Note: Descriptions are shown in the official language in which they were submitted.
CA 02039901 1999-09-10
1
PROCESS OF PHOSPHATING METAL SURFACES USING
NICI~:EL FREE PHOSPHATING SOLUTIONS
DESCRIPTION
This invention relates to a process of phos-
phating metal surface:> by a treatment with aqueous acid
phosphating solui~ions which contain zinc ions, manganese
ions, phosphate ions, and oxidizers, also to the use of
that process fo:r pre~treating the metal surfaces for a
succeeding painting, particularly by electro-dipcoating,
and to the phosphating of steel, aluminum and its alloys.
Metals are phosphated to form metal phosphate
layers which are firmly intergrown with the metal surface
and which in the~r~selve:> improve the resistance to corrosion
and in combination wii:h paints and other organic coating
swill contribute to a substantial increase of the adhesion
~f1 anr3 of the resistance
~0~9~0:~
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to subsurface corrosion. Phosphate layers serve also
as an insulation against a passage of electric cur-
rents and in combination with lubricants will reduce
sliding friction.
1:
A pretreatment bef ore a painting
operation may particularly be effected by low-zinc
phosphating processes, in which phosphating solutions
are employed which have relatively low contents of
zinc ions amounting, e.g., to 0.~ to l.j g/1. The
phosphate layers formed on steel under such conditions
will have a high content of phosphophyllite (Zn2Fe(P04)2
.4H~0), which has a much higher resistance to corrosion
than the hopeite (Zn3(P04)2.4H20) deposited from rela-
tively high-zinc phosphating solutions. If the low-zinc
phosphating solutions contain aluo nickel ions and/or
manganese ions, the protection afforded in the presence
of paints will be increased further. Zow-zinc processes
in which, e.g.9 0.5 to 1.5 g/1 manganese ions and' e.g.,
0.3 to ~.0 g/1 nickel Tone are employed are widely used
as so-called three-cation processes to prepare metal
surfaces for a painting operation, e.g., bef ore a catho-
dic electro-da.pcoa.ting of automobile car bodies.
i3ut disadvantages are due to the high
comcent of nickel ions in the phosphating solutions used in
the three-canon processes and to the high content of
nickel and of nickel compounds in the resulting phos-
phate layers because nickel and nickel compounds are
undesirable from the aspects of work place hygiene and
CA 02039901 1999-09-10
3
pollution of the environment.
It is <~n object of the invention to provide for
the phosphating of metals, particularly of steel,
galvanized steel, zinc alloy-plated steel and aluminum and
its alloys, a process by which phosphate layers can be
formed which have approximately the same quality as the
layers produced by the three-cation processes using Zn-Mn-
Ni but are free cf nic~:el and of nickel compounds.
To accomplish that object the process of the kind
described first hereinbefore is carried out in accordance
with the invention in such a manner that the metal surfaces
are contacted with phosphating solutions which are
substantially free of nickel and contain:
0.3 to 1.7 g/1 Zn
0.2 to 4.0 g/1 Mn
0.001 to 0.030 g/1 Cu
5 to 30 g/1 phosphate
(calculated as P205)
with a weight ra~~io of copper to phosphate of 1 . (170 to
30,000) in whi~~h by oxygen and/or other equivalent
oxidizers the concentration of Fe(II) is kept below 0.1 g/1
and which are adjusted to a pH value from 3.0 to 3.8.
The process in accordance with the invention is
particularly used to treat steel, galvanized steel,
.~,._
zinc alloy-plated steel, aluminum and its alloys. The
term steel covers soft plain carbon steels, relatively
higl2_strength and high--strength steels (which may be,
e.g., microalloyed, or dual-phase or phosphorus-alloyed
steels), and low-alloy steels. The zinc layers may be
formed, e.g., by electrolysis, hot dipping and vapor
deposition.
Typical qualities of zinc include
pure zinc and alloys containins;, a.g., Fe, Ni, Co, A1,
Cr. The term aluminum and aluminum alloys covers the
cast and wrought materials which are used in the metal-
working industry and which may contain, e.g., N!g, Mn,
Cu, Si, Zn, Fe, Cr, Ni, Ti as alloying elements.
