[0001] The invention relates to an acidic aqueous composition for preparing a corrosion
resistant coating on a metal substrate, a method of providing a corrosion resistant
coating on a metal substrate, as well as a post-treatment composition, in particular
for use in said method.
[0002] In the art mechanical and chemical treatment of metal surfaces for enhancing (bare)
corrosion resistance, as well as for improving bonding to a subsequently applied coating
such as an adhesive layer, paint layer, lacquer layer or other finishing layer and
thereby enhancing the corrosion resistance of the thus coated final product is well
known. E.g. mechanical treatment such as grit blasting has been used to improve adhesion,
when chemical treatment steps were not practical to apply. Chemical treatment of metal
surfaces of zinc (alloy) coated steel, mild steel, or aluminium and their alloys with
aqueous chromate (chromium VI) solutions results in a so called "chromate conversion
layer", which offers corrosion resistance and improved adhesion.
[0003] It has been recognized that these chromate based aqueous solutions suffer from the
toxicity of the Cr
6+ component thereof. Cr
6+ is classified as carcinogenic and will be banned from most industrial applications
involving high exposure risks for the operating staff. Disposal of the toxic treatment
composition is also a problem, although to a lesser extent if the hexavalent chromium
is converted into the comparatively innocuous trivalent chromium. However, such a
conversion brings about additional costs and expenses.
[0004] Therefore, in the art there is a need for treatments that are substantially free
of hexavalent chromium compounds, that offer corrosion resistance and bonding performance
to the metal surfaces treated similar to those obtained by treating these metal surfaces
with conventional solutions comprising hexavalent chromium. Proposals for satisfying
this need, which are based on Cr(III) typically in combination with one or more other
active components such as zirconium, are known.
[0005] From
US 2011/0293841 A1 an aqueous solution for forming a protective coating on a metal surface is known
that includes Cr
2(GF
6), in which G is a Group IV-B element (Zr, Ti or Hf), in particular Cr
2(ZrF
6)
3 at least one polymer having a plurality of carboxylic acid groups such as polyacrylic
acid and copolymers of methyl vinyl ether and maleic acid, and at least one polymer
having a plurality of hydroxyl groups for example polyvinyl alcohols and homopolymers
or copolymers of hydroxyethyl methacrylate, and/or at least one polymer having a plurality
of both carboxylic acid and hydroxyl acid groups exemplified by free-radical copolymers
of hydroxyl-ethyl methacrylate and methacrylic acid, wherein the composition contains
less than 500 ppm of alkali metal ions and less than 200 ppm of halide ions relative
to chromium. Application of a high purity Cr
2(ZrF
6)
3 solution, is said to improve the corrosion resistance of the metal substrate. The
Zr : Cr molar ratio is typically determined by the stoichiometry of the compound,
but due to the optional presence of other components the weight ratio Zr : Cr is typically
in the range of 2.4 : 1 - 3.0 : 1, most typically 2.6:1 -2.8:1. Any metal may be treated,
with apparently good results being obtained on zinc, zinc alloy, aluminium and aluminium
alloy surfaces. The addition of organo-functional silanes such as aminoproyl triethoxysilane
may improve adhesion of subsequently applied coatings such as paints) to the treated
surfaces, while maintaining good corrosion results. For a working bath the pH of a
treatment solution comprising the above polymers is in a range from 2.5 - 4.0, more
typically 2.8-3.2.
[0006] US 6375726 B1 has disclosed an acidic aqueous solution for the protection and surface treatment
of aluminium, aluminium alloys and coated aluminium substrates against corrosion.
The solution comprises at least one trivalent chromium salt such as trivalent chromium
sulphate, at least one alkali metal hexafluorozirconate in combination with at least
one water soluble or dispersible thickening agent and a water soluble surfactant.
The corrosion resistant aluminium substrates of this invention have improved adhesion
for overlaying coatings e.g. paints and a lower electrical resistance contact.
[0007] WO 2006/088519 A2 discloses an acidic aqueous solution for treating metal substrates, such as aluminium
alloy or iron alloy or a metal substrate having a pre-existing metal coating for example
anodized aluminium to improve the adhesion bonding and corrosion protection, which
comprises water soluble trivalent chromium compounds, fluorozirconates, fluorometallic
compounds, zinc compounds, thickeners, surfactants, and at least about 0.001 mole
per litre of the acidic solution of at least one polyhydroxy and/or carboxylic compound
as the stabilizing agent for the aqueous solution. The carboxylic compounds contain
one or more carboxylic functional groups having the formula R-COO- wherein R is hydrogen
or a lower molecular weight organic radical or functional group, and can be used in
the form of their acids or salts. The stabilizers are said to result in improved shelf-life
and working stability of the solutions. According to this document after treating
with the acid aqueous solution, application of a strong oxidizing solution can yield
a film having additional corrosion resistance, which is presumed to be due to the
formation of hexavalent chromium in the film derived from the trivalent chromium.
[0008] Furthermore an acidic, aqueous composition that contains a trivalent chromium compound,
an organo-functional silane, and a compound of a group IV-B element, is known from
US 2009/0280253 A1. The composition is said to protect metal surfaces, preferably aluminium and aluminium
alloys, against corrosion and improves their paint adhesion. The trivalent chromium
compound may comprise chromium fluoride and optionally others, such as chromium nitrate.
The organo-functional silane is preferably an aminopropyltriethoxy silane, and the
compound of a group IV-B element is preferably fluorozirconic acid. The composition
can either be dried-in-place or rinsed before a further coating layer is applied.
The composition may also include at least one polymer having a plurality of both carboxylic
functional groups, alone or with hydroxyl groups. The document also discloses a process
using the aqueous composition either with or without the organo-functional silane
along with a sealing step following the application of the aqueous composition; wherein
the sealing step involves applying a sealing composition, including an organo-functional
silane, to the metal surface.
[0009] From
US 2015/0020925 A1 a process for surface treatment of a part made of aluminium, magnesium, or one of
the alloys thereof, is known in order to protect the part from corrosion. The method
comprises consecutively immersing the part in a first aqueous bath containing a corrosion-inhibiting
metal salt such as trivalent chromium salt and an oxidizing compound such as K
2ZrF
6, and a second aqueous bath containing an oxidizing compound like hydrogen peroxide
and a corrosion-inhibiting rare-earth salt. The method can be carried out for the
chemical conversion of aluminium or the alloys thereof, and of magnesium or the alloys
thereof, on parts that have not been previously treated, or after anodizing the part
to seal the anodic layer.
