Passive corrosion mechanisms

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

For this study by Thompson et al (42), ion implantation and RBS were combined with more traditional electrochemical measurements to help establish the corrosion mechanisms of alloys in which a noble metal (Pt) was combined with an active/passive base metal (Ti). The alloys were created by ion implantation of Pt into pure Ti and were not of uniform bulk composition. Such surface alloys offer the possibilities of using a very small amount of a noble material to create a corrosion resistant coating on an otherwise chemically unstable but inexpensive metal or alloy. 

Since these hydrides are thermodynamically stable in the metal, the passive oxide can only be considered as a transport barrier, not as an absolute barrier. Various electrochemical techniques including EIS and photoelectrochemical measurements have been used to identify the mechanism by which the Ti02 may be rendered permeable to hydrogen, and to identify the conditions under which absorption is observable (31). These determinations show that H absorption into the Ti02 (and hence potentially into the metal) occurs under reducing conditions when redox transformations (Ti1 —> Tim) in the oxide commence. However, the key measurement, if H absorption is to be coupled to passive corrosion, is that of the absorption efficiency. 

The agitation of the medium or the level of agitation of the electrolyte has a great influence on the corrosion performance of most of the metallic alloys since agitation causes acceleration of diffusion of aggressive species or destruction of the passive layer mechanically. 

Macdougall, B., Graham, M.J., Growth and stability of passive films, in Corrosion mechanisms in theory and practice, P. Marcus and J. Oudar (eds.), Marcel Dekker, Inc., pp. 143-173, 1995. 

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

Refs. [i] Strehblow HH (2003) Passivity of metals. In Alkire RC, Kolb DM (eds) Advances in electrochemical science and engineering. Wiley-VCH, Weinheim, pp 271-374 [ii] Vetter KJ, Gorn F (1973) Electrochim Acta 18 321 [Hi] Strehblow HH (2002) Mechanisms of pitting corrosion. In Marcus P (ed) Corrosion mechanisms in theory and practice. Marcel Dekker, New York, pp 243-285 [iv] Strehblow HH (2003) Pitting corrosion. In Bard AJ, Stratmann M, Frankel GS (eds) Corrosion and oxide films. Encyclopedia of electrochemistry, vol. 4. Wiley, Weinheim, 337... 

The observations can be generalized. Besides the homogeneous oxide growth by Reaction (2), a dissolution/precipitation mechanism may contribute to the passivation. If it takes place by corrosion of an ion with lower valency (e.g. Fe " "), the following reaction (inverse of Reaction 17) includes oxidation and the necessary ETR (15). Then, the supporting passive film has to be electronically conducting. On the other hand, if it consists of passive corrosion (11) and following chemical precipitation (inverse of 18) without a redox process, electronic conduction is not necessary. 

The diffusion constant D is determined by the concrete quality. At the carbonation front there is a sharp drop in alkalinity from pH 11-13 down to less than pH 8. At that level the passive layer, which we saw in Chapter 2 was created by the alkalinity, is no longer sustained so corrosion proceeds by the general corrosion mechanism as described in the Chapter 2. 

The corrosion mechanism for Nd-Fe-B-based magnets has been studied in detail. Chin et al. (198 8) investigated passivating currents of Nd-Fe-B-based magnets in various aqueous solutions, and found that the current density between a Nd-Fe-B sintered magnet and a Ni-Cr reference electrode alloy was more than double that of Sm-Co magnets. 

It is suggested that the redox process prevents the corrosion (i.e., formation of rust or hydrated iron oxides) by catalyzing the formation of passivating coating of Fe Oj via chemical reactions shown in Figure 1.55. However, above mechanism remained doubtful and represent chemical unbalance. Therefore, more systematic studies are required to probe exact anti-corrosion mechanism. The efforts have been made to form CP-based copolymers (to improve solubility) and blends (to provide both... 

B. MacDougall and M. J. Graham, Growth and Stability of Passive Films, in Corrosion mechanisms in Theory and Practice. Edited by P. Marcus (Marcel Dekker, Inc., New York, 2002), pp. 189-216. 

That the phase separation of the surface oxide is thermodynamically driven has important consequences for corrosion. Given a passivating MgO film on Mg, there wiU be a thermodynamic driving force to the separation and subsequent renewal of exposed magnesium. The renewal of magnesium will result in increased reactivity and corrosivity of the previously passivated magnesium surface regardless of the corrosion mechanism that is active, any... 

Tang et al. studied these effects using DFT (with a complementaiy in situ STM study) [95]. They found similar trends for the Pt nanoparticles studied compared to the bulk in relation to metal/hydroxide/oxide stability but found a very strong dependence on particle size. For the smallest particle size studied (radius of 0.25 nm), at low pH, there is a direct dissolution pathway and hydroxides and oxides are not predicted to form prior to dissolution. Consequently, it can be surmised that under fuel cell conditions, there is a crossover in particle size below which the particles do not develop a passivating oxide layer and instead are subject to direct dissolution, thus decreasing their aheady compromised stability. This study again only looks at surface occupation of OH and O without surface/particle reconstruction but provides very useful information for the stability of Pt nanoparticles and the conditions that lead to oxide formation at the nanoscale. In addition, it shows how DFT can not only give information on phase stability but also point to corrosion mechanisms under different conditions. 

H.-H. Strehblow, V. Maurice, P. Marcus, Passivity of metals, in Corrosion Mechanisms in Theory and Practice, 3rd Edition, P. Marcus (Ed.), CRC Press, Taylor and Francis, New York, 2011, pp. 235-326. 

Corrosion Mechanisms—Self-stress in the Breakdown of Passive Films and Protective Scales... 

Pitting is a mode of localized dissolution leading to the formation of cavities within a passivated surface that is boldly exposed [47]. Pitting is a probabilistic or stochastic corrosion mechanism that is dependent on electrochemical... 

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