Personnel need to be aware of the conditions for hydrogen embrittlement and its formation process when selecting materials for a reactor plant. This chapter discusses the sources of hydrogen and the characteristics for the formation of hydrogen embrittlement.

EO 1.21 DESCRIBE hydrogen embrittlement, including the two required conditions and the formation process.

EO 1.22 IDENTIFY why zircaloy-4 is less susceptible to hydrogen embrittlement than zircaloy-2.

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 (H_Z) or methane gas (CH₄), which forms at the grain boundaries. 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.

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 Embrittlement of Stainless Steel

As shown in Figure 10, 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 is a primary reason that the reactor coolant is maintained at a neutral or basic pH in plants without aluminum components.

Figure 10 Hydrogen Embrittlernent

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 transgranular. 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.

Hy~ro~en Embrittlement of Zirconium Allow

Hydrogen embrittlement is a problem with zirconium and zirconium alloys, which often are used as cladding materials for nuclear reactors. Zirconium reacts with water as follows.

Part of the hydrogen produced by the corrosion of zirconium in water combines with the zirconium to form a separate phase of zirconium hydride (ZrH_1.5) platelets. 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. Zircaloy-2 (a zirconium alloy), which has been used as a fuel rod cladding, may absorb as much as 50% of the corrosionproduced hydrogen and is subject to hydrogen embrittlement, especially in the vicinity of the surface. Studies at Westinghouse, Batelle, and elsewhere have revealed that the nickel in the zircaloy-2 was responsible for the hydrogen pickup. This has led to the development of zircaloy4, which has significantly less nickel than zircaloy-2 and is less susceptible to embrittlement. In addition, the introduction of niobium into zircaloy-4 further reduces the amount of hydrogen absorption.

Summary

The important information in this chapter is summarized below.

Hydrogen Emlbrittlemerlt Summary

Hydrogen embrittlement

The conditions required for hydrogen embrittlement in steel is the presence of hydrogen dissolved in the water and the carbon in the steel. The hydrogen dissolved in the water comes from:

Making of steel

Processing parts

Welding

Storage or containment of hydrogen gas

Related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion.

Hydrogen embrittlement is the result of hydrogen that 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.

Zircaloy-4 is less susceptible to hydrogen embrittlement than zircaloy-2 because:

Zircaloy-4 contains less nickel

The introduction of niobium into zircaloy-4 reduces the amount of hydrogen absorption in the metal.