[0010] US 6648986 B1 teaches an acidic aqueous solution that contains a water soluble trivalent chromium
compound, a water soluble fluoride compound, an alkaline pH adjustment reagent and
provided with a solution stability additive for reducing precipitation of trivalent
chromium over time. The solution stability additive comprises a complexing agent that
is selected from the group consisting of organic acids (single coordination acids
and bidentate chelating compounds) and amino acids. The concentration of the solution
stability additive varies based on the complexing capability of the additive.
[0011] US 2010/0132843 A1 discloses a low sludge trivalent chromium based conversion coating bath that forms
corrosion resistant coatings on aluminium and aluminium alloys by immersion in aqueous
solutions containing trivalent chromium ions and fluorometallate ions followed by
optional rinsing. Trivalent chromium coated aluminium also serves as an effective
base for paint primers.
[0012] US 2006/240191 A1 discloses a process for treating metal substrates to improve the corrosion protection
and adhesion bonding strength which comprises treating the metal substrates with an
acidic aqueous solution having a pH ranging from 1.0 to 5.5, wherein the acidic solution
comprises a trivalent chromium compound, a fluorozirconate compound and at least one
stabilizing agent compound selected from the group consisting of polyhydroxy compounds,
carboxylic compounds and mixtures thereof.
[0013] It is an object of the invention to provide a stable treatment solution based on
Cr(III) for a substrate of aluminium, aluminium alloys, anodized aluminium, aluminized
steel, zinc or zinc alloy coated steel with a layer, that protects the substrate against
corrosion and/or offers adhesion for a subsequently applied (typically organic) coating
such as paint, and/or for adhesive bonding system, or at least an alternative treatment
solution.
[0014] Another object is to provide such a stable treatment solution that offers an enhanced
layer formation.
[0015] A further object is to provide a method of applying a treatment composition to such
a metal substrate in order to achieve corrosion resistance and/or adhesion.
[0016] Another object is to provide such a method that does not result in Cr
6+ species in the deposited layer that are easily released from the treated product
under severe conditions. Yet another object is to provide such a method that is easy
to apply in maintenance and repair of aerostructural parts.
[0017] According to a first aspect the invention provides an acidic aqueous composition
for preparing a corrosion resistant layer on substrates of aluminium, aluminium alloys,
anodized aluminium, zinc or zinc alloy coated steel, aluminized steel, wherein the
composition comprises:
trivalent chromium (Cr3+): |
0.04 - 6 |
g/l |
zirconium (Zr4+): |
0.08 - 8 |
g/l |
total fluoride (F-): |
0.1 - 9 |
g/l |
stabilizing agent comprising a hydroxyl carboxylic acid or corresponding bases thereof
(calculated as the acid): |
0.2 - 9 |
g/l |
wherein
the molar ratio Zr4+ : Cr3+ is in the range of 0.8 : 1 to 2.0 : 1;
the molar ratio Zr4+ : F- is in the range of 1: 5.5 to 1.0 : 2.0; and
pH is in the range of 3.0 - 5.0.
[0018] It has been shown that the aqueous composition according to the invention when used
as a bath is stable in time and does not form deposits and/or sludge in the bath.
When applied to a surface of aluminium, aluminium alloys and anodized aluminium, zinc
or zinc alloy coated steel, the surface, which is essentially free of Cr
6+, shows good corrosion resistance and if present, durable bonding to a subsequently
applied paint system or adhesive bonding system.
[0019] The acidic aqueous composition according to the invention comprises as main active
components trivalent chromium, tetravalent zirconium and fluoride in specific concentrations
and ratios, as well as one or more hydroxyl carboxylic acids as stabilizing agent.
Thereby a stable solution is provided with less fluoride with respect hexafluozirconic
acid (H
2ZrF
6) or ((alkali) metal) salt thereof, wherein the ratio Zr : F = 1 : 6.
[0020] Trivalent chromium is present in an amount of 0.04-6 g/l, preferably 0.2-1.0 g/l
for dipping and spraying applications. For touch-up, maintenance and repair applications
using e.g. brushing and wiping a broader range of 0.5-5.0 g/L is advantageous. If
the amount of chromium (III) is lower, then within industrially acceptable processing
times, the rate of layer formation is low resulting in a layer having an insufficient
thickness and thus inadequate protection and/or bonding.
[0021] The trivalent chromium can be derived from organic chromium salts, in particular
citrate, glycolate, tartrate, and combinations thereof. Another attractive route for
obtaining trivalent chromium is by reducing chromic acid (H
2CrO
4) with chemical agents that can be oxidized by chromic acid like methanol or hydrogen
peroxide leaving no residual products in the starting solution after heating. Another
attractive source for trivalent chromium and fluoride is using OF
3.4H
2O as a starting material. This compound is hardly soluble in water, but accompanied
by acidic components like HF, acidic hydroxyl carboxylic stabilizing agents and water
soluble acidic polymers or combinations thereof, it is. HF is preferably used as it
does not introduce extraneous anions other than those required.
[0022] Tetravalent zirconium is present in an amount of 0.08-8 g/l, preferably in the range
of 0.2-2.0 g/l. Suitable starting materials comprise hexafluozirconic acid and its
ammonium salt, zirconium salts of lactate, carbonate, glycolate and citrate, ammonium
zirconium carbonate and zirconium triethanol amine, and combinations thereof. The
high zirconium content is believed to enhance co-precipitation of Cr and Zr as Cr(OH)
3 and Zr(OH)
4 and thereby layer formation.
[0023] F- ions are present in the range of 0.1-9 g/l, preferably 0.2-2.0 g/l. The fluoride
can be obtained from HF, alkali metal fluoride like sodium fluoride, ammonium bi fluoride
(ABF), chromium fluoride, hexafluo zirconic acid and its ammonium salt, as well as
combinations thereof. If hexafluo zirconic acid is used as a starting material, at
least one other zirconium component is present, otherwise the ratio of Zr : F is not
within the required range. High fluoride contents compared to Cr and Zr results in
excessive etching hindering layer formation. Without being bound to any theory it
is believed that fluoride contents in the solution according to the invention will
prevent over-etching during layer formation and might prevent accumulation of fluorides
in the layer that could weaken the conversion layer during exposure in corrosive environment.
