U.S. patent application number 10/275504 was filed with the patent office on 2004-05-27 for electrochemically produced layers for providing corrosion protection or wash primers.
Invention is credited to Mayer, Bernd, Schweinsberg, Mattias, Wiechmann, Frank.
Application Number | 20040099535 10/275504 |
Document ID | / |
Family ID | 7640989 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040099535 |
Kind Code |
A1 |
Schweinsberg, Mattias ; et
al. |
May 27, 2004 |
Electrochemically produced layers for providing corrosion
protection or wash primers
Abstract
Use of a layer on an electrically conductive surface as a
corrosion protection layer and/or as a primer for an organic
coating, which may be obtained in a stage (a), in which a layer of
at least one inorganic compound of at least one metal A having a
weight per unit area of 0.01 to 10 g/m.sup.2 is deposited
electrochemically on the said surface from a solution containing
the metal A in dissolved form, wherein the metal A is a different
metal from the main component of the electrically conductive
surface and wherein the inorganic compound contains less than 20
wt. % phosphate ions; process for producing a coating comprising at
least two layers, at least one layer of an organic polymer being
applied to the layer deposited in stage a); metal component
comprising a coating comprising at least two layers obtainable in
this manner.
Inventors: |
Schweinsberg, Mattias;
(Langenfeld, DE) ; Mayer, Bernd; (Duesseldorf,
DE) ; Wiechmann, Frank; (Duesseldorf, DE) |
Correspondence
Address: |
HENKEL CORPORATION
THE TRIAD, SUITE 200
2200 RENAISSANCE BLVD.
GULPH MILLS
PA
19406
US
|
Family ID: |
7640989 |
Appl. No.: |
10/275504 |
Filed: |
June 9, 2003 |
PCT Filed: |
April 27, 2001 |
PCT NO: |
PCT/EP01/04780 |
Current U.S.
Class: |
205/170 |
Current CPC
Class: |
C25D 9/04 20130101; C25D
13/22 20130101 |
Class at
Publication: |
205/170 |
International
Class: |
C25D 005/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 6, 2000 |
DE |
100220746 |
Claims
1. Use of a layer on an electrically conductive surface as a
corrosion protection layer and/or as a primer for an organic
coating, may be obtained in a stage (a), in which a layer of at
least one inorganic compound of at least one metal A having a
weight per unit area of 0.01 to 10 g/m.sup.2 is deposited
electrochemically on the said surface from a solution containing
the metal A in dissolved form, wherein the metal A is a different
metal from the main component of the electrically conductive
surface and wherein the inorganic compound contains less than 20
wt. % phosphate ions.
2. Use as claimed in claim 1 wherein the compound deposited in
stage (a) is an oxide.
3. Use as claimed in one or both of claims 1 and 2 wherein the
metal A is selected from Mg, Ca, Sr, Ba, Al, Si, Sn, Pb, Sb, Bi,
Ti, Zr, V, Nb, Ta, Mo, W, Mn, Fe, Co, Ni, Zn, Cu.
4. Use as claimed in one or more of claims 1 to 3 wherein the
inorganic compound is deposited on the electrically conductive
surface at a potential relative to a standard hydrogen electrode of
between .+-.0.1 and .+-.300 V or a current density of from .+-.0.1
to .+-.10000 mA per cm.sup.2 of electrically conductive
surface.
5. Use as claimed in one or more of claims 1 to 4 wherein the
inorganic compound is X-ray crystalline.
6. A process for producing a coating comprising at least two layers
on an electrically conductive surface, characterised in that, in a
stage (a), a layer of at least one inorganic compound of at least
one metal A having a weight per unit area of 0.01 to 10 g/m.sup.2
is electrochemically deposited on the electrically conductive
surface from a solution containing the metal A in dissolved form,
wherein the metal A is a different metal from the main component of
the electrically conductive surface and wherein the inorganic
compound contains less than 20 wt. % phosphate ions, and in a
subsequent stage (b), at least one layer of an organic polymer is
applied to the layer deposited in stage (a).
