Filamentous corrosion

Introduction

This type of corrosion, despite its apparent dissimilarity to pitting corrosion, is considered a type of pitting corrosion. Pitting corrosion occurs on metal surfaces that are covered by organic materials, glazes, varnishes, and protective layers. For this reason, it is sometimes called undercoating corrosion. This type of corrosion is common and is observed under various coatings and on metal surfaces such as steel, zinc, aluminum, magnesium, and chrome-plated nickel.

Filamentous corrosion does not weaken the part, but only distorts its appearance. For example, filamentous corrosion occurs in the food packaging industry on the outer surface of a can. Studies have shown that this type of corrosion does not have any biological effect on the internal contents of the can and only causes damage to the appearance of the can.

The most important environmental factor in this type of corrosion is the high relative humidity in the atmosphere, so that if there is a humidity of 65-90 percent, this type of corrosion occurs and does not occur at less than this amount, if the percentage of humidity is higher, blisters are formed on the surface.

Filamentous corrosion is seen under transparent coatings as a network of corrosion products in which the corrosion product appears in the form of short filaments. The main activity in this type of corrosion is carried out by filaments. The width of the filaments is 0.1-0.5 and even 2 mm. The growth rate of each filament is constant and about 0.4 mm per day. The filament consists of an active end and an inactive colored tail, which are the corrosion products, and corrosion occurs only at the end of the filament. The filaments start at the edges and proceed in a straight line. They do not intersect the inactive tails as they move.

The filaments are reflected at the angle of impact when they hit an obstacle or the inactive tail of another filament. Whenever an active filament hits another filament at a 90-degree angle, it may become inactive or break into two other filaments that are reflected at a 45-degree angle. Whenever two filaments hit each other at a very low angle, they may join together.

Mechanism

Filamentary corrosion is considered a special type of crevice corrosion, but the exact mechanism of this type of corrosion is not completely known. Numerous studies on steel coatings show that the mechanism and stages of filament corrosion in steel are as follows:

In the initial stage of corrosion, moisture and water are absorbed from the surrounding area to the filament tip due to the high concentration of iron ions, and moisture continuously penetrates from the inactive tail to the active end of the filament. Oxygen penetrates into the coating through surface defects, and the oxygen concentration at the interface between the tail and the filament tip increases. With the appearance and hydrolysis of corrosion products, the pH decreases, the accumulation of Fe2+ and OH– ions (resulting from the formation of an electrochemical cell) causes the initiation and development of corrosion at the filament tip, and ultimately FeO and then hydrated Fe2O3 are formed.

Anodic reaction:           Fe → Fe2++2e–

Cathodic reaction:                O2+2H2O+e– → 4OH–

Methods to reduce or prevent filamentary corrosion

There is no specific method to prevent filamentary corrosion. Perhaps the simplest method is to reduce the percentage of ambient humidity, which means that if possible, we avoid exposing coated metal surfaces to humid environments. (This method is not always practical) Or we can use brittle surfaces as metal coatings because under the brittle coating, at the active end of the filament, the coating is cracked and with the penetration of oxygen from the rupture of the coating, the difference in oxygen concentration is eliminated and thus filamentary corrosion is stopped.

Of course, one of the disadvantages of brittle coatings is that they break with the slightest mechanical damage such as impact. With the advancement of nanomaterials science, the development and application of nanocoatings that have very low moisture permeability can be considered a very effective and important method for preventing filamentary corrosion.