U.S. patent application number 10/176308 was filed with the patent office on 2003-03-27 for method for cleaning and passivating a metal surface.
Invention is credited to Franz, Wolf-Dieter.
Application Number | 20030056807 10/176308 |
Document ID | / |
Family ID | 8177771 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030056807 |
Kind Code |
A1 |
Franz, Wolf-Dieter |
March 27, 2003 |
Method for cleaning and passivating a metal surface
Abstract
A surface of a light-metal alloy is anodically polarized in a
cleaning process. The surface is cleaned with a solution containing
phosphoric acid, alcohol, and optionally fluoride. Additionally,
the surface is passivated in an oxidizing process using, for
example, fluoride ions or an aqueous oxidizing agent. Preferably,
the light-metal alloy has a relatively high Al or Mg content. This
method is suitable for preparing the surface for a subsequent
coating operation.
Inventors: |
Franz, Wolf-Dieter;
(Geretsried, DE) |
Correspondence
Address: |
SENNIGER POWERS LEAVITT AND ROEDEL
ONE METROPOLITAN SQUARE
16TH FLOOR
ST LOUIS
MO
63102
US
|
Family ID: |
8177771 |
Appl. No.: |
10/176308 |
Filed: |
June 20, 2002 |
Current U.S.
Class: |
134/3 ; 134/28;
134/29; 134/41 |
Current CPC
Class: |
C25F 1/04 20130101; C25F
1/00 20130101 |
Class at
Publication: |
134/3 ; 134/28;
134/29; 134/41 |
International
Class: |
C23G 001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 20, 2001 |
EP |
01114981.2 |
Claims
What is claimed is:
1. A method for cleaning and passivating a metallic surface
comprising the steps of: cleaning a surface of a light-metal alloy
with a solution comprising phosphoric acid and an alcohol, wherein
the alloy is anodically polarized; and passivating the surface in
an oxidizing step.
2. The method according to claim 1 wherein the cleaning solution
has a phosphoric acid content which ranges from about 30 to about
90 percent by volume.
3. The method according to claim 2 wherein the phosphoric acid
comprises H.sub.3PO.sub.4 in a concentration ranging from about 50
to about 95 percent by weight.
4. The method according to claim 3 wherein the cleaning solution
consists essentially of phosphoric acid and alcohol.
5. The method according to claim 1 further wherein the surface is
passivated by contacting the surface with fluoride ions.
6. The method according to claim 5 wherein the light-metal alloy
has a Si content of at least about 0.1 percent by weight.
7. The method according to claim 5 wherein the light-metal alloy
has an Mg content of at least 50 percent by weight.
8. The method according to claim 5 wherein the cleaning solution
further comprises fluoride ions.
9. The method according to claim 8 wherein the cleaning solution
has a fluoride content which ranges from about 0.1 to about 10
percent by weight.
10. The method according to claim 5 wherein the cleaning solution
has a phosphoric acid content which ranges from about 30 to about
90 percent by volume.
11. The method according to claim 10 wherein the phosphoric acid
comprises H.sub.3PO.sub.4 in a concentration ranging from about 50
to about 95 percent by weight.
12. The method according to claim 11 wherein the cleaning solution
consists essentially of phosphoric acid, alcohol, and fluoride.
13. The method according to claim 2 wherein the surface is
contacted with a second solution comprising phosphoric acid and
fluoride ions subsequent to the cleaning step.
14. The method according to claim 13 wherein the second solution
further comprises an alcohol.
15. The method according to claim 13 wherein the solution has a
fluoride content which ranges from about 0.1 to about 10 percent by
weight.
16. The method according to claim 13 wherein the solution has a
phosphoric acid content which ranges from about 30 to about 90
percent by volume.
17. The method according to claim 13 wherein the phosphoric acid
comprises H.sub.3PO.sub.4 in a concentration ranging from about 50
to about 95 percent by weight.
18. The method according to claim 1 wherein the light-metal alloy
has an Al content of at least about 60 percent by weight and
wherein the surface is passivated by oxidizing the surface with an
aqueous oxidizing agent subsequent to the cleaning step.
19. The method according to claim 1 further comprising a final
alkaline rinsing step.
20. The method according to claim 1 wherein the alloy is anodically
polarized with a current density which ranges from about 10 to
about 500 A/m.sup.2.
