Environmental Engineering Reference
In-Depth Information
involving boiling water containing chromate is still commonly used. By
means of the hot water sealing (HTS) treatment, the hydration of the
aluminium oxide results in a volume expansion of the i lm resulting in
an occludation of the surface pores. Additives in the sealing bath are
sometimes used to increase the ei ciency of treatment. h e hazard of
this treatment is due to the dif erential thermal dilatations between the
substrate and the i lm, which might lead to the crazing of the i lm when
thermally loaded [15].
Cold sealing techniques of anodic i lms via CCC (Chromate
Conversion Coating) may of er advantages concerning the integrity of
the anodized coating. Since the beginning of the 1990s, the high toxicity
associated with Cr(VI) has imposed restrictions on its use in industrial
applications and consequently has led to the development of Cr(VI)-free
surface treatments. A number of promising “green” alternatives have
been studied with the objective of replacing chromates, and one of the
most promising alternatives to chromates in the metal-i nishing indus-
tries are silanes [16-18]. Silicon compounds improve the properties of
materials such as chemical resistance, mechanical strength and electrical
performances.
Further improvement of the corrosion resistance is possible by applying
HTS; dif erent silane conversion coatings at er TSA anodization with and
without HTS were tested within this work.
2.2
Silanes for Surface Treatment
h e goal of replacing the use of chromate for the surface treatment of
metals with silanes has a lot of advantages from an environmental point
of view, economical aspects, employment security and as far as the corro-
sion inhibitive ef ects on metals are concerned. Van Ooij and coworkers
have studied the corrosion performances of a variety of silane i lms on
dif erent metals substrates [19]. Silane coatings can be used for various
metals such as aluminium and its alloys [11, 20-22], copper [23], iron and
steel [24], zinc [25] and magnesium [26] coating alloys and could ei -
ciently protect metals against dif erent forms of corrosion. Plueddemann
[27] used the application of silicon compounds as coupling agents to
promote adhesion in the surface treatment of metals. Silanes have the
general chemical formula (RO) 3 Si(CH 2 ) n R, where RO is a hydrolyzable
alkoxy group (methoxy - OCH 3 ; ethoxy - OC 2 H 5 or acetoxy OCOCH 3 );
R ' is an organofunctional group (vinyl - C=C, amino - NH 2 ), and the
value of n is around 3 [28].
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