Note: Descriptions are shown in the official language in which they were submitted.
CA 02041892 1999-11-29
1
This invention relates to a process of forming
conversion coatings on surfaces of zinc and zinc alloys,
comprising a first stage of contacting the surfaces with a
solution containing at: least two different polyvalent metal
ions and complexing agent in such an amount that the
polyvalent metal ions are kept in solution, and a pH value
of at least 11, and a following stage of rinsing the
surfaces with a post rinsing solution, wherein the post
rinsing contains aluminum, zirconium, and fluoride having a
Al . Zr . F molar ratio of (0.15 to 8) . 1 . (5 to 52) and
a pH value not i:n excess of 5.
Such a process of forming a conversion layer is
known from DE-C-1 521 854 published on 15 July 1971
particularly as a pretreatment before a painting or film
laminating operation.
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The layer is Formed on the surface of
zinc or zinc alloy by a treatment with aqueous alka-
line solutions, which contain so-called non-alkali
metal ions consisting of ions of one or more of the
metals silver, magnesium, cadmium, aluminum, tin, tita-
nium, antimony, molybdenum, chromium, cerium, tungsten,
manganese, cobalt, iron, and nickel. It has been empha-
sized that particularly suitable solutions contain non-
alkali metal ions consisting of ions of iron or of
cobalt together with another of tre stated metals. mhe
solutions also contain organic complexing agents in an
amount which is 7ufficient to keep the non-alkali metal
ions in solution. The conversion coatings formed by
means of said ions will increase the resistance to
corrosion and will improve the adhesion of subsequently
applied organic coatings.
'fhe resistance to corrosion and the bond
strength will be further improved if -the surfaces on
which the conversion layer has been formed are post-
rinsed with an ~zd solwtion which contains hexavalent
chromium and optionally contains also trivalent chro-
mium.
'Whereas chat kno~avn process produces
good. results as regards the resistance to corrosion
and the bond strength, the use of triZalent chromium
ions and particularly of hexavalent chromitun ions i: the
pa.ssivatin~; post rinse solution is most undesirable
_3_
because hexavalent chromium is toxic and requires a
special disposalo
It is an object of the invention zo
provic;e far the formation cf conversion coatings on
surfaces of zinc or zinc alloys a process by which the
disadvantages of ~he known Frocess are avoided and, in
particular, the environment is not polluted or is only
very slightly ,polluted and which produces at least equally
good results as regards the resistance to corrosion and
the adhesion of paint,
Tha t object is accomplished in that the
process of the kind described first hereinbef ore is
carried out in such a manner in accordance with tl~ in-
vention that the solution used for post rinsing contains
aluminum, zirconium, and fluoride and has been adjusted
to a pH value not in excess of 5 0
''he process in accordance with the in-
ventior~ can be used to treat all surfaces which contain
zinc or zinc alloys, such as materials of massive zinc
or massive zinc alloys, but also to treat surfaces which
have been =~la~ted vrith zinc or zinc alloys electrolyti-
cally, by deposition from the gas phase or by hot dip
coating. Elements which can be alloyed with zinc include
particularly aluminum, silicon, lead, iron, r.ickel~ cobalt
and manganese, Sheet materials may be plated with zinc or
:with a zinc alloy on one side or on both sidesm
~~~~~~a
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.I.~ the surface of zinc or zinc alloy is
only slightly greased or soiled, there may be no need
for a preceding cleaning and degreasing of the surface.
In said cases, surfactants may b~ added in the first
stage so that the cleaning and degreasing are effected
witYx the treating sclution used also to form the con-
version coatingo Thai embodiment affords mainly the ad-
vantage that the entire pretrearmenr c.t the sr_2rf_a.ce can
be carried out in fewer stages because ~'he separate
cleaning and the associated rinsing with water are
omittedo
The alkaline solution used in the first
stage may be applied, e.g., by spraying, dipping or
f lOOdin f, a
It has been found that alkaline solu-
tions which contain iron(III) ions and additionally
contain cobalt ions and/or nickel ions and/or chromium
(III) ions and/or aluminum ions in a total content bet-
ween Oo3 and 3 g/1, preferably betwesn G.4 and 1.2 g/l,
ire part,icula 1y suitable. The polyvalent metal ions may
be used as salts of inorganic acids, such as nitric acid,
or as salts of organic acids, such as formic acid and
particularly also of acetic acid. It is also possible
to use salts of such organic acids which also
serve as complexing agents. Ampho~eric metals, such as
aluminum, may b~ dissolved in the form of the hydroxy
complex even without an additional anion or complexing
agento
~~~~l~a
C;~ ~:~ :,~
->_,
Due to the pickling attack during
t he treatment in the first stage, additional polyvalewt
cations, which are .pr-~sent .i.r, the surface to be treated
and are not contained in the freshly prepared bath solu-
tion, may enter the treating solutian from 'the surface
of zinc or zinc alloy. On surfaces plated with zinc by
hot dipping such cations consist of e.g. zinc, aluminium and
lead. The total concentration of such cations may increase
up to a few grams per liter and in general this.will not
disturb the format~.on of the conversion coating.
