Analyzing Method of Hydrogen Trapping Sites in Metal Crystals and the Relationship between Surface Defects and Hydrogen Absorption

Gen KatanoKEK, High Energy Accelerator Research Organization, Japan

Introduction about researches of Hydrogen Embritllement

Principal and Experimental procedure of Tritium Autoradiography

Results of Autoradiography observation High-Strength Steel and Ni3Al

Investigation of trapped hydrogen feature

Summary

Four steps of investigation of the cause of Hydrogen Embrittlement

• Where are hydrogen paths? Hydrogen can enter into metal along tunnel structure at grain boundary, matrix or other routes.

• What role does hydrogen play at the cracking?

• What kinds of hydrogen trap sites exist? What is the hydrogen bonding energy?

• How to prevent hydrogen embrittlement? Improvement?

Formally structure in metal crystalsDislocation Grain boundary

tunnel structure

Interstitial atoms

arrangement of atoms at cross section

Cracking mechanism supported by hydrogen

stress stress

1 3

Slip

face

Hydrogen gatheringOther slip face is active.Cracking direction change.

Makes atom dislocation.easier

Slip face

A slip face is active.

2

One after another cracking direction change And cracking progress.

4

Slip face

Approaches to solve the problem of Hydrogen Embrittlement

Gathering

Cracking

Cracking process

Hydrogen location analysis

Analysis of Hydrogen dynamics and bonding Energy to trapping sites

Computer simulationAutoradiographyLuminographyMicroprint MethodChannelling

crack Observation

Hydrogen Embrittlement has two stages

Silver particleDevelopment

tritiumβ - ray

Exposed emulsion particlePhoto emulsion

Collodion protection film

Use tritium instead of Hydrogen

Tritium decays

β-rays hits photo emulsion

After development silver particles remain

H → He + e + ν3 3 __

Observe silver particle using SEM(scanning electron microscopy) or TEM(transmission electron microscopy)

Principles of Tritium Autoradiography

Similar process as photography

1 Doping tritium to sample

Make the photo emulsion layer on the specimen2

3

456

Typical photograph of the Tritium Autoradiography

SEM ImageObservation of Autoradiography on High Strength Steel

Experimental procedure of doping tritium into metal structure

＋

－

sample

Copper wire

Pt anode

DC power supplycathod

NaOH aqueous solution (0.10 kmol/m )3

Tritium decay rate 3.7 10 Bq/m 315

Current density 25A/m2

Dorping time 2.0 h

e

e

e

e

e

e

e

e

OxygenHydrogenTritium

Gas bubble

2H + 2e → H2↑H + e → H (in Metal)

++

__

(HHS or Ni3Al)

Electrochemical cathodic charging

Observation of the Autoradiography by SEM

Grain boundariesOther types

SEM ImageTritium atoms entered into the metal along the grain boundaries and at several other types of entry paths.

SEM and TEM

Electron beam

Other phases

Grain boundariesDislocations

Electron beam

With SEM we can only get hydrogen distribution Information. With TEM

we can see not only silver particles but also metal structure.This method have no information

about internal structure.

Observation of the Autoradiography by TEM on HSSs

Silver grain

Ferrite

Ferrite

Ferrite

Cem

entite

Cem

entite

Cem

entite

TEM Image (photo) TEM Image (Enhanced)

Tritium atoms entered the metal along the boundaries line of “Sandwich structure”.

In shown photo, we can see that hydrogen enters HSS at boundaries and in Cementite structure.

Ferrite-ferrite boundaries trap many hydrogen atoms

Boundary line

Ferrite

Silver particle

Structure of intermetallic compound Ni3Al

Ni atomsAl atoms

Octahedral site

Tetrahedral site

Volume large in comparison to Tetrahedral site

L12 type crystal structure has two type interstitial sites.

Observation of trapped hydrogen distribution in Ni3Al

Grain boundaryTrapped tritium is uniformly distribution over the sample (not only at grain boundary). Silver particle

Hydrogen in Ni3Al dislocation

Dislocation lines

Silver particle

Dislocation trap tritium !

Hydrogen in damaged structure

Ni3Al structure was “damaged” by high energy (4MeV) Ni atoms.Damaged structure was doped by tritium and autoradiography was made.

Damaged structure traps hydrogen well !

Same doping procedure as before but the HSS sample was left to anneal at 100 C for 1h, before making Autoradiography

Prevention of Hydrogen embrittlement by baking.

Ferrite

Ferrite

Ferrite

Ferrite

Cementite

Cementite

Cementite

Cementite

Ferrite-Cementite boundary sampleOne tritium remains

Most of the hydrogen escaped from trap sites!

Anneal 100 C for 1h, before making Autoradiography

Ferrite-ferrite boundary sample

All tritium left the sample

Only one silver particle cannot be seen in this area. Ferrite

Silver particle

Summary of Autoradiography observation at High-Strength Steel

1

2

3

We observed hydrogen atoms trapping.grain boundaries ferrite-cementite boundarycementiteferrite-ferrite boundary.

Boundary structure traps many hydrogen atoms, but they leave after annealing at 100 C for 1h.

Therefore bonding energy is low at these trap sites.

Ni3Al

uniform

dislocations

damaged structure

High-strength steel

Non-uniform

boundaries of “sandwich structure”

grain boundaries

Hydrogen distribution

Hydrogen trap site

bcc structure fcc structure

Two types trapping sites

Low bonding energy trapping site

100 C 300 - 400 CHigh bonding energy trapping site

Typical peaks of hydrogen in thermal analysisby N.Suzuki etc.

Suggestion to Niobium or metals in general

Grain boundaries are important to hydrogen path and trapping sites by bcc metals.

Metal crystal have plural hydrogen trapping sites.

Low energy bonding sites release hydrogen easily.Therefore these hydrogen have greatly effect to hydrogen gathering.

Thank you very much for your kind attention.