A multipurpose anodising plant for non ferrous metals - Aluplanet

MultiAnodHK
Notes:
1. In the present paper no didactic or scientific aspect will be dealt; who is interested can
refer to some previous work of the same Authors. (1-7)
2. Part of the same subject is included in AAC Annual Conference Proceedings of 2007 (8)
3. Part of the same subject is included in Alumotive 2007 Conference Proceedings (9)
1- Foreword
Magnesium as explained in a recent congress can have a very broad horizon (8) assuming
suitable treatment is used to protect it against corrosion. As a regulation in force (10) limits the use /
presence of chromium salts in automotive and electronics, the chemical conversion treatments
known as Dow 1, 19, 21 or anodising ones like Dow 17 and Dow HAE, because all using chromic
acid in the treatment solutions and chromium (VI) must be substituted by something more
environmental friend. Such treatments, dating back to ‘40s – ‘50s, are well known and still included
in ASTM and MIL Specs, but cannot be applied in the mentioned fields.
Chrome free conversion coating on magnesium are not always completely satisfactory on
magnesium especially when the base material is not so good as casting quality or because the alloy
is aluminium free.
There is an important demand to have a good quality treatment for cast aluminium materials
especially for high silicon containing alloys. It’s true that in a medium term, magnesium should
replace a wide part of cast aluminium, but at the present time even aluminium products are
available on the market and need. A good corrosion and aesthetic finishing of high silicon content
aluminium alloys are not easy but the method proposed by us is really well promising.
We observed the same solution can be used for titanium materials and the results are
interesting even from an aesthetic point of view.
Originally, our process was studied for magnesium but its extension to cast aluminium appeared
innovative and giving excellent results even on cast aluminium, allowing an innovative white
finishing on it.
In our description we follow the trend of our research starting from magnesium and passing
to aluminium and to titanium.
2 – Anodising magnesium
Even when alloyed with other metals, magnesium remains a very reactive metal, especially
in presence of acids or in corrosive environments, in general. In these situations, therefore, a
conversion coating, even when correctly applied and followed by a good paint, is not always
suitable to protect the materials against the corrosion. To protect better magnesium materials and
improve the adhesion of a subsequent paint, a method to form, under current, a “composite” layer,
which can be over 20 micron thick, has been studied.
When a magnesium piece is immersed in an alkaline solution containing ions able to react
with such a metal, and connected to a positive pole of a current rectifier, a dense scantly conductive
layer is formed on the metal. Its composition depends on the composition of the solution and its
thickness is related to the passed current.
Magnesium is anodised in an alkaline solution.
In theory, any alkaline solution can form a layer on magnesium alloys, but in the industrial practice
the chosen anions are those able to form chemical complex or salts with magnesium. The cation has
a minor influence and is chosen among the alkaline ions or ammonia.
When using certain compositions sparks occur in the solution.
The sparks formation is quite impressive as shown in figure 1.
The photo shows a “light effect” produced in the anodising solution caused by the sparks between
anode and cathode. The amount of energy formed and partly “wasted” in these conditions is
2