Dissolution spontaneous

Rozenfd d, I. L. and Danilov, I. S., Electrochemistry of Pitting Corrosion, 1 Pit Formation During Spontaneous Dissolution of Stainless Steels , Zashch. Metal.,1, 134(1966) C.A.,65, 10114a... 

Different reactions (anodic and cathodic) can occur simultaneously at an electrode, even when there is no net current flow. In Section 2.5.1 we mentioned the example of an iron electrode in HCl + FeCl2 solution where anodic iron dissolution (2.24) and cathodic hydrogen evolution (2.25) occur simultaneously these are the reactions of spontaneous dissolution of iron not requiring a net current. 

Figure 1T2 shows anodic d cathodic polarization curves for the partial CD of dissolution 4 and deposition 4 of the metal and for the partial CD of ionization 4 and evolution 4 of hydrogen, as well as curves for the overall reaction current densities involving the metal (4) and the hydrogen (4). The spontaneous dissolution current density 4 evidently is determined by the point of intersection. A, of these combined curves. 

When the metal is in contact with an electrolyte solution not containing its ions, its equilibrium potential theoretically will be shifted strongly in the negative direction. However, before long a certain number of ions will accumulate close to the metal surface as a result of spontaneous dissolution of the metal. We may assume, provisionally, that the equilibrium potential of such an electrode corresponds to a concentration of ions of this metal of about 10 M. In the case of electrodes of the second kind, the solution is practically always saturated with metal ions, and their potential corresponds to the given anion concentration [an equation of the type (3.35)]. When required, a metal s equilibrium potential can be altered by addition of complexing agents to the solution (see Eq. (3.37)]. 

Under the effect of oxidizing agents, a metal may become passivated even when not anodically polarized by an external power source. In this case, passivation is evident from the drastic decrease in the rate of spontaneous dissolution of the metal in the solution. The best known example is that of iron passivation in concentrated nitric acid, which had been described by M. V. Lomonosov as early as 1750. Passivation of the metal comes about under the effect of the oxidizing agent s positive redox potentiaf. 

FIGURE 22.2 Schematic polarization curves for spontaneous dissolution (a) of active metals (h) of passivated metals. (1,2) Anodic curves for active metals (3) cathodic curve for hydrogen evolution (4) cathodic curve for air-oxygen reduction (5) anodic curve of the passivated metal. 

Sometimes anodic protection is used, in which case the metal s potential is made more positive. The rate of spontaneous dissolution will strongly decrease, rather than increase, when the metal s passivation potential is attained under these conditions. To make the potential more positive, one must only accelerate a coupled cathodic reaction, which can be done by adding to the solution oxidizing agents readily undergoing cathodic reduction (e.g., chromate ions). The rate of cathodic hydrogen evolution can also be accelerated when minute amounts of platinum metals, which have a stroug catalytic effect, are iucorporated iuto the metaf s surface fayer (Tomashov, 1955). 

The best known way of lowering the corrosion of iron is by its alloying with chrominm, nickel, and other metals. The corrosion resistance of the corresponding stainless steels is dne to the fact that chrominm is readily passivated. This is a quality that is fonnd even in alloys with relatively low chromium contents. Hence, stainless steels are practically always strongly passivated, and their spontaneous dissolution rates are very low. 

Corrosion (spontaneous dissolution) of the catalyticaUy active material, and hence a decrease in the quantity present. Experience shows that contrary to widespread belief, marked corrosion occurs even with the platinum metals. For smooth platinum in sulfuric acid solutions at potentials of 0.9 to 1.0 V (RHE), the steady rate of self-dissolution corresponds to a current density of about 10 A/cm. Also, because of enhanced dissolution of ruthenium from the surface layer of platinum-ruthenium catalysts, their exceptional properties are gradually lost, and they are converted to ordinary, less active platinum catalysts. 

Although the details of the equilibrium model are still uncertain, the general trends are likely reliable. As shown in Figme 5.16, most of the Fe(III) in seawater is predicted to be in the form of the FeL complex. The equilibrium model also predicts that this degree of complexation should enhance iron solubility such that 10 to 50% of the iron delivered to the ocean as dust will eventually become dissolved if equilibrimn is attained. If this model is a reasonable representation for iron speciation in seawater, uptake of [Fe(III)]jQjgj by phytoplankton should induce a spontaneous dissolution of additional particulate iron so as to drive the dissolved iron concentrations back toward their equilibrium values. 

Penicaud A, Poulin P, Derre A et al (2005) Spontaneous dissolution of a single-wall carbon nanotube salt. J Am Chem Soc 127 8-9... 

During the last decades, a great number of papers dealing with anodic or spontaneous dissolution of polycrystalline nickel were published. Only some illustrative examples could be mentioned here. In [55], the anodic dissolution of nickel was studied galvanostaticaUy in hydrochloric acid solutions of various concentrations. The reaction orders with respect to chloride ion and hydrogen ion concentrations were determined. 

