Mechanical factors/corrosion

Type of attack Environmental Cause of attack Mechanical factors Corrosion product... 

Closed sections and entrapment areas Mechanical factors Corrosion awareness... 

Although the mechanism of corrosion is highly complex the actual control of the majority of corrosion reactions can be effected by the application of relatively simple concepts. Indeed, the Committee on Corrosion and Protection concluded that better dissemination of existing knowledge was the most important single factor that would be instrumental in decreasing the enormous cost of corrosion in the U.K. 

Localised attack or fracture due to the synergistic action of a mechanical factor and corrosion... 

The essential features of the electrochemical mechanism of corrosion were outlined at the beginning of the section, and it is now necessary to consider the factors that control the rate of corrosion of a single metal in more detail. However, before doing so it is helpful to examine the charge transfer processes that occur at the two separable electrodes of a well-defined electrochemical cell in order to show that since the two half reactions constituting the overall reaction are interdependent, their rates and extents will be equal. 

Mechanical effects Corrosion can often be initiated or intensified by the conjoint action of mechanical factors. Typical examples include the presence of inherent or applied stresses, fatigue, fretting or cavitation effects. Inhibitors that are effective in the absence of some or all of these phenomena may not be so in their presence. In fact it may not always be possible to use inhibitors successfully in these situations and other methods of corrosion prevention will be required. 

The potential of the electrified interface of a metal immersed in an aqueous solution is of fundamental importance in studying the mechanism of corrosion reactions and in corrosion testing and monitoring, and it is, therefore, of some importance to consider the factors that determine the potential of a metal in a practical environment. The determination of the potential can be achieved without difficulty, but the significance of the potential is far more complex and some of the factors that affect the potential are as follows ... 

At present the iron-based alloys diffusion saturation by nitrogen is widely used in industry for the increase of strength, hardness, corrosion resistance of metal production. Inexhaustible and unrealized potentialities of nitriding are opened when applying it in combination with cold working [1-3], It is connected with one of important factors, which affects diffusion processes and phase formation and determines surface layer structure, mechanical and corrosion properties, like crystal defects and stresses [4, 5], The topical question in this direction is clarification of mechanisms of interstitial atoms diffusion and phase formation in cold worked iron and iron-based alloys under nitriding. 

Six forms of corrosion can be identified based on the apparent morphology of corrosion, the basic factor influencing the mechanism of corrosion in every form. The six forms are given in Table 6.1. 

Modeling fretting corrosion. An equation has been used for steel to evaluate the loss of weight W caused by fretting corrosion based on a model that combines the chemical and mechanical effect of the corrosion by fretting. The chemical factor concerns the oxidation that occurs at the time of wear, corresponding to adsorption of oxygen to form the oxide. The mechanical factor concerns the loss of particles, at the asperities on the opposite surface. 

The objective of investigation is to clarify the mechanisms of corrosion-induced pitting and cracking of the aluminum compressed air cylinder for the purpose of determining key factors. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Commercial HTS catalysts are mechanically quite strong. However, in an industrial process environment, the HTS catalyst can suffer from mechanical factors that deteriorate its performance, such as steam condensation leading to a gradual disintegration of the catalyst pellets, and deposition of foreign components (e.g. particulate matter from corrosion of the process equipment). Increase in the pressure drop across the catalyst bed resulting from these factors is yet another factor determining the catalyst lifetime. 

Mechanochemical wear is an intricate process in which corrosive damage and mechanical wear are interrelated [1,2]. The role of corrosive and mechanical factors in corrosion-induced wear of metals in electrodes is estimated in different ways. Some authors consider this process as purely corrosive, others... 

The study of SCC is typically multidisciplinary, i.e. the description of the three groups of factors and their effects belong to three different technological disciplines, namely electrochemistry/corrosion, physical metallurgy and fracture mechanics. The relative importance of electrochemical, metallurgical and mechanical factors varies strongly from one material-environment system to another. 

The thickness of the liner is a factor affecting permeation. For general corrosion resistance, thicknesses of 0.010-0.020 in. are usually satisfactory, depending on the combination of lining material and the specific corrodent. When mechanical factors such as thinning to cold flow, mechanical abuse, and permeation rates are a consideration, thicker linings may be required. 

When corrosion is caused by mechanical factors, say, vibration, all the noticeable points at which vibration would affect the facility have to be supported by the use of dampers in order for the facility not to have scratches, which would be the major source of corrosion. On joints and at points where pressure surges would occur, the use of expansion bellows should be encouraged in order to allow for high enough pressure that could burst a pipeline, and also to reduce the effects of the pressure that would have in turn caused much of the vibration. 

There are some corrosion cases where other factors, i.e. stress, or other mechanical factors are involved. For example, stress corrosion cracking, corrosion fatigue, hydrogen embrittlement, erosion corrosion, cavitation corrosion, fretting corrosion, etc. can be mentioned. 

This paper focuses on how to model the deterioration of static pressurized process equipment to enable efficient inspection and maintenance planning. Such equipment tends to gradually deteriorate over time from erosion, corrosion, fatigue and other mechanisms, and at some point of time inspection, repair or replacement is expedient with respect to safety, production and costs. The deterioration of the equipment is influenced by many factors such as type of equipment, system design, operation and process service, material and environment. For hydrocarbon systems, the most critical deterioration mechanisms are corrosion due to CO2 and H2S, microbially influenced corrosion, sand erosion and external corrosion (DNV 2002). In general, CO2 is the most common factor causing corrosion in oil and gas system of low alloy steel (Singh et al. 2007). 

Several deterministic corrosion models have been developed, some purely theoretical while others incorporate empirical results. For a review, see for example Nyborg (2002) or Nesic (2007). Many researchers have studied the mechanisms of corrosion on carbon steel in hydrocarbon environments. The purpose of the experiments has mainly been to study the effects of different influencing factors on the corrosion rate. With a theoretical background and mathematics based on physical understanding of the corrosion principles, these models often perform well under controlled experiments. However, their vahdation is limited outside the laboratory. This limited validation for field applications is mainly due to two reasons ... 

Tomashov has produced a detailed scheme of control based on the electrochemical mechanism of corrosion, which has been set out in an abridged and modified form in Table 0.2. However, although more fundamental than Table 0.1, it has several limitations, since it is not always possible to define the precise controlling factor, and frequently more than one will be involved. Thus removal of dissolved oxygen (partial or complete) from an aqueous solution reduces the thermodynamics of the reaction and also increases the polarisation of the cathodic reaction, and both contribute to the decrease in the corrosion rate although the latter is usually the more significant. 

In view of the electrochemical mechanisms of corrosion, the tendency for a metal to corrode can also be expressed in terms of the electromotive force (emf) of the corrosion cells that are an integral part of the corrosion process. Since electrical energy is expressed as the product of volts by coulombs Ovules, J), the relation between AG in joules and emf in volts, E, is defined by AG = -nFE, where n is the number of electrons (or chemical equivalents) taking part in the reaction, and F is the Faraday (96,500C/eq). The term AG can be converted from calories to joules by using the factor 1 cal = 4.184 absolute joules. 

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