HYDROGEN PERMEABILITY
Hydrogen embrittlement (HE) also known as hydrogen assisted cracking (HAC) and hydrogen-induced cracking (HIC), describes the embrittling of metal after being exposed to hydrogen . It is a complex process that is not completely understood because of the variety and complexity of mechanisms that can lead to embrittlement.
hydrogen embrittlement
For hydrogen embrittlement to occur, a combination of three conditions are required:
- The presence and diffusion of hydrogen
- A susceptible material
- Stress
Hydrogen is often introduced during manufacture from operations such as forming, coating, plating or cleaning. Hydrogen may also be introduced over time (external embrittlement) through environmental exposure (soils and chemicals, including water), corrosion processes (especially galvanic corrosion and acidic corrosion) including corrosion of a coating and cathodic protection . The following photos shows the effect of hydrogen on fracture surface (on the left ) of otherwise a ductile material(on right)
Mechanisms of Embrittlement
formation of brittle hydrides, the creation of voids, and enhanced de-cohesion or localized plasticity.
hydrogen permeation testing per ASTM G148
This electrochemical technique for evaluation of hydrogen uptake, permeation, and transport in metals is based on the work of Devanathan and Stachurski.
The procedures described in ASTM G148, can be used to evaluate the severity of hydrogen charging of a material produced by exposure to corrosive environments or by cathodic polarization. It can also be used to determine fundamental properties of materials in terms of hydrogen diffusion (for example, diffusivity of hydrogen) and the effects of metallurgical, processing, and environmental variables on diffusion of hydrogen in metals.
The data obtained from hydrogen permeation tests can be combined with other tests related to hydrogen embrittlement or hydrogen induced cracking to ascertain critical levels of hydrogen flux or hydrogen content in the material for cracking to occur.
Requirements
Hydrogen permeation testing is a complex process that requires the following if the results are to be reliable:
- Reproducibility
- Should be diffusion controlled and not surface controlled
- If diffusivity and barrier properties are of interest, both surfaces should be coated with palladium to prevent oxide formation to control the permeation process. The surfaces should be free of oxides prior to coating palladium
- Proof for diffusion controlled process is to plot 1/L vs steady state permeation for three coating thickness.
Significance & Use
The procedures described, herein, can be used to evaluate the severity of hydrogen charging of a material produced by exposure to corrosive environments or by cathodic polarization. It can also be used to determine fundamental properties of materials in terms of hydrogen diffusion (for example, diffusivity of hydrogen) and the effects of metallurgical, processing, and environmental variables on diffusion of hydrogen in metals.
The data obtained from hydrogen permeation tests can be combined with other tests related to hydrogen embrittlement or hydrogen induced cracking to ascertain critical levels of hydrogen flux or hydrogen content in the material for cracking to occur.
