Crevice corrosion cases

Copper alloys often show only weak crevice corrosion. This is especially the case if the copper alloy is coupled to a less noble alloy such as steel. The corrosion of the steel is stimulated by the galvanic effect caused by the coupling of dissimilar metals. Hence, the sacrificial corrosion of the steel protects the copper alloy (Fig. 2.9). See Chap. 16, Galvanic Corrosion. ... 

In all cases partial or total hulls of aluminum or stainless steel must be provided with cathodic protection. This also applies to high-alloy steels with over 20% chromium and 3% molybdenum since they are prone to crevice corrosion underneath the coatings. The design of cathodic protection must involve the particular conditions and is not gone into further here. 

Oxidation and tarnishing active dissoiution in acids anodic oxidation and passivity chemicai and eiectrochemical polishing atmospheric and immersed corrosion in certain cases Crevice corrosion hiiform corrosion deposit attack bimetaliic corrosion intergranuiar corrosion weid decay... 

Most cases of crevice corrosion take place in near-neutral solutions in which dissolved oxygen is the cathode reactant, but in the case of copper and copper alloys crevice corrosion can occur owing to differences in the concentration of Cu ions however, in the latter the mechanism appears to be different, since attack takes place at the exposed surface close to the crevice and not within the crevice in fact, the inside of the crevice may actually be cathodic and copper deposition is sometimes observed, particularly in the Cu-Ni alloys. Similar considerations apply in acid solutions in which the hydrogen ion is the cathode reactant, and again attack occurs at the exposed surface close to the crevice. 

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Pitting may be defined as a limiting case of localised attack in which only small areas of the metal surface are attacked whilst the remainder is largely unaffected, and this definition is applicable irrespective of the mechanism involved dezincification, crevice corrosion and impingement attack can all result in pitting, although the mechanisms of these three processes are quite different. 

The second degree of freedom is to design-out crevices where possible, although it must be remembered that crevice corrosion can go on underneath deposits. Crevice corrosion at a butt weld with incomplete root penetration is a common case (Fig. 9.7a). Where internal inspection is not possible and crevice corrosion is recognised as likely, A"-radiography of each weld can be specified. 

Weld corrosion (Section 9.5) Crevice corrosion at butt welds due to poor penetration has already been discussed and was shown in Fig. 9.1(a). Conversely, if there is a large weld bead protruding in the pipe bore, erosion/cor-rosion can occur downstream due to the turbulence produced over the weld bead (Fig. 9.8). In either case, the fault probably lies in the incorrect spacing of the butts at welding. 

Polarization techniques have also been used to determine mechanisms by which microorganisms induce localized corrosion in the forms of pitting or crevice corrosion. In most cases itpit was determined in the presence and absence of bacteria, itpit provides data as to the tendency for pitting, but not the rate for pit propagation. Salvarezza et ah " and De Mele... 

Anodic undermining has not been studied as extensively as cathodic delamination because there do not appear to be any mysteries. Galvanic effects and principles which apply to crevice corrosion provide a suitable explanation for observed cases of anodic undermining. 

The alloy Haynes 6B is resistant to corrosion in organic acids, but subject to pitting and crevice corrosion and SCC in chloride media. The corrosion rate of 0.3 mm/yr or 12mpy has been observed in 30 wt % of NaOH it is likely that caustic cracking will occur at high concentrations of NaOH and temperatures in the case of all the cobalt alloys. The nominal composition of high-temperature cobalt alloys is given in Table 4.54. 

Crevice corrosion is yet another example of corrosion caused by a difference in oxygen concentration between two areas on the metal surface. In this case, the region of low oxygen concentration lies inside a crevice caused by the overlapping of a piece of metal or other material, e.g. the crevice which exists under a washer pressed onto a metal surface (Fig.6). Even if the washer is insulating (e.g. nylon) as is used for mounting test coupons, corrosion will still occur. 

Solution flow typically enhances corrosion rates, by increasing the transport of dissolved oxygen to the metal surface, by increasing the rate of removal of protective corrosion products, and, in extreme cases, by physically removing the corrosion products or even metal (in the case of erosion by suspended particles or cavitation) (Fig. 4). In a few situations flow can be beneficial thus for stainless steels in chloride solutions, flow can prevent the development of the acidification that is necessary for pitting and crevice corrosion. 

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]. 

The sites for the oxidation reactions are called anodes, and the sites for the reduction reactions are called cathodes. Anodes and cathodes can be spatially separated at fixed locations associated with heterogeneities on the electrode surface. Alternatively, the locations of the anodic and cathodic reactions can fluctuate randomly across the sample surface. The former case results in a localized form of corrosion, such as pitting, crevice corrosion, intergranular corrosion, or galvanic corrosion, and the latter case results in nominally uniform corrosion. 

Several examples of the results of crevice corrosion are shown in Figs. 3 to 6. The similarities in topography amongst the examples include the accelerated attack of the substrate under the crevice former and the virtual absence of attack on the fully exposed surface. The accelerated attack within the crevice usually appears as uniform corrosion or pitting. In some cases, it is thought that the attack starts as metastable pits that coalesce into a more uniform attack. 

This section describes the balance of these opposing mass transport forces for the geometry of crevice corrosion. Mass transport of species in aqueous solution can occur by three processes migration, diffusion, or convection. In most cases of crevice corrosion, convection can be ignored owing to the restricted geometry involved. 

Ignored by most implementations of the CCS framework, ohmic drop can not only lead to passive-to-active transitions, but also can, in the context of environmental cracking, make hydrogen evolution, and therefore embrittlement, more viable at the crack tip. The IR framework has been successfully demonstrated in several model metal/environment systems [34, 35], and has been invoked to rationalize the practically important case of the crevice corrosion of Alloy 625 in chlorinated seawater [32, 33]. 

Fig. 14 Electrochemistry of propagating crevice corrosion. Two cases are shown (a) the material in the crevice corrodes actively and (b) the material in the crevice undergoes an active-passive transition.

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