Paint scratch corrosion

Zinc phosphate coatings resist corrosion creep which can occur underneath paint scratches. Painting should be carried out as soon as possible after the initial drying of the coating otherwise re-absorption of water may reduce paint adhesion. 

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. 

Various strategies are employed to prevent corrosion. The use of paint as a protective coating is described in our chapter introduction. A metal surface can also be protected by coating it with a thin film of a second metal. When the second metal is easier to oxidize than the first, the process is galvanization. Objects made of iron, including automobile bodies and steel girders, are dipped in molten zinc to provide sacrificial coatings. If a scratch penetrates the zinc film, the iron is still protected because zinc oxidizes preferentially ... 

Because corrosion is electrochemical, we can use our knowledge of redox reactions to combat it. The simplest way to prevent corrosion is to protect the surface of the metal from exposure to air and water by painting. A method that achieves greater protection is to galvanize the metal, which involves coating it with an unbroken film of zinc (Fig. 12.16). Zinc lies below iron in the electrochemical series, so if a scratch exposes the metal beneath, the more strongly reducing zinc releases electrons to the iron. As a result, the zinc, not the iron, is oxidized. The zinc itself survives exposure on the unbroken surface because, like aluminum, it is passivated by a protective oxide. 

In the case of scratches in paint films that cover the metal the principle is the same (Fig. 16.9). There is oxide formation at the scratch site, the corrosion continuing, therefore, in a direction parallel to the metal surface. 

Silicic acid esters, Si(OR)4, are produced by the reaction of SiCl4 with the appropriate alcohols. The most important representative of this group is tetraethoxysilane (tetraethyl orthosilicate) Si(OC2H5)4, which is used directly, or after hydrolysis to ethylpolysilicates, as a binder for ceramic pastes, for inorganic zinc dust paints (corrosion protection), for the surface treatment of glass and for the modification of silicates. Silicic acid esters are further used for rendering polymer surfaces scratch-resistant. 

A number of methods have been devised to protect metals from corrosion. Most of these methods are aimed at preventing rust formation. The most obvious approach is to coat the metal surface with paint. However, if the paint is scratched, pitted, or dented to expose even the smallest area of bare metal, rust will form under the paint layer. The surface of iron metal can be made inactive by a process called passivation. A thin oxide layer is formed when the metal is treated with a strong oxidizing agent such as concentrated nitric acid. A solution of sodium chromate is often added to cooling systems and radiators to prevent rast formation. 

The (physically wetting) polyaniline primer CORRPASSIV was compared with a fast-drying two-component epoxy resin primer (Messrs. Finalin, Type 144) and a metal-reactive two-component primer with adhesion-promoting properties (Type 918). All three primers were coated with a two-component polyurethane paint (B412). CORRPASSIV, both with and without top coat, exhibited no rust formation beneath the primer. The slight undermining receded after the test, and even within the cross and scratch injuries, it was virtually impossible to detect any corrosion. 

Fig. 42 Potential distribution measured with an SKP of a half side plasma modified zinc surface, which was subsequently painted with a clear coat and exposed to a corrosive environment. The low potential indicates the delaminated area starting from a scratch.

Crevice corrosion normally exists in areas such as lap joints, gaskets, and around bolts and rivets. Crevices are also created by deposits of corrosion products on the surface, scratches in the paint film, and so forth. The environmental conditions in the crevice can change over time which become suitable for the nearby clean surface. Crevice corrosion is sometimes attributed to one or more of the following ... 

Chem. Descrip. Tetraethyl orthosilicate CAS 78-10-4 EINECS/ELINCS 201-083-8 Uses Binder in precision investment castings as sec. backup casting coating mfg. of electronics, computer components silicon dioxide source in films deposited on silicon substrates catalyst intermediate component in zinc-based primer coatings for corrosion-resist, paints crosslinking agent in mfg. of sealants raw material in sol-gel systems scratch resist, hard coat on glass and plastic lens... 

The major problem with protective coatings is the rapid, localized corrosion which can occur when the coating is damaged, and several examples (e.g. scratched paint, tinplate) were given in Section 9.4. 

Polymer films form a barrier between the metal substrate and the environment. This protects the metal provided that the barrier is impermeable to corrosive agents and remains free of defects. Paint films rarely fulfill these conditions. Firstly, they may contain defects such as pores and scratches that perforate the barrier. Secondly, even intact paint films are permeable to oxygen and water to an extent that varies with their structure, thickness and chemical composition. 

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