21. The method according to claim 1 wherein the solution has a
temperature which ranges from about 10.degree. C. to about
40.degree. C.
22. The method according claim 1 wherein the cleaning and
passivating steps are performed in a time period ranging from about
10 seconds to about 5 minutes.
23. The method according claim 1 wherein the light-metal alloy
surface has not undergone a preliminary chemical treatment prior to
the cleaning and passivating steps.
24. The method according claim 1 wherein the light-metal alloy is a
regenerated material.
25. A method for coating a metallic surface comprising the steps
of: cleaning a surface of a light-metal alloy with a solution
comprising phosphoric acid and an alcohol, wherein the alloy is
anodically polarized; passivating the surface in an oxidizing step;
and coating the surface after the cleaning and passivating
steps.
26. The method according to claim 25 in which the coating is
carried out by means of electroless metal plating.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a methods for cleaning and
preserving a metal surface, particularly a light-metal alloy
surface. This invention further relates to methods for preparing a
metal surface for subsequent coating processes.
[0002] Many known methods for cleaning light-metal alloy surfaces
have the disadvantage that they require a relatively large number
of consecutive treatment steps and thus entail a comparatively high
degree of complexity and cost. In other cases, these known methods
lead to an unsatisfactory cleaning action as far as certain
substances are concerned. For example, some processes do not
reliably remove polysilanes.
[0003] After undergoing a cleaning procedure, surfaces of
light-metal alloys may be passivated, and thus be preserved to a
certain extent, by means of known oxidizing treatments. In its
general chemical context, such oxidation includes reaction with
oxygen, typically used on alloys with a high Al content, as well as
reaction with fluoride ions, typically used on alloys with a higher
Mg content.
[0004] Some of the known cleaning and passivating methods employ
substances which pose a health risk, such as nitric acid, which may
release nitrous gases. Furthermore, using conventional methods, it
is difficult to ensure the quality of a cleaned surface prior to
passivation if the cleaning and passivating steps are combined.
SUMMARY OF THE INVENTION
[0005] Among the several objects of the present invention,
therefore, may be noted the provision of a method for cleaning and
passivating a light-metal alloy surface which is effective in the
sense that it leads to good cleaning properties and is resistant to
the composition of the alloy and in the sense that it is also
economically efficient. A further object of the present invention
is the provision of a method for cleaning and passivating a
light-metal alloy surface in preparation for a subsequent plating
process.
[0006] Briefly therefore, the present invention is directed to a
method for cleaning a metallic surface in which a surface of a
light-metal alloy is cleaned and passivated. The cleaning is
performed with a solution comprising phosphoric acid and an
alcohol, and the surface is passivated in an oxidizing step. The
invention is further directed to a method for coating a metallic
surface in which a surface of a light-metal alloy is cleaned and
passivated prior to coating. The cleaning is performed with a
solution comprising phosphoric acid and an alcohol, and the
light-metal alloy surface is passivated in an oxidizing step.
[0007] Other objects and features of the present invention will be
in part apparent and in part pointed out hereinafter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0008] In metallurgy and materials science, light-metal alloys are
understood to encompass a variety of metal mixtures comprising
"light metals," such as Al, Be, Mg, and Ti. The most common
light-metal alloys contain Al or Mg. Preferably, the method
according to this invention is performed on light-metal alloys with
a relatively high Al content or those with a relatively high Mg
content.
[0009] It has been discovered that an anodic cleaning method
according to the present invention ensures a very thorough and, at
the same time, broad cleaning action of light-metal alloy surfaces.
Owing to the anodic operation, the cleaning method according to the
present invention has a certain inherent inhibiting effect in that
the anodic oxygen reactions of the light-metal surface prevent an
excessive erosion of the material.
[0010] Useful anodic current densities on the anodically polarized
light-metal alloy surface range from about 50, 30, or even 10
A/m.sup.2 as the lower limit to about 500 A/m.sup.2 as the upper
limit. An optimal current density is a function of the composition
of the alloy, the justifiably acceptable material erosion, and the
cleaning action required. It is possible to optimize the overall
cleaning and passivating process through anodic current density
without changing the composition of a treatment solution selected
to passivate a light-metal alloy in an oxidation step. Thus, it is
possible to obtain optimum results for different alloys with the
same solution composition. It is, of course, also possible to
optimize the composition of the solution according to the alloy
being treated.
