Localized corrosion inclusion sites

Localized corrosion of passivating metals initiates at local heterogeneities, such as inclusions and second-phase precipitates as well as grain boundaries, dislocations, flaws, or sites of mechanical damage. In the case of stainless steel surfaces, pit initiation occurs at sites of MnS inclusions. Exclusion of inclusions and precipitates, nonequilibrium... 

The harmful effect of sulfur [16] as an alloy impurity has been mainly attributed to the presence of reactive sulfide inclusions (usually MnS), which constitute very efficient sites for initiation of localized corrosion (see later). Low sulfur or Ti alloying improves the alloy resistance to crevice corrosion [16]. 

In accelerated formulations, this reaction is mediated by ferricyanide, and accelerated by fluoride as is discussed below. First stage coating growth results in round nanometer-sized nodules that nucleate quickly cover the surface [101]. Preferential nucleation and growth on local cathodic sites defined by local impurity element enrichment, and second-phase particle inclusions has also been reported [102, 103]. Coatings formed in just several seconds do confer useful levels of corrosion resistance, but are not as corrosion resistant as coatings formed by longer immersion times. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

For pit nucleation, defects within the metal surface also have to be considered. Inclusions like MnS may prevent the formation of a continuous, protecting passive film locally. They are preferential sites for a breakdown of passivity. Even in this situation, the special chemical properties of aggressive anions to start corrosion pits and to cause their continuous growth have to be included in the proposed mechanisms. These properties should be explained along with the ability of halides to form stable and fast-dissolving complexes of the cations of the metals under study with the related breakdown of their passive layers. 

In the first part of this chapter, experimental results illustrate how local probe techniques (SVET, microcapillary cell, pH probe) can be applied to quantify the corrosion on local electrochemical sites such precipitates, inclusions, intermetallics or defects on coated materials where galvanic coupling is the driving force. 

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