It is a basic requirement f or the
process in accordance with the invention that the
aqueous acid phosphating solutions must be substantially
free of nickel. This means that under technical condi-
tions the Ni concentration in the phosphating baths
must be lower than 0.0002 to 0.0'1 g/1 and is preferably
below 0.0001 g/1.
Another essential feature of the
invention calls f or the presence of the three metal cations
Zn, Mn, and Cu in the stated amounts. Particularly in the
treatment of Steel, a zinc concentr~.tion below 0.3 g/1
will result in the formation of a layer of much lower
quality, :Cf the zinc content exceeds 1.7 g/1, the phos-
phate layers formed on steel will have a much lower phos-
_5_
phophyllite content and the resulting phosphate
layers will have a much lower quality in combination
with a paint. Vith a content below Oo2 g/1 Ivln the
addition of said cacions will not result in detectable
advantages and a further improvement of the detected
quality cannot be expected with concentrations in
excess of 4 g/1. The Cu concentration is between 0.00'1
and 0.030 g/1. Below that ran~~e the favorable effect
on the formation of the layer and on the quality of
the layer will be lost, and a copper cor6ent above
0.030 g/1 will~increasingly result in a disturbing
cementation of Cu.
In the phosphating of steel, Fe
will be dissolved as Fe(II) ions. The phosphating bath
must contain Buff icient oxygen and/or other oxidizers
to prevent a steady_st~ate Fe(II) ion concentration in
excess of 0.1 g/1 that is to say any Fe in excess of the
above has to be transformed to Fe (III) and precipitated as iron
phosphate sludge.
To ensure the formation of a
satisfactory phosphate layer the phosphating solution
must be adjusted to a pH value between 3.0 and 3.80
The higher (lower) pH values will be employed in case
of lower (higher) bath temperatures and in case of
lower (higher) bath concentrations. In case of need
the pH value of the bath may be adjusted by a co-us~
of additional cations, such as alkali ions (Na, IC, NH4a
_6_
etc.), and/or alkaline earth metal ions (IJIg, Ca) and/or
additional anions (Id03, Cl, SiF6, 50,x, BF4, etc.). The
pH value of the phosphating solution may be corrected
during the preparation and use of the solution by an
addition of basic compounds (NaOH, Na2C039 ZnO~ ZnC03,
MnC03 etc.) or of acids (HN03, H3P0~,, H2SiF6, HCl, etc.)
as may be required.
The quality of the phosphate
layers formed by the process in accordance with the
invention may be improved by an addition of up t0 3 g/1
NIg ancy' or up to 3 g/1 Ca. The concentration of each of
said cations lies preferably in the range from Ooh to
1.3 g/l. The cations may be introduced into the phos-
phating solution, e.g., as a phosphate or as a salt in-
cluding the above-mentioned anions. The oxides, hydroxides
and carbonates mhy also be used as a source of Pfig and Ca.
If the process in accordance with
the invention is carried out by a spraying operation, the
Zn concentr;~tion wil~. preferably lie between 003 and
1 g/1 whereas for a spraying-dipping operation and fox
a dipping operation the bath is preferably adjusted to an
Zn content between 0.9 and 1.'~ g/1. The preferred A4n
concentration is between 0.4 and 1,3 g/1, regardless' of
the mode of application.
~~.ccording to a preferred feature of
the invention the medal surfaces are contacted with a
phosphating solution which contains 0.003 to 0.020 g/1 Cu.
Particularly good results will be produced if a phosphat-
ing bath is used in which the
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weight ratio of Cu to phosphate, calculated as P205'
is 1 . (1'70 to 30,000), and Cu and P205 are replenished
in a weight ratio of 1. : (5 to 2000).
To limit the Fe{II) concentration the
phosphating solution is contacted with oxygen, such as
atmospheric oxygen, and/or suitable oxidizers are added.