7. A process as claimed in claim 6 wherein the compound deposited
in stage (a) is an oxide.
8. A process as claimed in one or both of claims 6 and 7 wherein
the metal A is selected from Mg, Ca, Sr, Ba, Al, Si, Sn, Pb, Sb,
Bi, Ti, Zr, V, Nb, Ta, Mo, W, Mn, Fe, Co, Ni, Zn, Cu.
9. A process as claimed in one or more of claims 6 to 8 wherein the
inorganic compound is deposited on the electrically conductive
surface at a potential relative to a standard hydrogen electrode of
between .+-.0.1 and .+-.300 V or a current density of from .+-.0.1
to .+-.10000 mA per cm.sup.2 of electrically conductive
surface.
10. A process as claimed in one or more of claims 6 to 9 wherein in
substage (b) a cathodically or anodically depositable
electrocoating lacquer is applied.
11. A process as claimed in one or more of claims 6 to 9 wherein
the process is performed as a coil process and in substage (b) an
organic polymer layer is applied by dipping or spraying or by
applicator rolls.
12. A process as claimed in one or more of claims 6 to 9 wherein in
substage (b) a powder coating is applied.
13. A process as claimed in one or more of claims 6 to 9 wherein in
substage (b) an adhesive is applied.
14. A metal component, the surface of which bears a coating
comprising at least two layers, which may be obtained according to
one or more of claims 6 to 13.
15. A metal component as claimed in claim 14 wherein the inorganic
compound of at least one metal A is X-ray crystalline.
Description
[0001] This invention relates to the coating of surfaces to protect
them against corrosion and/or to provide them with a primer for a
subsequent organic coating. To this end, the surfaces have to be
electrically conductive, i.e. they may for example be surfaces of
metal or surfaces of glass or plastics made conductive by
appropriate treatment.
[0002] A very common industrial task involves providing metallic or
non-metallic substrates with a first coating, which has a
corrosion-inhibiting effect and/or which constitutes a primer for
the application thereon of a coating containing organic polymers.
An example of such a task is the pretreatment of metals prior to
lacquer coating, for which various processes are available in the
art. Examples of such processes are layer-forming or
non-layer-forming phosphating, chromating or a chromium-free
conversion treatment, for example using complex fluorides of
titanium, zirconium, boron or silicon. Technically simpler to
perform, but less effective, is the simple application of a primer
coat to a metal prior to lacquer-coating thereof. An example of
this is the application of red lead. An alternative to "wet"
processes are processes, in which a corrosion-protection or
coupling layer is applied by gas phase deposition. Such processes
are known, for example, as PVD or CVD processes. They may be
assisted electrically, for example by plasma discharge.
[0003] A layer produced or applied in this way may serve as a
corrosion-protective primer for subsequent lacquer coating.
However, the layer may also constitute a primer for subsequent
bonding. Metallic substrates in particular, but also substrates of
plastics or glass, are frequently pretreated chemically or
mechanically prior to bonding, in order to improve adhesion of the
adhesive to the substrate. For example, in vehicle or equipment
construction, metal or plastics components may be bonded metal to
metal/plastics to plastics or metal to plastics. At present, front
and rear windscreens of vehicles are as a rule bonded directly into
the bodywork. Other examples of the use of coupling layers are to
be found in the production of rubber/metal composites, in which
once again the metal substrate is as a rule pretreated mechanically
or chemically before a coupling layer is applied for the purpose of
bonding with rubber.
[0004] The conventional wet or dry coating processes in each case
exhibit particular disadvantages. For example, chromating processes
are disadvantageous from both an environmental and an economic
point of view owing to the toxic properties of the chromium and the
occurrence of highly toxic sludge. However, chromium-free wet
processes, such as phosphating, as a rule also result in the
production of sludge containing heavy metals, which has to be
disposed of at some expense. Another disadvantage of conventional
wet coating processes is that the actual coating stage frequently
has to be preceded or followed by further stages, thereby
increasing the amount of space required for the treatment line and
the consumption of chemicals. For example, phosphating, which is
used virtually exclusively in automobile construction, entails
several cleaning stages, an activation stage and, generally a
post-passivation stage. In all these stages, chemicals are consumed
and waste is produced which has to be disposed of.
