Mechanical stress, liquid metals

Cracking mechanisms in which corrosion is implicated include stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement. Purely mechanical forms of cracking such as brittle failure are not considered here. 

Univariant Systems.—Equilibrium between liquid and vapour. Vaporisation curve. Upper limit of vaporisation curve. Theorems of van t Hoff and of Le Chatelier. The Clausius-Clapeyron equation. Presence of complex molecules. Equilibrium between solid and vapour. Sublimation curve. Equilibrium between solid and liquid. Curve of fusion. Equilibrium between solid, liquid, and vapour. The triple point. Complexity of the solid state. Theory of allotropy. Bivariant systems. Changes at the triple point. Polymorphism. Triple point Sj—Sg— V. Transition point. Transition curve. Enantiotropy and monotropy. Enantiotropy combined with monotropy. Suspended transformation. Metastable equilibria. Pressure-temperature relations between stable and metastable forms. Velocity of transformation of metastable systems. Metastability in metals produced by mechanical stress. Law of successive reactions. 

Since technical pure liquids usually contain very fine solid particles, it is often difficult in practice to differentiate between pure fluid erosion and so-called hydroabrasive wear. As with jet wear, the number, hardness, and velocity of entrained particles determine the extent to which the protective covering layer is removed and the extent of the pure metal wear. If mechanical stressing due to entrained particles is predominant, then hardness and strength are of decisive importance for metals (Sick 1972). 

Stress-sorption cracking is the basic mechanism applying to stress cracking of plastics by specific organic solvents [41, 42] and to liquid-metal embrittlement—the cracking of solid metals by specific liquid metals. It is also the mechanism proposed earlier by Fetch and Stables [43] to account for stress cracking of steel induced by interstitial hydrogen (see Section 8.4). 

Liquid metal embrittlement (LME) is not a very common mechanism. Only a small number of liquid metals can cause LME in any given alloy system. LME is an inter- and intra-granular cracking mechanism produced by a liquid metal on the surface of a sensitive solid metal in the presence of a tensile stress, either applied or residual. The mechanism occurs only above a threshold stress vtdue and has an incubation period. Table 3 shows a few of the solid and liquid systems exhibiting LME. More alloys systems exhibiting LME are tabulated in the ASM Metals Handbook, 9th edition, Vol. 11, p. 238. 

Corrosion of metals corrosion testing in liquids under laboratory conditions without mechanical stress) (in German) Beuth Verlag GmbH, Berlin... 

During dynamic tests with thermal, load, and shutdown-start-up cycling, the amount and the vapor pressure of the water product varies and formation of liquid water might be involved. In addition, the shutdown-start-up or/and temperature cycling cause thermal and mechanical stresses to the membranes and cell components as well as volume expansions and contractions of the acid in the MEAs. Another important mechanism of the cell degradation involved in these dynamic tests is the corrosion of carbon supports and growth of noble metal catalysts [10]. Some dynamic test results are summarized in Table 22.10. 

Cavitation corrosion or cavitation-erosion is associated with the implosion of bubbles in a corrosive environment. In a negative pressure zone caused by high velocity the local pressure falls below the vapor pressure and forms bubbles. The bubbles spread in the liquid and implode in higher pressure zones on the metal surface. Material cavitation consists of a complex combination of mechanical stressing and electrochemical reaction ... 

It is postulated that specific ions are absorbed and interact with strained bonds at the surface of the crack tip, thus reducing the bond strength, and permitting continued brittle fi acture. This theory has been supported by observations in SCC. By chemisorption of the environmental species on the crack tip, the local fracture stress of the metal lattice is reduced. The theory has been applied to hydrogen embrittlement and liquid metal embrittlement. The adsorption phenomenon may be used to interpret the crack propagation mechanism of alloys which fail by hydrogen embrittlement, such as aluminum alloy 7075. 

This book will introduce the materials considered for the different structural components of the Generation IV systems, under high doses of irradiation such as fuel cladding, wrapper tubes, internal structures, lower doses such as pressure vessel, or no irradiation such as the power conversion systems. It will deal with the behavior in the different environments encountered, liquid metals, molten salts, supercritical water, and gas, as well as the behavior under mechanical stress and irradiation. Subsequently, the different classes of materials for in-core and out-of-core applications will be discussed. 

---