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Table 9.1
Steel sheets chemical composition (%)
Elements
C
Mn
Si
P
S
Al
Cu
wt%
0.003
0.31
0.03
0.025
0.04
0.025
0.032
Table 9.2 Composition of enamel frits in wt% of oxides
Oxides
SiO
2
B
2
O
3
Na
2
OK
2
O
CaO
BaO
MnO
CoO
NiO
CuO
TiO
2
Li
2
O
Frit 1
58
10
12
3
3
4
2.6
2.8
2.4
1.2
1
-
Frit 2
69
4
10
4
-
-
2.8
2.2
-
-
13
5
The wet enamel was prepared by milling, in an alumina ball mills, the two frits
with the proper additions of water, clay, and metal oxides for color control. The
milling process takes 4 h in order to obtain wet blends containing particles of
controlled size (*45 ± 6 lm). The steel sheets for the wet enamel deposition
were pre-treated according to the following seven steps: (i) degreasing at 60 C
with alkaline degreasing bath, (ii) acid attack in a 5 vol% solution of sulfuric acid
at 60 C, (iii) room temperature water bath, (iv) nickel deposition by immersion in
a 1.2 % of NiSO
4
bath, (v) room temperature water bath, (vi) immersion in a
0.3 % neutralizer solution at 60 C, and (vii) drying up at 110 C.
The dry enamel was prepared by milling, in an alumina ball mills, the two frits
with the proper additions of silicone oil. The milling process takes 9 h in order to
obtain a dry powder with a controlled size (*28 ± 5 lm). The steel sheets used to
prepare the samples by the dry enamel raw material were polished to take out all
superficial impurities due to sheets machining.
After enamels deposition over the steel substrates, the all systems enamel-
substrate were fired at the same conditions:
• heating phase, from room temperature to 850 C, in 6 min;
• maturing phase, at a constant temperature of 850 C, for 6.5 min;
• cooling phase from 850 C to room temperature in 7 min.
In Fig.
9.6
, two examples of micrographics images of the transversal section of
the enameled steel are reported.
In particular, in Fig.
9.6
a the transversal section of the coating obtained by means
of the wet enamel is reported, and in Fig.
9.6
b the coating obtained by means of the
dry enamel. From these images, it is possible to note that in the case of wet enamel
the distribution of the bubbles in the coating section is almost uniform; on the
contrary, in the case of the dry enamel the bubbles are dispersed near the external
face of the coating. This fact is due to the raw material formulation and in particular
to the presence of the clay in the wet enamel formulation. The usage of clays in the
wet enamel manufacturing aims to control both the slips rheology as well as
the bubbles distribution in the final coating. Moreover, in both images, it is possible
to note the presence of dendrites between the coating and the substrate.
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