[0005] Although dry coating processes entail fewer waste problems,
they have the disadvantage of being technically complex to perform
(for example requiring a vacuum) or of having high energy
requirements. The high operating costs of these processes are
therefore a consequence principally of plant costs and energy
consumption.
[0006] For this reason, there is a need for new coating processes
for producing corrosion-protection or primer layers, which require
less expenditure on apparatus than dry processes and are associated
with lower chemicals consumption and a smaller volume of waste than
wet processes.
[0007] It is known from the prior art that thin layers of metal
compounds, for example oxide layers, may be produced
electrochemically on an electrically conductive substrate. For
example, the article by Y. Zhou and J. A. Switzer entitled
"Electrochemical Deposition and Microstructure of Copper(I) Oxide
Films", Scripta Materialia Vol. 38, No. 11, pages 1731-1738 (1998),
describes the electrochemical deposition and microstructure of
copper(I) oxide films on stainless steel. The article investigates
above all the influence of deposition conditions on the morphology
of the oxide layers; it does not disclose any practical application
of the layers.
[0008] The article by M. Yoshimura, W. Suchanek, K- S. Han entitled
"Recent developments in soft solution processing: one step
fabrication of functional double oxide films by
hydrothermal-electrochemical methods", J. Mater. Chem. Vol. 9,
pages 77-82 (1999), investigates the production of thin films of
double oxides by a combination of hydrothermal and electrochemical
methods. The production of ceramic materials is given as an example
of application. The article does not contain any indication as to
the usability of such layers for corrosion protection or as a
primer.
[0009] Electrochemical formation of an oxide layer also occurs in
the processes known as anodic oxidation. The present invention
differs from these in that layers of metal compounds are deposited
on a substrate, wherein the metal of the metal compound constitutes
substantially a different metal from that which makes up the
optionally metallic substrate.
[0010] It is also known to assist the formation of crystalline zinc
phosphate layers electrochemically. However, the disadvantages of
phosphating (several substages, such as activation, phosphating,
post-passivation; occurrence of phosphating sludge) are not
overcome thereby. Electrochemical promotion of the formation of
zinc phosphate layers does not fall within the scope of the present
invention.
[0011] The present invention relates, in a first embodiment, to the
use of a layer on an electrically conductive surface as a corrosion
protection layer and/or as a primer for an organic coating, which
may be obtained in a stage (a), in which a layer of at least one
inorganic compound of at least one metal A having a weight per unit
area of 0.01 to 10 g/m.sup.2 is deposited electrochemically on the
said surface from a solution containing the metal A in dissolved
form, wherein the metal A is a different metal from the main
component of the electrically conductive surface and wherein the
inorganic compound contains less than 20 wt. % phosphate ions.
[0012] The solution, which contains the metal A in dissolved form,
is hereinafter designated "electrolyte". If the salt of the metal A
is dissolved in water, the conductivity of this solution is as a
rule sufficient for the purpose according to the present invention.
Should a non-aqueous solvent be used or the conductivity of an
aqueous solution not be adequate, a conducting salt, such as
tetraalkyl ammonium halide, may be added. The ions in the
conducting salt are not incorporated into the layer, or are
incorporated to only a minor extent, but they increase the
electrical conductivity of the electrolyte.
[0013] The electrically conductive surface may be an intrinsically
conductive surface, such as a metallic surface. However, the layer
may also be deposited on the surface of an electrically less
conductive or a non-conductive material, if suitable measures are
used to make the surface electrically conductive. In the case of
plastics, this may be achieved, for example, in that first of all
an electrically conductive metal layer is deposited by chemical
means, which then constitutes the basis for the electrochemical
deposition of a compound of the metal A. A glass surface may be
made electrically conductive, for example, in that it is dusted
with a powder of an electrically conductive substance or a
conductive layer is applied via the gas phase, for example by
chemical vapor deposition (CVD). However, for the present use it is
preferred for the electrically conductive surface to be a metal
surface.
