Localized corrosion filiform

The two major types of localized corrosion discussed are pitting corrosion, and crevice corrosion including filiform corrosion. In spite of the different morphological appearance of these two types of corrosion (Figure 6.15), the electrochemical basis of these two types are almost the same. The difference may rise from different causes in the initiation step of pitting or crevice corrosion.25... 

Types of Corrosion Phenomena. The major categories of phenomena ( include uniform, localized, and pitting corrosion selective dissolution and corrosion acting together with a mechanical phenomenon. In uniform corrosion, all areas corrode at the same rate. Examples of uniform corrosion include tarnishing and active dissolution of metals in acids. In localized corrosion some areas corrode more readily than others crevice corrosion and filiform... 

Filiform corrosion is characterized by formation of interconnecting filaments of corrosion under a paint film upon exposure to a humid environment. Filiform corrosion typically occurs only when the relative humidity exceeds about 65%. The mechanism is complicated and has been the subject of considerable discussion in the literature (, 1A, 2 1, 2, 24). Basically, a localized corrosion mechanism is responsible (Figure 8). The head of the growing filament is anodic, and as a result the filiform corrosion process has been termed a specialized form of anodic undermining... 

Fig. 10.16 Types of localized corrosion initially associated with the environment, (a) Crevice corrosion, (b) Deposit corrosion, (c) Waterline attack, (d) Filiform corrosion (e) Erosion corrosion, (f) Drop corrosion, (g) Turbulent-flow corrosion, (h) Fretting.

The first three forms of corrosion uniform or quasi-uniform general corrosion, galvanic corrosion and localized corrosion (pitting, crevice and filiform) have no clear separation. The oxide-hydroxide passive layer can play the... 

The importance of occluded cells cannot be overemphasised, and Brown considers that pitting, crevice corrosion, intergranular attack, filiform corrosion and hydrogen cracking are characterised by local acidification due to hydrolysis of metal ions, and that this phenomenon is of major significance in the overall mechanism. 

The corrosion phenomena commonly observed on painted metals include cathodic delamination, anodic undercutting, and filiform corrosion. Cathodic delamlnatlon results when the alkali produced by the cathodic corrosion reaction disrupts the paint-metal interface. This phenomenon has long been observed on cathodically protected painted steel (18) and has also been demonstrated to be responsible for the loss of paint adhesion that often occurs adjacent to corrosion sites on painted steel (19). The localization and separation of anodic and cathodic sites associated with corrosion at a break in a paint film on steel are schematically illustrated in Figure 7. 

By using the SKP (see Sect. 5.4.2.5), it was possible to measure the local potentials underneath an organic coating in situ without the deterioration of the corroding system [178]. The mechanism of FFC consisting of an anodic reaction at the corrosion front is reflected in rather different electrode potentials around the filament s head. Whereas, for cathodic undermining, the delamination front is positively polarized with respect to the already delaminated zone and the head of the filiform filament shows a negative potential with respect to the tail (Fig. 35) [178]. Therefore, the tip can be identified as the local anode and the local cathode is situated behind the anode within the tail. 

The electrochemical mechanism of filiform corrosion is described by a differential aeration cell between the front (low oxygen concentration) and the back (open to air through the cracked/porous tail of dry corrosion products) of the filament s active head . Therefore, the head is the local anode and the tail the local cathode, the opposite to the cathodic delamination described earlier. This is shown in Fig. 7-67 where an optical photograph of the filiform head is compared to the potential distribution in air (center) and in nitrogen (right) (Schmidt and Stratmann, 1998). 

Williams [73] carried out similar experiments for a 1 cm HP Mg (100 ppm Fe) sample in 5% NaCl, pH 6.5 and SVET maps were obtained at 2, 8.4, 17 and 28.3 h after specimen immersion. This sample took significantly longer than the CP Mg to show the first sign of breakdown - about 2 h as opposed to a few minutes for CP Mg. The point of breakdown was visually identified as a black dot on the surface with some associated hydrogen evolution. The local current density was over an order of magnitude lower than that obtained with CP Mg at initial breakdown. Thereafter the HP Mg showed features akin to filiform corrosion. In situ SVET showed that the... 

Polyurethane paints have been widely used in marine applications worldwide. However, these paints are quite brittle and tend to chip and crack, and for these reasons many prefer solvent-drying acrylic paints for the exterior finish. These paints can be removed locally by chemical solvents down to the primer and are reported to be easier to touch up. Whichever paint system is selected for exterior use, it is usual to qualify the system on the basis of its ability to prevent filiform corrosion. Topcoats main fimctions are to provide... 

Such thin films have to act as a barrier between corrosive ions and the metal and have to inhibit electron as well as ion transfer reactions. Moreover, they should be stable over a quite wide pH range since corrosive reactions especially under polymer films change the local pH at the front of deadhesion ranging from acidic pH values (e.g., during filiform corrosion) to very alkaline ones (e.g., cathodic delamination on steel) [140]. 

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