Although the phenomenon of hydrogen embrittlement has been observed and studied for more than a century, the mechanism which causes the embrittled condition is not yet understood. Hydrogen embrittlement develops in both ferrous and non-ferrous metals and alloys where such materials come in contact with a suitable hydrogen environment. By some unascertained means, the hydrogen atoms or ion invades the crystalline structure of the metal or alloy and disrupt the lattice such that the toughness of the metal or alloy is substantially reduced. Strength reduction can be so severe that the embrittled metal may be advanced to the point of fracturing without any attendant applied stress. Experimentation has demonstrated that high quality steel placed in an acid or electrolysis bath and charged for a period of time to extreme stages of hydrogen embrittlement can be manually crumbled as a result of the hydrogen embrittlement alone. The origin of the hydrogen may be acid solution, electrolysis processes, H.sub.2 environment or virtually any material which develops free hydrogen or hydrogen ion. Thus, once the hydrogen has been absorbed into the crystalline structure, knowing its origin is of little importance in predicting the effects of embrittlement.
The severity of these effects will vary with the attendant conditions. Obviously, the longer the duration of exposure of the metal to the hydrogen environment and the greater the surface area of exposed metal, the greater will be the extent of hydrogen absorption. It has also been demonstrated that harder metals tend to be more susceptible to the destrcutive effects of hydrogen embrittlement than metals of a more flexible nature. Furthermore, the process of embrittlement is enhanced by intermediate temperatures and low strain. The occurrence of these favorite conditions can increase the rate of hydrogen absorption into metals by a factor of as much as 10.sup.5 times that of normal hydrogen solubility.
Hydrogen embrittlement is to be distinguished from "hydrogen attack." The latter occurs at elevated temperatures and pressures, causing decarburization in steels. This results in the formation on intergranular fissures, blistering and other structural damage. Hydrogen embrittlement, on the other hand, does not necessarily involve such severe disruption of the crystalline structure, although some interstitial displacement by the hydrogen may occur. If an alloy is subjected to hydrogen attack, however it appears to be more susceptible to hydrogen embrittlement at lower temperatures--a problem well recognized by refineries, power plants and paper mills.
Practically speaking, hydrogen embrittlement is of great concern in many phases of industry and commercial life. It arises during the fabrication and processing of metals where hydrogen is present in meaningful concentrations in the surrounding environment. It occurs in electroplating and electrolysis processes. Welding is another source of hydrogen embrittlement due to the high temperature/low stress in proximity to the liquid metal in the fusion zone.
The primary public concern regarding hydrogen embrittlement is the fact that there is little warning as to pending catastrophic failure where the embrittlement is not accompanied by severe structural deformation such as that normally associated with fatigue dislocations and voids. The consequence is a delayed occurrence of failure in the metal which heretofore was unpredictable. Whereas current techniques enable the estimation of useful-life of metal being regularly subjected to known stress and fatigue, the only processes previously understood as useful to make such predictions concerning the extent hydrogen embrittlement have essentially required the destructive analysis of the subject metal. Such procedures defeat the purpose of prediction since the metal must be replaced in part to make the analysis.
The lack of a nondestructive method which reveals the embrittled condition caused by hydrogen absorption has precluded the development of a technique which enables the monitoring of progressive states of hydrogen embrittlement as the metal approaches the point of failure. Furthermore, the development of such a technique would permit the prediction of expected failures, enabling replacement within known safety limits and at lower cost due to the resultant controlled replacement.
The positron annihilation technique for analyzing crystalline structure and lattice defects has been understood since 1950; however, it has never been applied to hydrogen embrittlement. Research by Joseph C. Grosskreutz, as disclosed in U.S. Pat. No. 3,593,025 entitled "Detecting Defects by Distribution of Positron Lifetimes in Crystalline Materials," discusses the application of this technique to analysis of fatigue and stress damage in metals, but fails to suggest potential use of positron annihilation in non-stress related problems such as hydrogen embrittlement.
The reason for such application to the area of stress and fatigue damage in metals was based on the theory that the resulting voids, cracks, and dislocations within the crystalline lattice caused extension of positron lifetime within the metal "due to trapping of the positrons within the voids caused by such crystal defects." The absence of electron density within these voids decreases the frequency of annihilation between such electrons and the invading positrons, thereby extending positron lifetime. The essence, therefore, of the positron annihilation techiques used by Grosskreutz and others in this field of metal analysis relies on the presence of voids within the metal. Indeed, their studies in this field have been directed to materials subjected to stress and fatigue because these conditions are known to cause such defects.
The referred patent, however, does not deal with the nonstress related condition known as hydrogen embrittlement. The two catagories of defects addressed in the specification are fatigue defects and microporosity. Quoting from column one of the patent, "Micropores are accumulation of vacancies (vacancy clusters) in the crystal . . ." The disclosure refers to such defects as arising from fabrication, molding and machining--all stress related processes. It should be noted, therefore, that 35 USC 112 limits the application of this patent to art dealing with detection of voids, dislocations, vacancies and similar defects resulting from fatigue and stress. This is not the unique analysis required for detection of non-stress condition of hydrogen embrittlement.
Therefore, the application of positron annihilation techniques to the detection of hydrogen embrittlement in metals and similar crystalline structures has not heretofore been conceptualized. Since hydrogen embrittlement apparently occurs where hydrogen ions are dispersed through the lattice prior to any formation of voids and other fatigue related defects, the theoretical basis disclosed in the above reference material does not suggest that positron annihiliation is a useful technique for hydrogen embrittlement analysis. In fact, the only prior methods conceivably useful for detection of hydrogen embrittlement have involved quantitative detection of hydrogen by either nondestructively extracting the hydrogen by subjection of the material to high heat or destructively extracting the hydrogen from a sample of the subject metal by fusion techniques. The very fact that these quantitative processes have been the sole method for detection of hydrogen embrittlement strongly suggests that the research of Grosskreutz, when viewed in connection with the state of the art, did not teach or even suggest the potential application of positron annihilation as a non-destructive method for detecting the presence of hydrogen embrittlement.
Technically, the occurrence of hydrogen embrittlement is uniquely distinct from the characteristics and consequences of the application of fatigue and stress to crystalline structure. Where fatigue defects are best developed at high strain rates and low temperatures, hydrogen embrittlement prefers low strain and higher temperatures. It will be further noted that the application of the positron annihilation technique in hydrogen embrittlement detection involves a more complicated positron lifetime response which appears to be inconsistent with the predictions of the Grosskreutz patent.
Instead of a simple increase in positron lifetime with increased defects as indicated by Grosskreutz, the progressive stages of hydrogen embrittlement are accompanied by an initial increase followed by a subsequent decrease lifetime duration. Such a response is incompatible with the Grosskreutz disclosure. This distinction is discussed in greater detail in the detailed description.