Active dissolution, mechanism

Fluang [246] investigated the SCC of AISI 321 stainless steel in acidic chloride solutions by the SSRT technique and fracture mechanics. It was found that the cleavage fracture characterizes the fracture surface. The active dissolution mechanism controls the SCC of AISI 321 stainless steel in acidic chloride solutions and can be inhibited by using KI. The inhibition effect of KI on the SCC is due to inhibition of the anodic reaction of the corrosion process. 

Impedance measurements have frequently been used in the study of the anodic behavior of metals in order to obtain fundamental information on active dissolution mechanisms and on active-to-passive transitions (Cachet et al., 1992 Gabrielli et al., 1975). For example, the behavior of iron in acidic media was extensively studied by Gabrielli et al. (1975,1976) Epelboin et al., (1975b) and Sehweikert et al., (1980, 1981). 

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

Since the hydroxyl anion is involved in the mechanism given before, the implication is that other anions may also take part in the dissolution process, and that the effect of various chemicals may be interpreted in the light of the effect of each anion species. Most studies have been in solutions of sulphuric and hydrochloric acids and typically the reaction postulated for active dissolution in the presence of sulphuric acid is ... 

Fig. 8.2 Strain-generated active path mechanisms, (a) Often referred to as the film rupture model and (b) the slip step dissolution model. In both cases growth is by dissolution film rupture is the rate controlling step, not the mechanism of crack growth...

The depth profiling technique used on samples with a barrier film before and after the addition of chloride to the buffering borate electrolyte showed no indication of either chloride penetration or significant reduction of the average oxide layer thickness.123 This, of course, does not rule out the possibility of the formation, by any of the mechanisms suggested above, of pinholes with radii much smaller than that of the ion-gun beam, through which the entire active dissolution could take place, or the possibility that the beam missed pits formed sporadically across the surface. If pinholes which are not visible were formed, the dissolution should proceed in them with extremely high true current densities. 

The fact that impurities do not affect the active dissolution in chloride solutions at current densities larger than 0.01mA/ cm2 shows that the inhomogeneity resulting in a pitting mechanism of dissolution is unrelated to impurities and is an inherent property of the metal. 

Surface Reactions. As we have seen from the dissolution of oxides the surface-controlled dissolution mechanism would have to be interpreted in terms of surface reactions in other words, the reactants become attached at or interact with surface sites the critical crystal bonds at the surface of the mineral have to be weakened, so that a detachment of Ca2+ and C03 ions of the surface into the solution (the decomposition of an activated surface complex) can occur. 

The theories proposed to explain the formation of passivation film are salt-film mechanism and acceptor mechanism [21]. In the salt-film mechanism, the assumption is that during the active dissolution regime, the concentration of metal ions (in this case, copper) in solution exceeds the solubility limit and this results in the precipitation of a salt film on the surface of copper. The formation of the salt film drives the reaction forward, where copper ions diffuse through the salt film into electrolyte solution and the removal rate is determined by the transport rate of ions away from the surface. As the salt-film thickness increases, the removal rate decreases. In the acceptor mechanism, it is assumed that the metal-ion products remain adsorbed onto the electrode surface until they are complexed by an acceptor species like water or anions. The rate-limiting step is therefore the mass transfer of the acceptor to the surface. Recent studies confirmed that water may act as an acceptor species for dissolving copper ions [22]. 

As /a was found to increase with temperature [40,41], step (51) is believed to be an activated process, in which the electron must first be thermally excited from the Xj surface state to the conduction band edge before injection can occur. The corresponding current density is therefore usually referred to as the electron excitation current density. The reaction step with which reaction (51) is assumed to compete is then step (15) in the DH dissolution mechanism, step (37) in the DX mechanism, step (46) (or (46 )) in the DXC mechanism, and equally step (46) or (46 ) in the DHC mechanism, which we have hitherto not considered, and which comprises the following steps ... 

Transition-state theory may be useful in testing the dissolution mechanisms presented above. According to TST, for any elementary chemical reaction the reactants should pass through a free-energy maximum, labeled the activated complex , before they are converted to products. It is assumed that the reaction rate-determining step is related to the decomposition of this activated complex ... 

Types of Corrosion Phenomena. The major categories of phenomena ( include uniform, localized, and pitting corrosion selective dissolution and corrosion acting together with a mechanical phenomenon. In uniform corrosion, all areas corrode at the same rate. Examples of uniform corrosion include tarnishing and active dissolution of metals in acids. In localized corrosion some areas corrode more readily than others crevice corrosion and filiform... 

There are various concepts about the aluminum silicates dissolution mechanism. Relatively recently a low rate of their dissolution was explained by inner diffuse regime. Currently more substantiated appears hydrolysis with the formation of activated complexes. According to this theory, the dissolution begins with the exchange of alkaline, alkaline-earth and other metals on the mineral surface of H+ ions from the solution (see Figure 2.26). At that, metals in any conditions are removed in certain sequence. In case of the presence of iron and other metals with variable oxidation degree the process may be accompanied with redox reaction. Hydrolysis is a critical reaction in the dissolution of aluminum silicates. It results in the formation on the surface of a very thin layer of activated complexes in Na, K, Ca, Mg, Al and enriched with H+, H O or H O. The composition and thickness of this weakened layer depend on the solution pH. These activated complexes at disruption of weakened bonds with mineral are torn away and pass into solution. For some minerals (quartz, olivine, etc.) the disruption of one inner bond is sufficient, for some others, two and more. The very formation of activated complexes is reversible but their destruction and removal from the mineral are irreversible. 

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