Hydrogen Embrittlement

The embrittlement of a metal or alloy by atomic hydrogen involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal.

Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions which inject hydrogen into the component. Presently this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys and aluminum alloys.

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 related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized below: (Defence Standard 03-30)

1. by some manufacturing operations such as welding, electroplating, phosphating and pickling; if a material subject to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any hydrogen is employed
2. as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction described here acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”
3. the use of cathodic protection for corrosion protection if the process is not properly controlled.

Hydrogen Embrittlement of Mechanical Fasteners

Hydrogen embrittlement of fasteners is a major factor in the choice of material or coating for such components. Hydrogen embrittlement may be a serious concern with high strength fasteners made of carbon and alloy steels for which is can be caused by the absorption of atomic hydrogen into the fastener's surface during manufacture and processing. The introduction of atomic hydrogen is particularly possible during acid pickling and alkaline cleaning prior to plating, and then during actual electroplating. (reference)

The metallic coating subsequently plated on the fastener entraps atomic hydrogen in the base metal and if the hydrogen is not relieved by a post-baking operation the hydrogen atoms may migrate towards points of highest stress concentration when load or stress is applied. Cracks will promulgate through the component surface, weakening the component due to the loss of cross-section area. The failure is usually completed by a ductile fracture.

The susceptibility of any material to hydrogen embrittlement in a given test is directly related to the characteristics of its trap population related to the material microstructure, dislocations, carbides and other elements present in the structure.

The greater the hydrogen concentration becomes, the lower the critical stress, or lower the hydrogen concentration, the higher the critical stress at which failure may occur. Products having Vickers hardness exceeding HV 320 require special care to reduce the risk of this phenomenon during the plating process or coating procedures. Some experts feel that hardness exceeding HV 390 is a threshold beyond which further steps to manage hydrogen embrittlement risk are required.

A stress relieving anneal should be considered for fasteners which have been work hardened during fabrication and are to be electroplated. Instances have been reported of fasteners failing by hydrogen embrittlement after many years in service with the cracks associated with corroded thread roots, providing thus an indication of the role of corrosion as a possible source of the hydrogen necessary to promote hydrogen embrittlement. (Defence Standard 03-30)

Assuming that the fasteners are not charged with hydrogen before entering service, there are some features connected with atmospheric corrosion that indicate that this should not normally promote hydrogen embrittlement; since the condensed water films will often be insufficiently acidic to support a significant hydrogen-production cathodic reaction described here. On the other hand, in the occluded zones of crevices, acidic conditions are more likely and a periodic visual inspection programme is recommended with a policy of replacement of high-strength fasteners that are exhibiting significant corrosion.