Page 121 - QRedition5.0 062021

Quality Requirements Edition 5.0 European Enamel Association

8.7 Valuation of enamelling defects therefore on enamellings unavoidable. The defect frequency will be reduced down to a
8.7.1 General hints minimum by a neat operation in good controlled plants, and utilization of defined and
checked base materials.

Definition
Defects in the enamelling are locally limited interruptions of the compactness or structure For the optical surface quality, real and to the functional purpose of the workpiece adjusted
of the glasslike coating. The local penetration of one layer by another is likewise to be seen control standards are in particular an important criterion. Relating to this, a too severe
as a defect on multilayer enamellings, as in this case it is also a structural interruption. standard reduces in general the entire product quality. The susceptibility to mechanical
Structural interruptions are not always visible by naked eye. Physical or chemical analyses damage (chipping, cracks) increases with the rise of enamel thickness. With each rework
may be used for the detection of defects. operation, the application thickness increases inevitably. To secure the corrosion
resistance on difficult accessible areas of articles with a high corrosion demand, special

By judging the defects one should consider, whether thereby a diminution of function of the testing methods can be helpful (e.g. measurement of the protective current requirement on
article or merely an optical detraction arises. The optical Valuation is independent of the enamelled hot water tanks).
functional application.

If by agreement with a client of certain articles, a certain number of defects (pores) is not to

Typical enamelling defects are : be infringed, porosity tests complying to the defined standards ISO 2746 (high voltage)
Burn off (EEA 8.7.2) respectively to EN ISO 8289, NEN 2709 (sponge test with low voltage) have to be carried
Cracks (EEA 8.7.3) out. Note: High voltage can cause open defects (pores) from big blisters.
Pores and Inclusions (EEA 8.7.4)
Chipping, Spalling (EEA 8.7.5)
Contamination, fallen-on (EEA 8.7.6)
in different production areas
Black Specks in the (EEA 8.7.7)
enamelling

The common characteristic of enamellings is the melting together of siliceous (glass-like)
and metallic components in a temperature range of 500 - 950 C. By this, reactions with
0
the formation of gasses occur (mainly H , CO and CO ). Detrimental secondary effects can
2 2
be suppressed but never entirely eliminated. Within an industrial process, contaminations,
also from different production lines are never entirely avoidable - their effects intensify by
the reaction with the molten enamel at elevated temperature. A certain part of defects is Illustration: Big blister in an enamel layer. Its thin glass-skin can be disturbed by the high
voltage test. (enlargement 200 : 1)

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