[0024] Low fluoride contents have the opposite effect and may reduce stability of the composition.
[0025] In addition to the concentration ranges the following molar ratio apply.
[0026] The molar ratio Zr
4+ : Cr
3+ is in the range of 0.8 : 1 to 2.0 : 1; preferably 0.9:1 - 1.3 : 1, such as 1.04:
1.0;
and
the molar ratio Zr
4+ : F
- is in the range of 1: 5.5 to 1.0 : 2.0.
[0027] Then the molar ratio Cr
3+ : F
- is in the range of 1 : 11 to 1 : 1.6.
[0028] The acidic aqueous comprises a stabilizing agent being a hydroxy carboxylic acid
or a corresponding base thereof in a concentration of 0.2-9 g/l, preferably in the
range of 0.2- 5 g/l, more preferably 0.3 - 3 g/l (calculated as the acid). The stabilizing
agent can be used as the acid or a water soluble salt thereof. Suitable hydroxyl carboxylic
acids are lactic acid, citric acid, malic acid, tartaric acid, glycolic acid, gluconic
acid and combinations thereof. The stabilizing agent may be introduced in the composition
by the chromium or zirconium compound as well as shown above in the form of the zirconium
and/or chromium (III) salt of the conjugated base of the hydroxy carboxylic acid,
or as a similar part of the pH adjusting agents as discussed below. The stabilizing
agent acts to stabilize the solution as a dipping bath having the composition according
to the invention in time, which bath would otherwise at the given concentrations and
ratios of its components deteriorate rapidly. It is assumed that the acid becomes
part of the resulting layer comprising the hydroxides and oxides of chromium and zirconium.
Higher contents of hydroxyl carboxylic acids results in coating layers that are sensitive
to dissolving. Too low contents will cause instability and sludge forming of the composition
during operation, typically a process bath for dipping, over time. The composition
according to the invention has a pH in the range of 3.0-5.0, preferably 3.4-4.4. If
the pH is too low then the formation of the protective layer on the substrate is limited
by the simultaneous attack of the formed layer by the acidic components. Thus the
composition as a whole offers a balance between components and the amount thereof,
their functional properties and processing possibilities. In order to set the acidity
at the required level the composition may contain pH adjusting agents, typically bases,
such as one or more alkali metal hydroxides like sodium hydroxide, potassium hydroxide
and ammonia. As said before, the pH adjusting agent may be a source of the base of
the hydroxyl carboxylic acid stabilizing agent. Examples include lactate, citrates
of ammonium and/or sodium and the like. Advantageously the pH adjusting agent is present
in amount of 0-1.0 g/l. The alkali metal cations do not - or to a negligible extent
- affect the formation of the protective layer. When the pH rises to values above
5, the composition can easily become instable. At pH less than 3.0 it is hard to form
substantial coating weight in view of corrosion resistance.
[0029] The composition may also comprise one or more water soluble surfactants for improving
the wetting properties, advantageously in a concentration of 0-1.0 g/l, preferably
0.001-0.5 g/l. Surfactants that can be used in the composition according to the invention
include acid stable low foaming anionic and non-ionic surfactants like alkaryl sulfonates
and poly ethylene glycol fatty amines. The surfactant provides uniform wetting of
the substrate. If the amount of surfactant is too high, it can cause excessive foaming
in the process.
[0030] Other optional components of the composition according to the invention comprise
water soluble polymers and organo-functional silanes and/or their hydrolysed oligomers.
The water soluble polymers include homopolymers and copolymers that preferably are
based on the following monomers: acrylic acid, methacrylic acid, vinylalcohol, vinylether,
maleic acid, monohydroxy acrylic ester, vinylphosphonic acid, vinylsulphonic acid,
methyl vinylether, monohydroxy methacrylic ester and combinations thereof, up to 4.0
g/l, preferably 0.01-4.0 g/l, more preferably 0.1-1 g/l. These polymers improve wetting
behaviour of the treatment composition, as well as adhesion of subsequently applied
organic coatings. Too high concentrations will reduce wet adhesion of an organic coating.
The concentration of the silanes or oligomers, if present, ranges up to 4.0 g/l. Advantageously
the reactive functional group is at least one selected from a mercapto group, an amino
group, a vinyl group, an epoxy group and a methacryloxy group, advantageously in an
amount of 1 to 40 mg/l based on Si.
[0031] According to a second aspect the invention relates to a method of treating a substrate
of aluminium, aluminium alloy, anodized aluminium, aluminized steel, zinc or zinc
alloy coated steel for corrosion protection, adhesion of an organic coating or adhesive
bonding system, which method comprises a step of applying the acidic aqueous solution
according to the invention as explained above onto a substrate of aluminium, aluminium
alloy, anodized aluminium, aluminized steel, zinc or zinc alloy coated steel.
[0032] Typically the metal surface to be treated with the composition according to the invention
is pre-treated using known mechanical and/or chemical pre-treatment processes or combination
thereof for obtaining a wettable surface, which typically requires the surface to
be roughened and to be substantially free of rust, fat, oil and the like. Mechanical
pre-treatment processes comprise grit blasting, shot peening, scuffing, scotch brite®
and abrading. Chemical pre-treatment includes e.g. (acidic/alkaline/solvent) degreasing,
de-oxidation, desmutting, pickling. Typically each chemical pre-treatment is followed
by a rinsing step using tap water or demineralised water. Combinations of mechanical
pre-treatment and chemical pre-treatment in any order is also possible. In case of
aluminium or its alloys the surface may be anodized. Typically treatment according
to the invention of bare aluminium will form a conversion layer. Treating anodized
aluminium according to the invention will result in a sealing layer.
[0033] The way of applying the composition according to the invention to the metal surface
is not limited. However, homogeneity and uniformity of the applied wet film on the
substrate before drying will be advantageous. Suitable application methods include
touch-up methods, spraying, dipping, wiping, brushing, roll coating and the like.
