Pitting corrosion active-passive alloys

Fig. 5.35 Schematic polarization curve for an active-passive alloy having susceptibility to localized corrosion (pitting) due to chloride ions. Pitting initiates at Eb,pit- Small-dashed section is observed when chloride ion concentration initiates penetration of the passive film.

E7.8. Electrokinetic parameters of an active-passive alloy are given in Table E7.5 and the anodic polarization curve of the active-passive alloy in the presence of chloride ions is shown in Fig. 7.23. Pitting corrosion initiation occurs at 0.045 V vs. SHE. 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

As expected from the data in Table 7.3, galvanic corrosion is one of the major practical corrosion problems of aluminium and aluminium alloys. The reason for this is that aluminium is thermodynamically more active (less noble) than most other common structural materials, and that the passive oxide which usually protects aluminium may easily be broken down locally when the potential is raised due to contact with a more noble material. This is particularly the case when aluminium and its alloys are exposed in waters containing chlorides or other aggressive species (see also Section 7.6, Pitting Corrosion). 

While in active range, the alloying elemaits go into solution in their lowest valencies, during corrosion above (/, iron(III) and chromium(VI) compounds are formed. If chlorides are now added to the corrosive medium, an increase in current is observed at Up, this being due to pitting corrosion (Figure 20.21). Chlorides as well as bromides and iodides thus decrease the passive range of the material. 

The chloride pitting resistance of this alloy is similar to that of type 316 stainless steel and superior to that of types 430 and 439L. Like all ferritic stainless steels, t)/pe 444 relies on a passive film to resist corrosion, but exhibits rather high corrosion rates when activated. This characteristic explains the abrupt transition in corrosion rates that occur at particular acid concentrations. For example, it is resistant to very dilute solutions of sulfuric acid at boiling temperature, but corrodes rapidly at higher concentrations. 

Potential monitoring Potential change of monitored metal or alloy (preferably plant) with respect to a reference electrode Measures directly state of corrosion of plant, e.g. active, passive, pitting, stress corrosion cracking, via use of a voltmeter and reference electrode Moderate... 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

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