The chemical effects of phosphating on the surfaceistoconvert the surface
to anonalkaline condition, protecting the surface from reactions with oils in
paint and to protect against the spread of corrosion from defects. Alkaline
residues on the surfaceo ft he base metal lead to underfilmcorrosion.
Phosphating increases the uniformityi nt he surfacet exturea nd surface
area that improves paint adhesion, that in turn, increases the service life of a
paint film.
7.12.2C hromate Coating
Chromate conversionc oatings are formed on aluminum and its alloys—
magnesium, zinc, and cadmium. These coatings provide good corrosion
protection and improve the adhesion of organic layers. Achromate coating is
composed of acontinuous layer consisting of insolubletrivalent chromium
compounds and soluble hexavalentc hromium compounds. The coating
structure provides as econdary barrier,i nhibiting action, and also good
adhesion for lacquer films.
Chromate coatings providet heir corrosionr esistanceb ased on the
following three properties:
1. Cr(III) oxide, that is formed by the reduction of Cr(IV) oxide, has
poor solubilityi na queous mediaa nd,t hereby,p rovidesa
barrier layer.
2. Cr(VI) willb ei ncluded in the conversion coating and willb e
reduced to Cr(III) to passivatet he surfacew heni ti sd amaged,
preventing hydrogen gas from developing.
3. The rate of cathodic oxygenreactions is strongly reduced.
Most chromate conversion coatings are amorphous gel-like precipitates,
so they are excellent in continuity.The service life is dependent on thickness,
characteristicsofthe base metal, coating conditions—particularly dry heat—
and the environmental conditions that the chromated products are used
under.
When ac hromated product is exposed to the atmosphere, hexavalent
chromium slowly leaches from the film, with the result that the surface
appearance changes from irridescent yellow to either ag reen color or to
clear.T he structure of the film consists of more of the insoluble trivalent
chromium compounds. Passivation is provided for any damaged areas by
the leached hexavalentchromium.
The longer the time of wetness, the shorter the service life of the coating
because chromate coatings absorb moisture, and moisture results in the
leaching of hexavalent chromium. The leaching behaviorofachromate film
is also affected by its aging process, drying process, and long-term storage.
Aging of ac hromate coating reduces its protective ability.
Coatings
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CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
to anonalkaline condition, protecting the surface from reactions with oils in
paint and to protect against the spread of corrosion from defects. Alkaline
residues on the surfaceo ft he base metal lead to underfilmcorrosion.
Phosphating increases the uniformityi nt he surfacet exturea nd surface
area that improves paint adhesion, that in turn, increases the service life of a
paint film.
7.12.2C hromate Coating
Chromate conversionc oatings are formed on aluminum and its alloys—
magnesium, zinc, and cadmium. These coatings provide good corrosion
protection and improve the adhesion of organic layers. Achromate coating is
composed of acontinuous layer consisting of insolubletrivalent chromium
compounds and soluble hexavalentc hromium compounds. The coating
structure provides as econdary barrier,i nhibiting action, and also good
adhesion for lacquer films.
Chromate coatings providet heir corrosionr esistanceb ased on the
following three properties:
1. Cr(III) oxide, that is formed by the reduction of Cr(IV) oxide, has
poor solubilityi na queous mediaa nd,t hereby,p rovidesa
barrier layer.
2. Cr(VI) willb ei ncluded in the conversion coating and willb e
reduced to Cr(III) to passivatet he surfacew heni ti sd amaged,
preventing hydrogen gas from developing.
3. The rate of cathodic oxygenreactions is strongly reduced.
Most chromate conversion coatings are amorphous gel-like precipitates,
so they are excellent in continuity.The service life is dependent on thickness,
characteristicsofthe base metal, coating conditions—particularly dry heat—
and the environmental conditions that the chromated products are used
under.
When ac hromated product is exposed to the atmosphere, hexavalent
chromium slowly leaches from the film, with the result that the surface
appearance changes from irridescent yellow to either ag reen color or to
clear.T he structure of the film consists of more of the insoluble trivalent
chromium compounds. Passivation is provided for any damaged areas by
the leached hexavalentchromium.
The longer the time of wetness, the shorter the service life of the coating
because chromate coatings absorb moisture, and moisture results in the
leaching of hexavalent chromium. The leaching behaviorofachromate film
is also affected by its aging process, drying process, and long-term storage.
Aging of ac hromate coating reduces its protective ability.
Coatings
455
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485