Passive alloys, crevice corrosion potential

Pitting corrosion is an insidious type of localized corrosion in which cavities appear on a smooth passivated surface. Crevice corrosion is another type of localized attack in which the attack is on metal surface that is not exposed to the bulk solution but is immediately below the border, under a creviced or occluded region (such as under a gasket). From the environmental point of view, both types of attack depend on the concentration of halides (especially chloride), the electrochemical potential, and the temperature. Table 2-7 shows the rate of corrosion of several alloys according to the standard ASTM G 28 tests and for the green-death and yellow-death solutions (see Section 2.1.1). 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibiUty for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibiUty depends on pH. The susceptibiUty temperature increases paraboHcaHy from 65°C as pH is increased from 2ero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equiUbrium potential into the passive region where a protective film is formed and maintained. 

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... 

ASTM F 746 Standard Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials Measurement of pitting or crevice tendency by measuring repassivation tendency after polarization at noble potential. Applicable only to passive alloys. 

The free corrosion potentials of these molybdenum-free CrNi steels in natural and aerated seawater are within the range Uh = 0.2 V to 0.4 V. These steels are sufficiently passive to avoid serious general corrosion, but they must be integrated in the structural corrosion protection measures due to their sensitivity to pitting and crevice corrosion. This is often realised automatically since the steels have electrically conductive connections to low-alloyed steels or other less noble materials. 

Conversely, there is some agreement that a critical value (usually referred to as protection potential) exists that is the maximum possible value of below which no crevice corrosion may propagate significantly. This is consistent with the fact that crevice corrosion of passive alloys in chloride environments does not occur in deaerated environments. 

For a given crevice geometry, the critical potentials for crevice initiation and repassivation decrease with increasing chloride content (Fig. 10) and increasing temperature (Fig. 11) of the bulk solution. This means that the susceptibility to crevice corrosion of passivated alloys increases with the chloride content and the temperature. For example, titanium alloys become sensitive to crevice corrosion only in hot concentrated chloride solutions around 100/150°C [9,10]. Propagation rates also increase with temperature. 

Figure 21 (a) Evolution of the anodic characteristic of passivated alloy when decreasing the pH and increasing the chloride content, (b) Activation inside a crevice when the corrosion potential is located in the activity peak due to ohmic drop in the crevice. 

On very resistant stainless alloys, the criterion of depassivation pH tends to be replaced by a criterion of critical crevice solution (CCS) [19,57] and Gartland assumes that crevice corrosion occurs when the solution is so aggressive that passivity is no longer possible over the whole potential range. 

The propagation of crevice corrosion can also be arrested by decreasing the potential of the outside surfaces below a critical value (see earlier). The existence of a repassivation or protection potential was recognized very early, in particular by Pourbaix et al. [81] for pitting corrosion. From a practical point of view, the existence of a protection potential below which no crevice corrosion is possible is of major importance because it guarantees the immunity of passivated alloys in near-neutral chloride solutions in the absence of oxidizing species and because it makes possible the cathodic protection of stmctuies. 

Second, the critical potential may be a repassivation potential. It has been shown in artificial active crevices that lowering the potential of the free surfaces causes the local environment to become less aggressive (see, for example, Pourbaix [36]). Thus, at some point, the environment becomes not aggressive enough for active dissolution to be sustained and the metal surface in the crevice becomes passive. In this case, a subsequent increase of the corrosion potential does not produce immediate reactivation inside the crevice. Starr et al. [82] observed this situation on 12% Cr stainless steels in near-neutral environments Dunn and Sridhar [83] observed the same behavior on alloy 825. However, the repassivation may be attributed to different environment changes an increase of local pH in the crevice [82,84], a destabilization of the salt film that controls the... 

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. 

Approaching a potential from more active potentials at a certain scan rate will create a surface structure different from that created when approaching the potential from more noble potentials. The positive hysteresis shown in Fig. 7.20 is caused by the polarization to more noble potentials making the surface more passive. The negative hysteresis in Fig. 7.19 is caused by a decrease in passivity, often produced by the initiation of localized corrosion. This latter phenomenon is usually a reflection of a propensity for localized corrosion in the form of either pitting or crevice corrosion. From a practical standpoint, a positive hysteresis usually signifies that the alloy will be more resistant to localized corrosion than does a negative hysteresis. ... 

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