Critical crevice solution

The self-sustaining nature of the process leads to a drastic reduction in the stability of the passive film. Stage HI is characterized by the breakdown of the passive film due to the attainment of what has become known as the critical crevice solution. This solution has a low pH [typically 1 or less (23)] and a high Cl"... 

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. 

In the third stage (Fig. 4.21), accelerated corrosion takes place due to the breakdown of the passive film, because the solution inside the crevice in the second stage is highly aggressive. The concentration of solution at which the passive film breaks dovm is called critical crevice solution. ... 

Critical crevice solution break down passivity... 

They are critical crevice solution (CCS), passive current (Jp) and composition of the alloy. 

Critical crevice solution [2,19,46,94] taking into account the pH and the chloride content but not the effect of metal cations Critical dissolved A1 content [95]... 

It is also an accepted fact that the crevice corrosion ceases to grow at potentials less positive than a certain critical potential resulting in crevice protection as shown for austenitic stainless steel in Figure 22.30 [59,61]. The critical potential, Ecrev, is called crevice protection potential or the critical crevice corrosion potential. It was found for a cylindrical crevice in austenitic stainless steel that the crevice protection potential shifts in the less positive direction as a logarithmic function of solution chloride concentration [61] ... 

FIGURE 22.30 Schematic polarization curves of a cyhndrical crevice in an anode of stainless steel in neutral solutions of three different chloride concentrations [59] h = crevice depth, Icrev — anodic crevice dissolution current, ccr = chloride concentration in the solution bulk, crcv = crevice protection potential, and /Crev— minimum crevice dissolution current at the critical crevice (protection) potential crcv. 

It is in fact the acidification of the occluded crevice solution that triggers the crevice corrosion. The critical acid concentration, < , , for crevice corrosion to occur corresponds to what we call the passivation-depassivation pH, beyond which the metal spontaneously passivates. This critical acidity determines the crevice passivation-depassivation potential, and hence the crevice protection potential Ecrev. The electrode potential actually measured consists of the crevice passivation-depassivation potential, E -ev, and the IR drop, A/iIR, due to the ion migration through the crevice. Assuming the diffusion current from the crevice bottom to the solution outside, we obtain AEm = icmv x h constant, where crcv is the diffusion-controlled metal dissolution current density at the crevice bottom and h is the crevice depth [62], Since anodic metal dissolution at the crevice bottom follows a Tafel relation, we obtain Eciev as a logarithmic function of the crevice depth ... 

FIGURE 22.31 Schematic potential-dimension diagrams for localized corrosion of stainless steel in aqueous solution [63] Epit — pitting potential, ER = pit repassivation potential, Ep = passivation potential in the critical pit solution, Emv — crevice protection potential, rj]13 = critical pit radius for pit repassivation, a = pit repassivation, and b = transition from the polishing mode to the active mode of localized corrosion. 

Figure 10.4 Critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT) for various stainless steels in 6% FeCb solution [10.9]. The figures below the columns show the contents of Cr, Ni and Mo, respectively (compare Tables 10.5 and 10.6).

To overcome these limitations. Method D measures the critical crevice corrosion temperature, using a multiple crevice assembly (MCA). The test solution is 6 wt. percent FeCl3 acidified with 1 wt. percent HCl. This test method is also referenced in ASTM G 157 for evaluating the corrosion properties of wrought iron- and nickel-based corrosion resistant alloys for the chemical process industries. 

An extension of the ECT method described above that has been used to evaluate alloys for service on North Sea oil platforms [97,98] htis been referred to as cyclic ther-mammetry [99]. The technique involves exposing a coupion fitted with an MCA to a test solution under eui applied potential that simulates a process environment. The current density of the crevice specimen is monitored eis the temperature of the test solution is slowly chemged (e.g., 4°C/24 h) in a cychc mEinner up to a critical vcJue smd then back towEird room temperature. The test results Eire in the form of a curve (Fig. 5) that looks very much like a conventional Emodic polEirization curve [97] with the exception that temperature rather than applied potential is the independent variable. VEdues for a critical crevice corrosion... 

Steinsmo, U., Rogne, T., Dmgli, J. A., and Gartltind P. O., High Alloyed Stainless Steels for Chlorinated Seawater Applications—Critical Crevice Temperatures, Engineering Solutions to Industrial Corrosion Problems, NACE International, Sandefjord, Norway, 1993. 

The materials Monit and Sea-Cure are characterised by good resistance to pitting, crevice and stress corrosion cracking in seawater. The critical pitting corrosion temperature in the FeCls test is 328 K (55 °C) and the critical crevice corrosion temperature is 318 K (45 °C). In Table 36, the pitting potentials of the two superferrites and the austenitic steels 1.4539 (SAE 904 L, XlNiCrMoCu25-20-5) and X3CrNiMol7-13-3 (SAE 316,1.4436) measured in 5% NaCl solution are presented. 

Figure 1-11. Critical crevice temperatures of stainless steels and nickel alloys in 10% FeClj solution (Heubner,...

Critical Crevice Corrosioir Temperatures m 10% Ferric Chloride Solution... 

The critical crevice corrosion temperature of an alloy is the temperature at which crevice corrosion is first observed when immersed in a ferric chloride solution. Table 1.3 lists the critical crevice corrosion temperature of several alloys in 10% ferric chloride solution. 

The magnitude of crevice corrosion also depends on the depth of the crevice, width of the gap, number of crevices and ratio of exterior to interior crevice. It has been shown for types 316 and 304 stainless steel that smaller the gap, the less is the predicted time for initiation of crevice corrosion (Fig. 4.14). The reason is that when the ratio of crevice solution volume to creviced area is small, the acidity is increased and the critical value for initiation of crevice is achieved rapidly. The ratio of the bold area to the creviced area also affects crevice corrosion (Fig. 4.15). Generally, the larger is the bold area (cathodic) and smaller the creviced area (anodic), the larger is the probability of crevice corrosion. This has been shown by work on types 304 and 316 stainless steel and... 

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