[0014] The inorganic compound of the metal A is deposited from a
solution containing the metal A in dissolved form. The solution may
be a single- or multi-component, aqueous or non-aqueous solution.
Examples of non-aqueous solvents having good dissolving power with
regard to suitable metal compounds are liquid ammonia, dimethyl
sulfoxide or organic phosphane derivatives. Examples of a
multi-component aqueous solution are water/alcohol mixtures.
[0015] The electrochemical deposition may be performed cathodically
or anodically, cathodic deposition being more universally usable
and therefore preferred. Deposition of the inorganic compound of at
least one metal A from a corresponding solution may proceed
according to two different mechanisms. On the one hand, deposition
may be coupled with a change in the oxidation state of the metal A,
wherein a layer of a sparingly soluble compound of the metal A, in
the oxidation state modified relative to the solution, grows on the
electrically conductive surface. For example, copper(I) oxide may
be deposited cathodically from an aqueous solution containing
copper(II) ions. Another deposition mechanism is based on the fact
that electrochemical processes performed on the electrically
conductive surface cause a shift in the pH in the vicinity of the
surface. As a consequence of this, an inorganic compound of at
least one metal A may grow on the electrically conductive surface,
which compound is sparingly soluble under the localised pH
conditions at the surface. It is not then process. A shift in the
pH at the electrically conductive surface may be effected, for
example, in that hydrogen ions are discharged, thereby causing the
pH to rise locally.
[0016] Where an inorganic compound of at least one metal A is
mentioned herein, it is meant that this compound has in any event
to contain the metal A. However, it may additionally contain
further metals B, C, etc. These further metals may be present in
the solution in addition to the metal A and deposited together
therewith. These other metals may, however, also be components of
the electrically conductive surface and be directly incorporated
into the inorganic compound of at least one metal A during
formation of the layer thereof. Examples of inorganic compounds,
which contain a further metal in addition to the metal A, are mixed
oxides, which may belong, for example, to the spine1 or perovskite
structural type. Examples are titanates or niobates.
[0017] Due to ease of performability and the possibility of using
water as the solvent, it is preferable for the compound deposited
in stage (a) to be an oxide, which may also be a mixed oxide of
various metals. However, the present use is not restricted to
oxides, but additionally extends to non-oxide inorganic compounds,
such as selenides, sulfides or nitrides, which may be deposited
from suitable, optionally water-free solvents.
[0018] It is not essential for the purposes of the present
invention, for the inorganic compound of at least one metal A to
consist of a merely binary or ternary compound. Rather, this
compound may also be of a more complex structure, for example by
also incorporating ions or molecules from the solution into the
compound. Hydrated or sulfated oxides are examples of this.
[0019] The present use does not involve pure electroplating, since
an electroplated layer does not constitute an "inorganic compound"
in the sense of the present invention. Rather, it is required of
the layer of at least one inorganic compound of at least one metal
A that at least part of the metal A is present in an oxidation
state>0.
[0020] In principle, any layer of at least one inorganic compound
of at least one metal A which may be electrochemically deposited
and is sufficiently chemically stable to act as a
corrosion-protection layer may be employed for the present use.
This means that the layer provides better corrosion protection with
or without lacquer applied thereto than the uncoated metal surface.
For reasons of price and availability, it is preferable for the
metal A to be selected from Mg, Ca, Sr, Ba, Al, Si, Sn, Pb, Sb, Bi,
Ti, Zr, V, Nb, Ta, Mo, W, Mn, Fe, Co, Ni, Zn, Cu. For practical
purposes, the most significant metals from this list are Al, Si,
Ti, Zr, Mo, W, Mn, Fe, Co, Ni, Zn and Cu.