[0011] The solution used in the anodic cleaning steps comprises
phosphoric acid in an amount which preferably ranges from about 30
to about 90 percent of the solution on a volumetric basis. Within
this range of volume fractions, the phosphoric acid can measure
from about 50 to about 95 percent H.sub.3PO.sub.4 by weight. The
solution further comprises an alcohol and, optionally, fluoride
ions.
[0012] The solution process temperature preferably ranges from
about 10 to about 40.degree. C. The overall treatment time for the
anodic cleaning steps (if there are several) may range, for
example, from about 10 seconds to about 5 minutes and depends to a
considerable degree on the current density selected, the
justifiably acceptable material erosion, and the degree of
soiling.
[0013] In one embodiment, a light-metal alloy surface is cleaned
and passivated by means of an oxidation step, characterized by a
treatment step with a solution which comprises phosphoric acid and
an alcohol, and in which the surface is anodically connected.
Useful alcohols comprise common alcohols, such as methanol,
ethanol, propanol, butanol, and polyhydric alcohols and derivatives
therefore, such as isopropanol. Diols, polyethers, and other
alcohols are also useful. Butanol and isopropanol are preferred
alcohols. Mixtures of two or more alcohols are also useful.
[0014] The anodic cleaning step with a solution comprising
phosphoric acid and alcohol is highly effective both with respect
to degreasing and etching the surface and, moreover, is also able
to remove problem residues, such as polysilane release agents. For
example, when light-metal alloys are produced by means of a
die-casting process, they are contaminated with release agents.
Even these agents are reliably and completely removed when the
cleaning method disclosed by this invention is used.
[0015] The cleaning action of the method according to the present
invention is sufficiently thorough and broad that preliminary
chemical treatment steps prior to placing a light-metal alloy into
the solution comprising phosphoric acid and the alcohol are
unnecessary. Thus, the surfaces to be treated can be dry and can be
placed directly into the solution.
[0016] In another embodiment fluoride ions are used to passivate
the surface. Accordingly the solution used to treat the light-metal
alloy surface comprises phosphoric acid and fluoride ions, wherein
the light-alloy surface is anodically connected during this
treatment step. Useful sources of fluoride ions comprise alkali
fluoride, ammonium-(bi)-fluoride, and hydrogen fluoride.
[0017] Treatment with a fluoride-containing solution is
particularly useful for light-metal alloys with a substantial Mg
content. The treatment step with fluoride ions is preferred for a
light-metal alloy having a Mg content of at least about 50 percent
by weight. MgF.sub.2 forms as the passivating layer or in the
passivating layer.
[0018] Treatment with fluoride ions is also preferred for
light-metal alloys with a Si content, more preferably for
light-metal alloys having a Si content of at least about 0.1, 0.5,
1 or 2 percent by weight or more. Alloys with relatively low Si
concentrations are preferably treated with solutions having
relatively low fluoride concentrations. A treatment step with the
fluoride ions is still useful for light-metal alloys with a low or
negligible Mg content.
[0019] Solutions having fluoride concentrations as low as about
0.5, 0.3, or even 0.1 percent by weight have been found to be
useful, as have solutions having fluoride concentrations as high as
10, 20, or even 30 percent by weight.
[0020] Combinations of the above embodiments may be performed. For
example, in a third embodiment, the treatment step is performed
with a solution comprising phosphoric acid, an alcohol, and
fluoride ions. Alternatively, the aforementioned embodiments may be
performed separately in succession. For example, a first treatment
step performed with a solution comprising phosphoric acid and an
alcohol may be followed by a second treatment step performed with a
solution comprising phosphoric acid and fluoride ions. To optimize
cleaning properties, the solution in the second treatment step may
further comprise an alcohol. This alcohol may be the alcohol used
in the first treatment step or a different alcohol.
[0021] In another embodiment the cleaning and passivating method
according to the present invention further comprises an additional
passivating/oxidizing step with an aqueous oxidizing agent. This
oxidizing agent may be, for example, a persulfate solution or a
solution of peroxomonosulfuric acid (Caro's acid). This additional
passivation/oxidation step is preferably performed wherein the
light-metal alloy has a substantial Al content, in particular an Al
content of at least about 60 percent.
[0022] It is preferred that other treatment steps, e.g. a treatment
step with a solution comprising phosphoric acid and especially a
treatment step with a solution comprising fluoride ions, are
performed before an additional passivation/oxidation step.
