Local corrosion described

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

Stress corrosion cracking (SCC) or embrittlement corrosion describes any of a number of corrosion processes where in a corrosive environment, localized stress accelerates the rate of corrosion that may occur under or within a deposit. 

More often the passive layer is broken down locally and then the steel is said to be attacked by localized corrosion, the most important forms being pitting, crevice corrosion, and corrosion cracking. Most often the localized corrosion is caused by halogen ions such as chloride, bromide, and iodide. Pitting or pitting corrosion is seen as small pinholes on the surface of the steel. This section describes electrochemical instrumental methods to investigate and measure this form of corrosion attack. 

Corrosion described as pitting is usually localized and can be in the form of small, deep pits as well as large, shallow pits. It can occur in areas of stagnation where an anode site can develop. 

A general scheme for the development of corrosion models based on electrochemical principles has been described, and a number of examples for active, passive, and localized corrosion has been given. This chapter is by no means comprehensive, and a search of the scientific and technical literature will unearth many additional examples. The value in using electrochemical methods both to develop understanding of the corrosion process and to measure the values of specific modeling parameters is obvious. However, their application alone would not provide all the elements and parameter values required for the development of corrosion models, so the use of supplementary techniques is necessary. It is necessary also to keep in mind that electrochemical techniques inevitably accelerate the corrosion process one is interested in. Consequently, the scaling of electrochemi-cally determined parameter values to the rates and time periods of interest in the corrosion process to be modeled should be undertaken carefully and with a full knowledge of the limitations involved. 

The inner surface of hydrogen gas vessels is susceptible to localized corrosion due to impurities that can exist in the steel and hydrogen gas." Interactions between localized corrosion and hydrogen embrittlement have not been specified however, impurities in the gas and steel are known to affect hydrogen embrittlement, as described in section 7.4. 

In developing the arguments that are presented later in this review, it is necessary to keep in mind the relative scales (dimensions) at which each phase occurs. This is important because the effect of flow on localized corrosion is largely (though not totally) a question of the relative dimensions of the nucleus and the velocity profile in the fluid close to the surface. However, the velocity profile is a sensitive function of the kinematic viscosity, which in turn depends on the density and the dynamic viscosity. Because the kinematic viscosity of water drops by a factor of more than 100 on increasing the temperature from 25 °C to 300 °C, the conclusions drawn from ambient temperature studies of the effect of flow on localized corrosion must be used with great care when describing flow effects at elevated temperatures. 

The term uniform corrosion describes the more or less uniform wastage of material by corrosion, with no pitting or other forms of local attack. If the corrosion of a material can be considered to be uniform, the life of the material in service can be predicted from experimentally determined corrosion rates. 

The model described here has been developed from a metallurgical standpoint, rather than an electrochemical one and originated as a cellular automata (CA) finite difference model. This approach dealt with the evolution of a representative concentration and electrical potential throughout the electrolyte only [1], The model was able to predict morphological features such as localized corrosion pits and capping but was limited to qualitative simulation. However the CA method has found alternative applications in the growth of corrosion pits... 

Etching of iron by the feedback mode of the SECM has recently been reported by Still and Wipf (26). They produced localized corrosion at passivating iron surfaces by generating chloride ions at an SECM tip. Here is a case in which the metal is covered with an oxide layer that needs to be removed in order to facilitate metal dissolution. These experiments are described in great detail in Chapter 12. 

For the determination of weight loss, the corrosion products have to be removed. Inhibited acids are normally used for this purpose, but it is important to be sure that such acids remove only the corrosion products and do not attack the base metal. For example, the following acids could be used inhibited hydrochloric acid for iron, sulfuric acid for copper, and chromium trioxide solutions for zinc. The chemical cleaning procedures for the removal of corrosion products are further described in ISO 8407 (3). The removal of corrosion products is also necessary to confirm homogenous corrosion and exclude possible local corrosion attack. 

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