17.3 Titanium Hydriding

Mechanistic Fundamentals & Crystallography of Titanium Hydriding

1. Phenomenological Description (API RP 571 Section 3.66)

Titanium Hydriding is an environmental degradation mechanism that causes severe embrittlement and premature mechanical failure of titanium and titanium alloys. When atomic hydrogen (H•) is absorbed into the metallic matrix, it precipitates as brittle, needle-like or plate-like titanium hydride (TiH2) within the alpha (α) crystal lattice. Over time, the accumulation of hydride needles destroys the material impact toughness and tensile ductility, causing components to shatter or snap under minor mechanical vibration, bundle pulling, or pressure shocks.

Titanium is widely selected in petroleum refining for its outstanding resistance to wet sour (H2S), ammonium bisulfide (NH4HS), and chloride-rich overhead streams where carbon steels, brasses, and stainless steels fail due to pitting, erosion-corrosion, or stress corrosion cracking. However, its susceptibility to hydriding represents a critical operational vulnerability that must be managed through strict metallurgical and electrochemical controls.

2. Crystallographic Transformation and Volumetric Lattice Expansion

To understand the embrittlement process, engineers must examine the crystallographic interaction between hydrogen and titanium:

• Hexagonal Close-Packed (HCP) Alpha Matrix: Commercially Pure (CP) titanium has an HCP crystal structure (α-phase) at temperatures below its beta transus (1620 °F / 882 °C).
• Extremely Low Solid Solubility: At ambient temperatures (20 °C / 68 °F), the solid solubility limit of hydrogen in alpha titanium is exceptionally low—typically only 20 to 50 ppmw (parts per million by weight). Any hydrogen absorbed beyond this solubility threshold must precipitate as a second phase.
• Precipitation of Titanium Hydride (TiH2): Absorbed atomic hydrogen combines with titanium to form face-centered cubic (FCC) or face-centered tetragonal (FCT) delta-hydride (TiHx, where x ≈ 1.5 to 2.0, nominally TiH2): Ti + 2H• -> TiH2.
• Massive Volumetric Mismatch: The density of commercially pure alpha titanium is approximately 4.51 g/cm³, whereas the density of titanium hydride (TiH2) is only 3.76 g/cm³. This corresponds to a massive volumetric expansion of 17% to 23% when hydride plates precipitate within the constrained alpha matrix.
• Internal Micro-Cracking: As hydride needles precipitate along preferential crystallographic planes (the {10-10} prism planes and {10-11} pyramidal planes), the enormous local dilation generates intense internal shear stresses. Micro-cracks initiate along the hydride-matrix interfaces. Under applied mechanical loads, cracks propagate rapidly along the brittle hydride platelets, converting a ductile metal capable of >25% tensile elongation into a brittle material exhibiting virtually zero plastic ductility.

                    TITANIUM HYDRIDING MECHANISM
  
  1. Atomic Hydrogen Generation      2. Interstitial Diffusion
  ┌─────────────────────────────┐    ┌─────────────────────────────┐
  │ Galvanic coupling (Fe-Ti),  │    │ Absorbed atomic H diffuses  │
  │ Over-CP (over-polarized),   │───▶│ into HCP alpha titanium     │
  │ Embedded iron smears, or    │    │ lattice (accelerates at     │
  │ reducing acid corrosion     │    │ temperatures >165 °F/74 °C) │
  └─────────────────────────────┘    └─────────────────────────────┘
                                                    │
                                                    ▼
  4. Severe Embrittlement            3. Hydride Precipitation
  ┌─────────────────────────────┐    ┌─────────────────────────────┐
  │ Brittle TiH2 platelets snap │    │ Hydrogen exceeds 20-50 ppmw │
  │ under vibration or pulling; │◀───│ solubility; needle-like     │
  │ tube wall cracks through    │    │ TiH2 precipitates with      │
  │ with zero ductile necking   │    │ 17-23% volume expansion     │
  └─────────────────────────────┘    └─────────────────────────────┘

Critical Factors and Refinery Environmental Drivers

1. Operating Temperature Kinetics

RP 571 Conditions for Hydriding

API RP 571 states that hydriding of titanium in refinery service generally requires a combination of:

• Metal temperature above about 165 °F (74 °C);
• pH below about 3 or above about 8, or neutral pH with high H2S content; and
• A galvanic couple to a more active metal such as carbon steel, or another source of cathodic hydrogen such as over-protective cathodic protection.

Hydriding has been found in sour water stripper and amine unit overhead condensers, heat exchanger tubes, piping, and other titanium equipment operating above 165 °F (74 °C).

• Diffusion Threshold: Temperature is the primary kinetic driver governing hydrogen diffusion through the titanium lattice. At temperatures below 165 °F (74 °C), hydrogen diffusion in alpha titanium is relatively sluggish. Any hydriding that occurs is generally restricted to a superficial surface layer (a few micrometers thick), which rarely impairs bulk structural integrity unless operating periods span decades.
• Accelerated Penetration: Above 165 °F (74 °C), the diffusion coefficient of hydrogen increases exponentially. Atomic hydrogen rapidly migrates into the bulk wall thickness of condenser tubes (typically 0.035 to 0.065 inch / 0.89 to 1.65 mm wall), precipitating dense networks of through-wall hydride needles.

