Corrosion mechanically activated

Warnings are noted in the literature to be careful in the interpretation of data from electrochemical techniques applied to systems in which complex and often poorly understood effects are derived from surfaces which contain active or viable organisms, and so forth. Rather, it is even more important to not use such test protocol unless the investigator fuhy understands both the corrosion mechanism and the test technique being considered—and their interrelationship. 

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. 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

For example, for NGZ, the loss of capacity is higher but comparable with the loss of graphite active mass (66.8 and 50.0% 94.7 and 56.5%). Thus, corrosion mechanism (3) is a predominant one for such an unstable to oxidation graphite class. 

For NGF, the loss of capacity is much higher (approximately 68%) than the loss of graphite active mass (approximately 2.5%). Thus, for this type of graphite the corrosion mechanisms (4) and (5) with formation of surface groups may be considered predominant. 

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. 

In the past ten years the number of chemistry-related research problems in the nuclear industry has increased dramatically. Many of these are related to surface or interfacial chemistry. Some applications are reviewed in the areas of waste management, activity transport in coolants, fuel fabrication, component development, reactor safety studies, and fuel reprocessing. Three recent studies in surface analysis are discussed in further detail in this paper. The first concerns the initial corrosion mechanisms of borosilicate glass used in high level waste encapsulation. The second deals with the effects of residual chloride contamination on nuclear reactor contaminants. Finally, some surface studies of the high temperature oxidation of Alloys 600 and 800 are outlined such characterizations are part of the effort to develop more protective surface films for nuclear reactor applications. ... 

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. 

Stirrer The stirrer (Fig. 4) consists of a 1-mm in diameter (B S gauge 18), corrosion-resistant wire bent into a series of three loops about 25 mm apart. It should be made so that it will move freely in the space between the thermometer and the inner wall of the sample container. The shaft of the stirrer should be of a convenient length designed to pass loosely through a hole in the cork holding the thermometer. Stirring may be hand operated or mechanically activated at 20 to 30 strokes/min. 

Addition of free water in the reaction mixture may also affect positively. However, the addition of free water cannot always be appropriate, since without sufficient chemical interaction with the substrate oxide it would not work or even can be harmful reducing the main effect of mechanical loading. Chemical interaction between solid surfaces and water under mechanical activation significantly differs from static conditions. During comminution, solids are subjected to dynamic loading that results in the extension and compression of chemical bonds. This process is believed to be similar to corrosion under pressure [29]. 

The temperature dependence of corrosion rate is given by the temperature dependence of all the parameters mentioned above and participating in the corrosion process. The main roles are played by the temperature dependence of the diffusion coefficient and that of viscosity which determines the convection rate. Solubility and the other characteristics are of lesser significance. As the parameters involved do not have the same temperature coefficienis, the activation energy evaluated directly from the corrosion kinetics is not reliable for interpretation of the corrosion mechanism. 

Unlike chemical reactions, tribochemical reactions are triggered by frictional force, resulting in wear. Tribochemical wear can be seen as one type of corrosive wear. When the corrosion is activated by mechanical interactions between the contacting surfaces, it produces activated surface sites and localized high temperatures sufficient for chemical reaction. Tribochemical wear involves surface charging of electrons, surface passivation, and surface film removal processes. 

In our concluding remarks we can emphasize that depending on the nature of interactions between the components that constitute the medium and the solid, as well as on a combination of external conditions, one may observe the effects of various types and intensity. These include the facilitation of plastic flow of solids, or, alternatively, brittle fracture due to the action of lowered stresses mechanochemical phenomena in the zone of contact mechanically activated corrosion (the stress corrosion) the processes that are close to the spontaneous dispersion (the so-called quasi-spontaneous dispersion), and the true spontaneous dispersion, leading to the formation of thermodynamically stable lyophilic system. A great variety of types of interactions that exist between the stressed solids and the medium in contact with it requires careful and thorough examination of conditions under which... 

EROSION, LIQUID - Removal of films or metal by mechanical action and corrosion of active metal. 

The LSP mechanism proposes that SCC results from the effect of the structure ahead of the crack tip [61]. This mechanism assumes that a galvanic corrosion between active sites (weakened passive site) and surroimding passive surfaces produces large anodic currents at the rupture site. Repassivation of the active sites is prevented by the presence of weakened passive films on the surface. It has been su ested that the weakened passive film... 

Electrochemical methods, in contrast to chemical methods, provide mainly qualitative data on corrosion systems. They are very suitable for studying corrosion mechanisms and the influence of parameters as well as for investigating the value and effectiveness of active corrosion protection... 

The studies carried out showed that, without the additive, processes of corrosion-mechanical wearing of all the metals contacting with the lubricant actively occur during the tests according to the PPA-2 method. In the pjresence of the additive P-1 these processes almost stopped after the fourth hour of op>eration. Probably, the additive formed chemiadsorption compounds on the metal surfaces which protect them from the interaction with the corrosion aggressive products. As a result, the concentration of metals in the exhausted lubricant sharply decreased and remained almost the same after the induction pjeiiod. 

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