Corrosion filiform

Crevice corrosion and pitting corrosion are similar in that both involve autocat-alytic propagation. The two modes differ in causes of initiation the morphology and the degree of penetration of pitting are different (17). 

This is a special form of filamentary corrosion occurring on metallic surfaces and is related to crevice corrosion also known as underfilm corrosion. This form of corrosion is generally apparent under painted body of some used cars. It appears as a blister under the paint. The filament propagation underfilm may appear split or joined together, as they propagate in direct lines, some of them reflecting because of obstacles such as adhesive parts of the organic film to the substrate that become trapped in a very narrow place (17). 

The filament occurs on metals covered by an organic film and because of a certain discontinuity in the film, air, and water penetrate through the coating and reach the underlying metal. This humid layer becomes saturated or rich in corrosive ions from soluble salts and forms a zone known as the active head of the filament. The 

Filiform corrosion of AZ91 magnesium alloy involves a corrosion mechanism different from the conventional mechanism. In this case, dissolved oxygen is not necessary, and the filiform corrosion propagation is fueled by hydrogen evolution at the filament head and is controlled by mass transfer because of the salt film on the tip of the filament (20, 21). 

Filiform corrosion is always shallow in depth and causes loss of product integrity only when it occurs on thin sheet (about 0.05 mm or thinner) and foil (defined as 0.15 mm in thickness), e.g., food containers, or a foil moisture barrier on insulation board or on foil-laminated paper packaging. 

On thicker painted sheet, as for aircraft or automobiles, filiform corrosion is primarily a cosmetic problem, but it causes loss of paint adhesion and can act as a site for initiation of pitting or other forms of corrosion. 

Filiform corrosion occurs independent of light, metallurgical factors in the steel, and bacteria. Although threads are visible only under clear lacquers or varnishes, they probably also occur under opaque paint films. They have been observed under various types of paint vehicles and on various metals, including steel, zinc, aluminum, magnesium, and chromium-plated nickel. This type of corrosion takes place on steel only in air of high relative humidity (e.g., 65-95%). At 100% relative humidity, the threads may broaden to form blisters. They may not form at all if the film is relatively impermeable to water, as is stated to be the case for paraffin [17]. The mechanism appears to be a straightforward example of a differential aeration cell. 

Filiform corrosion is specific to lacquered metal. This is mainly an alteration of surface appearance. The underlying metal only sulfers a very superficial attack, not exceeding a depth of a few tens of microns. 

Filiform corrosion always starts at coating defects, such as scratches, and weak points beards, cut edges or holes. It can be seen after several years of service. 

This type of corrosion, described for the first time in 1944 [32], was observed on lacquered steel first and then on high-resistance aluminium alloys for aerospace applications [33]. 

Coil-coated strip in aluminium alloys is not prone to this type of corrosion. After the introduction of new lacquering techniques by electrostatic deposition of powders on profiles or sheets in aluminium alloys of the 3000,50(X) and 60(X) series, filiform corrosion appeared in the 1980s, mainly in the coastal areas of Northern Europe. 

Several mechanisms have been proposed in order to explain this special type of corrosion. 

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Filiform Corrosion-type of corrosion that takes place under a film in the form of randomly distributed hairlines. 

The various types of localised corrosion have been enumerated in Table 1.2 in Section 1.1, and many of them are dealt with in some detail in other sections of this volume. For this reason this section will be confined to a consideration of the factors that give rise to crevice corrosion, filiform corrosion, pitting, selective leaching and erosion-corrosion and of the mechanisms of these forms of localised attack. 

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. 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

The main factor in causing filiform corrosion is the relative humidity of the atmosphere, and if this is below 65% (the critical relative humidity for the atmospheric corrosion of most metals, see Section 2.2) it will not occur. As the relative humidity increases the thickness of the filaments increases at 65-80% relative humidity they are very thin, at 80-95% relative humidity they are much wider and at approximately 95% relative humidity they broaden sufficiehtly to form blisters. 

