgrades, and shutdown drain-and-rinse procedures add single-digit thousands, most of them once.

Failure side. A hydriding failure is rarely a leak you patch. The realistic outcome is unplanned replacement of the exchanger ($25,000–110,000 as above), plus 1–4 weeks of production loss. On a modest circuit producing 10 t/day of cathode at a $500/t processing margin, one week of outage costs roughly $35,000 of margin; four weeks cost $140,000. Add emergency freight, contractor premiums, and secondary damage to adjacent equipment, and a single hydriding failure commonly retires $100,000–300,000 against a prevention program costing a small fraction of that annually. The comparison is not close, which is why we write the inspection cadence into the specification rather than the maintenance backlog. For current pricing on corrosion-resistant filtration and exchanger hardware, request a quote with your liquor analysis.

Case File: Plate Heat Exchanger Failure in Copper SX Spent Electrolyte

The following is a synthetic worked example, internally consistent and typical of real failure patterns; it does not describe any specific plant.

A Grade 2 titanium plate heat exchanger cooled spent electrolyte from a copper SX–EW circuit — approximately 180 g/L H₂SO₄ with trace chloride and low residual Fe³⁺. Service life: 14 months versus an expected many years. Table 2 reconstructs the investigation.

Table 2 — Synthetic case file: Grade 2 plate exchanger in copper SX spent electrolyte (worked example)

The arithmetic closes: the coupon slope (85 ppm in 6 months) versus the port-region reading (480 ppm after 14 months) is consistent with crevice-accelerated uptake running several times faster than the bulk rate, and 480 ppm sits far beyond the ~100–150 ppm embrittlement band for Grade 2. The equipment did not fail because titanium was the wrong material; it failed because three preventable causes stacked.

FAQ

Can titanium hydriding be reversed or repaired?

No. Titanium hydriding is irreversible: once hydrogen has precipitated as TiH₂ inside the metal, no heat or chemical treatment economically removes it from a finished component. The practical options are to remove the hydrogen source, monitor the remaining margin through periodic hydrogen analysis, and plan replacement before the embrittled zone compromises the equipment.

What hydrogen level makes Grade 2 titanium unsafe?

For Grade 2, embrittlement concern typically begins when bulk hydrogen exceeds roughly 100–150 ppm, which is also the acceptance ceiling for new plate under ASTM B265. Above that range, bend ductility falls measurably and impact energy declines. The threshold shifts with section thickness and temperature, so confirm critical applications with bend testing on your own material.

Is titanium suitable for hydrochloric acid service?

Only within narrow limits. Unalloyed titanium tolerates dilute HCl at ambient temperature with dissolved oxygen present, but hot or concentrated HCl dissolves the passive film and hydrides the metal rapidly. For genuine HCl duties, palladium-bearing Grade 7 is the standard choice. Above roughly 80 °C, even upgraded grades need test-work confirmation. Hydrofluoric acid is a hard no-go for all grades.

Why are graphite gaskets a problem against titanium?

Graphite is more noble than titanium, so every graphite-to-titanium contact is a small galvanic couple with titanium as the cathode — exactly the geometry where hydrogen evolves on the titanium surface. The gasket edge also forms a crevice where acid stagnates and the film starves. Use isolated, non-graphitic gasket systems or an insulating interlayer.

Does cathodic protection always damage titanium?

No, but misapplied cathodic protection always does. Titanium at moderate protected potentials in oxidizing liquor keeps its film; titanium forced into strongly cathodic potentials — typically steel-tuned impressed-current settings — becomes a hydrogen evolution surface and hydrides. Where shared structures require protection, use coatings on the steel and verified potential limits so the titanium never enters the charging window.

How often should titanium equipment in acid service be inspected?

A practical cadence: visual inspection with deposit sampling annually; companion coupon retrieval every 6–12 months; hydrogen analysis plus metallography on representative components at every major shutdown after the second year. Tighten the interval when hot reducing acid, crevices, or galvanic contacts coexist, and after any excursion where hot acid sat stagnant against the metal.

Is titanium-clad steel a safe choice for acid vessels?

Yes, with conditions. The facing behaves like solid titanium wherever it is continuous and sound. Risk concentrates at discontinuities — weld seams, nozzles, repairs — where the steel substrate can polarize the titanium locally and hydriding can initiate. Specify full bond testing, rigorous weld and heat-tint procedures, steel corrosion allowance at discontinuities, and periodic liner integrity checks.

Conclusion

Titanium hydriding is not a mystery; it is the predictable consequence of letting hydrogen reach titanium through a breached film, an overdriven potential, or hot reducing acid — and every cause here has a known fix. Keep the TiO₂ film fed with oxidant, isolate the galvanic paths, eliminate crevices, control fabrication quality, and trend bulk hydrogen against the ~100–150 ppm band for Grade 2, and titanium remains one of the most durable materials in hydrometallurgy. If you are specifying titanium equipment or investigating a suspected hydriding failure, request a materials review with your liquor analysis, and our process team will grade-match the duty and set an inspection cadence. More troubleshooting guides are available in our resources hub.

References

1. R. W. Schutz and D. E. Thomas, “Corrosion of Titanium and Titanium Alloys,” in ASM Handbook, Volume 13B: Corrosion: Materials, ASM International, Materials Park, OH.
2. ASM Handbook, Volume 13B: Corrosion: Materials — section on hydride formation and hydrogen damage in titanium and titanium alloys, ASM International.
3. ASTM B265, Standard Specification for Titanium and Titanium Alloy Strip, Sheet, and Plate, ASTM International, West Conshohocken, PA.
4. ASTM B338, Standard Specification for Seamless and Welded Titanium and Titanium Alloy Tubes for Condensers and Heat Exchangers, ASTM International, West Conshohocken, PA.
5. ASTM B348, Standard Specification for Titanium and Titanium Alloy Bars and Billets, ASTM International, West Conshohocken, PA.
6. ASME Boiler and Pressure Vessel Code, Section VIII, Pressure Vessels — materials and fabrication requirements applicable to titanium pressure parts, The American Society of Mechanical Engineers, New York.
7. TIMET, Corrosion Resistance of Titanium — technical publication covering grade selection, temperature limits, and reducing-acid behavior, Titanium Metals Corporation, Denver, CO.

Changelog

• 2026-11-07: Initial publication.