Corrosion mechanisms analysis

This book systematically summarizes the researches on electrochemistry of sulphide flotation in our group. The various electrochemical measurements, especially electrochemical corrosive method, electrochemical equilibrium calculations, surface analysis and semiconductor energy band theory, practically, molecular orbital theory, have been used in our studies and introduced in this book. The collectorless and collector-induced flotation behavior of sulphide minerals and the mechanism in various flotation systems have been discussed. The electrochemical corrosive mechanism, mechano-electrochemical behavior and the molecular orbital approach of flotation of sulphide minerals will provide much new information to the researchers in this area. The example of electrochemical flotation separation of sulphide ores listed in this book will demonstrate the good future of flotation electrochemistry of sulphide minerals in industrial applications. 

There is a special importance in the mechanism of 02 reduction on iron because of its relevance as the counter-cathodic reaction in corrosion mechanisms that involve Fe more often than other metals. Many of the practical costs of Fe corrosion occur in neutral solution, so that the pH range in the study described here (Jovancicevic, 1986) is between 6 and 9. The experimental methods involved the use of ring-disk analysis (see Section 7A. 14) to detect H202, an obvious possible intermediate in the measurement of the log /—potential relation (Fig. 7.101) to give Tafel constants and the reaction order with respect to 02 and pH. 

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. ... 

Determination of residual stress of a failed component is one of the most important steps in failure analysis. The determination of residual stress is useful when failed components experience stress concentration, overload, distortion or the formation of cracks in the absence of applied loads, subjected to corrosive environments as in stress corrosion, mechanical or thermal fatigue due to cyclic loading, or when faults in processing such as shot peening, grinding, milling and improper heat treatment such as stress relief, induction hardening, thermal strains, exposure temperature are involved. 

The present work confirmed the layer by layer assembly of ZMP nanocontainers and the release mechanism of corrosion inhibitor. FTIR and TEM study confirms the successful formation of ZMP nanocontainer as a layer by layer system with the aid of ultrasonic irradiation. Zeta potential and particle size analysis also shows the formation of layers and shows appropriate change in the surface charge, which could be responsible for the release mechanism initiated by the change in pH. Release study and corrosion results from Tafel plot and corrosion rate analysis showed significant improvement in the anticorrosion properties of coatings due to the optimum loading of the ZMP nanocontainers. 

Analysis of corrosion mechanisms from electrochemical noise (ECN)... 

Although not a true surface technique, SEM-EDS often provides useful information in regard to surface corrosion mechanism. The ubiquitous nature, low cost, and ease of use of this technique cause it to be used as a tool in many failure analyses involving corrosion. Because its analysis depth is much larger (approximately a micrometer) than the true surface techniques, it is not necesseiry to analyze samples that are high-vacuum compatible. This results in the necessity of almost no sample preparation for many different kinds of samples. 

The most common test method is the coupon immersion test, usually conducted under static or near-static conditions. Other methods, used to a lesser degree, include rotating electrodes, of which the best arrangement for water is probably the rotating cylinder, and small recirculating systems. Electrochemical tests and polarization studies are used primarily to elucidate the corrosion mechanism. Surface analysis is also used, primarily for mechanistic studies. 

The importance of surface and chemical analysis techniques in electronics corrosion testing cannot be overstated. These powerful tools contribute to solving problems and elucidating corrosion mechanisms in simple and complex systems. Chemical analysis techniques include infrared (IR), ultraviolet (UV), and RAMAN spectroscopy X-ray diffraction atomic adsorption emission and mass spectroscopy gas and liquid chromatography and optical and transmission electron microscopy. Surface analytical techniques include electron spectroscopy for chemical analysis (ESCA), Auger, secondary ion mass spectroscopy (SIMS), and ion scattering spectroscopy (ISS). These important techniques used in conjunction with corrosion tests are described in another section of this manual. 

In this work, the role of NO2 in the atmospheric corrosion of zinc was analyzed from a detailed characterization of corrosion products. Laboratory tests with exposru e parameters close to the conditions observed in real atmospheres were performed, with the aim of simulating close-to-reality corrosion mechanisms. Low-pollutant concentrations and shortterm exposmes were carried out. For these reasons, XPS were used for the analysis of very thin corrosion layers formed. 

It should be noted that photoelectrochemical investigations [15b] suggest that the last two reactions, involving the formation and then dismutation of S2O33-and resulting in the formation of adsorbed sulfur, are operating in some cases. Photoelectrochemical microscopy shows the deposition around the inclusions of a ring of material deduced to be sulfur, which is consistent with the results of several other studies [6,10b]. A detailed analysis of the fundamental aspects of the sulfur-assisted corrosion mechanisms is presented in Chapter 9. 

After the "A and B alloys were removed from the autoclave, they were subjected to electrochemical impedance spectroscopy (EIS) measurements under ambient conditions in pH 10 solutions. It was anticipated that such measurements might help to support the corrosion mechanisms hypothesized from the surface analysis of alloys exposed to actual boiler conditions. Analy.sis of the ambient EIS data gave film resistances of 1.2 x 10 and 1.1 x 10 Q-cm 2 for the A and B alloys, respectively, at their corrosion potential. Both are very high resistances, which suggest that a passive film can form on either alloy under these electrochemical conditions. However, in the light of their significantly different behavior under simulated boiler conditions, it can be concluded that these particular EIS tests may not be pertinent to an understanding of the corrosion problem at hand. 

Cabinet tests. Cabinet testing refers to tests conducted in closed cabinets where the conditions of exposure are controlled and mostly designed to accelerate specific corrosion situations while trying to emulate as closely as possible the corrosion mechanisms at play. Cabinet tests are generally used to determine the corrosion performance of materials intended for use in natural atmospheres. In order to correlate test results with service performance, it is necessary to establish acceleration factors and to verify that the corrosion mechanisms are indeed following the same paths. Modem surface analysis techniques can be quite useful to ascertain that the corrosion products have the same morphologies and crystallographic stmctures as those typically found on equipment used in service. There are basically three types of cabinet tests ... 

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