Metallic repassivation

Film is locally removed by dissolution, surface shear stress or particle/bubble impact but it can repassivate. Erosion corrosion rate is a function of the frequency of film removal, bare metal dissolution rate and subsequent repassivation rate. 

Whatever the source of this type of contamination, it must be quickly and effectively removed from the boiler to prevent the problem from recurring. The standard procedure is to blow down the boiler to the maximum extent possible until the oil is removed. In more serious cases, it may be necessary to drop the boiler and provide a boil-out program. A boiler boil-out uses a simple formulation (usually based on caustic or strongly alkaline phosphates) and is employed to re-expose and repassivate the metal surfaces. 

Ti, Ta, Zr, and Nb are preferred, because of their ability of forming stable compact oxides film during anodic polarization. Compared with other metals, titanium has the lowest density, easy machinability, high anticorrosion and quick repassivation electrochemical performance, and relatively low cost (Leyens and Peters 2003). It becomes a preferred choice as the substrate material of diamond film electrode (Drory and Hutchinson 1994 Chen andLin 1995 Chen et al. 2003 Hian et al. 2003 Gerger et al. 2004 Chen et al. 2005 Guo and Chen 2007a). 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

E, the repassivation potential, causes the metal to be passivated. Between E and the breakdown potential E, the system can be unstable, as indicated by the bursts of current. 

The initiation of a pit is assoeiated with the breakdown of the protective film on the metal surface. In cases where pit depths increase rapidly, the environment is usually such that no repair or repassivation of the protective layer can be accomplished. In situations where many shallow pits form, the environment is usually one where repassivation of the damaged film can be made, but initiation of new sites occurs on a regular basis. 

Figure 59. Potential-titne curves measured at two different positions along a metal wire during (a) Fe dissolution in 1 N H2SO4, (b) Au corrosion in 1 N HCl/2 N NaCl, (c) Zn dissolution in 4 N NaOH, (d) Activity wave of Fe in 12 N HNO3 with successive repassivation and (e) passivation wave during Co dissolution in 1.3 M Cr03 + 1 N HCl with successive reactivation. The distance between the potential probes corresponds to the distance given under each curve. (After Franck, reprinted with permission from VCH Publishers.)...

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

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