Engineering personnel need to be aware of the conditions for hydrogen embrittlement and its formation process when selecting materials for a certain application. This article discusses the sources of hydrogen and the characteristics for the formation of hydrogen embrittlement.
Concern
Another form of stress-corrosion cracking is hydrogen embrittlement. 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) or methane gas (CH4), which forms at the grain boundaries. In zirconium alloys, hydrogen embrittlement is caused by zirconium hydriding.
Sources of Hydrogen
Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and relating to hydrogen as a contaminant in the environment that is often produced as a by-product of general corrosion. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating.
Hydrogen Embrittlement of Stainless Steel
As shown in the image below, hydrogen diffuses along the grain boundaries and combines with the carbon (C), which is alloyed with the iron, to form methane gas. The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks.
Hydrogen Embrittlement
If the metal is under a high tensile stress, brittle failure can occur. At normal room temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking under these conditions, the path is trans granular. At high temperatures, the absorbed hydrogen tends to gather in the grain boundaries and stress-induced cracking is then intergranular. The cracking of martensitic and precipitation hardened steel alloys is believed to be a form of hydrogen stress corrosion cracking that results from the entry into the metal of a portion of the atomic hydrogen that is produced in the following corrosion reaction.
Hydrogen embrittlement is not a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can rediffuse from the steel, so that ductility is restored.
To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low or no embrittlement plating or coating processes, and restricting the amount of in-situ (in position) hydrogen introduced during the service life of a part.
Summary
The important information in this section is summarised below.
Hydrogen Embrittlement Summary
The conditions required for hydrogen embrittlement in steel are the presence of hydrogen and the presence of carbon in the steel. Hydrogen comes from:
- Manufacturing of steel.
- Processing parts.
- Welding.
- Storage or containment of hydrogen gas.
- Hydrogen as a contaminant in the environment (often produced as a by-product of general corrosion).
Hydrogen embrittlement is the result of hydrogen that diffuses along grain boundaries and combines with 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.