Audi TT 2016 Introduction Manual page 15

Safety precautions against contact corrosion
Aluminum has a naturally occurring (passivating) surface
oxide layer that protects the underlying material from
corrosion. This is why an unpainted aluminum component
does not normally corrode.
However, if aluminum comes into contact with a metal
which has a positive electrical potential relative to alu-
minum and is in an electrolyte, such as salt water, contact
corrosion will occur.
The greater the difference in electrical potential, the
greater the corrosion will be. Since aluminum is usually the
less noble metal, it is degraded.
Flow-drill screwing
Contact corrosion can also occur if unsuitable connecting
elements (screws, nuts, washers, etc.) are used. Therefore,
a number of precautions must be taken in order to prevent
this process from occurring during body production and
repair work. Audi uses only connecting elements with a
special surface coating for the aluminum/steel hybrid
construction used in the TT. In addition, all rubber and
plastic parts and all adhesives are made of non-conductive
materials. As a further precaution, all hybrid metal connec-
tions are sealed with polyvinyl chloride (PVC) or with wax
after cataphoretic dip primer coating. The cataphoretic
coating (sometimes referred to as E-coat) is an electrically
induced surface treatment that provides resistance to
corrosion and oxidation. See the complete definition on the
following page.
Solid punch riveting
MIG/MAG welding
Clinching
630_075
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