Environmental Engineering Reference
In-Depth Information
examples of domestic applications. Glass coatings are susceptible to mechanical
damage and cracking by thermal shocks and as such care should be taken against
such eventualities.
Chemical Conversion Coatings
Chemical conversion coatings are different from other types of coatings in that
these are formed in situ by chemical reaction with the metal surface. Coatings
comprise inorganic compounds rather than surface alloying (cf. chemical vapor
deposition). Sometimes such coatings form naturally when exposed to corrosives
and further corrosion is prevented. The formation of lead sulfate coating on expo-
sure to sulfuric acid is an example. In other cases, such coatings are artificially
developed for protection against subsequent corrosive exposures.
Anodizing
of aluminum is a prominent example of chemical conversion coat-
ing. Aluminum is inherently associated with a surface oxide film of a few mi-
crometers thickness. The film is thickened (1-7 mils) by anodic oxidation of
aluminum in a suitable electrolyte, usually dilute sulfuric acid or chromic acid,
with the passage of current. Anodized coatings are hard and tightly adherent to
the base. The hardness approaches that of corundum. The corrosion resistance
does not improve much on anodizing, as the oxides are tubular and porous. How-
ever, the pores can be ''sealed'' by exposure to boiling water or steam for several
minutes and the corrosion resistance of the scaled anodized layer will improve
in a wide variety of corrosives. The anodized layer also provides a good base
for application of paints, which is otherwise difficult on aluminum. Magnesium
can also be anodized.
Phosphatizing
of steel is another important example of chemical conversion
coating. The steel surface is treated with a cold or hot solution of dilute zinc or
magnesium phosphate or orthophosphate with phosphoric acid. A network of
porous metal phosphate is produced on the metal surface. The phosphate coating
does not provide much corrosion protection, but it also provides a good base
for subsequent paint application. Phosphatizing also ensures protection against
corrosion during the interval between fabrication of the part and application of
paint. Automobile bodies are the best known examples of phosphatizing treat-
ment.
Oxide coatings
are produced on steel by controlled high-temperature oxidation
in air or by treatment in hot alkali solutions containing some oxidizing additives
such as nitrates, chlorates, or persulfates. Black, brown, or blue coatings are de-
veloped, depending on film thickness. The oxide coatings are not protective but
are made so by rubbing with inhibitor-containing oils or waxes. Gun barrels pro-
vide example for oxide-coated utility.
Chromate
coatings are produced on zinc, cadmium, and on coatings of these
metals on steel. An immersion in sodium dichromate solution acidified with sulfu-