The requirement that AG be negative for spontaneous dissolution is readily met if the first term, AH, is less than the second term, — 7A5. Because the polymer chain units are bound through primary, covalent bonds, the units are not free to move independent of their neighboring units. Thus the — TAS term is lower for polymer solution than for the solution of smaller molecules. Most approaches for correlating structure with polymer solubility have focused on the AH term. Hildebrand has proposed the following relationship,... 

Because the CBPC process is based on slow dissolution of the components, spontaneous dissolution of oxides is not desirable in the ceramic formation. This implies thatEq. 6.18 is a requirement for a dissolution reaction that is useful in forming a ceramic. Consider, for example, dissolution of MgO in a neutral medium given by Eq. 5.9c... 

Finally, the critical relative humidities are dependent on the nature of the solid. For example, the spontaneous dissolution process has been observed for many water-soluble substances at relative humidities significantly below that associated with a saturated solution of the substance in water.Van Campen, Amidon, and Zograf have examined the moisture sorption kinetics of deliquescent solids at relative humidities above what they term the critical relative humidity (RHo), where adsorbed water takes on the character of condensed water and serves as a solvent. It is important to recognize that a highly undesirable process such as deliquescence can occur when it may not be expected (e.g., when RHo < RH < RHs). 

The considerations dealt with so far have been concerned with gas bubbles of larger sizes (above about 10" mm). Bubbles of smaller sizes (called seed ) exhibit specific behaviour which may lead to spontaneous dissolution even without the effect of refining agents. The surface tension acting in the bubble walls produces increased pressure in the enclosed gas, given by the relationship... 

The stability of the tooth surface to spontaneous dissolution and accretion is primarily mediated by statherin, a 43 amino acid polypeptide encoded by a single gene (Stath) on chromosome 4. Its two N-terminal serine residues spontaneously become phosphory-lated in saliva, and they attach statherin tightly to exposed tooth enamel, preventing dissolution or calcium phosphate accretion. Nevertheless, all proteins in whole saliva adhere to some extent to enamel, forming an acquired pellicle that replaces abraded enamel cuticle (Chap. 9). Differences in amount or composition of the proteins in whole saliva cause differences in acquired pellicle composition. 

Thus, in Refs, and 209-214) been suggested that electrochemical reactions of cathodic reduction and the processes of spontaneous dissolution of metals in aqueous media proceed via an intermediate stage of formation of hydrated electrons. This means that the electrons leave the electrode and go into the solution and then the hydrated electrons react in the solution bulk, reducing, for instance, the proton donors. The theoretical and experimental argumentation of these authors, as regards the possibility of the mechanism itself, was disputed by other researchers... 

The short-term potential for solid solution formation is low for minerals with small solubility products, such as aluminum oxides and aluminosilicates, because spontaneous dissolution and recrystallization is very slow in these minerals. Without recrystallization, trace metals cannot be incorporated into the mineral structures. Movement of metal ions into these mineral crystals by solid diffusion is not possible on the time scale of adsorption experiments ionic diffusion into most crystalline solids is negligibly slow at all but extremely high temperatures. Nevertheless, metals could diffuse into imperfect solids along interstices, pores, or other structural defects. 

Both Mn and Mn are stable only in the solid phase of soils, where they form insoluble oxide and hydroxide minerals of variable structure and oxidation state (e.g., Mn02, MnOOH, Mn304, Na4Mn,4027). The Mn ion is released from these solids by spontaneous dissolution or cation exchange, especially under acidic or reducing conditions. Release of Mn can also be detected in the presence of pyrophosphate. 

The spectroscopic verification of a stable alloy surface in UHV (ultrahigh vacuum conditions) has to be done at an open circuit, where, in the case of a single crystal surface, it means a stable double layer. However, when the single crystal has to be transferred to the electrochemical cell to perform adsorption or electrode reactions, the persistence of this situation is not obvious. A constant correlation between the open circuit potential and the electron binding energy of the double layer components (anion, solvent, etc.) has to be maintained [50]. When the rest potential lies at the onset of the oxide formation, there is a further stabilization, but this can produce spontaneous dissolution of the less noble metal of the alloy. Thus, air contact has to be avoided. 

Under particular conditions the thermodynamically stable micellar dispersions may form as a result of spontaneous dissolution of crystalline or liquid surfactant macroscopic phase. Even though the state of substance in a micelle is not always equivalent to that in a macrophase, a rather high degree... 

H. Konno, M. Kawai, M. Nagayama, The mechanism of spontaneous dissolution of the air-formed oxide film on iron in a deaerated neutral phosphate solution, Surf. Technol. 24 (1985) 259—271. 

Another important aspect to consider when looking at the mechanical properties of cementitious bone substitutes is that the mechanical properties may vary quite extensively upon implantation. For example, since gypsum and brushite are soluble in physiological conditions, the mechanical properties of these materials rapidly decrease upon implantation. This spontaneous dissolution is also the reason why these materials are often combined with less soluble bone substitute such as / -TCP or... 

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