hydrogen-embrittlement

name	hydrogen-embrittlement
description	Hydrogen embrittlement (HE) — mechanisms (HEDE/AIDE/HELP), threshold stress intensity K_IH, susceptibility by material (high-strength steels, titanium, nickel), plating-induced HE, hydrogen charging, prevention (baking, coatings, material selection), ASTM F519/F326.
metadata	{"priority":7,"promptSignals":{"phrases":["hydrogen embrittlement","HE steel","hydrogen induced cracking","delayed fracture hydrogen","plating hydrogen embrittlement","ASTM F519"],"minScore":3}}

Hydrogen Embrittlement (HE) — Complete Skill

Mechanisms of Hydrogen Embrittlement

HE definition: absorption of atomic hydrogen into metallic lattice → reduces ductility, toughness, and crack growth resistance; most dangerous at high-strength levels and under sustained tensile stress

HEDE (Hydrogen Enhanced Decohesion)

Mechanism: H atoms preferentially segregate to grain boundaries and crack tips → reduce atomic bonding energy → decohesion at lower stress Cohesive energy reduction: ΔE_cohesion = -A × θ_H [θ_H = local hydrogen coverage from Langmuir isotherm] Manifestation: intergranular fracture (IG) at grain boundaries; Auger spectroscopy shows H at grain boundary fracture surface

AIDE (Adsorption-Induced Dislocation Emission)

Mechanism: H adsorption at crack tip → reduces energy for dislocation nucleation → increased dislocation emission → easier crack advance at lower stress Result: quasi-cleavage fracture or mixed intergranular/transgranular

HELP (Hydrogen Enhanced Localized Plasticity)

Mechanism: H reduces dislocation-dislocation interaction → localized plasticity increases → macroscopic strain concentrates → premature failure in narrow bands Paradox: local plasticity increases but global ductility decreases → macroscopic brittleness

Most steels: HEDE dominant at high strength; HELP contributes at moderate strength; both often coexist

Susceptibility by Material

High-Strength Steels (Primary Concern)

Threshold for HE concern: σ_y > 1,000 MPa (HRC ≥ 32) → significant HE risk

Threshold stress intensity K_IH (hydrogen-assisted cracking): K_IH / K_IC: 0.1–0.5 for martensitic steels at σ_y = 1,400–1,900 MPa AISI 4340 (σ_y = 1,500 MPa, HRC 48): K_IC = 60 MPa√m; K_IH = 15–25 MPa√m in cathodic environment Lower K_IH → more susceptible; fracture at lower applied stress intensity

Strength-HE trade-off for bearing steels: 52100 at HRC 60: extreme brittleness → K_IH ≈ 5–10 MPa√m; cathodic protection removes bearing protection 300M (σ_y = 1,862 MPa): K_IH = 10–20 MPa√m → extreme care required in any plating or pickling

Ultra-high strength alloys: Maraging 300 (σ_y = 1,900 MPa): K_IH ≈ 10–15 MPa√m in H₂ environment AF1410 (σ_y = 1,586 MPa): better HE resistance due to Ni+Co+Mo composition

Titanium Alloys

Ti-6Al-4V (alloy): H solubility in α phase limited → titanium hydrides TiH₂ form → embrittlement Threshold hydrogen content: > 150 ppm H → hydride formation → K_IH < K_IC H content from manufacturing: 50–100 ppm typical; limit per AMS 4928: ≤ 150 ppm Beta-processed Ti: higher H tolerance; α-β near transus: more sensitive

Titanium HE applications: welding (use H-free shielding gas); acid pickling; electroplating

Nickel-Based Superalloys

Ni alloys: relatively resistant at high strength; HE occurs at grain boundaries (HEDE) IN718 (σ_y = 1,034 MPa): K_IH / K_IC ≈ 0.7 in H₂ → significant but moderate sensitivity Waspaloy: K_IH / K_IC ≈ 0.8 → low sensitivity Inconel 625: excellent HE resistance → preferred in H₂ service

Austenitic Stainless Steels

Low-to-moderate HE risk: austenite (FCC) slows H diffusion (D_H in FCC ≈ 10⁻¹¹ m²/s vs. 10⁻⁹ m²/s in BCC) 304/316: moderate HE at high pressure H₂; K_IH / K_IC ≈ 0.5–0.8 Cold-worked 304: martensitic transformation → much higher HE susceptibility (BCC martensite)

Hydrogen Sources

Electroplating (most common industrial source): H⁺ + e⁻ → H_atomic → absorbed into steel during plating Cadmium plating: highest H absorption risk Chromium plating (hard chrome): significant H absorption; F519 test required Zinc, zinc alloy, nickel electroplating: moderate H absorption

Pickling (acid cleaning): Fe + 2HCl → FeCl₂ + 2H_atomic → H absorbed during acid cleaning before plating Strong acids (HCl, H₂SO₄): high H generation; inhibited acid reduces H uptake

Cathodic protection: Too-negative potential: water electrolysis → H evolution → H absorption Safe limit: E < -1,000 mV SCE in seawater (below this → H generation accelerates)

