FILIFORM CORROSION

16.6 FILIFORM CORROSION

Metals with organic coatings may undergo a type of corrosion resulting in numer- ous meandering thread - like fi laments of corrosion product. This is sometimes known as underfi lm corrosion, and it was called fi liform corrosion by Sharmon

[15] (Fig. 16.1 ). It has been described by several investigators and reproduced in the laboratory [16] . According to reported descriptions, the fi laments, or threads, on steel are typically 0.1 – 0.5 mm wide. The thread itself is red in color, character-

istic of Fe 2 O 3 , and the head is green or blue, corresponding to the presence of

(a)

(b)

Figure 16.1. Filiform corrosion. ( a ) Lacquered tin can. 1 × . ( b ) Clear varnish on steel, 10 × (86% R.H., 840 h). [Reprinted with permission, M. Van Loo, D. Laiderman, and R. Bruhn, Corrosion 9 , 279 (1953). Copyright NACE International 1953.]

ORGANIC COATINGS

ferrous ions. Each thread grows at a constant rate of about 0.4 mm/day in random directions, but threads never cross each other. If a head approaches another thread, it either glances off at an angle or stops growing.

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 fi lms. 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 cor- rosion 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 fi lm is relatively impermeable to water, as is stated to be the case for paraffi n [17] . The mechanism appears to be a straightforward example of a differential aeration cell.

16.6.1 Theory of Filiform Corrosion

Various schematic views of a fi liform thread or fi lament are shown in Fig. 16.2 . The head is made up of a relatively concentrated solution of ferrous salts, as was shown by analysis [17] . Hence, water tends to be absorbed from the atmosphere in this region of the thread. Oxygen also diffuses through the fi lm and reaches higher concentrations at the interface of head and body and at the periphery of the head, compared to lower concentrations in the center of the head. This sets up a differential aeration cell with the cathode and accumulation of OH − ions in

all regions where the fi lm makes contact with the metal, as well as at the rear of the head. The anode is located in the central and forward portions of the head attended by formation of Fe 2+ . * The liberated OH − ions probably play an impor-

tant role in undermining the fi lm in view of the well - known ability of alkalies to destroy the bond between paints and metals (cathodic delamination). In addition,

they diffuse toward the center of the head, reacting with Fe 2+ to form FeO · n H 2 O, which, in turn, is oxidized by O 2 to Fe 2 O 3 · nH 2 O. The precipitated oxide assumes

a typical V shape because more alkali is produced in the region between the head and the body (more O 2 ), compared to the periphery of the head. Behind the V - shaped interface, Fe 2 O 3 exists predominantly, and, since it is less hygroscopic than ferrous salt solutions, water again diffuses out through the fi lm, leaving this portion relatively dry. Oxygen continues to diffuse through the fi lm, serving to keep the main portion of the fi lament cathodic to the head.

If a head should approach another fi lament body, the previous participation of the fi lm in fi liform growth will have depleted the fi lm of organic and inorganic anions necessary to the accumulation of high concentrations of ferrous salts in the head and also of cations necessary to build up a high pH at the periphery.

* The accumulation of alkali at the periphery can be demonstrated by placing a large drop of dilute sodium chloride solution (1 – 5%), preferably deaerated, containing a few drops of phenolphthalein and about 0.1% K 3 Fe(CN) 6 on an abraded surface of iron. Within a few minutes, the periphery turns pink and the center turns blue

PL ASTIC LININGS

Figure 16.2. Schematic views of fi liform fi lament on iron showing details of differential aeration cell causing attack.

This serves to discourage further growth of the fi lament in the direction of the old fi lament. Furthermore, and perhaps more important, the previous accumula-

tion of OH − added to that being formed, plus still greater abundance of oxygen, tends to ensure that the old fi lament body remains cathodic, encouraging the approaching anode to veer off in another direction. If the head should lose its electrolyte because of delaminated fi lm at the old thread which it approaches, the fi lament would stop growing, a situation sometimes observed.

Phosphate surface treatments and chromate prime coats of paint serve to limit fi liform corrosion, but apparently do not prevent its occurrence. Wholly adequate remedies have not yet been found.