Stable pit growth

Two main points should be remembered from this lab. First, the breakdown potential is not necessarily the best measurement of pitting resistance. This is because pitting can occur at potentials below EM, as was demonstrated by metastable pitting in test 4. Ebi corresponds to the potential for stable pit growth and propagation only. Pits can nucleate, however, at any potential above the repassivation potential. Secondly, the effects that additional anions have on the pitting behavior is concentration dependent and not mass dependent. 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

In the potential range anodic to stable pit growth occm. The value of is shifted to lower anodic potentials... 

From an electrochemical viewpoint, stable pit growth is maintained as long as the local environment within the pit keeps the pit under active conditions. Thus, the effective potential at the pit base must be less anodic than the passivation potential (f ) of the metal in the pit electrolyte. This may require the presence of voltage-drop IR-drop) elements. In this respect the most important factor appears to be the formation of a salt film at the pit base. (The salt film forms because the solubility limit of e.g. FeCl2 is exceeded in the vicinity of the dissolving surface in the hishlv Cr-concentrated electrolyte.)... 

In the potential range cathodie to one frequently observes so-ealled metastable pitting. A number of pit growth events are initiated, but the pits immediately repassivate (an oxide film is formed in the pit) beeause the eonditions within the pit are such that no stable pit growth can be maintained. This results in a polarization cmwe with strong current oscillations aiU <... 

An attempt has been made to explain the requirement for a minimum concentration of aggressive anions to maintain stable pit growth [29]. With the assumption that a salt film of thickness S has to be maintained at the growing pit surface, one obtains... 

The stabilization of the growth of corrosion pits may be influenced by several complicating factors depending on the state of their development. However, the presence of halides within the electrolyte is a necessary condition. Pit growth will stop when halide-containing electrolytes are replaced by halide-free solutions [70]. One needs a bulk concentration of at least 0.0003 M to cause stable pit growth [6,... 

Metastable pitting may be considered as a precursor of stable pit growth which occurs above the pitting potential. The charge associated with the formation and repassivation of metastable pits can be calculated by integrating the corresponding current spikes. This allows one to estimate the extent of pit growth before they... 

All these discussed factors for stable pit growth are effective for a later stage of pit growth. The influence of the chemistry of the so-called aggressive anions is missing in most models for the mechanism of pitting. In discussions, the question What makes the halides so aggressive is sometimes encountered. It is apparently the chemistry of... 

Stable pit growth processes are not presented in this chapter. The key point is that conditions are established in which repassi-... 

The details of the mechanisms also depend on the metal or the composition of the alloys as well as on the electrolyte and other environmental conditions. Metallic and norunetallic inclusions often play a decisive role in the start of a corrosion pit. In most cases the presence of aggressive anions is necessary for breakdown of passivity and stable pit growth. The discussion in this chapter explains the effect of these anions by their tendency to form complexes with metal ions. It concentrates on the behavior of some pure metals, such as pure Fe and Ni, in simple electrolyte solutions. Some basic concepts are the center of interest, although it is known fi om the technical applications of the difieient materials that the appearance and the... 

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