Localized corrosion definition

Chapter 1 includes definitions of different corrosion mechanisms that are classified as general corrosion and localized corrosion. A full description and detailed scientific approach of each corrosion mechanism under the above classification is not included since books on this topic are available in the literature. 

A general definition of the term acute toxicity is The adverse effects occurring within a given time, following a single exposure to a substance. The term usually excludes local irritant or corrosive effects arising from a single application of a substance to the skin or eye (Section 4.5) (EC 2003). 

It must be stressed that the primary mechanism of many topical irritants (e.g., organic solvents, corrosives) is the impairment to the stratum corneum barrier properties discussed earlier. If the stratum corneum barrier is perturbed, a feedback response may be initiated whereby regeneration of the barrier occurs. This reaction is mediated by cytokines (especially TNF-a) originating locally within the epidermis. However, additional responses to these inflammatory mediators may in themselves launch an irritation response mediated by the keratinocytes. Thus, regardless of the initiating mechanism, the sequelae to many irritants is the same, making the definition of unique dermal computational toxicology models difficult. 

Chemical any element, eompound or mixture of elements and/or compounds. A substance that a) possesses potentially hazardous properties (including, but not limited to flammability, toxicity, corrosivity, reactivity) or b) is included on any federal, state, or local agency regulatory list or c) is assoeiated with an MSDS and is not an Article as defined in 29CFR1910.1200. For the purposes of this document this definition also applies to chemical product (see definition). 

As the p-Cl-BHA was the most effective, it was subjected to a thorough study. The results show that the p-Cl-BHA definitely reduced the copper dissolution rate in 0.5 M NaCl solution (Fig. 9-15). The mass of the electrode increased due to the adsorption of the inhibitor and formation of an inhibitor-corrosion-products complex on the electrode surface. This protective film accounts for the inhibition efficiency of p-Cl-BHA against localized attack on the copper electrode in NaCl (Shaban et al., 1998 a, b). 

One of the key factors in any corrosion situation is the environment. The definition and characteristics of this variable can be quite complex. One can use thermodynamics, e.g., Pourbaix or -pH diagrams, to evaluate the theoretical activity of a given metal or alloy provided the chemical makeup of the environment is known. But for practical situations, it is important to realize that the environment is a variable that can change with time and conditions. It is also important to realize that the environment that actually affects a metal corresponds to the microenvironmental conditions that this metal really sees, i.e., the local environment at the surface of the metal. It is indeed the reactivity of this local environment that will determine the real corrosion damage. Thus, an experiment that investigates only the nominal environmental condition without consideration of local effects such as flow, pH cells, deposits, and galvanic effects is useless for lifetime prediction. 

Pitting arises when localized/aggressive environments break down the nominally passive and corrosion-resistant film. Pits may form at scratches, mechanical defects, second phase particles, or stochastic local discontinuities in the oxide film. Since by definition pitting refers to local loss passivity, pitting only occurs in the near-neutral pH range for Al. 

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