Preferred oxidizers include nitrite, chlorate, bromate,
peroxi compounds (H202, perborate, percarbonate, per-
phosphate, etc.), and organic nitro compounds, such as
nitrobenzene self onates. Such oxidizers may be used
alone or in combination, optionally also with weaker
oxidizers such as nitrate. Suitable combinations are,
e.g., nitrite-nitrate, nitrite,.chlorate(-nitrate), peroxt
compound-N03, bromate-nitrate, chlorate-nitrobenzene
sulfonate(-nitrate), bromate-nitrobenzene sulfonate(-ni-
trate). Said oxidizers serve not only to oxidize Fe(II)
ions but will also accelerate thf: formation of the phos-
phate layer. Examples of typical concentration range s
of said, oxidizers in the phosphating bath will now be
stateds 0.04 to 0.5 g/1 nitrites 0.5 to 5 g/1 chlorate;
a.3 to 4 g/1 bromatey 0.005 to 0.1 g/h peroxi compound,
calculated as H202; 0.05 to 1 g/1 nitrobenzene sulfonate,
In accordance with a further preferred
feature of the invention the metal. surfaces are contacted
~.vith phosphating solutions which additionally contain
modifying compounds of the group consisting of sur-
factants, hydroxycarboxylia acid, tartrate, citrate,
..g_
simple fluoride, borofluoride, silico-fluoride. An ad-
dition of a surfactant (e. g., 0.05 to 0.5 g/1) will
improve t he phosphating of slightly greased metal sur~-
faces. Hydroxycarboxylic acids, such as tartaric acid,
citric acid and/or their salts, in concentrations in
the range from, e.g., 0.03 t o 0.3 g/1, will distinctly
decrease the weight of the phosphate layer. A simple
fluoride will promote the phosphating of metals which
can be attacked only with difficulty andvvill reduce
the minimum phosphating time and increase the surface
area covered by the phosphate layer. This may be accomp-
lisped, e.g., by F contents of O.l to 1 g/l. Resides,
the controlled addition of simple fluoride will permit
a formation of crystalline phosphate layers on aluminum
and its alloys. HF4 and SiF6 will also increase the
aggressiveness of the phosphating baths and this will
becoma distinctly apparent in the treatment of hot-
galvanized surfaces. Such additions are made, e.g., in
amounts of 0,4 to 3 g/l.
The phosphating process in accordance
with, the invention can be carried out by spraying
spraying-dipping and dipping operations. The bath tempe-
ratures lie usually between 40 and 60 °C.
For a treatment of steel and aluminum,
contacting times of, e.g., 1 to 5 minutes will be suffi-
cient for a deposition of uniformly covering phosphate
layers. On the other hand, contacting times shorter than
seconds are often sufficient in the treatment of gal-
vanized steel so 'that the process can also be carried
out in coril coating plants.
Before being contacted with, the phos-
phating solution, the surfaces are :usually caeaned- and
rinsed and are often treated with activators based on
titanium phosphateo
The phosphate layers formed by the
phosphating process in accordance with the invention are
finely crystalline and effect a uniform coverage. The
weight per unit of area is usually between l05 and 4.5
g/m2 in the treatment of steel, galvanized steel and
zinc alloy--plated steel and between Oo5 and 2.5 g/m2 in
the treatment of aluminum and ~.ts alloys.
During the phosphating treatment,
components of the phasphating solution are consumed,
e.g., by incorporation in the phosphate layer, sludge
formation, mechanical losses of bath solution remaining
on the treated metal surfaces or in the sludge to be carried off, or by
oxidation-reduction reactions and also decomposition. For
that reason the analysis of the phosphating solution
must be monitored and the deficient components must be
replenished.
'fhe phosphate layers may be used to
advantage, inter alia, for protection against corrosion,
to facilitate cold working and for electri-
2~~9'~ ~~.
