Note: Descriptions are shown in the official language in which they were submitted.
DESCRIPTION 1 7
This invention relates to a process of applying
phosphate coatings to surfaces consisting of aluminum
and/or zinc and/or its or their alloys by means of
phosphating solutions used in low-zinc technology.
Phosphating solutions used in low-zinc technology
are, by definition, solutions which predominantly contain
zinc in a concentration of about 0.4 to 1.5 g/l as a layer-
forming cation and in which the ratio of Zn to PO4 is lessthan about 0,08.
It is known that thin phosphate coatings can be
applied to metal surfaces consisting of steel, galvanized
steel or aluminum in order to provide a suitable base for
a finish to be applied by a subsequent electrophoretic dip
coating process. The phosphating solutions used for that
process usually contain zinc, nickel, manganese, magnesium,
cadmium, copper, cobalt, alkali and/or ammonium ions, also
phosphate ions, accelerating additives, such as nitrite,
chlorate, bromate, peroxide, m-nitrobenzene sulfonate,
nitrophenol or combinations thereof. For a treatment of
aluminium, its alloys and steel which has been galvanized
by electrodeposition, the solutions preferably contain in
addition simple and/or complex fluorides. Additional
anions, such as chloride, nitrate and sulfate, serve to
maintain electron neutrality. The phosphating solutions
may optionally contain grain-refining additives for
improving the layer, such as hydroxycarboxylic acids,
aminocarboxylic acids or condensed phosphates.
The metal surfaces consist in a major proportion
of steel and in changing proportions of galvanized steel
and in a small proportion consist of aluminum materials and
are treated by spraying and/or dipping processes.
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, If surfaces consisting only of zinc and/or
aluminum are contacted with the phosphating solution, it
will be found that the free acid points and/or the ratio of
free acid to total acid in the phosphating solution being
used will increase and the phosphating solution will get
out of equilibrium. As a result, the formation of the
coating will be deteriorated and will finally be suppressed
as the throughput increases. The surplus acid cannot be
neutralized with sodium hydroxide, ammonium hydroxide or
other alkalies because this would result in a co-
precipitation of a part of the bath component zinc.
It is an object of the invention to provide for
the application of phosphate coatings to surfaces
consisting of zinc and/or aluminum and/or its or their
alloys by means of phosphating solutions used in low zinc
technology a process in which a rise of the free acid
content and/or of the acid ratio is prevented so that the
disadvantages of the known processes will be avoided
whereas the process can be carried out in a simple manner
and is economical.
To accomplish that object the process of the kind
described firæt hereinbefore is carried out according to
the invention in such a manner that the surface is
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contains iron-III in a concentration of at least 1 mg/l and
to which 50 to 2000 mg iron-III-ions are added per square
meter of treated surface area.
More particularly, the invention provides a
process of applying a phosphate coating to a surface
consisting of a metal compound selected from the group
consisting of aluminum, aluminum alloys, zinc, zinc alloys
and alloys of zinc and aluminum, by contacting said surface
with a phosphating solution as is used in low-zinc
technology, said solution cont~;ning
0.4 to 1.5 g/l zinc, and
lo to 26 g/l P2O5
and having a weight ratio of Zn to P205 adjusted to a value
in the range of 0.075 : l to 0.015 : 1, characterized in
that said phosphating solution with which the surface is
contacted, also contains iron-III-ions in a minimum
concentration of 1 mg/l, the amount of iron-III-ions that is
added to the solution being ranging from 50 to 2000 mg iron-
III-ions per s~uare meter of treated area. .~
The iron ions may be introduced into the
phosphating solution in the form of FeIII-ions, but also in
the form of iron-II-ions together with an oxidizing agent
25 ! oxidizing iron-II to iron-III, e.g. chlorate, nitrite,
peroxide. Suitable compounds are, e.g., the nitrates,
chlorides and fluorides of di- or trivalent iron as well as
iron(III) complexes of hydroxycarboxylic acids, amino-
carboxylic acids and the like.
