Filiform Corrosion

8/3/2019 Filiform Corrosion

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Filiform corrosion occurs under surface layers such as

paint.

It depends on the relative moisture of the air and thequality of the surface treatment preparation prior to

coating.

Filiform corrosion has the appearance of thin

threadlike attacks progressing along the surfacebeneath a surface layer.


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The mode of attack issimilar to pittingcorrosion in that thefront of the attack issupported by moisturewhich penetrates thesurface layer andbecomes depleted ofoxygen making the areaanodicBut the corrosionproducts formed maycause deformation innarrow crevices ordelamination of surfacetreatment.


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According to one recent

investigation, a reduction of the

Cu content in the aluminium alloy

below 0,3% reduces filiformcorrosion substantially[56].

But the corrosion products formedmay cause deformation in narrow

crevices or delamination of surface

treatment.


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This type Filiform Corrosion


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Filiform corrosion normally starts at small, sometimes

microscopic, defects in the coating.

Lacquers and "quick-dry" paints are most susceptible

to the problem

Their use should be avoided unless absence of an

adverse effect has been proven by field experience.

Wheere a coating is required, it should exhibit low

water vapor transmission characteristics and excellentadhesion. Zinc-rich coatings should also be considered

for coating carbon steel because of their cathodic

protection quality.


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The picture on the left shows filiform corrosion causing bleed-

through on a welded tank. The picture on the right shows "worm-

like" filiform corrosion tunnels forming under a coating at the

Atmospheric Test Site.

Filiform corrosion is minimized by careful surface preparation prior

to coating, by the use of coatings that are resistant to this form ofcorrosion (see above), and by careful inspection of coatings to

insure that holidays, or holes, in the coating are minimized.


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PREVENTI0N

A variety of actions can help in the prevention offiliform corrosion. Probably the best strategy is to

make certain that there is excellent adhesion of the

coating over a salt-free metal substrate.

In addition, using multiple coating layers that have

low water and electrolyte permeability, sealing of cut

or drilled edges, adequate coverage of sharp corners

and edges, and prevention of surface defects such as

pinholes, cratering, and dewetting all help. Zinc-rich

primers reduce the tendency for filiform to occur

over steel.


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Testing for filiform corrosion resistance usually

involves contamination of scribed-painted panels by

exposure to salt spray or a salt solution

(hydrochloric acid and hydrogen chloride gas also

have been used) followed by placement of thespecimens in a humidity chamber for a period of time

Deally, the test panels will have been produced on

the customer's line. Filiform corrosion test standards

include ASTM D 2803, "Standard Guide for Testing

Filiform Corrosion Resistance of Organic Coatings on

Metal" and ISO 4623, "Paints and varnishes--

Determination of resistance to filiform corrosion

Filiform Corrosion

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