Excess of treatment fluid on parts with intricate geometries can be removed with water
rinsing, compressed air or wiping. After applying the acidic aqueous composition one
or more rinsing steps with demi water are performed, preferably the last rinsing step
with demi water having an electrical conductivity in the range of 5-200 microS, and
a drying step of drying the thus rinsed substrate, in particular at a temperature
in the range of 10-50 °C.
[0034] Advantageously the method according to the invention is performed with the composition
having a temperature in the range of 10-80°C, preferably 15-50 °C. Typically the processing
time ranges from 1-30 minutes, preferably from 3-15 minutes. Processing times of less
than 3 minutes are practically not feasible on industrial scale in view of reproducible
coating results. Processing times of more than 30 minutes may interfere with other
operations on a continuous production line.
[0035] The coating weight, as measured by XRF after drying, is typically in the range of
5-200 mg Cr/m
2, such as 15-100 mg Cr/m
2. For a composition according to this invention coating weight can be controlled by
adjusting concentration, pH, bath temperature and immersion time. Too high coating
weights will give corrosion resistance, but may reduce adhesion properties and increase
the surface electrical resistivity. Low electrical resistivity of the metal surface
is important for certain applications as it prevents build-up of static electricity
and will not influence welding properties.
[0036] It has appeared also that the method according to the invention is very suitable
for maintenance and repair of damaged aluminium surfaces, like aerostructural parts,
in particular where electrical resistivity of the resulting protective layer should
be low, e.g. as detailed in MIL-DTL-5541F Class 3 coatings.
[0037] For certain alloys, in particular those copper containing aluminium alloys as used
in the aerospace technical field such as 2024 and 7075, it has appeared advantageously
to conduct a further treatment step. This further treatment step comprises post-treating
the substrate that has been treated with the acidic aqueous composition, with a second
acidic aqueous composition comprising an oxidizing agent and an acidifying component,
which second composition has a pH in the range of 1-5, preferably 1-3. It has been
shown that the risk of formation of Cr
6+ in the formed protective layer is reduced by using the acidic second solution. Cr
6+ was less than the detection limit (<0.03 µg/cm
2) in the used second solution and in the thus obtained protective layer.
[0038] Advantageously the oxidizing component is a water soluble peroxide, preferably hydrogen
peroxide. Hydrogen peroxide functions as a reductor with respect to Cr
6+ in an acidic environment. Thus trivalent chromium will not be converted into hexavalent
chromium. The concentration of the oxidizing component ranges typically from 10-100
g/l, preferably from 25-100 g/l.
[0039] The acidifying component is present in a concentration of 0.2-20 g/l, preferably
0.5-5.0 g/l. The acidifying component is advantageously a non-halogenated inorganic
acid such as nitric acid, or a metal salt thereof excluding rare earth metals, preferably
a salt of aluminium, zirconium and/or trivalent chromium, preferably the nitrate salt
thereof. These acidic components should not dissolve the applied coating by the first
composition nor etch aluminium. Nitric acid is a strong acid but has shown not to
attack the coating from the first solution. Other strong acids like hydrogen chloride
and sulphuric acid are too aggressive towards the first deposited layer. The second
composition should be acidic enough to avoid formation of hexavalent chromium.
[0040] The temperature of the second composition is preferably in the range of 10-50 °C,
more preferably at ambient temperature, like 20-30 °C. Such a low temperature is advantageous
in order to avoid fast decomposition of peroxides.
[0041] Treating time is typically 1-30 minutes, preferably 3-15 minutes.
[0042] Suitable application methods for the second composition include spraying, dipping,
wiping, brushing, roll coating and the like.
[0043] The invention is illustrated by the following Examples and Tests.
[0044] Composition 1 and 2 as indicated in Table 1 were prepared from commercially available
compounds, as well as a Comparative Example without any hydroxy carboxylic acid. It
appears that the composition of the Comparative Example is not stable.
Table 1. Examples starting Compositions according to the invention and Comparative
Example and their stability
|
Composition 1 (wt.%) |
Composition 2 (wt.%) |
Comparative Example (wt.%) |
Zirconium triethanolamine (13.8 wt.% Zr) |
4 |
|
|
Hexafluoro zirconium acid (19.8 wt.% Zr) |
1.41 |
|
|
Ammonium zirconium carbonate (14.8 wt.%Zr) |
|
4.5 |
4.5 |
Chromium trifluoride *4 H2O (28,7 wt.% Cr) |
1.51 |
1,03 |
1.03 |
Hydrogen fluoride (20%) |
|
0.9 |
0.9 |
Malic acid (99%) |
0.9 |
0.7 |
|
Remainder water |
|
|
|
Zr : Cr mol ratio |
1.1 : 1 |
1.3 : 1 |
1.3:1 |
Zr : F mol ratio |
1 : 4.8 |
1 : 3.6 |
1:3.6 |
|
|
|
|
Stability (no suspension or gel formed) |
> 240 days |
> 240 days |
< 1 day |
[0045] From these starting Compositions first treatment solutions were prepared by dilution
with demi water as indicated in Table 2. Table 3 lists compositions of the second
treatment solutions.
Table 2 First treatment solutions (treatment 1 = TR1)
Examples TR1 |
[Zr] g/L |
[Cr] g/L |
[F] g/L |
pH |
C1 |
20 wt.% of Composition 1 |
1.66 |
0.87 |
1.65 |
3.9 |
C2 |
8 wt.% of Composition 2 |
0.53 |
0.24 |
0.4 |
3.9 |
C3 |
65 wt.% of Compositon 2 |
4,31 |
1,95 |
3.2 |
3.6 |
Table 3 Second treatment solutions (treatment 2 =TR2)
Examples TR 2 |
composition |
pH |
P1 |
Al(NO3)3 *9H2O, 4.33 g/l + H2O2 (30%) 50 g/l |
2.5 |
P2 |
Sulphuric Acid (1M ) 5.33 g/L H2O2 (30%) 50 g/l |
2.1 |
P3 |
HNO3 (1M) 34.63 g/L H2O2 (30%) 50 g/l |
1.9 |
[0046] Below Table 4 shows the various (optional) method steps and the conditions thereof,
which are carried out according to the invention.