Fig. 1.52 Mechanism of filiform corrosion showing how atmospheric oxygen and watCT enter the active head through the film (lacquer) and how water leaves through the inactive tail. This results in a high concentration of oxygen at the V -shaped interface between the tail and the head, and to a differential aeration cell (after Uhlig )...

This appears as a random non-branching white tunnel of corrosion product either on the surface of non-protected metal or beneath thin surface coatings. It is a structurally insensitive form of corrosion which is more often detrimental to appearance than strength, although thin foil may be perforated and attack of thin clad sheet (as used in aircraft construction) may expose the less corrosion resistant aluminium alloy core. Filiform corrosion is not commonly experienced with aluminium, as reflected by the insignificance afforded it in reviews on the phenomena (Section 1.6). 

A patchy form of rust that attacks paint films from underneath, can be caused by sweaty hands, residues from fluxes, etc. Examples of the latter include residues from phosphating and soluble salts (including those from unsuitable rinsing water) and they can manifest themselves on steel in the form of a creeping filiform corrosion, i.e. as progressing threads of rust which loosen the coating. This can be followed visually through transparent films. It occurs, however, only when the relative humidity of the surround-... 

Even small traces of certain corrosion stimulants, notably soluble chlorides and sulphates, can maintain a continuing corrosion process under a paint film because the salts accelerate the initial dissolution of ferrous iron (and other metal ions) but are not immobilised in the hydrated oxide corrosion products. Filiform corrosion is the most spectacular example of this phenomenon, but progressive spread, preceded by blistering, is also observed from scratches or other breaks in a coating, for example during salt spray tests. 

Filiform Corrosion a form of corrosion under paint coatings on metals characterised by a thread-like form advancing by means of a growing head or point. 

D 2803 1982 (1987) Test method for filiform corrosion resistance of organic coatings... 

General rules for corrosion tests applicable to storage conditions Filiform corrosion test on steel Liquefied petroleum gases-corrosiveness to copper. Copper strip test Road vehicles-brake linings-resistance to water, saline solution, oil and brake fluid-test procedure... 

Filiform Corrosion corrosion in the form of hairs or filaments progressing across a metal surface. 

It resists pinpoint rust formation before enameling and filiform corrosion after enameling. 

Filament winding, 26 767, 768 of thermosetting resins, 19 558 Filament-winding resin-impregnated glass rovings, 20 117 Filament yarns, 11 250 acrylic, 11 212 Filiform corrosion, 7 174 Filled fibers, encapsulation spinning of, 16 26... 

Figure 1. The assignments of FTIR-RA spectra of unaged epoxy free and coated films on steel have been given previously ( ). It should be noted that this aged specimen exhibited extensive filiform corrosion after 7 months exposure. Figures 2 show the spectral changes at different exposure periods. These spectra have been normalized for the baseline shift resulting from the reflection change of the steel substrate due to exposure and corrosion.

Sease C, Selwyn LS, Zubiate S, Bowers DF, Atkins DR (1997) Problems with coated silver whisker formation and possible filiform corrosion, Stud Conserv 42 1-10271. 

The various modes of degradation encountered are (i) general or uniform corrosion (ii) galvanic corrosion (iii) pitting corrosion (iv) crevice corrosion (v) filiform corrosion (vi) granular corrosion (viii) stress corrosion cracking (viii) corrosion fatigue. 

Pitting corrosion crevice corrosion filiform corrosion Intergranular corrosion sensitization exfoliation dealloying... 

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... 

Filiform Corrosion. This consists of the filamentary corrosion occuring on metallic surfaces and is a special type of crevice corrosion, sometimes called underfilm corrosion. It is frequently observed under the painted body of some used cars. It appears as a blister... 

In addition to SO2 damage of synthetic and natural polymers, another destructive mechanism called "filiform" corrosion should... 

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. 

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... 

Figure 8. Schematic illustration of filiform corrosion. (Reproduced with permission from Ref. 4. Copyright 1971 Wiley)...

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