296
Fundamentals of Corrosion
Boric acid electrolytes produce a film that is iridescent and oxides in the
range of 2500–t5oo Å. The coating is essentially nonporous.
Oxalic and other organic acids are electrolytes that are used to produce
both protective and decorative films. Unsealed coatings are generally yellow in color. These films are harder and more abrasion resistant than conventional sulfuric acid films. However, the specially hard coatings produced
under special conditions in sulfuric acid are superior.
The anodized coating consists of two major components: the nonporous
barrier layer adjoining the metal and a porous layer extending from the barrier layer to the outer portion of the film. Sulfuric, chromic, and oxalic acid
electrolytes form both barrier and porous layers while boric acid electrolytes
produce only barrier films.
Anodizing of aluminum provides long-term corrosion resistance and a
decorative appearance. Corrosion of the anodized film is induced by SO x
gas and depositions of grime, sulfates, and chlorides. These depositions promote corrosion because they tend to absorb aggressive gases and moisture,
thereby increasing the time of wetness and decreasing the pH of the electrolyte at the interface between the depositions and the surface.
Although rain increases the time of wetness, it has the effect of cleaning
the surface rather than making the surface corrosive. Some of the depositions are removed by rain. Cleaning with water is one method that helps
protect the anodized aluminum from corrosion. In marine atmospheres, the
depositions can be removed with water because the depositions are primarily soluble chlorides. However, in industrial atmospheres, detergents are
needed because the deposits are greasy.
SO x gas is the most aggressive pollutant for anodic films. The corrosive
effect depends on the concentration, with the corrosion area increasing linearly with concentration.
8.6 Eclipse Molding Framework (EMF) Control Protection
The types of coatings belonging in this category are paints and other organic
coatings. Surface conditions are converted to more stable states by coating
with organic compounds. This method delays the generation of the electromotive force causing the corrosion of base metal.
The service life of an organic coating is determined by the durability of
the coating itself and its adhesive ability on the base metal. The former is the
stability of a coating layer as exposed to various environmental factors, and
the latter is determined by the condition of the interface between the organic
film and the interface.
Details referring to the protective abilities and causes of corrosion of organic
polymer films were given in Chapter 6, while the details of the protective
abilities and causes of corrosion of paint films were given in Chapter 7.
Fundamentals of Corrosion
Boric acid electrolytes produce a film that is iridescent and oxides in the
range of 2500–t5oo Å. The coating is essentially nonporous.
Oxalic and other organic acids are electrolytes that are used to produce
both protective and decorative films. Unsealed coatings are generally yellow in color. These films are harder and more abrasion resistant than conventional sulfuric acid films. However, the specially hard coatings produced
under special conditions in sulfuric acid are superior.
The anodized coating consists of two major components: the nonporous
barrier layer adjoining the metal and a porous layer extending from the barrier layer to the outer portion of the film. Sulfuric, chromic, and oxalic acid
electrolytes form both barrier and porous layers while boric acid electrolytes
produce only barrier films.
Anodizing of aluminum provides long-term corrosion resistance and a
decorative appearance. Corrosion of the anodized film is induced by SO x
gas and depositions of grime, sulfates, and chlorides. These depositions promote corrosion because they tend to absorb aggressive gases and moisture,
thereby increasing the time of wetness and decreasing the pH of the electrolyte at the interface between the depositions and the surface.
Although rain increases the time of wetness, it has the effect of cleaning
the surface rather than making the surface corrosive. Some of the depositions are removed by rain. Cleaning with water is one method that helps
protect the anodized aluminum from corrosion. In marine atmospheres, the
depositions can be removed with water because the depositions are primarily soluble chlorides. However, in industrial atmospheres, detergents are
needed because the deposits are greasy.
SO x gas is the most aggressive pollutant for anodic films. The corrosive
effect depends on the concentration, with the corrosion area increasing linearly with concentration.
8.6 Eclipse Molding Framework (EMF) Control Protection
The types of coatings belonging in this category are paints and other organic
coatings. Surface conditions are converted to more stable states by coating
with organic compounds. This method delays the generation of the electromotive force causing the corrosion of base metal.
The service life of an organic coating is determined by the durability of
the coating itself and its adhesive ability on the base metal. The former is the
stability of a coating layer as exposed to various environmental factors, and
the latter is determined by the condition of the interface between the organic
film and the interface.
Details referring to the protective abilities and causes of corrosion of organic
polymer films were given in Chapter 6, while the details of the protective
abilities and causes of corrosion of paint films were given in Chapter 7.