Table 4. General process conditions, preferred conditions and actual test values.
|
Product |
Concentration [wt.%] |
Treatment time [min] |
Temp. [°C] |
pH |
|
|
|
|
|
|
Alkaline degreasing |
(Cleaner ABF) |
3.0 |
3-7 (5) |
50-55 (53) |
9-10 (9.5) |
Tap water rinsing |
|
|
|
|
|
Acidic deoxidation/ desmutting |
(Adeox 11) |
3.5 |
3-7 (5) |
15-25 (20) |
1-3 (1) |
Tap water rinsing |
|
|
|
|
|
Demi water rinsing |
|
|
|
|
|
First treatment solution |
(C2) |
5-20 |
2-20 [5--15] (10) |
10--80 [30 - 45] (40) |
3-5 [3.4- 4.4] (3.9) |
Demi water rinsing |
|
|
|
|
|
Forced hot air drying (optionally) |
|
|
|
10-- 80 [40 --60] |
|
|
|
|
|
|
|
Second treatment solution |
(P1) |
|
0 -60 [3--7] (5) |
5--60 [15-- 30] (20) |
1-5 [1-3] (2.5) |
Demi water rinsing |
|
|
|
|
|
Forced hot air drying (optionally) |
|
|
(10) |
10-- 80 [40 -- 60] (60) |
|
[..] represent preferred ranges; (..) actual test value in below Example 2. |
Test 2
[0047] In this test set-up panels of aluminium alloys AA2024-T3 and AA2024-T81 were treated
according to the steps in table 4 and actual values listed therein for C2. Neutral
salt spray resistance according ASTM B117 was compared on the same alloy with the
most common used hexavalent chrome conversion coating Alodine 1200S.
[0048] For the Alodine 1200S treated samples the aluminium substrate panels were subjected
to the steps as summarized in below Table 5.
Table 5. Pre-treatment and treatment steps for Alodine 1200 treated samples
|
Product |
Immersion time [min] |
Temperature [°C] |
Alkaline Predegreasing |
Turco 6849 |
10 |
60 |
Alkaline degreasing |
Turco 4215 |
10 |
65 |
Tap water rinsing |
|
|
|
Acidic deoxidation/desmutting |
Socosurf 1858 |
5 |
45 |
Tap water rinsing |
|
|
|
Demi water rinsing |
|
|
|
Conversion Coating |
Alodine 1200S |
2 |
RT |
Demi water rinsing |
|
|
|
[0049] Per conversion coating, 5 panels were treated and subjected to the neutral salt spray
test described in ASTM B117. After 168 hours of exposure the panels were evaluated
according to MIL-DTL-81706. The test results are summarized in below Table 6.
Table 6. Test results
Al substrate |
Pit count after 168 hrs NSS |
|
|
Alodine 1200S |
C2/P1 |
1 |
AA2024-T3 |
3 |
5 |
0 |
3 |
2 |
3 |
1 |
3 |
1 |
2 |
2 |
AA2024-T81 |
7 |
5+ |
|
6 |
5+ |
|
5 |
3 |
|
6 |
3 |
|
6 |
3 |
[0050] Corrosion resistance according to MIL-DTL-81706 requirements:
- (a) No more than 5 isolated spots or pits, none larger than 787 µm (0.031 in) diameter,
per test specimen.
- (b) No more than 15 isolated spots or pits, none larger than 787 µm (0.031 in) in
diameter, on the combined surface area of five test specimens, subjected to salt spray
testing.
[0051] Table 7 shows data of the resulting layer weight of a conversion coating applied
to AA2024 steel using 8 wt% solution of composition 2 (C2) under varying process time,
using varying immersion (dipping) times, pH and T conditions.
[0052] Tthe aluminium was pre-treated as listed in Table 4.
Table 7 Effect of varying process conditions on the resulting layer weight of the
conversion coating on AA2024 (expressed in mg Cr/m
2) using 8 wt.% of composition 2
|
pH=4.2; T= 40°C |
Immersion t [min] |
2 |
5 |
10 |
Coating weight {mg Cr/m2} |
12.5 |
18.8 |
24.1 |
|
|
|
|
|
pH=4.2; t= 5 min. |
Temperature bath 1 [°} |
30 |
35 |
40 |
Coating weight {mg Cr/m2} |
13.7 |
16.3 |
18.8 |
|
|
|
|
|
t= 5 min; T=40°C |
pH bath 1 |
3.8 |
4 |
4.2 |
Coating weight {mg Cr/m2} |
11.6 |
13.3 |
18.8 |
Test 3
[0053] AA2024-T3 panels were treated with first treatment solution C1 under the same conditions
as listed in Table 4 for C2 and various second treatment solutions.
[0054] Below Table 8 shows the corrosion test results of 2024 aluminium alloy, as well as
the Cr
6+ content present in conversion coating determined by boiling water extraction.
Table 8. Aluminium alloy 2024 treated by C1 at 40 °C during 10 minutes, followed by
a post rinse using various second treatment solutions.
Second treatment solution |
Corrosion rating Salt Spray Test ASTM B117 after 168 hours |
Hexavalent chromium measured after boiling water extraction |
None |
>5 pits |
|
P1 |
2 pits |
< 0.03 µg/cm2 |
P2 |
2 pits |
< 0.03 µg/cm2 |
H2O2 (30%) 50 g/l |
1 pits |
0.09 µg/cm2 |
P3 |
3 pits |
< 0.03 µg/cm2 |
[0055] From the test data it appears that the corrosion resistance of an AA2024-T3 substrate
that is treated with a (trivalent chromium) first treatment solution according to
the invention and a (hydrogen peroxide) second treatment solution according to the
invention is improved compared to that of the same Al substrate that is only treated
with the first treatment solution. Moreover the test data indicate that less hexavalent
chromium is formed on the substrate due to the acidic, but non-aggressive nature of
the second treatment solution.
Test 4
[0056] Different Al alloys were treated according to actual conditions listed in Table 4.
The below Table 9 lists the test results.
