Autocatalytic corrosion

Pitting occurs witli many metals in halide containing solutions. Typical examples of metallic materials prone to pitting corrosion are Fe, stainless steels and Al. The process is autocatalytic, i.e., by initial dissolution, conditions are established which furtlier stimulate dissolution inside tire pit tire metal (Fe in tire example of figure C2.8.6 dissolves. 

Hydrolysis is a significant threat to phosphate ester stabiHty as moisture tends to cause reversion first to a monoacid of the phosphate ester ia an autocatalytic reaction. In turn, the fluid acidity can lead to corrosion, fluid gelation, and clogged filters. Moisture control and filtration with Fuller s earth, activated alumina, and ion-exchange resias are commonly used to minimise hydrolysis. Toxicity questions have been minimised ia current fluids by avoiding triorthocresyl phosphate which was present ia earlier natural fluids (38). 

The electrochemistry of single-crystal and polycrystalline pyrite electrodes in acidic and alkaline aqueous solutions has been investigated extensively. Emphasis has been laid on the complex anodic oxidation process of pyrite and its products, which appears to proceed via an autocatalytic pathway [160]. A number of investigations and reviews have been published on this subject [161]. Electrochemical corrosion has been observed in the dark on single crystals and, more drastically, on polycrystalline pyrite [162]. Overall, the electrochemical path for the corrosion of n-EeS2 pyrite in water under illumination has been described as a 15 h" reaction ... 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

The crevice corrosion mechanism is very similar to that of pitting with respect to the autocatalytic propagation. However, the causes of initiation, the morphology and the penetration of pitting are quite different from that of crevice corrosion (Figure 6.21).16... 

The insoluble corrosion product Fe(OH)2 can help bacterial film to control the diffusion of oxygen to the anodic sites in the pit. This forms a typical tubercle. If chlorides are present in the aqueous solution, the pH of the solution trapped in the tubercle can become very acid due to the autocatalytic propagation mechanism of localized corrosion due to deposit formation and generation of hydrochloric acid. 

Sweeping away corrosive ionic species. The attack of stainless steel decreases as the solution flow speed increases. Stainless steel in nitric acid without agitation, is attacked autocatalytically because of the formation of nitrous acid, as a cathodic reaction product. 

Consider a pit formed in a piece of aluminum that is in contact with seawater. As we shall show, the pH of the solution inside a pit can become quite low, leading to an increased rate of corrosion, which further lowers the pH, and so on. Thus, pitting corrosion can be considered to be an autocatalytic process, with its rate increasing with time. 

It can be seen that at low pH the oxidation is most likely to occur but the potential of persulfate is always in the passivation region. It was observed by Lumieres and Seyewetz [139] that reduction of the image density with persulfate had a notable induction period and the reaction appeared to be autocatalytic. It was suggested that silver ions themselves catalyze the reaction. It is possible that the buildup of silver ions lowered the silver potential sufficiently for corrosion to occur alternatively the production of silver(III) compounds might be responsible for the acceleration. 

Figure 10 Photographic development mechanism. The reduction potential, E"(Ag+/Agn), ofthe latent image clusters, when in contact with a solution. Increases with the number of atoms n. Therefore a nuclearity threshold for developmen t is created by the redox poten tial of the developer E°(CP/D). Above the critical nuclearity n, the potential E°(Ag yAg ) is higher than E°(CA/D), and alternate electron transfer toward A g, and Ag adsorption on Err allows the cluster to grow autocatalytically. On the contrary, when l"(Ag, /Agg is lower than E°(E>-/D), corrosion ofsubcritical clusters takes place by oxidizing molecules, such as D or Ox [7],...

Thus, in the presence of dissolved oxygen, the mechanism is autocatalytic in that the corrosion product of the anodic reaction, Eq 7.6, through the reaction of Eq 7.7, progressively supports the cathodic reaction, Eq 7.8. Not only is the cathodic reactant regenerated, but also the concentration increases such that the corrosion rate tends to increase with time. 

Cover art represents autocatalytic processes occurring in a corrosion pit. The metal, M, is being pitted by an aerated NaCl solution. Rapid dissolution occurs within the pit, while oxygen reduction takes place on the adjacent surfaces. Source U.R. Evans, Corrosion, Vol 7 (No. 238), 1951... 

IV) Initiation is an autocatalytic increase in the exposed Mg surface area. Removing MgiOl Iq increases the exposed surface area, but so could corrosion of a clean spot on Mg/ to form a more convoluted surface. 

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

The autocatalytic nickel deposition can also be used for alloy deposition (Ni-Co, Ni-Fe) and for the deposition of nickel dispersion coatings (Ni/SiC). The nickel film can be heat-treated, below or well above 280 °C, the transition temperature amorphous/crystalline. The hardness increases up to 400 °C (1000-1200 HV) and then decreases again. Especially the corrosion protection... 

According to Weissenrieder et al. [4], in the absence of either NO2 or O3, iron passivates in 200 ppb SO2. In the presence of oxidants, such as NO2 or O3 in humidified air, locahzed corrosion was detected by the authors and was described as sulfate nests [4]. The observed corrosion is autocatalytic in nature with a rapid dissolution of iron and production of Fe and Fe cations. Localized corrosion promotes the catalytic conversion of SO2 to sulfate anions, which enhance the sulfate-induced corrosion of iron, thus creating more dissolved iron cations. Next, the sulfate nests distribute through increased deposition of SO2 at high pH cathodic sites and locally create new anodic sites by decreasing the pH. 

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