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Fundamentals of Corrosion
3.1.5 Passive Film on aluminum
Aluminum forms a thin, compact, adherent oxide film on the surface that limits further corrosion. When formed in air at atmospheric temperatures, it is
approximately 5 mm thick. If formed at elevated temperatures or in the presence of water or water vapor, it will be thicker. The oxide film is stable in the
pH range from 4 to 9. With a few exceptions, the film will dissolve at lower or
higher pH ranges. Exceptions are concentrated nitric acid (pH 1) and concentrated ammonium hydroxide (pH 13). In both cases, the oxide film is stable.
The oxide film is not homogeneous and contains weak points. Breakdown of
the film at weak points leads to localized corrosion with increasing alloy content;
and on heat-treatable alloys, the oxide film becomes more nonhomogeneous.
3.1.6 Passive Film on Titanium
Titanium forms a stable, protective, strongly adherent oxide film. This film
forms instantly when a fresh surface is exposed to air or moisture. Addition
of alloying elements to titanium affects the corrosion resistance because
these elements affect the composition of the oxide film.
The oxide film of titanium is very thin and is attacked by only a few substances, the most notable of which is hydrofluoric acid. Because of its strong
affinity to oxygen, titanium is capable of healing ruptures in this film almost
instantly in any environment when a trace of moisture is present.
3.1.7 Passive Film on Tantalum
When exposed to oxidizing or slightly anodic conditions, tantalum forms
a thin impervious layer of tantalum oxide. This passivating oxide has the
broadest range of stability with regard to chemical attack or to thermal
breakdown compared to other metallic films. Chemicals or conditions that
attack tantalum, such as hydrofluoric acid, are those that penetrate or dissolve the film.
3.1.8 uniform Corrosion rates
When exposed to a corrosion medium, metals tend to enter into a chemical union with the elements of the corrosion medium, forming stable compounds similar to those found in nature. When metal loss occurs in this
manner, the compound is referred to as the corrosion product and the surface
is referred to as having been corroded. An example of such an attack is that
of halogens, particularly chlorides. They will react with and penetrate the
film on stainless steel, resulting in general corrosion. Corrosion tables were
developed to indicate the interaction between a chemical and a metal. This
type of attack is called general or uniform corrosion. It is one of the most easily measured and predictable forms of corrosion. Many references exist that
Fundamentals of Corrosion
3.1.5 Passive Film on aluminum
Aluminum forms a thin, compact, adherent oxide film on the surface that limits further corrosion. When formed in air at atmospheric temperatures, it is
approximately 5 mm thick. If formed at elevated temperatures or in the presence of water or water vapor, it will be thicker. The oxide film is stable in the
pH range from 4 to 9. With a few exceptions, the film will dissolve at lower or
higher pH ranges. Exceptions are concentrated nitric acid (pH 1) and concentrated ammonium hydroxide (pH 13). In both cases, the oxide film is stable.
The oxide film is not homogeneous and contains weak points. Breakdown of
the film at weak points leads to localized corrosion with increasing alloy content;
and on heat-treatable alloys, the oxide film becomes more nonhomogeneous.
3.1.6 Passive Film on Titanium
Titanium forms a stable, protective, strongly adherent oxide film. This film
forms instantly when a fresh surface is exposed to air or moisture. Addition
of alloying elements to titanium affects the corrosion resistance because
these elements affect the composition of the oxide film.
The oxide film of titanium is very thin and is attacked by only a few substances, the most notable of which is hydrofluoric acid. Because of its strong
affinity to oxygen, titanium is capable of healing ruptures in this film almost
instantly in any environment when a trace of moisture is present.
3.1.7 Passive Film on Tantalum
When exposed to oxidizing or slightly anodic conditions, tantalum forms
a thin impervious layer of tantalum oxide. This passivating oxide has the
broadest range of stability with regard to chemical attack or to thermal
breakdown compared to other metallic films. Chemicals or conditions that
attack tantalum, such as hydrofluoric acid, are those that penetrate or dissolve the film.
3.1.8 uniform Corrosion rates
When exposed to a corrosion medium, metals tend to enter into a chemical union with the elements of the corrosion medium, forming stable compounds similar to those found in nature. When metal loss occurs in this
manner, the compound is referred to as the corrosion product and the surface
is referred to as having been corroded. An example of such an attack is that
of halogens, particularly chlorides. They will react with and penetrate the
film on stainless steel, resulting in general corrosion. Corrosion tables were
developed to indicate the interaction between a chemical and a metal. This
type of attack is called general or uniform corrosion. It is one of the most easily measured and predictable forms of corrosion. Many references exist that