Table 9
|
Coating weight [mg Cr/m2] |
contact resistance MIL-DTL-81706 B Class 3 conversion coating < 5 mΩ/in2) [mΩ/in2] |
corrosion resistance MIL-DTL-5541F ASTM-B117 >168 hrs [hrs] |
AA2024-T3 |
23 |
0.4 |
>168 |
AA2024-T81 |
15 |
0.5 |
>168 |
AA6061-T6 |
30 |
1.6 |
>3000 |
AA7075-T73 |
30 |
1.5 |
>168 |
AA5083-H111 |
41 |
2.3 |
>168 |
Test 5
[0057] The method and compositions according to the invention are also suitable for maintenance
and repair purposes, such as maintenance of aeroplanes, using the so-called touch-up
or brush (wipe) methods, wherein the respective Al parts are degreased and any oxide
skin is removed mechanically. Thereafter the first and second treatment solutions
according to the invention are applied using a double (vertical and horizontal) wipe
technique with dust free cloth.
[0058] Various Al alloys were subjected to an acidic etching with Adeox 8 (100 g/L), rinsed
with water and then treated with first treatment solution C3 and again rinsed. For
AA2024-T3 thereafter second treatment solution P3 was applied, followed by a rinsing
step with water. Electrical resistivity and general corrosion performance were tested
as shown in below Table 10.
Table 10
Alloy |
contact resistance MIL-DTL-81706 B Class 3 conversion coating |
Corrosion resistance |
|
(asis < 5 mΩ /in2) |
168 hrs Salt Spray ASTM B117 (< 10 mΩ/in2) |
336 hrs Salt Spray ASTM B117 |
[mΩ/ in2] |
[mΩ / in2] |
[pass/fail] |
AA2014-T3 |
0.2 |
2.1 |
pass |
AA5754 |
0.1 |
1.7 |
pass |
AA6061-T6 |
0.1 |
0.5 |
pass |
AA7075-T73 |
0.4 |
0.4 |
pass |
[0059] Table 11 presents further corrosion results of various metal substrates when treated
according to the invention with C2 and optionally P1.
Table 11. Overview corrosion test results various aluminium alloys treated with C2,
optionally followed by post-rinse P1 at room temperature during 5 minutes.
Aluminium Alloy |
C2 |
P1 |
Neutral Salt Spray < 5 pits ASTM B117 |
AA2024-T3 |
no |
no |
< 8 hr |
AA2024-T3 |
yes |
no |
>168 hr |
AA2024-T3 |
yes |
yes |
>500 hr |
AA5005 |
yes |
no |
>1000 hr |
AA5754 |
yes |
no |
>1000 hr |
AA6060 |
yes |
no |
>1000 hr |
AA6061-T6 |
yes |
no |
>1500 hr |
AA6061-T6 |
yes |
yes |
>3000 hr |
AA6063 |
yes |
no |
>1000 hr |
Test 6
[0060]
Table 12. Standard boric acid/sulphuric acid anodizing procedure on 2024 aluminium
alloy 2024. Anodized coating thickness 5 µm. Anodized layer is treated /sealed with
Composition 2 at 2 different concentrations (T=40 °C, t= 10 minutes) without post
rinse.
Example |
Immersion time [min] |
Bath temperature [°C] |
Conc. Cr3+ [g/L] |
Sealed according to this invention Measured Cr |
Corrosion rating Salt Spray Test ASTM B117 |
1 |
- |
- |
- |
- |
completely corroded in 24 h |
2 |
10 |
40 |
0,5 |
80 Cr mg/m2 |
no pits after 168 h |
3 |
10 |
40 |
0,25 |
40 Cr mg/m2 |
3 pits after 168 h |
Test 7
[0061]
Table 13. HDG (hot dip galvanized) Steel experiment
Substrate fresh hot dipped galvanized steel cooled down to 80 °C |
Corrosion test Humidity 100% RH, 40°C Time to 5% white rust formation |
Corrosion test: ASTM B117 Neutral salt spray test Time to 5% white rust formation |
No treatment |
< 12 hours |
< 2 hours |
10 wt.% Composition 2, Room Temperature, 10 minutes |
500 hours |
48 hours |
1. An acidic aqueous composition for preparing a corrosion resistant layer on substrates
of aluminium, aluminium alloys, anodized aluminium, zinc or zinc alloy coated steel,
or aluminized steel, wherein the composition comprises:
trivalent chromium (Cr3+): |
0.04 - 6 |
g/l |
zirconium (Zr4+): |
0.08 - 8 |
g/l |
total fluoride (F-): |
0.1 - 9 |
g/l |
stabilizing agent comprising a hydroxyl carboxylic acid or corresponding base(s) thereof
(calculated as the acid): |
0.2 - 9 |
g/l |
wherein
the molar ratio Zr4+ : Cr3+ is in the range of 0.8 : 1 to 2.0 : 1;
the molar ratio Zr4+ : F- is in the range of 1: 5.5 to 1.0 : 2.0; and
pH is in the range of 3.0 - 5.0.
2. The composition according to claim 1, wherein the concentration of trivalent chromium
(Cr3+) is in the range of 0.2-1.0 g/L or in the range of 0.5-5.0 g/L.
3. The composition according to claim 1 or 2, wherein the concentration of zirconium
(Zr4+) is in the range of 0.2-2.0 g/L.
4. The composition according to any one of the preceding claims, wherein the concentration
of fluoride ions (F-) is in the range of 0.2-2.0 g/L.
5. The composition according to any one of the preceding claims, wherein the concentration
of stabilizing agent comprising a hydroxyl carboxylic acid or corresponding base(s)
thereof (calculated as the acid) is in the range of 0.2-5 g/L, preferably 0.3-3 g/L.
6. The composition according to any one of the preceding claims, wherein the pH is in
the range of 3.4-4.4.
7. The composition according to any one of the preceding claims comprising one or more
of water soluble surfactants in a concentration of 0-1.0 g/L, preferably 0.001-0.5
g/L, pH adjusting agents in a concentration of 0-1.0 g/L, water soluble polymers,
organo functional silanes or oligomers in a concentration of up to 4.0 g/L.
8. A method of treating a substrate of aluminium, aluminium alloy, anodized aluminium,
aluminized steel, zinc or zinc alloy coated steel for corrosion protection, adhesion
of a coating or adhesive bonding system, which method comprises a step of applying
the acidic aqueous solution according to any one of the preceding claims 1-7 to a
substrate of aluminium, aluminium alloy, anodized aluminium, aluminized steel, zinc
or zinc alloy coated steel.
