Corrosion rates influencing factors

Corrosion Theory 1259. Forms of Corrosion Attack 1268. Factors Influencing Corrosion Rate 1292. Corrodents in Drilling Fluids 1300. Corrosion Monitoring and Equipment Inspections 1312. Corrosion Control 1323. Recommended Practices 1340. 

The effect of temperature on corrosion rate is influenced by the following factors [188] ... 

From the temperature factor A, an activation energy may again be calculated which gives a useful indication of the influence of flow-rate on corrosion rate. 

It has long been recognized that local environmental characteristics influence the rates of material corrosion. After two years of measurements at 39 sites in Europe and North America, significant relationships have been shown between corrosion rates of building materials and atmospheric pollutants( 5). While direction of exposure relative to weather and other factors such as frequency and duration of wetting significantly influence corrosion, Kucera (46) has shown that sulphur oxides are strongly correlated with deterioration of structural materials. 

Corrosion rate is a function of time of wetness, considered as the time during which corrosion occurs, but in general it should not be a linear function because corrosion rate changes with time. There are different factors influencing, for example, the protective properties of the corrosion products, the increase or decrease of the acceleration caused by contaminants, increase or decrease of the thickness and conductivity of the electrolyte layer,... 

The environmental factors that influence the corrosion rate vary with the depth in seawater as detailed8 in Table 4.3. The variation of the factors in seawater at different global locations has been documented in the literature.9... 

Implantations of rare gases did produce slightly higher corrosion rates than unimplanted iron. It was noteworthy that the reproducibility of the measurements of implanted samples was worse than of unimplanted ones. Oxide layers did not play the important role in this study which they did in the work of Ashworth et al. because in H2SO4 solution immediate cathodic reduction of the oxide takes place. Au implantation enhanced the corrosion rate of iron by a factor of 10 lead implantation inhibited the corrosion rate by the same factor. Copper, finally, did not have much influence, a behavior that is similar to conventional iron-copper alloys with a copper content of less than 10 %. These findings proved that ion implantation can vary the corrosion resistance of iron not only in neutral but also in acid solutions. 

The geometric surface area is only a basic parameter for the corrosion rate expression in normalized unit g/m2 within normalized test duration. An actual surface area is mentioned [2], but not defined in standards [1, 2] therefore we use the geometric surface and try to evaluate influence of various factors that increase the real surface area. The corrosion rate depends on such surface parameters as surface roughness and its anisotropy, evenness of material composition on the surface, pretreatment of surface, storage of samples before the corrosion test, and so on. Inasmuch as these parameters have no proper units for quantitative estimation expression, it was impossible to evaluate their contribution to the combined uncertainty of the tested surface area. The nominal value of surface roughness in the standard method is indicated as i a=1.3 0.4 pm, but in our case 7 a=0.67 0.01 mm, with an anisotropy Rai=0.60 pm (along the plate) and i aj=0.74 pm (across the plate). 

The influence of mainly SO on the corrosion rate of several materials has been shown in numerous national exposure programs. During the last decades a number of empirical relations have been derived from measurements of atmospheric corrosion rates of the most important structural metals and from measurements of environmental factors. The results are usually presented in form of equations including pollution and meteorological parameters (5.). 

Even if there is a significant correlation between corrosion rates of copper and aluminum and SO pollution there are other factors than SO that influence significantly the corrosion rate of the two metais also at exposure within the temperate climate zone. 

It is important to study the potential distribution inside the crevice and its relation to the polarization curve in order to obtain a better understanding of the mechanism by which the crevice corrosion occurs. Factors that influence both the potential distribution and the polarization curve include the metal environment reaction products and the crevice geometry as well as the well-known promoters of crevice corrosion acidification and chloride ion build-up within the crevice. When excess oxygen entered the crevice via flushing air-saturated solution through it, the corrosion rate decreased which was consistent with the proposed need to keep the cathodic reaction outside the crevice, separate from the anodic reaction, for a stable crevice corrosion to occur [151]. 

It is emphasized that while following the above procedure to determine whether a metal/environment combination is susceptible to corrosion, no information is provided on the rate of corrosion, the physical nature of the attack (i.e., uniformity of attack), the influence of corrosion products, or factors relating to the environment, such as fluid velocity and uniformity of fluid composition. 

Simple but pedagogically useful theories of electrode kinetics are presented in Chapter 3. This permits discussion of models for anodic and cathodic reactions at the metal/environment interface and for diffusion of species to and from the interface. Mathematical models of these theories lead to so-called kinetic parameters whose values govern the rate of the interface reaction. The range of values that these parameters can have and some of the variables that can influence the values are emphasized since these will relate to understanding the influence of such factors as surface conditions (roughness, corrosion product films, etc.), corrosion inhibitors and accelerators, and fluid velocity on corrosion rates. This chapter also introduces electrochemical measurements to determine values of the kinetic parameters. 

Most metals tend to corrode in an environment of air and/or water, forming metal oxides or hydrated oxides. Whether or not such a reaction is possible is dictated by the thermodynamics of the corrosion reaction. If the reaction has a negative Gibbs free energy of formation, then the reaction is thermodynamically favoured. While thermodynamics determines whether a particular reaction can occur or not, the rate of the corrosion reaction is determined by kinetic factors. A number of variables can affect the corrosion rate, including temperature, pH and passivation, which is the formation of a thin protective film on a metal surface. Passivation can have a tremendous influence on the corrosion rate, often reducing it to a negligible amount. 

The corrosion rate of zinc in neutral salt solution varies with the type of anions. Factors influencing the solubility of zinc in the solution affect the corrosion rate. The solubility of zinc in the solution is increased by... 

McNeil and Odom [16] developed a thermodynamic model to predict metal susceptibility to MIC by 8RB. If the reaction to produce the sulfide from the oxide has a negative Gibbs free energy, the reaction will take place. If the value is positive, the metal is immune to derivation by sulfides and will not be vulnerable to corrosion by 8RB. The model is limited to thermodynamic predictions as to whether a reaction will take place and does not consider metal toxicity to the organisms, tenacity of the resulting sulfide or others factors that influence corrosion rate. The following is a summary of mineralogical products... 

The steel corrosion rate in concrete will be higher in the presence of chloride ions, which reduce the anodic polarization and enhance the conductivity of the paste [226]. Other factors are influencing also concrete resistivity, namely the low w/c ratio, low humidity and temperature decrease, all result in resistivity increase. Therefore, in dry concrete or immersed in water, its corrosion is negligible. 

Factors influencing (FAC) are Fluid chemistry (pH < 9.0, 02< 5 ppb, AVT (R)), Fluid temperature (70-3000 C), Flow turbirlence (Geometry), Metal composition (CS Low alloy steels). FAC is higher at around 130 - 150 Deg C, see Figme. 10, showing corrosion rate at varioits temperature pH. Single... 

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