Hydrogen Damage

There are a number of different forms of hydrogen damage to metallic materials, resulting from the combined factors of hydrogen and residual or tensile stresses. Hydrogen damage can result in cracking, embrittlement, loss of ductility, blistering and flaking, and also microperforation.

Hydrogen Induced Cracking

Hydrogen induced cracking (HIC) refers to the cracking of a ductile alloy when under constant stress and where hydrogen gas is present. Hydrogen is absorbed into areas of high triaxial stress producing the observed damage.

Hydrogen Embrittlement

Hydrogen embrittlement is the brittle fracture of a ductile alloy during plastic deformation in a hydrogen gas containing environment.

Loss of Tensile Ductility

The loss of tensile ductility occurs with metals exposed to hydrogen which results in a significant reduction in elongation and reduction in area. It is most often observed in low strength alloys and has been witnessed in steels, stainless steels, aluminum alloys, nickel alloys, and titanium alloys.

High Temperature Hydrogen Attack

High pressure hydrogen will attack carbon and low-alloy steels at high temperatures. The hydrogen will diffuse into the metal and react with carbon resulting in the formation of methane. This in turn results in decarburization of the alloy and possibly cracks formation.

Blistering

Blistering occurs primarily in low strength metals. It is a result of atomic hydrogen diffusion into defect areas of the alloy. The monotonic atoms combine into gas molecules in voids within the metal. Then, the high pressure of H2 entrapped within the metal causes the material to blister or rupture. This form of attack has been observed in low strength steels exposed to H2S or when cleaned in pickling baths.

Shatter Cracks, Flaking, and Fish Eyes

These forms of hydrogen damage are similar to blistering and are seen primarily during processing. Hydrogen is more soluble at the melting temperatures of metals allowing it to enter defect areas. The decreased solubility of hydrogen when cooled then produces the damage features.

Microperforation

Microperforation has been seen in steels in a high pressure hydrogen and room temperature environment. The hydrogen produces fissures in steel alloys such that gases and liquids can permeate the material.

Degradation in Flow Properties

An increase in creep rates occurs in iron alloys and steels under ambient conditions in hydrogen environments, and in several alloys at elevated temperatures.

Hydride Formation

The precipitation of metal hydride phases in magnesium, tantalum, niobium, vanadium, uranium, zirconium, titanium, and their alloys, in the presence of hydrogen produces a degradation of mechanical properties and cracking.

Hydride Formation

The precipitation of metal hydride phases in magnesium, tantalum, niobium, vanadium, uranium, zirconium, titanium, and their alloys, in the presence of hydrogen produces a degradation of mechanical properties and cracking.

Metals’ Susceptible to Hydrogen Damage

Table 17 lists susceptible metals to the various types of hydrogen attack..

Managing Hydrogen Damage

Methods to deter hydrogen damage are to:

• Limit hydrogen introduced into the metal during processing.
• Limit hydrogen in the operating environment.
• Structural designs to reduce stresses (below threshold for subcritical crack growth in a given environment)
• Use barrier coatings
• Use low hydrogen welding rods