aluminum. Aluminum alloys containing silicon or manganese and the
heterogeneous aluminum–magnesium alloy yield coatings that rangef rom
gray to brown and may be patchy in some cases. The adsorptive power of
these coatings make them excellentb ases for dyes, especiallyi ft hey are
sealedi nn ickel or cobalt acetate solution.
It is not recommended to use sulfuric acidasthe electrolyte for anodizing
work containing jointsthat can retain the sulfuric acid after removal from the
bath. The retained electrolyte will provide sites for corrosion.
When chromic acid is used as the electrolyte, the coatings produced are
generally opaque, gray,and irridescent, with the quality being dependenton
the concentration and purity of the electrolyte. These are unattractive as
compared to those formed usingsulfuric acidasthe electrolyte. When 0.03%
sulfate is added to the electrolyte, colorless and transparent coatings are
formed. These coatings are generally thin, of low porosity,a nd difficult to
dye. Blackc oatings can be obtained in concentrated solutions at elevated
temperatures.A ttractive opaques urfacesc an be obtained by adding
titanium, zirconium, and thallium compounds to the electrolyte.
The chromic acid anodizing process is the only one that can be used on
structures containingb lind holes, crevices, or difficult to rinsea reas.
Chromic acida nodizing generally increases fatigues trength, whereas,
sulfuric acid anodizing may produce decreases in fatigue strength.
Boric acidelectrolytes produce afilm that is irridescent and oxides in the
range of 2500–7500A ˚ .The coating is essentially nonporous.
Oxalic and other organic acids are electrolytes used to produce both
protective and decorative films. Unsealed coatings are generallyy ellowi n
color.T hese films are harder and more abrasion resistant than the conventional sulfuric acid films. However, the specially hard coatings produced
under special conditions in sulfuric acid electrolytes ares uperior.
The anodized coating consists of two major components: the nonporous
barrierl ayer adjoiningt he metal and ap orous layer extending from the
barrierl ayer to the outer portion of the film. Sulfuric, chromic, and oxalic
acid electrolytes form both barrier and porous layers, whereas, boric acid
electrolytes produce onlybarrier films.
Anodizing of aluminum provides long-term corrosionr esistance and
decorative appearance. Corrosion of the anodizedfilm is induced by SO x gas
andd epositionso fg rime,s ulfates, andc hlorides.T hese depositions
promote corrosion because they tend to absorb aggressive gases and
moisture, thereby,i ncreasing 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 surfacerather than making the surfacecorrosive.Some of the depositions
are removed by the rain. Cleaning with water is one method that helps to
protect the anodized aluminum from corrosion. In marinea tmospheres,
the depositions can be removed with water because the depositions are
primarily soluble chlorides. However, in industrial atmospheres, detergents
are needed because the deposits areg reasy.
Corrosion of Linings and Coatings
458
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
heterogeneous aluminum–magnesium alloy yield coatings that rangef rom
gray to brown and may be patchy in some cases. The adsorptive power of
these coatings make them excellentb ases for dyes, especiallyi ft hey are
sealedi nn ickel or cobalt acetate solution.
It is not recommended to use sulfuric acidasthe electrolyte for anodizing
work containing jointsthat can retain the sulfuric acid after removal from the
bath. The retained electrolyte will provide sites for corrosion.
When chromic acid is used as the electrolyte, the coatings produced are
generally opaque, gray,and irridescent, with the quality being dependenton
the concentration and purity of the electrolyte. These are unattractive as
compared to those formed usingsulfuric acidasthe electrolyte. When 0.03%
sulfate is added to the electrolyte, colorless and transparent coatings are
formed. These coatings are generally thin, of low porosity,a nd difficult to
dye. Blackc oatings can be obtained in concentrated solutions at elevated
temperatures.A ttractive opaques urfacesc an be obtained by adding
titanium, zirconium, and thallium compounds to the electrolyte.
The chromic acid anodizing process is the only one that can be used on
structures containingb lind holes, crevices, or difficult to rinsea reas.
Chromic acida nodizing generally increases fatigues trength, whereas,
sulfuric acid anodizing may produce decreases in fatigue strength.
Boric acidelectrolytes produce afilm that is irridescent and oxides in the
range of 2500–7500A ˚ .The coating is essentially nonporous.
Oxalic and other organic acids are electrolytes used to produce both
protective and decorative films. Unsealed coatings are generallyy ellowi n
color.T hese films are harder and more abrasion resistant than the conventional sulfuric acid films. However, the specially hard coatings produced
under special conditions in sulfuric acid electrolytes ares uperior.
The anodized coating consists of two major components: the nonporous
barrierl ayer adjoiningt he metal and ap orous layer extending from the
barrierl ayer to the outer portion of the film. Sulfuric, chromic, and oxalic
acid electrolytes form both barrier and porous layers, whereas, boric acid
electrolytes produce onlybarrier films.
Anodizing of aluminum provides long-term corrosionr esistance and
decorative appearance. Corrosion of the anodizedfilm is induced by SO x gas
andd epositionso fg rime,s ulfates, andc hlorides.T hese depositions
promote corrosion because they tend to absorb aggressive gases and
moisture, thereby,i ncreasing 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 surfacerather than making the surfacecorrosive.Some of the depositions
are removed by the rain. Cleaning with water is one method that helps to
protect the anodized aluminum from corrosion. In marinea tmospheres,
the depositions can be removed with water because the depositions are
primarily soluble chlorides. However, in industrial atmospheres, detergents
are needed because the deposits areg reasy.
Corrosion of Linings and Coatings
458
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485