9. The method according to claim 8, wherein the composition has a temperature in the
range of 10-80 °C, preferably 15-50 °C.
10. The method according to claim 8 or 9, wherein the treatment time is 1-30 minutes,
preferably 3-15 minutes.
11. The method according to any one of the preceding claims 8-10, further comprising one
or more steps of rinsing the treated substrate with demineralized water, wherein preferably
the last rinsing step is carried out using demineralized water having a conductivity
in the range of 5-200 microS, and more preferably a step of drying the rinsed substrate,
in particular at a temperature of 10-50 °C.
12. The method according to any one of the preceding claims 8-11, wherein the coating
weight is in the range of 5-200 mg chromium/m2, as measured by XRF after drying.
13. The method according to any one of the preceding claims 8-12, further comprising a
step of after-treating the substrate that has been treated with the acidic aqueous
composition according to any one of claims 1-7 with a second acidic aqueous composition
comprising an oxidizing agent and an acidifying component and having a pH in the range
of 1-5, preferably 1-3.
14. The method according to claim 13, wherein the concentration of the oxidizing agent
is in the range of 5-100 g/L, preferably 8-50 g/L, and/or wherein the oxidizing agent
is a water soluble peroxide, preferably hydrogen peroxide,
15. The method according to any one of the preceding claims 13-14, wherein the concentration
of the acidifying component is in the range of 0.2-20.0 g/L, preferably 0.5-5.0 g/L,
and/or wherein the acidic component is a salt of aluminium, zirconium or trivalent
chromium, preferably a nitrate salt thereof.
1. Saure wässrige Zusammensetzung zur Herstellung einer korrosionsbeständigen Schicht
auf Substraten aus Aluminium, Aluminiumlegierungen, eloxiertem Aluminium, mit Zink
oder Zinklegierung beschichtetem Stahl oder aluminiertem Stahl, wobei die Zusammensetzung
umfasst:
dreiwertiges Chrom (Cr3+): |
0,04 - 6 |
g/l |
Zirkonium (Zr4+): |
0,08 - 8 |
g/l |
Gesamtfluorid (F-): |
0,1 - 9 |
g/l |
Stabilisierungsmittel, umfassend eine Hydroxylcarbonsäure oder entsprechende Base(n)
davon (berechnet als die Säure): |
0,2-9 |
g/l |
wobei
das Molverhältnis Zr4+ : Cr3+ im Bereich von 0,8 : 1 bis 2,0 : 1 liegt;
das Molverhältnis Zr4+ : F- im Bereich von 1 : 5,5 bis 1,0 : 2,0 liegt; und
der pH-Wert im Bereich von 3,0 - 5,0 liegt.
2. Die Zusammensetzung nach Anspruch 1, wobei die Konzentration an dreiwertigem Chrom
(Cr3+) im Bereich von 0,2 - 1,0 g/L oder im Bereich von 0,5 - 5,0 g/L liegt.
3. Die Zusammensetzung nach Anspruch 1 oder 2, wobei die Konzentration von Zirkonium
(Zr4+) im Bereich von 0,2 - 2,0 g/L liegt.
4. Die Zusammensetzung nach einem der vorstehenden Ansprüche, wobei die Konzentration
an Fluoridionen (F-) im Bereich von 0,2 - 2,0 g/L liegt.
5. Die Zusammensetzung nach einem der vorstehenden Ansprüche, wobei die Konzentration
des Stabilisierungsmittels, das eine Hydroxylcarbonsäure oder entsprechende Base(n)
davon umfasst, im Bereich von 0,2 - 5 g/L, bevorzugt 0,3 - 3 g/L (berechnet als Säure)
liegt.
6. Die Zusammensetzung nach einem der vorstehenden Ansprüche, wobei der pH-Wert im Bereich
von 3,4 - 4,4 liegt.
7. Die Zusammensetzung nach einem der vorstehenden Ansprüche, umfassend ein oder mehrere
wasserlösliche Tenside in einer Konzentration von 0 - 1,0 g/L, bevorzugt 0,001 - 0,5
g/L, pH-Einstellmittel in einer Konzentration von 0 - 1,0 g/L, wasserlösliche Polymere,
organofunktionelle Silane oder Oligomere in einer Konzentration von bis zu 4,0 g/L.
8. Verfahren zum Behandeln eines Substrats aus Aluminium, Aluminiumlegierung, eloxiertem
Aluminium, aluminiertem Stahl, mit Zink oder Zinklegierung beschichtetem Stahl zum
Korrosionsschutz, zur Haftung eines Beschichtungs- oder Klebebindungssystems, wobei
das Verfahren einen Schritt des Auftragens der sauren wässrigen Lösung nach einem
der vorhergehenden Ansprüche 1 - 7 auf ein Substrat aus Aluminium, Aluminiumlegierung,
eloxiertem Aluminium, aluminiertem Stahl, mit Zink oder Zinklegierung beschichtetem
Stahl umfasst.
9. Das Verfahren nach Anspruch 8, wobei die Zusammensetzung eine Temperatur im Bereich
von 10 - 80 °C, bevorzugt 15-50 °C, aufweist.
10. Das Verfahren nach Anspruch 8 oder 9, wobei die Behandlungszeit 1 - 30 Minuten, bevorzugt
3 - 15 Minuten beträgt.
11. Das Verfahren nach einem der vorhergehenden Ansprüche 8 - 10, weiter umfassend einen
oder mehrere Schritte des Spülens des behandelten Substrats mit entmineralisiertem
Wasser, wobei bevorzugt der letzte Spülschritt unter Verwendung von entmineralisiertem
Wasser mit einer Leitfähigkeit im Bereich von 5 - 200 mikroS durchgeführt wird, und
mehr bevorzugt einen Schritt des Trocknens des gespülten Substrats, insbesondere bei
einer Temperatur von 10 - 50 °C.
12. Das Verfahren nach einem der vorhergehenden Ansprüche 8 - 11, wobei das Beschichtungs-
gewicht im Bereich von 5 - 200 mg Chrom/m2 liegt, gemessen durch RFA nach dem Trocknen.
