Other Factors Affecting Corrosion

As has been noted, temperature can have a significant influence on the corrosion process. This is not surprising because it is an electrochemical reaction, and reaction rates increase with increasing temperature. There are additional influences on corrosion other than the corrodent itself. 

The surface finish of the component also has an impact on the mode and severity of the corrosion that can occur. Rough surfaces or tight crevices can facilitate the formation of concentration cells. Surface cleanliness can also be an issue with deposits or films acting as initiation sites. Biological growths can behave as deposits or change the underlying surface chemistry to promote corrosion. 

Effects of environment and alloy content on anodic polarization behavior. 

Increased corrodent velocity can shift the cathodic polarization curve such that passive behavior can be induced. 

Other variations within the metal surface on a microscopic level influence the corrosion process. Microstructural differences, such as secondary phases or grain orientation, will affect the way corrosion manifests itself. For corrosive environments where grain boimdaries are attacked, the grain size of the material plays a significant role in how rapidly the material s properties can deteriorate. Chemistry variations in the matrix of weld deposits are also factors. 

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Apart from the effects of alloying and impurities, there are some other factors affecting corrosion, for example, changes in microstructure by thermal or mechanical treatments and ensuing changes of the surface condition. 

Under dissolution-controlled conditions, corrosion by liquid metals should increase with increasing temp)erature. For example, assuming all other factors affecting corrosion are fixed, the corrosion rate-temperature relationship can be expected to follow the classical Arrhenius expression, log k exp(-0/RT), where k is corrosion rate, Q is the activation energy, R is the gas constant, and T is the absolute temperature. This is shown graphically in Fig. 4 for type 316 stainless steel in sodium. In this case, the corrosion rate can be related directly to mass loss, which can be expressed m terms of wall thinning. 

Other factors affecting corrosion include temperature, relative velocities between the metal and the media, surface finish, grain orientation, stresses, and time. 

Other Factors Affecting Corrosion Inhibitor Requirements... 

Other factors affecting corrosion include bacteria, scaling, mechanical or chemical treatment of lines prior to commissioning, such as treatment of the pipe during storage, and completion methods and... 

In addition to the chemical requirements, a number of other factors affect the susceptibility of the system to on-load corrosion, particularly operation and design. 

The chosen meaningful parameters are the following reaction heat, flammability, explosiveness, toxicity, corrosiveness, chemical interaction, inventory, process temperature and pressure, equipment safety and safe process structure (see Table 5). This does not mean that other factors affecting the inherent safety of a process are meaningless. On the contrary they should be considered more detailed in further design stages. 

Reference point for the impact of corrosion on other factors affecting the economy. 

Some of the other factors affecting galvanic corrosion are area ratios, distance between electrically connected materials, and geometric shapes. Galvanic corrosion of the anodic metal takes the form of general or localized corrosion, depending on the configuration of the couple, the nature of the protective films formed and the nature of the metals. 

There are in addition several other factors that accelerate corrosion and must betaken into account these include crevices, galvanic coupling, tensile stress, aeration, presence of impurities, surface finish, etc. If these were also taken into consideration then several million experiments would have to be performed to compile such data. There are many instances where two or more chemicals exert a marked synergistic action such that low dissolution rates obtained in either environment become much greater in the presence of both. Further, the corrosiveness of a chemical will be affected by the presence of certain impurities, which may act as either accelerators or inhibitors. To take all these factors into account would add to an already impossible task and as Evans has remarked, There are not enough trained investigators in the world to obtain the empirical information to cover all combinations of conditions likely to arise . Unfortunately corrosion science has not yet reached the stage where prediction, based on a few well established laws, allows selection of materials to be made without recourse to a vast amount of data. 

The corrosion rate will be dependent on the temperature and concentration of the corrosive fluid. An increase in temperature usually results in an increased rate of corrosion though not always. The rate will depend on other factors that are affected by temperature, such as oxygen solubility. 

Active microbiologically influenced corrosion (MIC) of metal due to bacteria is quite complex. It can involve several species of microorganisms and is affected by temperature, TOC, pH, and other factors. Examples of bacterial species which are associated with MIC are described below ... 

Some other factors, including the metallurgical condition of the material (such as composition and heat treatment) and the loading mode (such as uniaxial), affect corrosion fatigue crack propagation. (Glaeser and Wright)5... 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

With respect to corrosion, the conventional classification of climates in marine, inland, industrial, etc. types is not sufficient. It should now be specified with respect to the actual chemical components in the atmosphere, as well as humidity and other factors. Recent research in this field has led to much more precise methods for estimating corrosion rates in polluted atmospheres (38). Economically, perhaps even more important problems are caused by the increased corrosion of water supply pipelines. Not only copper is dissolved, but also cadmium from soldered joints, and larger steel and cement pipelines may also be affected. 

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