The service life of ac hromate coating depends on the coating thickness.
Chromate coatings absorb moisture, and moisture results in the leaching of
hexavalent chromium. Therefore, the longer the time of wetness, the shorter
the life of the coating. However, as long as the leaching of the hexavalent
chromium continues, the basem etal is protected.
Environmental conditions, particularly time of wetness and temperature,
determine the leaching rate. In natural environments, the leaching rate is
commonly low.Pollutants in the atmosphere, particularly chlorideions, also
increase the rate of deteriorationoft he film. Chromate conversion coatings
provide good corrosionr esistance in am ilda tmosphere, sucha si ndoor
atmospheres, and surfacea ppearance. They also provide ag ood base for
organic films.
Chromate conversionc oatings are usually appliedt oz inc and its alloy
coated sheets, to protect against staining during storage,and to products of
zinc-die castings, aluminuma nd its alloys, and magnesium and its alloys.
7.12.3O xideCoatings
Iron or steel articles to be coated are heatedinaclosed retort to atemperature
of 16008 F/871 8 C; afterwards, superheated steam is admitted.T his results
in the formation of red oxide( Fe 2 O 3 )a nd magnetic oxide( Fe 3 O 4 ). Carbon
monoxideisthen admitted to the retort, reducing the red oxide to magnetic
oxidet hat is resistant to corrosion. Each operation takes approximately
20 min.
Iron and steel may also be oxide coated by electrolytic means. The article
to be coatedi sm ade the anode in an alkaline solution (anodic oxidation).
These coatings areprimarily for appearance such as for cast iron stove parts.
7.12.4A nodized Coatings
The electrochemical treatment of am etal serving as an anode in an
electrolyte is known as anodizing. Since aluminum’se lectrode potential is
negativea nd its oxide film is stablei nn aturale nvironments, surfacet reatments have been developed for the purpose of producing more stable oxide
films. The anodic films formed can be eitherporous or nonporous depending upon which electrolyte is used.
Porous films result when electrolytes such as sulfuric acid, oxalic acid,
chromic acid, and phosphoric acid are used. These films have the advantage
of being able to be dyed.
Sulfuric acid is the most widely used electrolyte. Al arge range of
operating conditions can be utilizedt op roduce ac oating to meet specific
requirements. Hard protective coatings are formed that serve as ag ood
base for dyeing. In order to obtain the maximum corrosion resistance, the
porous coating must be sealed after dyeing. The anodic coating formed,
usings ulfuric acid as the electrolyte, is clear and transparent on pure
Coatings
457
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
Chromate coatings absorb moisture, and moisture results in the leaching of
hexavalent chromium. Therefore, the longer the time of wetness, the shorter
the life of the coating. However, as long as the leaching of the hexavalent
chromium continues, the basem etal is protected.
Environmental conditions, particularly time of wetness and temperature,
determine the leaching rate. In natural environments, the leaching rate is
commonly low.Pollutants in the atmosphere, particularly chlorideions, also
increase the rate of deteriorationoft he film. Chromate conversion coatings
provide good corrosionr esistance in am ilda tmosphere, sucha si ndoor
atmospheres, and surfacea ppearance. They also provide ag ood base for
organic films.
Chromate conversionc oatings are usually appliedt oz inc and its alloy
coated sheets, to protect against staining during storage,and to products of
zinc-die castings, aluminuma nd its alloys, and magnesium and its alloys.
7.12.3O xideCoatings
Iron or steel articles to be coated are heatedinaclosed retort to atemperature
of 16008 F/871 8 C; afterwards, superheated steam is admitted.T his results
in the formation of red oxide( Fe 2 O 3 )a nd magnetic oxide( Fe 3 O 4 ). Carbon
monoxideisthen admitted to the retort, reducing the red oxide to magnetic
oxidet hat is resistant to corrosion. Each operation takes approximately
20 min.
Iron and steel may also be oxide coated by electrolytic means. The article
to be coatedi sm ade the anode in an alkaline solution (anodic oxidation).
These coatings areprimarily for appearance such as for cast iron stove parts.
7.12.4A nodized Coatings
The electrochemical treatment of am etal serving as an anode in an
electrolyte is known as anodizing. Since aluminum’se lectrode potential is
negativea nd its oxide film is stablei nn aturale nvironments, surfacet reatments have been developed for the purpose of producing more stable oxide
films. The anodic films formed can be eitherporous or nonporous depending upon which electrolyte is used.
Porous films result when electrolytes such as sulfuric acid, oxalic acid,
chromic acid, and phosphoric acid are used. These films have the advantage
of being able to be dyed.
Sulfuric acid is the most widely used electrolyte. Al arge range of
operating conditions can be utilizedt op roduce ac oating to meet specific
requirements. Hard protective coatings are formed that serve as ag ood
base for dyeing. In order to obtain the maximum corrosion resistance, the
porous coating must be sealed after dyeing. The anodic coating formed,
usings ulfuric acid as the electrolyte, is clear and transparent on pure
Coatings
457
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485