Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen. Hydrogen is present practically everywhere, several kilometres above the earth and inside the earth. Engineering materials are exposed to hydrogen and they may interact with it resulting in various kinds of structural damage. Damaging effects of hydrogen in metallic materials have been known since 1875 when W. H. Johnson reported: “some remarkable changes produced in iron by the action of hydrogen and acids”. During the intervening years many similar effects have been observed in different structural materials, such as steel, aluminium, titanium, and zirconium. Because of the technological importance of hydrogen damage, many people explored the nature, causes and control measures of hydrogen related degradation of metals. Hardening, embrittlement and internal damage are the main hydrogen damage processes in metals. Hydrogen may be picked up by metals during melting, casting, shaping and fabrication. They are also exposed to hydrogen during their service life. Materials susceptible to hydrogen damage have ample opportunities to be degraded during all these stages.
Classifications
Hydrogen damage may be of four types: solid solution hardening, creation of internal defects, hydride embrittlement, and hydrogen embrittlement. Each of these may further be classified into the various damaging processes.
Metals like niobium and tantalum dissolve hydrogen and experience hardening and embrittlement at concentrations much below their solid solubility limit. The hardening and embrittlement are enhanced by increased rate of straining.
Hydride embrittlement
In hydride forming metals like titanium, zirconium and vanadium, hydrogen absorption causes severe embrittlement. At low concentrations of hydrogen, below the solid solubility limit, stress-assisted hydride formation causes the embrittlement which is enhanced by slow straining. At hydrogen concentrations above the solubility limit, brittle hydrides are precipitated on slip planes and cause severe embrittlement. This latter kind of embrittlement is encouraged by increased strain-rates, decreased temperature and by the presence of notches in the material.
Creation of internal defects
Hydrogen present in metals can produce several kinds of internal defects like blisters, shatter fracture, flakes, fish-eyes and porosity. Carbon steels exposed to hydrogen at high temperatures experience hydrogen attack which leads to internal decarburization and weakening.
Blistering
Atomic hydrogen diffusing through metals may collect at internal defects like inclusions and laminations and form molecular hydrogen. High pressures may be built up at such locations due to continued absorption of hydrogen leading to blister formation, growth and eventual bursting of the blister. Such hydrogen induced blister cracking has been observed in steels, aluminium alloys, titanium alloys and nuclear structural materials. Metals with low hydrogen solubility are more susceptible to blister formation. While in metals with high hydrogen solubility like vanadium, hydrogen prefer to induce stable metal-hydrides instead of bubbles or blisters.
Shatter cracks, flakes, fish-eyes and micro perforations
Flakes and shatter cracks are internal fissures seen in large forgings. Hydrogen picked up during melting and casting segregates at internal voids and discontinuities and produces these defects during forging. Fish-eyes are bright patches named for their appearance seen on fracture surfaces, generally of weldments. Hydrogen enters the metal during fusion-welding and produces this defect during subsequent stressing. Steel containment vessels exposed to extremely high hydrogen pressures develop small fissures or micro perforations through which fluids may leak.
Porosity
In metals like iron, steel, aluminium, and magnesium, whose hydrogen solubilities increase with increasing temperature, liberation of excess hydrogen during cooling from the melt, produces hydrogen gas porosity.
Hydrogen embrittlement
By far, the most damaging effect of hydrogen in structural materials is hydrogen embrittlement. Materials susceptible to this process exhibit a marked decrease in their energy absorption ability before fracture in the presence of hydrogen. This phenomenon is also known as hydrogen-assisted cracking, hydrogen-induced blister cracking. The embrittlement is enhanced by slow strain rates and low temperatures, near room temperature.
Brittle delayed failure of normally ductile materials when hydrogen is present within is called hydrogen stress cracking or internal hydrogen embrittlement. This effect is seen in high strength structural steels, titanium alloys and aluminium alloys.
Hydrogen environment embrittlement
Embrittlement of materials when tensile loaded in contact with gaseous hydrogen is known as hydrogen environment embrittlement or external hydrogen embrittlement. It has been observed in alloy steels and alloys of nickel, titanium, uranium and niobium.
Hydrogen lowers tensile ductility in many materials. In ductile materials, like austenitic stainless steels and aluminium alloys, no marked embrittlement may occur, but may exhibit significant lowering in tensile ductility in tensile tests.
Hydrogen may also affect the plastic flow behaviour of metals. Increased or decreased yield strengths, serrated yielding, altered work hardening rates as well as lowered fatigue and creep have been reported.
Control of hydrogen damage
The best method of controlling hydrogen damage is to control contact between the metal and hydrogen. Many steps can be taken to reduce the entry of hydrogen into metals during critical operations like melting, casting, working, welding, surface preparation, like chemical cleaning, electroplating, and corrosion during their service life. Control of the environment and metallurgical control of the material to decrease its susceptibility to hydrogen are the two major approaches to reduce hydrogen damage.
Detection of hydrogen damage
There are various methods of adequately identifying and monitoring hydrogen damage, including ultrasonic echo attenuation method, amplitude-based backscatter, velocity ratio, creeping waves/time-of-flight measurement, pitch-catch mode shear wave velocity, advanced ultrasonic backscatter techniques, time of flight diffraction, thickness mapping and in-situ metallography – replicas. For hydrogen damage, the backscatter technique is used to detect affected areas in the material. To cross-check and confirm the findings of the backscatter measurement, the velocity ratio measurement technique is used. For the detection of and, time of flight diffraction is a suitable method to use.