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cal insulation. They are preferably used to prepare
metal surfaces for a painting operation, particularly
by elextro-dipcoating, and particularly good results
will be produced in a combination with a cathodic elec-
tro-dipcoating. Bef ore the painting operation the phos-
phate layers axe preferably treated with passivating
postrinsing agents based, e.g., on Cr(VI), Cr(VI)-
Cr(III), Cr{III)> Cr(III)-f luozirconate, Al(III), AI(IIT)-
fluozirconate, because this will further improve the
adhesion of the paint and its resistance to migration
under the paint.
The invention will be expl~.ned more
in detail with reference to the following a~camples:
Examples
Sheets of steel, galvanized steel,
and aluminum were degreased with an alkaline cleaning agent
and rinsed with grater and after an optional activating pre-
treatment with a solwtion which contained titanium phos-
phate were phosphated by a 'treatment with phosphating so-
lutions 1 to 12 at 50oC. In all cases, uniformly covering
phosphate layers were formed, which in combination with
a paint applied by cathodic electro-dipcoating and further
painting (automobile-type treatment) provided a high resistance to corrosion
and migration under the paint.
2U3~~~~.
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1 2 3 4 5 6
Zn (g/1) 0.7 0.7 0.9 0.8 0.8 0.7
114n (g/1) 1 Ue8 1 1 1
Mg (g/1) - 0.8 - '1 - _
Cu mg/1 5 3 5 3 5 4
rda (g/1) 3.47 2e13 4.68 2086 3,67 5.82
Ca (g/1) - 1.3 _ _ _ _
P205 (g/1) 12 12 14 14 13 13
No2 (g/1) 0.07 0.07 0.09 0.09 - 0.17
Clo3 (g/1) - _ _ _ _ _
rdBS (g/1) - -. .. _ - ...
H202 (g/1) - - _ - 0.03 -
rro3 (g/1) 3 3 4 4 3 7
Cl (g/1) - _ _ _ _ _
F (g/1) - oe3 0.3 - 0.1
SIFG (g/1) ' ~ _ o., _ let
V
PH 3.4 3.4 3e4 3.4 3p4 3.4
TA 20.1 20.3 23.4 23.7 2~:.8 25.0
Application S S S S S S
ZW (g/m2) on stee l 2.7 2e3 2.5 2.3 1.8 2.5
ZZ9 ( g/m2 ) on
galvanized steel 2.8 2.5 2.3 2.4 1.9 2~8
LW (g/m2) on
aluminum - - 2a2 2.0 - 0.8
_12_
8 9 l0 11 12
~n (g/1) 0.7 1.3 105 1.3 1.3 1.4
Ntn (g/1) 1 l05 0.7 1 1 let
1,4g (g/1) '~ _ 1.2 ~ 0.8
C~u mg/1 4 5 4 3 4 5
Na (g/1) 3.69 3.92 1.80 4.3g x..78 5.4
Ca (g/1) _ _ _ _ ~ -
P205 (g/1) 6 10 10 11 16 18
N02 (g/1) 0.17 0.1'7 0.15 0.11 - -
clo3 (g/1) - _ _ _ 2 3
S (g/1) _ _ _ - 0.6 0.5
g2p2 (g/1) - _ _ _ _ _
No3 (g/1) 7 8 7 6 3 -
C1 (g/1) - _ _ _ _ 4
P (g/1) o.l '_ _' 0.1 _
SIF6 (g/1) 1.2 - .- 1.2 - -
pH 3.6 3.3 3.3 3.3 3~3 3.3
TA 14.6 18.6 18.3 22.8 26.9 30.6
Application S D D D D D
~~ly (g/m2) on steel 2.3 302 300 ~.3 2.1 2.0
Z'R ( g/m2 ) on
galvanized steel 2.7 3.4 3.2 3.5 2.0 2.C
LW (g/m2) on .
aluminum 0.7 - _ 0.6
2p~~~p:~.
_13_
Ex~lanati.ons
NBS - nitrobenzene sulf onate sodium salt
TA - total acid = consumption of Oo1 N NaOH in ml
for 10 ml bath sample against
phenolphtalein
ZW - weight of layer
S .- spraying
D - dippi.x~g
The Fe(II) concentration is less than Ool g/1 in all baths,
