Forms of Metallic Corrosion
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temperature, a decrease in pH, or an increase in the concentration of the corrodent leads to aggravation of corrosion fatigue.
3.13.1 Preventive Measures
Corrosion fatigue can be reduced or eliminated by:
1. Lowering of the stress
2. Controlling the environment
3. Use of coatings
4. Cathodic protection
5. Shot peening
3.14 Filiform Corrosion
Metals with semipermeable coatings or films may undergo a type of corrosion resulting in numerous meandering threadlike filaments of corrosion
beneath the coatings or films. The essential conditions for this form of corrosion to develop are generally high humidity (65 to 95% relative humidity
at room temperature), sufficient water permeability of the film, stimulation
by impurities, and the presence of film defects (mechanical damage, pores,
insufficient coverage of localized areas, air bubbles, salt crystals, or dust
particles).
The threadlike filaments of corrosion spread in a zig-zag manner. The filaments are 0.1 to 0.5 mm wide and grow steadily, but do not cross each other. Each
filament has an active head and an inactive tail. If an advancing head meets
another filament, it gets diverted and starts growing in another direction.
On steel, the tail is usually red-brown and the head is blue, indicating the
presence of Fe 2 O 3 or Fe 2 O 3· nH 2 O at the tail and Fe 2+ ions in the head as corrosion product. The growth formation is explained by the formation of a differential aeration cell. The head absorbs water from the atmosphere because of
the presence of a relatively concentrated solution of ferrous salts, and hydrolysis creates an acidic environment (pH 1 to 4). Oxygen that diffuses through
the film tends to accumulate more at the interface between the head and the
tail. Lateral diffusion of oxygen serves to keep the main portion of the filament cathodic to the head.
Filiform corrosion has been observed on aluminum, steel, zinc, and magnesium, usually under organic coatings such as paints and lacquers. It has
also been found under tin, enamel, and phosphate coatings. The attack does
not damage the metal to any great extent but the coated surface loses its
appearance. Filiform corrosion is always shallow in depth and causes loss
75
temperature, a decrease in pH, or an increase in the concentration of the corrodent leads to aggravation of corrosion fatigue.
3.13.1 Preventive Measures
Corrosion fatigue can be reduced or eliminated by:
1. Lowering of the stress
2. Controlling the environment
3. Use of coatings
4. Cathodic protection
5. Shot peening
3.14 Filiform Corrosion
Metals with semipermeable coatings or films may undergo a type of corrosion resulting in numerous meandering threadlike filaments of corrosion
beneath the coatings or films. The essential conditions for this form of corrosion to develop are generally high humidity (65 to 95% relative humidity
at room temperature), sufficient water permeability of the film, stimulation
by impurities, and the presence of film defects (mechanical damage, pores,
insufficient coverage of localized areas, air bubbles, salt crystals, or dust
particles).
The threadlike filaments of corrosion spread in a zig-zag manner. The filaments are 0.1 to 0.5 mm wide and grow steadily, but do not cross each other. Each
filament has an active head and an inactive tail. If an advancing head meets
another filament, it gets diverted and starts growing in another direction.
On steel, the tail is usually red-brown and the head is blue, indicating the
presence of Fe 2 O 3 or Fe 2 O 3· nH 2 O at the tail and Fe 2+ ions in the head as corrosion product. The growth formation is explained by the formation of a differential aeration cell. The head absorbs water from the atmosphere because of
the presence of a relatively concentrated solution of ferrous salts, and hydrolysis creates an acidic environment (pH 1 to 4). Oxygen that diffuses through
the film tends to accumulate more at the interface between the head and the
tail. Lateral diffusion of oxygen serves to keep the main portion of the filament cathodic to the head.
Filiform corrosion has been observed on aluminum, steel, zinc, and magnesium, usually under organic coatings such as paints and lacquers. It has
also been found under tin, enamel, and phosphate coatings. The attack does
not damage the metal to any great extent but the coated surface loses its
appearance. Filiform corrosion is always shallow in depth and causes loss