Hydrogen embrittlement is one of many forms of stress-corrosion cracking. Hydrogen embrittlement results from the combined action of an applied tensile stress and a corrosive hydrogen environment, both influences are necessary. In this case the corrosive agent is hydrogen in its atomic form (H as opposed to the molecular form, H2), which diffuses interstitially through the crystal lattice, and concentrations as low as several parts per million can lead to cracking. Although embrittlement of materials takes many forms, hydrogen embrittlement in high strength steels has the most devastating effect because of the catastrophic nature of the fractures when they occur. Hydrogen embrittlement is the process by which steel loses its ductility and strength due to tiny cracks that result from the internal pressure of hydrogen (H2), which forms at the grain boundaries. In case of steels, hydrogen than diffuses along the grain boundaries and combines with the carbon to form methane gas. The methane gas collects in small voids along the grain boundaries, where it builds up enormous pressures that initiate cracks and decrease the ductility of the steel. If the metal is under a high tensile stress, brittle fracture can occur.
See also: Fracture Toughness
It is a complex process that is not completely understood because of the variety and complexity of mechanisms that can lead to embrittlement. A number of mechanisms have been proposed to explain hydrogen embrittlement. Mechanisms that have been proposed to explain embrittlement include the formation of brittle hydrides, the creation of voids that can lead to bubbles and pressure build-up within a material. Hydrogen is introduced to the surface of a metal and individual hydrogen atoms diffuse through the metal structure. Because the solubility of hydrogen increases at higher temperatures, raising the temperature can increase the diffusion of hydrogen.
For hydrogen embrittlement to occur, a combination of three conditions are required:
- the presence and diffusion of hydrogen
- a susceptible material
In zirconium alloys, hydrogen embrittlement is caused by zirconium hydriding. At nuclear reactor facilities, the term “hydrogen embrittlement” generally refers to the embrittlement of zirconium alloys caused by zirconium hydriding.
Special Reference: U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Hydrogen Embrittlement of Zirconium Alloys
Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. There are various fuel failure root causes, that have been identified in past. In the early dates of PWR and BWR operations, these causes were predominantly fabrication defects or fretting. One of possible causes is also:
- Internal Hydriding. Inadvertent inclusion of hydrogen-containing materials inside a fuel rod can result in hydriding and thus embrittlement of fuel cladding. Hydrogen sources were mainly residual moisture or organic contamination in fuel pellets/rods. This cause of failure has been practically eliminated through improved manufacturing.
- Delayed hydride cracking (DHC). Delayed hydride cracking is time-dependent crack initiation and propagation through fracture of hydrides that can form ahead of the crack tip. This type of failure can be initiated by long cracks at the outer surface of the cladding, which can propagate in an axial/radial direction. This failure mechanism may potentially limit high burnup operation.
The aggressive agent in this respect is primary circuit water, at a temperature of some 300° C. This oxidizes zirconium according to the reaction:
Zr + 2H2O→ZrO2 + 2H2
resulting in formation of solid oxide on the metal’s surface.
Part of the hydrogen produced by the corrosion of zirconium in water combines with the zirconium to form a separate phase of zirconium hydride (ZrH1.5) platelets. Hydrogen migrates under the effect of the thermal gradient to accumulate in the less hot regions, forming hydrides that are liable to cause brittleness in the cladding, as the fuel cools down. The metal then becomes embrittled (ductility decreases) and it fractures easily. Cracks begin to form in the zirconium hydride platelets and are propagated through the metal.
Hydrogen embrittlement is also of very high importance for high temperature steam oxidation of zirconium alloys.