2. Galvanic Coupling (The Refinery Galvanic Cell)

The single most prevalent cause of titanium hydriding in petroleum refineries is galvanic coupling with active carbon steel or low-alloy steel components:

• The Galvanic Couple: In the galvanic series in seawater or sour refinery process water, titanium is extremely noble (cathodic, passive), exhibiting an open-circuit potential of approximately +0.05 to -0.15 V SCE. In contrast, carbon steel is highly active (anodic), exhibiting a potential of approximately -0.60 to -0.75 V SCE.
• Electrochemical Action: When titanium tubes are expanded directly into a carbon steel tubesheet, or when titanium tube bundles are housed inside a carbon steel shell without electrical isolation, a severe galvanic couple is established in the presence of an aqueous electrolyte (sour water, cooling water, desalter brine):
  • Anode (Carbon Steel): Undergoes accelerated anodic dissolution: Fe -> Fe2+ + 2e-.
  • Cathode (Titanium): The electrons flow to the noble titanium surface, driving cathodic reduction of hydrogen ions: 2H+ + 2e- -> 2H•. Because titanium absorbs atomic hydrogen, this continuous cathodic charging drives hydrogen directly into the titanium tube ends, leading to catastrophic hydriding and cracking at tube-to-tubesheet roller expansions.

3. Cathodic Protection (CP) Over-Protection

In sea water-cooled or cooling tower-cooled surface condensers tubed with titanium, cathodic protection systems (sacrificial zinc/aluminum anodes or Impressed Current Cathodic Protection [ICCP]) are frequently installed to protect carbon steel water boxes and tubesheets:

• Hydrogen Reduction Boundary: If the cathodic protection system polarizes the titanium to potentials more negative than -0.90 V to -1.00 V vs Copper/Copper Sulfate Electrode (CSE) (or approximately -0.85 V SCE), vigorous cathodic hydrogen evolution occurs on the titanium surface.
• Under these over-protected conditions, atomic hydrogen charging accelerates dramatically, causing rapid through-wall hydriding of tube ends even at moderate cooling water temperatures (70 °F to 100 °F / 21 °C to 38 °C).

4. Embedded Iron Surface Contamination

During vessel fabrication, tube bundling, or mechanical cleaning, if carbon steel tools, steel wire brushes, iron shot-blast media, or steel lifting slings come into abrasive contact with titanium, microscopic particles of metallic iron are smeared into the titanium surface:

• In subsequent wet or humid service, each embedded iron smear forms an autonomous galvanic micro-cell. The iron particle dissolves anodically, while the adjacent titanium acts as a micro-cathode, driving atomic hydrogen into the titanium lattice directly beneath the smear.
• This produces localized "hydride blisters" or pit-like hydrided craters that initiate through-wall fatigue or vibration cracks.

5. Susceptible Materials Comparison

Affected Equipment, Prevention & Inspection Protocols

1. Affected Refinery Equipment and Units

• Crude Unit Atmospheric Tower Overhead Systems: Overhead condensers where titanium Grade 2 tubes are exposed to condensing water containing hydrochloric acid (HCl), hydrogen sulfide (H2S), and ammonia (NH3). Failures concentrate at carbon steel tubesheet joints and baffle supports.
• Sour Water Stripper (SWS) Overhead Condensers: Severe environments with high concentrations of H2S, NH3, and cyanides. Galvanic coupling between titanium bundles and carbon steel shells drives rapid tube end hydriding.
• Fluid Catalytic Cracking (FCC) & Delayed Coker Gas Recovery Overhead Coolers: Exposure to wet sour streams containing cyanides, which destroy passive scales and accelerate hydrogen flux.
• Desalter Effluent Brine Coolers: Titanium plate-and-frame or shell-and-tube exchangers handling hot (>200 °F / 93 °C) chloride brines.

2. Engineering Prevention and Mitigation Protocols

• Electrical Isolation (Dielectric Separation):
  • Never expand titanium tubes directly into carbon steel tubesheets. Use solid titanium tubesheets, titanium-clad (explosively bonded or roll-bonded) carbon steel tubesheets, or high-integrity non-conductive dielectric sleeves/ferrules.
  • Install non-conductive PTFE or ceramic-sleeved flange insulation kits on piping connections to electrically isolate titanium bundles from carbon steel shells and interconnecting piping.
• Cathodic Protection Voltage Restriction:
  • For seawater-cooled condensers, design and monitor cathodic protection so the titanium is never over-polarized. Published potential limits differ by reference electrode, so confirm which electrode a limit refers to.
• Thermal Oxidation (Oxide Barrier Thickening):
  • Thermally oxidize titanium components by heating in air at 1100 °F to 1200 °F (593 °C to 649 °C) for 30 to 60 minutes.
  • This process grows a dense, stable rutile titanium dioxide (TiO2) ceramic film with a thickness of 1 to 2 µm (compared to the natural ambient film thickness of only 2 to 5 nm). This thickened oxide film provides an outstanding, durable physical barrier that dramatically reduces hydrogen permeation.
• Post-Fabrication Acid Pickling (Iron Decontamination):
  • Following all welding, bending, and bundle fabrication, titanium assemblies must undergo chemical pickling in an aqueous solution of 10% to 20% nitric acid (HNO3) + 2% to 4% hydrofluoric acid (HF) at room temperature for 10–20 minutes, followed by demineralized water rinsing.
  • The nitric acid passivates the titanium while the HF selectively dissolves all embedded carbon steel particles, tramp iron smears, and surface oxides, eliminating localized micro-galvanic sites.

3. Inspection Methodologies and Metallurgy Testing