[0021] The electrochemical deposition may be performed
potentiostatically or galvanostatically. Galvanostatic deposition
is technically simpler to perform and is therefore preferred. Layer
formation preferably proceeds in that the inorganic compound is
deposited on the electrically conductive surface at a potential
relative to a standard hydrogen electrode of between .+-.0.1 and
.+-.300 V or a current density in the range of from .+-.0.1 to
.+-.10000 mA per cm.sup.2 of electrically conductive surface. The
procedure is preferably performed at potentials of between .+-.0.1
and .+-.100 V or at a current density of from .+-.0.5 to .+-.100 mA
per cm.sup.2. The signs preceding the voltage and current density
express the fact that deposition may proceed cathodically or
anodically. Cathodic deposition, i.e. a negative potential relative
to the standard hydrogen electrode, is preferred.
[0022] It is known from the literature cited above that the
morphology, chemical composition and crystal structure of the
deposited layer depend on deposition conditions and thus may be
influenced by the choice of conditions. For example, the
above-mentioned layer parameters depend on the concentration of
metal ions A and optionally further components in the solution, the
flow rate of the solution relative to the electrically conductive
surface, the potential established and/or the current density
established. The layer characteristics may thus be deliberately
modified by the choice of these parameters. Deposition is
preferably performed under such conditions that the inorganic
compound is deposited in X-ray crystalline form. X-ray crystalline
means that the inorganic compound produces sharp X-ray reflections
when subjected to an X-ray diffraction experiment. The resultant
highly textured surface is particularly suitable as a primer for an
organic coating.
[0023] Thorough mixing of the electrolyte and/or relative movement
of the electrolyte relative to the metallically conductive surface
may accelerate layer formation and influence the morphology of the
layer. This may proceed in that the electrolyte is stirred or
pumped around in the electrolysis vessel. In addition, the
electrolyte may be thoroughly mixed and moved by blowing in a gas,
in particular air.
[0024] Mention was made above of deposition at a certain potential
relative to a standard hydrogen electrode. Stating a potential in
this manner presupposes the use of a reference electrode located as
close as possible to the electrically conductive substrate surface.
However, it is simpler in practice to operate galvanostatically and
to establish the desired current density by varying the terminal
voltage of the electrically conductive surface as the working
electrode and of any desired counter-electrode. Examples of
suitable counter-electrodes are those which are stable for
sufficiently long periods under the selected electrolysis
conditions, for example stainless steel, gold, silver, platinum,
graphite or glassy carbon.
[0025] In another embodiment, the present invention relates to a
process for producing a coating comprising at least two layers on
an electrically conductive surface, characterised in that, in a
stage (a), a layer of at least one inorganic compound of at least
one metal A having a weight per unit area of 0.01 to 10 g/m.sup.2
is electrochemically deposited on the electrically conductive
surface from a solution containing the metal A in dissolved form,
wherein the metal A is a different metal from the main component of
the electrically conductive surface and wherein the inorganic
compound contains less than 20 wt. % phosphate ions, and in a
subsequent stage (b), at least one layer of an organic polymer is
applied to the layer deposited in stage (a).
[0026] A "coating comprising at least two layers" means that, as
described above, a layer of at least one inorganic compound of at
least one metal A is applied to the electrically conductive surface
and at least one layer of an organic polymer is in turn applied to
the said first layer. It goes without saying that a plurality of
different layers of organic polymers may be applied to the layer of
an inorganic compound. This is known from automobile construction,
for example, in which, according to the prior art, at least three
different layers of organic polymers are generally applied to the
phosphate layer serving as inorganic corrosion-protection layer and
coupling layer. These layers may comprise an electrocoating
lacquer, a filler and a topcoat, for example.
[0027] The layer of at least one inorganic compound of at least one
metal A may consist of a layer, the formation, properties and
composition of which have been described above.
[0028] In an embodiment of substage (b) of the present process for
producing a coating comprising at least two layers, a cathodically
or anodically depositable electrocoating lacquer may be applied.
However, this presupposes that the layer is sufficiently
electrically conductive for an electrocoating lacquer to be
deposited. This is the case, for example, with a layer of copper(I)
oxide having a weight per unit area lower than 10 g/m.sup.2.