Suitable complexing agents. include parti-
cularly organic chelatin~, agents of various kinds, for
instance, dicarboxylic acids (malonic acid, fumaric acid
etc.); amino acids (e. g., glycine); hydroxycarboxylic
acids (e. g., citric acid, glad onic acid, lactic acid);
1,3-diketones (e. g., acet~laaetoine); aliphatic polyal-
cohols (e. go, sobitol, 1,2-ethamediol); aromatic carbo~cy-
lic acids (e. g., salicylic acid, phthalic acid); amino-
carboxylic acid (e. g., ethylenediamine-tetraacetic acid).
Other complexing agents, such as methanephosphonic acid
diethanolamide, may also be used, The amount of the
complexing agent conta~.ned in the solution must be suffi-
cient for a complex binding of all polyvalent metal ions
which are present. For this reason the ccntent of complex-
ing agent must be increased too when the content of poly-
valent metal fans in :,he solution increases. Because in-
creasing amounts of certain complexing agents, which are
inherently acidic, ma;~ decrease the alkalinity of the
CA 02041892 1999-11-29
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solution, it is preferred to use complexing agents in the
form of neutral salts, particularly of the alkali metal
salts. It has been found that no advantage will be afforded
by the use of co:mplexing agents in a surplus.
Particularly desirable results will be produced
by the use of complexing agents consisting of salts of
gluconic acid, ;particularly of hexahydroxyheptanoic acid.
The solution should contain complexing agents in an amount
between 0.05 and 10 g,~l, in most cases between 1.5 and 5.5
g/1 (based on th~~ sodium salt of hexahydroxyheptanoic acid)
The aqueous solution must have a pH value of at
least 11. Best results will be produced with pH values in
the range from 12.2 to 13.3. The pH value may be adjusted,
e.g., by triethanolamine, alkali hydroxides, alkali
carbonates, al~:ali phosphates, alkali polyphosphates,
alkali pyrophosphates, alkali borates, alkali silicates or
mixtures thereof. But the use of alkali hydroxides,
particularly sodium hydroxide, is most desirable.
The temperature of the solution used in the first
stage may generally lie between 20 and 90°C. The preferred
temperature range is about 45 to 65°C.
The treating time is generally about 2 to 60
seconds, as a rule, and preferably 5 to 30 seconds. It will
depend, inter al:ia, on the application
_~_
technology that is employed. For instance, the treat-
ing time will be shorter in a spraying process than
in a dipping process under conditions which are equal
in other respectso
In general, solutions having a lower
metal ion concentration must be used at higher tempe-
ratures and for longer treating times than solutions
having a higher metal ion concentration.
~Nhen a conversion coating has. been form-
ed, any surplus treating solution should be removed as
completely as possible from the surface of zinc or zinc
alloy. That may be effected, e.g., by dripping, squee gee-
ing, blasting or rinsing with water or with an aqueous
solution, which may have been acidified, e.g., by an in-
organic of organic acid (hydrofluoric acid, boric acid.,
nitric acid, formic acid, acetic acid, etc.).
The post rinse solution mar be applied,
e.g., by dippin~;~, flooding, spraying or roller coating.
In a preferred embodiment of the inven-
tion the solution employed f or the aftertreatment contains
aluminum,, zirconium and fluoride in a total concentra-
tion of A1 -~ Zr a- F between 0.1 and 8 g/1 preferably
between 0.2 and 5 g/1> The Al ; Zr s F mole ratios
should desirably be adjusted to (0.15 to 8) . 1 : (5 to
52), particularly to (0.15 t o 2.0) . '1 . (5 to 16). In
a particularly preferred embodiment of the process the
CA 02041892 1999-11-29
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Al . Zr . F ratio in the post rinsing solution is (0.15 to
0.67) . 1 . (5 to T). According to a further preferred
feature of the invention the pH value should be adjusted to
2 to 5.
The post rinsing solutions used in the process in
accordance with the invention contain, inter alia, acid
aluminum fluoride zirconates and in case of a surplus of
aluminum additionally contain other salts of aluminum, such
as fluorides, t:etraf_Luoroborates, nitrates. They may be
produced, e.g., in that zirconium metal or zirconium
carbonate is initially dissolved in aqueous hydrofluoric
acid to form complex fluoro'zirconium acid. Aluminum metal
or aluminum hydroxide or an aluminum salt, such as the
nitrate, fluoride, tEa rafluoborate, formate, acetate, is
then added, preferably in dissolved form, and is optionally
dissolved. A possible slight cloudiness of the solution
will not adversely affect its effectiveness. Whereas the
preparation is preferably effected on the described route,
the solutions ma.y alternatively be prepared in a different
manner.
The pH value of the solution is preferably
adjusted with c:ations of volatile bases. These include
particularly ammonium, ethanolammonium and di- and
triethanolammoni~am. The adjustment particularly to
relatively high pH value in the stated pH range and to
relatively high concentrations in the range stated for
4
rL J c.~ ~J
-9-
the total concentration of A1 + Zr + F may result in
a cloudiness of the solution but this wily not adversely
affect the effectiveness of the process.
