Aluminium Casting Alloys - Aleris

Surface treatment: corrosion
and corrosion protection
Aluminium casting alloys – like wrought
aluminium alloys – owe their corrosion
resistance to a thin, tenacious coating
layer of oxides and hydroxides. In the pH
range from 4.5 to 8.5, this oxide layer is
practically insoluble in aqueous media
and aluminium casting materials suffer
only negligible mass disappearance.
This passivity can, however, be annulled
locally at weak points in the oxide layer
due to the action of water containing
chloride. Since the aqueous medium,
e.g. weather, only acts periodically, a
protective oxide layer forms again at
small, local corrosion sites, e.g. repassivation
occurs. Deep pitting corrosion
can only arise when there is a long-term
effect from aggressive water containing
chloride (e.g. sea water). Beside the
chloride content, the amount of oxygen
in the water also plays a role; corrosion
reaction can only occur in neutral media
(pH = 4.5-8.5) in the presence of
oxygen. The remedy for this can come
in the form of passive protection by
coating or by means of active cathodic
corrosion protection using a sacrifi cial
anode, for example.
Magnesium as an alloying element
causes the formation of a thicker oxide
layer containing MgO and, consequently,
provides greater corrosion protection
against water containing chlorides and
48
Aluminium Casting Alloys
slightly alkaline media (e.g. ammonia
solutions) since magnesium oxide in
contrast to aluminium oxide is insoluble
in alkaline solutions.
Copper as an alloying element causes
a deterioration in corrosion properties.
This increases slightly with a rising Cu-
content in the range below 0.2 % copper,
above 0.2 to 0.4 % more strongly.
Already with a Cu-content of 0.2 %,
permanent action from aqueous solutions
containing chlorine can have a very
negative effect on corrosion behaviour.
The negative infl uence of iron on cor-
rosion behaviour is not as distinctive
as that of copper. With an Fe-content
of up to 0.6 %, there is no signifi cant
deterioration in the corrosion behaviour
of casting alloys.
The surface treatment of aluminium cast
products is carried out to improve their
corrosion resistance, for decorative
purposes or to increase the strength
of the components.
A homogeneous, non-porous cast struc-
ture free from shrink holes and cracks
makes coating easier. The quality of the
coating is infl uenced decisively by the
pre-treatment.
Wiping, immersion and steam degreasing
(in that order) produce increasingly
grease-free surfaces without removing
the surface oxide fi lm. Grinding, brush-
ing, abrasive blasting or polishing do not
remove the oxide fi lm completely and,
as a rule, act as a preparation to further
surface treatment. Possible sources of
defects leading to subsequent faults
comprise the use of brushes made of
brass or non-stainless steel as well as
sand or steel shot.
When grinding, the use of ceramic
grinding elements without further pretreatment
frequently leads to good paint
adhesion. One precondition is that no
fi nes from the grinding elements are
pressed into the surface of the casting.
Chemical degreasing agents with
a pickling or etching effect remove the
oxide layer and, as a consequence, all
impurities. It is also worth mentioning
that there is also matt or bright pickling
before anodic oxidation to produce a
special surface fi nish.
Following the alkaline pickling of AlMg
or AlSi casting alloys, the pickling fi lm
must be removed by means of an acid
after-treatment with nitric acid, nitric/
hydrofl uoric acid or sulphuric/hydrofl
uoric acid. Instead of alkaline pickling
with fi nal dipping, it is more benefi cial
to use an acidic fl uoride-containing
pickling solution immediately.