Passive alloys, crevice corrosion passivity breakdown

On passivated alloys, and particularly on stainless alloys (Fe-Ni-Cr-Mo-... alloys), crevice corrosion occurs when the local conditions in the crevice cause passivity breakdown and the onset of active dissolution. As a consequence, cievice corrosion exhibits two different stages an incubation period during which the local environment in the cievice evolves toward critical conditions and... 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

In the case of the nickel alloys, the stability of the passive layer is a problem. The alloys depend on the oxide films or the passive layers for corrosion resistance and are susceptible to crevice corrosion. The conventional mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects such as the acidification or the migration of the aggressive ions into the crevice solution [146]. These solution composition changes can cause the breakdown of the passive film and promote the acceleration and the autocatalysis of the crevice corrosion. In some cases, the classic theory does not explain the crevice corrosion where no acidification or chloride ion build up occurs [147]. 

Ogawa, H., Denpo, K., and Miyasaka, A., Criteria for Passivity Breakdown of High Alloy Materials in Relation to Crevice Corrosion Nucleation, Corrosion Science, Vol. 31, No. 1, 1990, pp. 459-464. 

Under certain special environmental conditions, the passive films, which were described earlier in this Chapter, are susceptible to localized breakdown. Passivity breakdown may result in accelerated local dissolution (localized corrosion) of the metal or alloy. There are two (related) major forms of localized corrosion following passivity breakdown localized corrosion initiated on an open surface is called pitting corrosion, and localized corrosion initiated at an occluded site is called crevice corrosion. In the presence of mechanical stress, localized dissolution may promote the initiation of cracks. 

The most widespread cases of crevice corrosion of passivated alloys are caused by aerated (or more generally oxidizing) chloride solutions such as sea or brackish water. In these chloride solutions, the environment in the crevice becomes progressively more acidic and more concentrated in chloride anions and metal cations. There are several possible causes of passivity breakdown, including low pH, high chloride content, presence of metallic chloride complexes, and pitting inside the crevice gap. Passivity breakdown occurs only if the corrosion potential of the free surface exceeds a critical value, but the relationship between the potential of the external surfaces and the evolution of the environment in the crevice is not completely understood. 

Metals Affected. Resistance to crevice corrosion can vary from one alloy-environment system to another. Although crevice corrosion affects both active and passive metals, the attack is often more severe for passive alloys, particularly those in the stainless steel group. Breakdown of the passive film within a restricted geometry leads to rapid metal loss and penetration of the metal in that area. 

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