HANDBUCH DER GUSSWERKSTOFFE - Honsel

28
treatment conditions therefore either promote or inhibit
corrosion. Local formation of a galvanic element is
frequently the cause of local corrosion. This results in
anodic dissolution of the base constituents.
During the assembly of components comprising different
materials care should be taken to ensure that no elec -
tri cally conductive medium comes between them. This
would otherwise produce a galvanic element and result
in the electrochemical dissolution of base metal. The
potential difference between the materials is the basic
cause for the dissolution of the metal, the density of the
anodic current being the decisive factor. Consequently,
the area ratio of both material partners (electrodes)
play a decisive role.
According to the requirements various methods are avail -
able for corrosion protection. The natural oxide skin can
be replaced by considerably thicker hard anodised layers
of up to 100 µm thickness. In so doing, the Si alloys be -
come grey in colour and not suitable for decorative purposes.
Technical anodising results in considerably better
corrosion protection and in the non-compacted condition
provides anex cellent base for paint systems. Further more,
chemically produced coatings such as phosphating and
chromating can be used for protection against corrosion.
A further field of the use of paint and coating systems is
also available for corrosion protection with a decorative
appearance but special advice is required here.
Results from standardised test methods for corrosion be -
haviour can only be compared under known con ditions.
Direct transfer into practical use is not admissible.
MAGNESIUM
In air, magnesium spontaneously covers itself with a thin
layer of magnesium oxide (hydroxide). Contrary to alu-
Example of the connection of Mg with other
components
Mg
Iron or
Al casting
To be avoided Better Recommendable
Al
minium, the layer is porous and does not form suffi cient
protection against corrosion. Magnesium is hardly attakked
by alkalis. Contrary to this it is strongly attacked by
nearly all acids, with the ex cep tion of concen trated
hydrofluoric acid. Elements such as Ni, Fe and Cu lead to
an increase in the corrosion rate, even when present as
impurities. For this reason alloys have been developed
with particularly low contents of these elements, e.g. a
maximum of only 10 ppm Ni. These alloys are generally
identified by the addition of HP (High Purity) and have
only a fraction of the usual rates of corrosion. EN alloys
comply with these requirements. Magnesium is one of
the electrically negative materials, forming a galvanic
element with almost all other metals and resulting in its
dissolution. This effect is put to practical use in boilers
and on ships by means of so-called „sacrificial anodes“.
The high negative electrical potential (- 2,3 V) leads to
the fact that special measures have to be taken when
assembling together with other components. Even the
design of a casting is a contributory decisive factor with
regard to the corrosion resistance.
As a basic principle, if there is no electrolyte between
the adjoining metals, then there is no corrosion. When
different materials need to be assembled, plastic caps,
wide flange bolts, Al bolts or interme diate layers of
plastic reduce the corrosion attack.
Protection of the surface against corrosion is achieved
by various processes. Mainly pro tection is by means of
trans formation layers such as chromating and anodising,
organic and metallic coatings as well as spraying pro -
cesses. In most cases coatings and paints are applied for
decorative require ments. Consultation with our engineers
is re commendable in order to ensure correct
implementation.
Salt spray corrosion resistance
(ASTM B 117)
Corrosion rate (mg/cm 2 day)
0,30
0,25
0,20
0,15
0,10
0,05
0,00
AZ91D AM60B AM50A AS21 AE42 AlSi9Cu3
Resistance of various Mg casting alloys according to the salt spray test