13. Das Verfahren nach einem der vorhergehenden Ansprüche 8 - 12, weiter umfassend einen
Schritt der Nachbehandlung des Substrats, das mit der sauren wässrigen Zusammensetzung
nach einem der Ansprüche 1 - 7 behandelt wurde, mit einer zweiten sauren wässrigen
Zusammensetzung, die ein Oxidationsmittel und eine säuernde Komponente umfasst und
einen pH-Wert im Bereich von 1 - 5, bevorzugt 1 - 3 aufweist.
14. Das Verfahren nach Anspruch 13, wobei die Konzentration des Oxidationsmittels im Bereich
von 5 - 100 g/L, bevorzugt 8-50 g/L liegt, und/oder wobei das Oxidationsmittel ein
wasserlösliches Peroxid, bevorzugt Wasserstoffperoxid, ist.
15. Das Verfahren nach einem der vorhergehenden Ansprüche 13 - 14, wobei die Konzentration
der säuernden Komponente im Bereich von 0,2 - 20,0 g/L liegt, bevorzugt von 0,5 -
5,0 g/L, und/oder wobei die saure Komponente ein Salz von Aluminium, Zirkonium oder
dreiwertigem Chrom ist, bevorzugt ein Nitratsalz davon.
1. Composition aqueuse acide destinée à la préparation d'une couche, résistante à la
corrosion, sur des substrats d'aluminium, d'alliages d'aluminium, d'aluminium anodisé,
d'acier revêtu de zinc ou d'alliage de zinc ou d'acier aluminisé, la composition comprenant
:
du chrome trivalent (Cr3+) : |
0,04 à 6 g/l |
du zirconium (Zr4+) : |
0,08 à 8 g/l |
du fluorure total (F-) : |
0,1 à 9 g/l |
un agent stabilisant comprenant un acide hydroxylcarboxylique ou au moins une base
correspondante de celui-ci (calculée en tant qu'acide): |
0,2 à 9 g/l |
le rapport molaire Zr4+:Cr3+ étant dans la gamme de 0,8:1 à 2,0:1 ;
le rapport molaire Zr4+: F- étant dans la gamme de 1:5,5 à 1,0:2,0 ; et
le pH étant dans la gamme de 3,0 à 5,0.
2. Composition selon la revendication 1, dans laquelle la concentration en chrome trivalent
(Cr3+) est dans la gamme de 0,2 à 1,0 g/l ou dans la gamme de 0,5 à 5,0 g/l.
3. Composition selon la revendication 1 ou 2, dans laquelle la concentration en zirconium
(Zr4+) est dans la gamme de 0,2 à 2,0 g/l.
4. Composition selon l'une quelconque des revendications précédentes, dans laquelle la
concentration en ions fluorure (F-) est dans la gamme de 0,2 à 2,0 g/l.
5. Composition selon l'une quelconque des revendications précédentes, dans laquelle la
concentration en agent stabilisant comprenant un acide hydroxylcarboxylique ou au
moins une base correspondante de celui-ci (calculée en tant qu'acide) est dans la
gamme de 0,2 à 5 g/l, de préférence 0,3-3 g/l.
6. Composition selon l'une quelconque des revendications précédentes, dans laquelle le
pH est dans la gamme de 3,4 à 4,4.
7. Composition selon l'une quelconque des revendications précédentes, comprenant au moins
un tensioactif soluble dans l'eau à une concentration de 0 à 1,0 g/l, de préférence
de 0,001 à 0,5 g/l, des agents d'ajustement du pH à une concentration de 0 à 1,0 g/l,
des polymères solubles dans l'eau, des silanes ou oligomères organo-fonctionnels à
une concentration allant jusqu'à 4,0 g/l.
8. Procédé de traitement d'un substrat en aluminium, alliage d'aluminium, aluminium anodisé,
acier aluminisé, acier revêtu de zinc ou d'alliage de zinc pour la protection contre
la corrosion, l'adhérence d'un système de revêtement ou de collage, lequel procédé
comprend une étape d'application de la solution aqueuse acide selon l'une quelconque
des revendications précédentes 1 à 7, à un substrat en aluminium, alliage d'aluminium,
aluminium anodisé, acier aluminisé, acier revêtu de zinc ou d'alliage de zinc.
9. Procédé selon la revendication 8, dans lequel la composition a une température dans
la gamme de 10 à 80 °C, de préférence de 15 à 50 °C.
10. Procédé selon la revendication 8 ou 9, dans lequel la durée de traitement est de 1
à 30 minutes, de préférence de 3 à 15 minutes.
11. Procédé selon l'une quelconque des revendications précédentes 8 à 10, comprenant en
outre au moins une étape de rinçage du substrat traité avec de l'eau déminéralisée,
la dernière étape de rinçage étant de préférence réalisée avec de l'eau déminéralisée
ayant une conductivité dans la gamme de 5 à 200 microS, et plus préférablement une
étape de séchage du substrat rincé, en particulier à une température de 10 à 50 °C.
12. Procédé selon l'une quelconque des revendications précédentes 8 à 11, dans lequel
le poids de revêtement est dans la gamme de 5 à 200 mg de chrome/m2, tel que mesuré
par XRF après séchage.
13. Procédé selon l'une quelconque des revendications précédentes 8 à 12, comprenant en
outre une étape de post-traitement du substrat, qui a été traité avec la composition
aqueuse acide selon l'une quelconque des revendications 1 à 7, avec une deuxième composition
aqueuse acide comprenant un agent oxydant et un composant acidifiant et ayant un pH
dans la gamme de 1 à 5, de préférence de 1 à 3.
14. Procédé selon la revendication 13, dans lequel la concentration en agent oxydant est
comprise entre 5 et 100 g/l, de préférence entre 8 et 50 g/l, et/ou l'agent oxydant
est un peroxyde soluble dans l'eau, de préférence du peroxyde d'hydrogène.
15. Procédé selon l'une quelconque des revendications précédentes 13 à 14, dans lequel
la concentration du composant acidifiant est dans la gamme de 0,2 à 20,0 g/l, de préférence
de 0,5 à 5,0 g/l, et/ou le composant acide est un sel d'aluminium, de zirconium ou
de chrome trivalent, de préférence un sel nitré de ceux-ci.