Corrosion mechanism carbon dioxide

Carbon dioxide dissolves in water to form a weak acid (carbonic acid), which reduces the pH of the solution and, consequently, increases its corrosivity. Corrosion caused by carbon dioxide is generally referred to as sweet corrosion, and results in pitting. The mechanism of carbon dioxide corrosion is as follows [197,198] ... 

In dry air the stability of zinc is remarkable. Once the protective layer of zinc oxide formed initially is complete, the attack ceases. Even under under normal urban conditions, such as those in London, zinc sheet 0 -8 mm thick has been found to have an effective life of 40 years or more when used as a roof covering and no repair has been needed except for mechanical damage. The presence of water does, of course, increase the rate of corrosion when water is present the initial corrosion product is zinc hydroxide, which is then converted by the action of carbon dioxide to a basic zinc carbonate, probably of composition similar to ZnCOj 3Zn(OH)2 . In very damp conditions unprotected zinc sometimes forms a loose and more conspicuous form of corrosion product known as wet storage stain or white rust (see p. 4.171). 

The mechanisms of corrosion by steam are similar to those for water up to 450°C, but at higher temperatures are more closely related to the behaviour in carbon dioxide. Studies at 100°C have demonstrated that uranium hydride is produced during direct reaction of the water vapour with the metal and not by a secondary reaction with the hydrogen product. Also at 100°C it has been shown that the hydride is more resistant than the metal. Inhibition with oxygen reduces the evolution of hydrogen and does not involve reaction of the oxygen with the uranium . Above 450°C the hydride is not... 

Steam traps are installed in condensate, mechanical return systems and are a frequently overlooked item for reducing operating costs. Large industrial process plants typically have many hundreds of steam traps installed to recover low-energy condensate and remove (potentially corrosive) air and carbon dioxide. 

In practice, the potential causes of boiler section corrosion are many and often commonplace. Initiators include oxygen, carbon dioxide, acid, caustic, copper plating, chelant, and even the water itself. In addition, mechanical problems may be an initiator of corrosion, which in turn may lead to boiler mechanical failure. 

In contrast to SCC of carbon and low-alloy steels in chloride, sulfide, and sulfuric acid environments by hydrogen-embrittlement mechanisms, cracking in several environments is attributed to passive-film cracking and/or active-corrosion-path anodic-dissolution penetration mechanisms (Ref 124). These environments include nitrates, hydroxides, ammonia, carbon-dioxide/carbonate solutions, and aqueous car-bon-monoxide/carbon-dioxide. Nitrate-bearing solutions are encountered in coal distillation and fertilizer plants hydroxide solutions in the production of NaOH and in crevices of steam boilers and ammonia cracking has occurred in tanks and distribution systems for agricultural ammonia applications. 

Other problems may arise if the modeler s objective is to explain or predict the results of an "applications level" experiment (one involving a relatively complex system) that is carried out in the field or, more commonly, the laboratory. First, the conditions assumed to prevail in the experiment may not be the actual ones. For example, the experiment may be thought to be a closed system, when in fact there is loss or gain of volatiles such as carbon dioxide. For another, the walls of the experimental vessel may be thought to be not a factor in the course of reaction when in fact they are via such mechanisms as diffusive absorption or corrosion. 

Rebar corrosion in concrete is considered to occur in two phases [8]. The first phase begins with construction of the structure and ends with corrosion initiation when depassivating species reach the reinforcement. The second phase is the active corrosion that destroys the structure. Controlling rebar corrosion in this phase is very difficult. Passive film corrosion is initiated when local pore solution at the concrete-rebar interface drops below the passivation pH due to the presence of atmospheric carbon dioxide (carbonation) or chloride penetration. The following mechanism controls the carbonation process ... 

Research has shown thaU in the absence of sulfate, SRB can switch to fermentation and ferment a variety of substances and produce hydrogen, carbon dioxide, and acetate. These products can then be used by other bacteria such as methane-producing bacteria (methanogenes), which, by consuming hydrogen, can accelerate corrosion through mechanisms such as cathodic depolarization. Therefore, fermentative capabilities of SRB allow them to use alternative substances other than sulfate. 

The calculations discussed above do not include the effects of graphite corrosion caused by impurities such as water, carbon dioxide, and other oxidants in the coolant. This corrosion, or "burnoff", of the graphite will result in some loss of strength and changes to the elastic modulus and other mechanical and thermal properties. The maximum burnoff occurs in the hottest element. 

The cryogenic process uses pelletized carbon dioxide at -73°C as a fluidized abrasive cleaning agent for surface preparation and removal of corrosion and old coatings. Not only does this provide an abrasion mechanism, but certain inorganic salts and organic contaminants can be dissolved with supercritical carbon dioxide. 

The behavior of zinc in distilled water has often been studied in connection with investigations on the mechanism of corrosion. Bengough et al. (1929) and Bauer and Krohnke (quoted by Wiederholt, 1965) found that zinc is not greatly corroded in pure water provided it is free from oxygen and carbon dioxide, even when boiling (Traube, 1885 Taboury and Gray, 1939). 

Though not considered a major problem in North America, carbonation of concrete is a major cause of steel corrosion in the world. Calcium hydroxide in the concrete can react with carbon dioxide or carbon monoxide to produce calcium carbonate. The calcium carbonate is not detrimental to the mechanical properties, but in the process the pH of the concrete drops below 10. At that point the corrosion rate of embedded steel can significanfly increase. This phenomenon is mostly found in concretes with low cement contents, high permeabiUly, and low concrete cover over the rebars. 

Zinc-coated steel and monolithic zinc objects corrode similarly a useful reference on the mechanism of corrosion of a galvanized coating is that of Daesen [3]. In dry air, a film of zinc oxide initially forms however, the presence of moisture and carbon dioxide changes the corrosion product to a basic zinc carbonate film. Similarly, a dilute presence of sulfur compounds can result in a production of a basic sulfate film. Zinc oxide, basic zinc carbonate, and basic zinc sulfate are stable protective layers if left undisturbed however, the pH of the environment can interfere with these layers, resulting in the formation of more soluble products. Because zinc forms an amphoteric oxide, both strong alkaline and acid conditions interfere with the formation of these protective layers. Attack is most severe at pH values below 6 and above 12.5. Within this range, corrosion is relatively slow. 

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