[0029] In this embodiment, rinsing with water is preferably
performed between deposition of the layer of inorganic compound and
application of the electrocoating lacquer. The said rinsing may
comprise immersion or spraying. It may be advantageous to rinse
using low-salt or completely deionised water, at least in the last
rinsing stage. Chemical post-passivation of the inorganic layer
prior to electrocoating, as is generally performed in the case of
phosphating for example, is not necessary in the present
process.
[0030] In a further embodiment, the present process is performed as
a coil process. In this case, in substage (b) an organic polymer
layer is applied by dipping or spraying or by applicator rolls. A
coil process implicitly presupposes a non-rigid substrate, such
that this process variant is preferably performed using metal
strips. The process preferably proceeds continuously. The
electrochemical layer formation in substage (a) and the application
of the organic polymer layer in substage (b) are thus performed on
a moving strip.
[0031] The application of an organic polymer layer to a moving
strip is known in the prior art as the "coil coating process". The
coating installations used therefor are also suitable for the
present process. The organic polymer layer may exhibit different
thicknesses and different functions, for example it may be only a
few .mu.m thick and serve as a forming aid and/or as a primer for a
subsequent lacquer coating. In such a case, the composition and
layer thickness of the primer are preferably so adjusted that
electric resistance welding is still possible. In addition, it may
preferably be possible to apply an electrically depositable dip
coating to the primer. Such organic primer layers on a chemically
produced inorganic layer on a metal surface are known in the art by
various trade names, depending on function and composition.
Examples are Durasteel.RTM. and Granocoat.RTM..
[0032] While, in the case of the above-described primer layers, the
layer thickness is below 10 .mu.m and amounts for example to 6 to 9
.mu.m, in the coil coating process a thicker organic lacquer
coating may also be directly applied, to which no subsequent
lacquer coating is applied. The layer thicknesses are then from 50
to 200 .mu.m.
[0033] In addition, a powder coating may be applied as the organic
polymer in substage (b). To this end, the inorganic layer on the
electrically conductive surface need no longer be as electrically
conductive as is required for subsequent electrocoating. A powder
coating is preferably applied to shaped articles which are not
exposed to any marked degree of corrosion. Examples thereof are
articles such as household equipment or electronic apparatus stored
in enclosed spaces.
[0034] The organic layer applied in substage (b) may also be an
adhesive layer. The inorganic layer of at least one metal A then
serves as a coupling layer between adhesive and the metallically
conductive substrate. For this embodiment of the process in
particular, the metallically conductive substrate may consist not
only of a metal itself, but also of surfaces of plastics or glass
which have been made electrically conductive. Therefore, the
inorganic layer may act as a coupling layer between one of the
substrates metal, plastics or glass and an adhesive, wherein the
adhesive may be used to join together either similar or different
substrates. Examples may be found in the construction of vehicles,
aircraft or household equipment, where metals are adhered to each
other or to plastics or glass. Bonding of plastics to plastics is
also feasible. In particular, glass panels may be bonded to vehicle
bodywork in this way.
[0035] In a particular embodiment, an adhesive is applied in
substage (b) with which a vulcanised or non-vulcanised rubber part
is joined to a metal part. The resultant component is generally
designated a "rubber/metal composite". As a rule, a non-vulcanised
rubber part is joined by an adhesive to the metallic substrate via
the inorganic layer serving as a coupling layer and then vulcanised
through a temperature increase, frequently with the simultaneous
exertion of pressure. Such process stages are familiar in the art,
wherein the metallic substrate is not coated electrochemically with
a layer of an inorganic compound, however, but rather is subjected
either to only mechanical or also to wet-chemical pretreatment.
[0036] Furthermore, the present invention relates in a further
embodiment to a metal component, the surface of which bears a
coating comprising at least two layers, which coating may be
obtained in one of the ways described above. The said metal
component may comprise, for example, vehicles or vehicle parts,
household equipment, housings for electronic apparatus, furniture
or architectural parts. Preferred materials for the metal
components are iron, zinc, aluminum, magnesium and alloys, of which
more than 50 atom % is one of these elements. Metals and alloys may
be selected which are currently conventionally used for the
above-mentioned metal components.