It will be particularly desirable to introduce the
iron ions in an aqueous solution into the phosphating
solution.
In a preferred embodiment of the invention, the
metal surface is contacted with a phosphating solution to
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which SO to 1000 mg iron-III-ions are added per square meter
of treated surface area. In that case, too, addition of
iron can be performed as mentioned above.
The process in accordance with the invention can
be used with all known phosphating solutions used in low-
zinc technology.
In another preferred embodiment of the invention
the solution that is used, further contains
0.4 to 1.5 g/l zinc
to 26 g/l P205 and
O to 1.3 g/l Ni
O to 1.3 g/l Mn
O to 1.3 g/l Mg.
In such solutions the weight ratio of Zn to P20s should be
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and - if applicable - the weight ratio of Ni, Mn and/or Mg
to Zn should be adjusted to a value up to 1.5 : 1.
In a further desirable embodiment of the
invention, phosphating solutions are used which contain as
an accelerator
2 to 25 g/l NO3
1 to 6 g/l C103
0.1 to 2 g/l organic nitro compound
0.05 to 0.5 g/l NO2
0.02 to 0.1 g/l peroxide (calculated as H2O2) or
mixtures thereof.
The phosphating solutions may be applied by
conventional processes. Particularly desirable modes of
application are spraying and/or dipping. A temperature in
the range from 30 to 70C is preferred.
During non-processing intervals, iron-III-ions
must be added to maintain a concentration of at least 1
mg/l.
The process in accordance with the invention will
be particularly suitable for a treatment of pure aluminum
and alloys, e.g., of the grades AlMgSi, AlMg and AlMgMn.
As to the formation of coatings on zinc surfaces,
materials consisting of solid zinc, particularly of steel
which has been galvanized by hot dip or- electrolytic
processes, may be treated. Other suitable materials are
alloys of zinc with, e.g., Ni, Fe or Al in the form of
s
X
_ 7 - l 3 3 4 3 7 1
By means of the process in accordance with the
invention, phosphate coatings which are entirely uniform and~
continuous can be formed even for a prolonged time. Such
coatings will be particularly suitable when an electrophoretic
5 dip coating process is to be performed thereafter.
The invention will be explained by way of example and
more in detail in the following Examples.
Example
Previously degreased and pickled sheet metal elements
10 of aluminum (70%) of the grades AlMgSi and AlMg3 and of
electro-galvanized steel (30%) were treated in a 10-liter
phosphating container with a phosphating solution which
contains
0.7 g/l Zn
0.7 g/l Ni
1.0 g/l Mn
3.4 g/l Na
11.5 g/l P2O5
3.0 g/l NO3
0.5 g/l F
0.1 g/l NO2
The temperature of the solution amounted to 55 to 60C.
The treatment was performed by spraying for 3 minutes. The
number of free acid points of the phosphating solution
25 amounted to 1Ø
X
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, By an addition of iron(III) citrate, the phosphating
solution had been adjusted to an initial iron-III-concentra-
tion of 2 mg/l.
During the processing, 250 mg iron(III) citrate (calc.
as Fe) per m2 of surface area were added to the lo liters of
phosphating solution. As is apparent from the following
Table, the number of free acid points was virtually constant.
The resulting phosphate covering was uniform and continuous
throughout.
Throughput (m2) 0.4 0.8 1.2 1.6 2.4 2.8 3.2
Free acid points l.o 1.1 1.1 1.0 l.o 1.1 l.o
5 Control Example
In a control example, sheet metal elements having the
same quality were treated in the same manner with the above-
mentioned phosphating solution, with the difference that said
solution did not contain iron-III-ions and was not replenished
lo by an addition of iron-III-ions. As the throughput increased,
the number of free acid points increased as stated below.
Whereas high-grade phosphate coatings were initially obtained,
the quality deteriorated as the number of points increased.
No coating was formed when the number of free acid points
15 amounted to 1.50 or more.
Throughput (m ) 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
Free acid points l.o 1.15 1.~5 1.3~ ~ o
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