Corrosion underfilm

Rust Creep-also called underfilm corrosion refers to corrosive action that results in damaged or uncoated areas and extends subsequently under the surrounding inert protective coating. 

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... 

When corrosion develops on painted steel the question is often raised as to whether corrosion was a result of paint failure or the paint failure was caused by corrosion. Several studies have shown that adhesion forces are reduced greatly after water soaking or even at very high humidity -and it has been argued that film detachment by water usually precedes underfilm corrosion . Against this view others have claimed that those paints known to have reduced wet adhesion, e.g. those based on alkyd resins, are not uniquely, or even especially, subject to underfilm corrosion Several factors should be considered in this discussion ... 

A continuous intact film of water-resistant paint forms an effective electrical resistance to the flow of a corrosion current (a resistance of over lO flcm through the film is easily achieved). Underfilm corrosion can then only occur if a channel of electrolyte connecting anode and cathode can be established by local adhesion failure between the coating and the metal substrate. 

After major surface contaminants have been removed, e.g. by wet abrasive blasting of hot-rolled structural steel, application of a thin coat of an etch primer greatly reduces the incidence of underfilm corrosion, presumably by eleminating localised areas of poor adhesion. Phosphate pretreatments followed by effective rinsing have a similar effect over cold reduced sheets. 

Conversion coating Conversion coatings are chemical solutions which react with the metal surface to create a corrosion-resistant layer onto which the coating can bond. For mild steel iron phosphate is used to attain good adhesion, but it does not give the underfilm corrosion resistance which can be obtained using zinc phosphate. Zinc coatings can be treated with either zinc phosphate or chromate. Aluminium is usually treated with chromate... 

Figure 54 LEIS admittance map of two scribe marks on 10 (im thick polyvinyl chloride/ polyvinyl acetate coating on A1 alloy 5182 (Al-2.5 Mg) after exposure to 3.5 wt% NaCl for 24 h. The map was collected using a 15 mV amplitude voltage perturbation at 500 Hz. Underfilm corrosion is occurring in association with the rightmost scribe, as indicated by the extended high admittance region located approximately at x = 2, y = 1. (From M. W. Wittmann, R. B. Leggat, S. R. Taylor. J. Electrochem. Soc. 146, 4071 (1999).)...

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... 

This primer can be applied by brush, spray, or dipping and it functions by both improving the adhesion of subsequently apphed top coatings and by reducing the risk of underfilm corrosion. Paints used as the final coats are referred to as finishes or top coats. They are based on binders selected to withstand the conditions likely ... 

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 three two-layer systems 1-3 were subjected to the salt spray test. The panels were scratched with a scratching tool (Sikkens). The back and the edges of the substrates were covered with an adhesive film. The prepared panels were placed in a spray chamber (Erichsen, Model 608/1000 1) and continuously sprayed with 5% NaCl solution (pH 7). During the test, the chamber was heated to 35°C. At certain times, the panels were removed and visually assessed for underfilm corrosion (DIN 53 167), blistering (DIN 53 209), and degree of rusting (DIN 53 210). 

The aim of this study was to investigate the compatibility between CORRPASSIV primer and topcoats based on various binders [82]. The selected topcoats have subjected to the salt spray test performed on scratched panels for the three coating systems. Underfilm corrosion, blistering, and degree of rusting were assessed. 

For intact coatings, the major deterioration mechanism is the transport of electrolyte solutions to the interface between the steel substrate and the coating. This transport results from the equalization of concentration and temperature differences. Water molecules disrupt the bonding and polar interactions that are responsible for a good adhesion at the interface [86]. Underfilm corrosion is possible because of the loss of adhesion. The coatings have been defined as intact when the potential of the steel panel was not measurable at the beginning of the test. 

Hayashi, K., Ito, Y., Kato, C., and Mytoshi, Y. (1990). Behavior at underfilm corrosion tips of 2ii-Fe alloy coated steel sheets. Tetsu To Hagane, 76(9), 1496-1503 (in Japanese). 

Strom, M., et al. (1992). Mechanism of underfilm corrosion propagation in painted automotive galvanized steels during exposure in the new Volvo high performance chamber. Galvatech 92, Verlag Stahleisen, Dusseldorf, Germany, pp. 521-527. 

J. R. Scully, Electrochemical Impedance Spectroscopy for Evaluation of Organic Coating Deterioration and Underfilm Corrosion— A State of the Art Technical Review, Report No. DTNSRDC/SME-86/006, D. W. Taylor Naval Ship Research and Development Center, Bethesda, Md., 1986... 

Any of the above defects can result in undercutting. After the corrosive penetrates to the substrate, corrosion will proceed to extend under the film areas that have not been penetrated or failed. Some linings are more resistant to undercutting or underfilm corrosion than others. Usually if the Uning exhibits good adhesive properties, and if the prime coat is chemically resistant to the corrosive environment, underfilm corrosion will be greatly retarded. 

The chemical effects of phosphating on the surface is to convert the surface to a nonalkaline condition, protecting the surface from reactions with oils in paint and to protect against the spread of corrosion from defects. Alkaline residues on the surface of the base metal lead to underfilm corrosion. 

Unlike the scanning Kelvinprobe (SKP), all the other scanning reference electrodes require electrolytic connection between the sample and the probe. It is therefore not really possible to study underfilm corrosion of polymer-coated metals. However, complimentary to the SKP, the activity of the defect including the inhibiting effect of pigments in the coating can be estimated. Since,... 

FIGURE 7.2 Natural log of underfilm corrosion, as a function of drying ratio for cold-rolled steel, electrogalvanized (20 giro Zn and 40 giro Zn), and hot-dipped galvanized (90 g/vo Zn). Data from Ito, Y., Hayashi, K., and Miyoshi, Y., Iron Steel J., 77, 280, 1991. 

However, if conditions include high R y, then underfilm corrosion is less on galvanized steel than on cold-rolled steel, for the following reasons ... 

Jordan, D.L., Influence of iron corrosion products on the underfilm corrosion of painted steel and galvanized steel, in Zinc-Based Steel Coating Systems Metallurgy and Performance, Krauss, G. and Matlock, D.K., Eds., The Minerals, Metals, Materials Society, Warrendale, PA, 1990, 195. 

In the literature two mechanisms are proposed to describe the propagation of underfilm corrosion in case of intact and defective coatings. These mechanisms are ... 

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