[0037] In a preferred embodiment, the above-described metal
component bears the inorganic compound of at least one metal A in
X-ray crystalline form. X-ray crystalline means that the inorganic
compound produces sharp X-ray reflections when subjected to an
X-ray diffraction experiment.
[0038] The advantages of the present use and of the present process
are in particular that the thickness, composition and internal and
external structure of the inorganic layer may be more readily
controlled by the selection of the deposition parameters than when
the process is performed purely chemically. Fewer process stages
are required for application of the layer than in the case of
phosphating and in general less sludge arises than in the case of
purely chemical layer formation. In comparison with gas phase
deposition processes, electrochemical deposition is faster and
associated with less expenditure on equipment and lower energy
consumption. Moreover, it is not necessary to prepare volatile
starting compounds, as with gas phase deposition.
[0039] Another advantage of electrochemical layer formation is that
growth of the layer may be controlled by means of the electrical
resistance at the metallically conductive surface. Provided that
the growing layer exhibits higher. electrical resistance than the
electrically conductive surface, which is generally the case, layer
growth slows down when the electrical resistance becomes too high
owing to layer formation. While there are points on the metallic
conductive surface which are still uncoated or the layer is still
thin enough for a current still to flow at the set voltage, layer
growth occurs at these points. If the metallically conductive
surface is covered virtually completely with a layer of such a
thickness that the electrical resistance rises markedly, the
process of layer formation may be stopped. In the case of
galvanostatically controlled layer growth, virtually complete layer
formation is revealed by a marked increase in terminal voltage. The
process may then be automatically terminated when the terminal
voltage reaches a preselected value.
EXAMPLE
Cathodic Deposition of Copper(I) Oxide on Steel from an Aqueous
Solution
[0040] A pilot corrosion protection process was performed on
cold-rolled steel by means of cathodic deposition of Cu.sub.2O
without an activation stage (shortening of the process sequence).
The following process parameters were set:
[0041] Cleaning: weakly alkaline (Ridoline.RTM. 1559, 2.5%,
75.degree. C., 5-10 min)
[0042] Rinsing: tap water, deionised water
[0043] Activation: NONE
[0044] Conversion:
[0045] Electrolyte: 0.4 M CuSO.sub.4+3 M lactic acid, pH 10,
60.degree. C., stirred at 400 revolutions per minute
[0046] Deposition both potentiostatically (0.2 V v. standard
hydrogen electrode) and galvanostatically (-0.8 to -2.6
mAcm.sup.-2)
[0047] Treatment time: 10-300 seconds
[0048] Post-rinsing: deionised water
[0049] Drying: Compressed air
[0050] Characterisation: scanning electron microscopy, X-ray
photoelectron spectroscopy, corrosion test (climate condition
test)
[0051] Lacquer coating: cathodic dip coat ED 5000
[0052] The layers formed are continuous after a treatment time of
about 50 seconds and consist of fine (<1 .mu.m) crystallites of
Cu.sub.2O.
[0053] The layer properties are very easy to determine owing to the
electrochemical nature of the process, even without interfering
with the electrolyte composition. Thus, for example, the layer
thickness at a constant total current may be precisely determined
by the total charge which has passed, e.g. for i=-800 mA:
1 Process time Layer weight (Seconds) (gm.sup.-2) 10 0.4 30 0.7 60
1.1 120 2.4 300 5.6
[0054] In corrosion tests (10 cycles of VDA climatic condition
test, cathodic dip coating), a marked improvement in corrosion
protection is revealed by the coating as a function of the
thickness of the layer applied:
2 Process time Climatic condition test: (Seconds) Creepage U/2
(mm)*.sup.) 10 4.8 30 4.5 60 3.9 120 3.6 300 2.6 *.sup.)half scribe
width
* * * * *