Factors Affecting Pitting

The tank failed by pitting corrosion from the inside. Notwithstanding the general corrosion over parts of the tank and the pitting that was observed, the compositions of both the steel and the oil were within specification. The corrosion observed could have been caused by water, but the source of such water and the method by which it entered the tank could not be established. 

The premature failure of the tank was most likely caused by water that entered the tank, either before or during service. Regardless of the source of water, the quantity and composition of water in the tank were sufficient for the corrosion to penetrate the tank in 2yr. 

As indicated by EDS analysis, the oxide scale consisted mainly of oxide with some areas containing chlorides and/or sulfides. These sulfides and chlorides were not spread 

Therefore, it is assumed that there was a small amount of water in the tank as a result of condensation, the water would react with the steel until the water was completely consumed by the reaction. When the water was consumed, the reaction stopped. When more water was available, then the reaction would start again. The rate-determining step was the water supply to the steel. 

High levels of sulfur and chlorine were found in the partially corroded and severely corroded samples. 

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Factors affecting pitting corrosion in austenitic chromium-nickel-molybdenum steels are listed in Table 20.2. 

Horvath, J. and Uhlig, H., Metallurgical Factors Affecting the Critical Potential for Pitting Corrosion of Cr-Fe-Ni Alloys , J. Electrochem. Soc., 114, 201c (1%7)... 

It was a century ago that researchers started to study the factors affecting the behaviour of water-oil-surfactant systems but it is only with the introduction of Winsor s R theory (1954) that the formulation effects could be interpreted. Winsor s R theory was the first qualitative description of the formulation, paving the way to an understanding of how intermolecular interactions among the different chemical species present in a system are related to its behaviour. Throughout the following decades, several empirical experimental correlations such as the phase inversion temperature (PIT), semiempirical ones such as the cohesive energy ratio (CER), and models based on thermodynamics such as the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [15, 143, 144] led... 

H. Bohni and H.H. Uhlig, Environmental Factors Affecting the Critical Pitting Potential of Aluminum, J. Electrochem. Soc., Vol 116, 1969, p 906-910... 

Comstock, G.L. and Cote, W.E., 1968. Factors affecting permeability and pit aspiration in coniferous sapwood. Wood Sci. Technol, 2 279-291. 

Factors Affecting THE Breakdown Potential (Pitting Potential)... 

How speed in gas transmission lines is a significant factor affecting corrosion. At low flow speeds (nsnally 3 mis or less), liquid drop-out, particularly water, can lead to a corrosive sitnation. Field contours can be critical - valleys followed by steep uphill sections of line lead to hqnid accnmnlation. The area of gas breakout (splash zone) is particularly vulnerable to pitting corrosion, leading to early line features. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

More details of other factors that affect the critical pitting potential have been discussed by Uhlig and his co-workers" . They indicated that for stainless steel the critical pitting potential decreased with increasing concentration of chloride ion. At a fixed chloride level, passivating ions in solution, such as sulphate and nitrate, etc., cause the pitting potential to become more positive at a sufficient concentration these ions totally inhibited pitting, as shown in Fig. 19.40 for SO and CIO . 

The surface of the silicon crystal, no matter how it is finished, will have a certain number of lattice defects, which tend to dissolve preferentially resulting in formation of etch pits and other features. Terraces and steps of various sizes are inevitable consequences of anisotropic dissolution of the surfaces misoriented from the (111) surface. Also, a silicon surface, whether initially smooth or not, in HF solutions, has an intrinsic tendency to roughen and form micropores governed by sensitivity of the electrochemical reactions on a semiconductor electrode to surface curvature. Furthermore, the two groups of factors shown in Fig. 7.57 may affect each other. For example, the initial lattice inhomogeneities may provide the sites for deposition whereas localized deposition may enhance the development of etch features such as pits or hillocks. 

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