Welding: Moisture in electrode coating → H₂O → H dissolution → diffusion into HAZ → delayed cracking 0–72 h post-weld Cellulosic (E6010) electrode: highest H; Low-H (E7018): ≤ 8 mL/100g weld metal; Ultra-low-H: ≤ 4 mL/100g

Hydrogen gas (industrial): H₂ storage and piping: atomic H at high pressure → K_IH concerns for high-strength steels ASME B31.12: hydrogen piping and pipeline code; material limits σ_y ≤ 483 MPa (70 ksi) for P > 5 bar

ASTM Testing Standards

ASTM F519 (Most Widely Used)

Purpose: evaluate susceptibility of hydrogen embrittlement from electroplating processes Specimen: notched cylindrical tensile specimen (Type 1a: 4340 steel, HRC 54–56; Type 2: cadmium plated) Test: apply sustained load at 75% of notch tensile strength (NTS); hold 200 hours (or until fracture) Acceptance: no fracture in 200 hours → plating process accepted

Plating process approval sequence:

1. Baseline: unplated Type 1a specimen; verify NTS > 2,000 MPa
2. Plated specimen (same heat treat): apply 75% NTS sustained load; 200 h test
3. If fracture in < 200 h: adjust plating process (reduce current density, add pH buffer, reduce time)
4. If no fracture: process qualified

ASTM F326 (Electronic Hydrogen Permeation)

Purpose: measure H permeation rate through steel foil → indication of H absorption during process Electrochemical method: sample as membrane between two cells; cathodic charging on one side; oxidation current measured on other side Output: permeation transient curve → D_H and C_H surface concentration → H flux J_H

ASTM F1624 (Threshold Stress Intensity K_IH)

Purpose: direct measurement of K_IH for material-environment combination Method: rising displacement test at slow strain rate; determine K at onset of hydrogen-assisted cracking Correlation: K_IH decreases with increasing yield strength for same alloy; H pressure increases effect

Prevention Strategies

Baking (Relief of Hydrogen)

Baking process (AMS 2759/9): Temperature: 191°C (375°F) ± 14°C Time: ≥ 23 hours (for HRC 39–53) Required for: all steels HRC > 39 plated with Cd, Cr, Zn, Ni, or other processes Time limit after plating to start baking: ≤ 4 hours (before H diffuses to critical locations and causes delayed fracture during handling)

Effectiveness at very high hardness: HRC > 55: baking may not fully relieve trapped H; must evaluate with F519 For HRC > 55 + critical parts: consider alternative coatings (HVOF, cold spray, PVD) instead of electroplating

Material Selection

Reduced HE risk by design: Use lower-strength steel if load allows (σ_y < 1,000 MPa → dramatically reduced HE) Use Ni alloys or Ti with controlled H content (Ti-3Al-2.5V for tubing → better HE tolerance than Ti-6Al-4V) Austex grades (austenitic): slower H diffusion → less HE from any exposure

Coating Alternatives

HVOF (high-velocity oxygen fuel) WC-Co: no H involvement → no plating HE; replaces hard chrome PVD coatings (CrN, TiN): vapor deposition → no H absorption Organic coatings (epoxy, PTFE): no electrochemical H generation; for lower-load applications Mechanical zinc (Geomet, Dacromet): mechanical zinc coating → no H from plating; cathodic protection from zinc

Process Controls

Reduced acid cleaning time: minimize pickling exposure; use alkaline or mild acid alternatives Inhibited acid: propargylic alcohol or phenylthiourea inhibitors → reduce H absorption by 70–90% Pulse plating: intermittent current reduces H incorporation vs. DC plating Lower current density: slower deposition → less H evolution at cathode

Delayed Fracture Testing

Sustained load test (ASTM F519): most relevant; simulates service Constant load (dead weight): simplest; applied for 200 hours Rising step load: progressively increase load; find threshold stress for HE in shorter time Slow strain rate test (SSRT): stretch specimen slowly in corrosive environment; compare ductility to air

Standards

Standard	Scope
ASTM F519	Hydrogen embrittlement from electroplating — standard test
ASTM F326	Electronic hydrogen permeation test
ASTM F1624	K_IH threshold stress intensity for HE
AMS 2759/9	Baking after plating — aerospace requirement
ASTM B849	Pre-treatments before cadmium plating
ASME B31.12	Hydrogen piping and pipelines

Output

Provide: material and heat treat (grade, HRC or σ_y [MPa]), hydrogen source (plating type/acid/welding/H₂ environment), HE susceptibility assessment (low/moderate/high with basis), K_IH / K_IC ratio (if known or estimated), ASTM F519 test requirement (yes/no), baking requirement (AMS 2759/9: T = 191°C, t ≥ 23 h — yes/no), H content limit [ppm] (for titanium), alternative surface treatment (HVOF/PVD instead of plating), process control measures (inhibited acid/pulse plating/low current density), delayed fracture time observed (if tested), and applicable standard (ASTM F519, AMS 2759/9, ASME B31.12).