Accordinc; to a further desirable
feature of the invention the surfaces provided with, a
conversion coating are rinsed with an aqueous
solution which additionally contains at ~e'st one of thn
anions benzoate, caprylate, ethyl hexoate, salicyate9
preferably in a total concentration of C~.05 to 0~5 g/1.
This will further increase the bare corrosion
protection. The anions may be introduced by means
of the corresponding acids or their saltso
According to a further desirable feature
of the invention the post rinsing ~ solution is applied
for betvJeen about 1 .and 1?_~ seconds, particularly for
between 1 and j0 seconds. The solution may be applied at
a temperature between 20oC and about 80°C. 'i'e:~~;sr:~.tnarF:;
between 20 and 50 °C are preferred.
Deionized ~Yvater or low-salt water is
preferably used to prepare the .post rinsing bath. 'hater
having a high salt content is less suitable for the pre-
paration of the bath.
After zhe passivating aftertreatment the
surface may be dried, e.g., on the air or in an oven and
optionally may be rinsed befora with deionized water.
Acco~~ding to a preferred embodiment of the invention the
surface which has been subjected to the passivating after-
~!~ ~.~a'
-lo-
treatment is subjected to an accelerated drying, e.g., with
hot air or by infrared radiation.
The process in accordance with the in-
vention serves primarily to pretreat surfaces of zinc or
zinc alloys before they are painted' laminated with a
f i1m or coated with an adhesive. It will improve the ad-
hesion of the organic films on the metallic substrate and
will also improve their resistance to a f ormat'._on of
blisters under corrosive action and will inhibit subsur-
face carrosion proceeding from ~.efects f.n the film.
The .process in accordance with the in-
vention,will be explained more in detail. and by way of
example with reference to the following example.
Example
fo forn a conver~;~ion coating on cleaned
and degreased sheets of hot galvanized steel they -,were
dipped for 30 seconds into a solution which w~:s a~ a
temperature of 55°C and had the following composition:
0~3 g/1
Fe5+ 0.2 g/1
rd03 1 ~ 3 g/1
sodium Salt of
hexahydroxyhepta-
noic acid 2~2 g/1
NaOH 2~.4 g/1
Thereafter the sheets were rinsed with
..x. ',l ."..~ Vo'
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water and were then subjected to a passivating af-ter-
treatment. To that end the sheets were dipped into 'the
post rinse solution for 5 seconds and were subse-
quently squeegeed for a removal of surplus solution.
After a drying at 75°C in an oven operated with circulat-
ing air for 0.5 minute the pretreated sheets were painted
with an epoxy .primer and an acrylate top coat, The
total coating had a thickness of about 25 dam.
The treated sheets were subsequently
subjected to the following testss
The adhesion of paint was determined
by the T-Bend Test, in which the sheets were bent through
180° and the various radii of curvature (Tn) were stated
as amounting to n times the sheet metal thickness (n==,
'1,2 ...). The test result is stated as the percentage of
the flaked-off paint surface area in the total curved
surface area.
In other treated sheets, scratches ex-
tending as far as to the metal sub~rate were made by means
of a metal needle and the sheets were formed with a
cut edge by plate shears. Thereafter the sheets were sub-
jected to the salt spray test in accordance with DIN 50021
SS for 1008 hours. The test result was stated as the
extent in mm of the migration under the paint
from the crack or the cut edge.
To prepare the post rinsing solution
to be employed, 1,6 g/1 and ZO g/1 respectively of an
aqueous concentrate, which contained
_ 12--
0.855% by weight A1 and 8.62% by weight Zr and 10.'7/
by weight F, was diluted with deionized watertp
yield post rinsing solution A
and post rinsing solution B. Both solutions
ware adjusted with ammonia to a pH value of about 3.6.
F'or comparison, a post rinsing
solution C was employed, which contained Cr('VI) arid
Cr(IIT) and had a pH value o2 about 3.3.
The post rinsing solutions had the
following composityons:
Post rinsing solution A:
A1 0.014 g/1
Zr 0.14 g/1
F o.17 g/1
NH4 0.016 g/1
Post rinsing solution B:
Al Ool? U/1
Zr 1.?2 g/1 y
F 2.14 g/1
NHS 0.40 g/1 .
Post rinsing solution C:
Cr6~ 2.0 g/1
Cr~+ G.8 g/1
F 0.2 g/1
Zn 0.3 g/1
~~~~~i
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The test results are stated in the follow-
ing Tables.
Table 1: Paint adhesion in T-Bend Test
Post rinsing Flaked off surface area (/) in
Solution case of radius of curvature Tn
T1 T2 T3 T4
A (invention) 100 55 15 5
B (invention) 100 65 25 5
C (comparison) 100 80 30 5
Table 2: Resistance to corrosion in salt spray test
Post rinsing
solution migration under paint (mm) after
1008 h
at the scratch at the edge
A (invention) z1 to 1 8 to g
B (invention) 0 to 1
C (comparison) 1 to 3 g to 10
A comparison of the tabulated data reveals
that the data obtained with the process in accordance
with the invention are just as good or better than those
of the also tested comparison process in which an post
rinsing solution waa used that contained Cr(VI) and
Cr(III).