[0023] An additional passivation/oxidation step is not necessary
for a surface of a light-metal alloy with a high Mg content which
is coated with fluoride. The fluoride passivation layer may be
harmed by such a step if it is carried out in an excessively acidic
range, for example at a pH of no greater than about 6.
[0024] A special advantage of the methods according to the present
invention is that the final oxidation of the Al alloy can be
carried out without having to use nitric acid since the metal
surface is untarnished. Thus, nitrous gases, such as are generated
in conventional processes do not form. Accordingly there is not
need to employ technically complex procedures for exhaustion and
purification of the waste gases, and it is not necessary to obtain
approval according to the relevant applicable regulations, e.g.
Bundes-Immissionsschutzgesetz, the German Federal Pollution Control
Act.
[0025] The cleaning and passivating method according to the present
invention may further comprise an alkaline rinsing step performed
in alkalized water, for example. Preferably, alkalized water with a
pH value at least about 10 is used. An alkaline rinsing step is
especially beneficial if the passivating surface is dominated by
MgF.sub.2 at preferred pH values but less beneficial if the
passivating surface is dominated by Al.sub.2O.sub.3.
[0026] A special advantage of this invention is that it is possible
to obtain good results even on regenerated light-metal alloys. In
particular, no sludge is generated. In conventional processes, the
metal impurities of regenerated materials have led to considerable
problems with cleaning and have frequently made it completely
impossible to clean and subsequently coat the surfaces. Even if the
Al content is relatively high, the metal surfaces in the anodic
baths according to the present invention remain untarnished,
allowing the subsequent oxidation mentioned earlier without having
to use nitric acid.
[0027] Since the quality of a coating depends largely on the
cleanliness of the surface, both as far as the appearance of the
coating and the load-bearing capacity of the coating are concerned,
this invention is particularly useful to prepare light-metal alloy
surfaces for subsequent coating steps of any type.
[0028] In particular, another embodiment comprises a subsequent
metal plating step, preferably electroless metal plating. The
components thus treated can also subsequently be chemically
zinc-plated, nickel-plated, or copper-plated, or they can be coated
with alloys thereof. In chemical conversion coating, the
passivating layers are dissolved or converted, thus ensuring that a
good and direct contact between the metals is obtained.
[0029] The following examples illustrate the invention.
EXAMPLE 1
[0030] Without any preliminary chemical cleaning, a dry, anodically
polarized AZ91 alloy, an alloy with a relatively high Mg content,
is introduced into a bath consisting of 60% phosphoric acid
(H.sub.3PO.sub.4) and 40 vol % of butanol. The current density is,
for example, 20 A/m.sup.2 at a temperature of 25.degree. C. and a
treatment time, for example, of 30 sec.
[0031] The AZ91 alloy is subsequently introduced into a second bath
which has a composition identical to the composition mentioned
above but which, in addition, also comprises 2 percent by weight of
ammonium bifluoride. A second anodic cleaning is carried out for
another 20 sec at the same current density.
[0032] Subsequently, the AZ91 components are rinsed in alkalized
water (with a pH value slightly higher than 10). The AZ91 surface
is now passivated with a fluoride coating and can be metal-plated
using a conventional method. In this particular practical example,
chemical conversion coating with Zn, Ni, or Cu or with an alloy
made thereof is used. Other alloys with a relatively high Mg
content, such as AM50 or AZ31 alloys, may be treated according to
the methods presented in this example.
EXAMPLE 2
[0033] An technical aluminum alloy with a high Al content, in this
case GdAlSi.sub.8Cu.sub.3, is used. Since this alloy comprises Si,
the fluoride bath described in the first practical example is used
here as well. The same quantitative parameters can be used, except
that the rinsing step in alkalized water is omitted. Instead,
neutral water having a pH of approximately 7 is used for rinsing,
which is followed by an additional oxidation with persulfate
solution to strengthen the passivating layer. This treatment can
also be used, for example, for GdAlSi.sub.9Cu.sub.3.
[0034] In view of the above, it will be seen that the several
objects of the invention are achieved.
[0035] As various changes could be made in the above material and
processes without departing from the scope of the invention, it is
intended that all matter contained in the above description be
interpreted as illustrative and not in a limiting sense.
* * * * *