Physicochemical mechanism, corrosion

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

The electrochemical, physicochemical and adhesional aspects of corrosion protection by organic coatings are shortly discussed. Attention is drawn to some inoonsist-ancies in the interpretation of protective mechanisms and suggestions are given how protective principles may be optimally realized in practical systems. 

Delay (train)—A pyrotechnic item, mostly of relatively slow reaction time (0.1 to 30 sec/in.), suitable for timing sequences of events. Delays may also be mechanical (clocks), electronic, physicochemical (actuation by corrosion, or by softening of a plastic), or physical (release of compressed air). 

Every kind of interaction between a solid phase and other solid, liquid, or gaseous phases naturally begins at surfaces (external or internal). The mechanism of all type of physicochemical processes at such boundaries will be influenced by the properties of the more or less unsaturated surface particles, their dipole moments, their migration capacity, and the conditions of electronic exchange between the phases in question. This also holds true for the rather badly defined group of processes which are broadly described as corrosion (Figs. 1-3). 

Corrosion is the destractive and unwanted attack of a corrosive environment on metals and alloys, and decreases the lifetime of materials used in aircraft and spacecraft manufacturing. As a result of the degradation of metal surfaces, materials can lose their mechanical, other physical, chemical and physicochemical properties and appearance. " Metal surface corrosion accounts for approximately 4% of an industrialized nation s gross national product (GNP) spent on its prevention, replacement of corroded parts, maintenance and environmental protection. This translates to nearly 300 bilhon direct and indirect costs to the US economy per year at 2003 prices. ... 

Other online techniques can be used to examine the change in electrode or electrolyte materials. If parts of the electrode or electrolyte are lost due to reaction, finite vapor pressure, or other reasons, analysis of the effluent product can often be correlated to the particular loss mechanism. For example, in PEFCs, one mode of physicochemical electrolyte degradation is accompanied by loss of the fluorine ion, which can be detected by measurement of the effluent condensed water fluorine content. Also, if carbon corrosion is occurring, carbon monoxide or carbon dioxide gas is produced, and this can be measured with a sufficiently sensitive device. In general, if there is a chemical reaction causing the degradation, the product species from this can be detected in the effluent and correlated with the measured loss. 

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