FR2540521A1 - Process for producing a matt or semi-matt chromium-containing deposit on a ferrous metal, and product obtained - Google Patents

Abstract

Process for producing a matt or semi-matt chromium-containing deposit on a ferrous metal, characterised in that the surface of the said metal, after having optionally been subjected to known degreasing and pickling treatments, is subjected successively: - to an anodic treatment, preferably alkaline; - and then, immediately after the said anodic treatment, to chromium plating carried out at a temperature greater than approximately 30 DEG C, preferably greater than 40 DEG C, with a current intensity greater than approximately 20 A/dm<2> and preferably greater than 30 A/dm<2>.

Description

Process for obtaining a mat or semi-mat chrome deposit on a ferrous metal, and product obtained.

The present invention relates to a process for obtaining a mat or semi-mat chrome deposit on a ferrous metal and the. product obtained.

A very large number of documents (patents, publications) describe processes making it possible to deposit shiny chromium on the surface of a ferrous metal. The hardness and brilliance of chrome spite (deposit which may contain chromium oxide) are sought.

It turned out, however, that for. a certain number of applications, the brilliance and the hardness of the chromium deposit no longer constitute essential qualities of the chromium layer because these qualities lead, on the other hand, to difficulties when it is desired to deposit and hang on said chromium layer a layer of paint, varnish (tin can) or a photosensitive layer (offset plates). The solution used to date to hang a layer of this type on the shiny and hard chrome layer is to use a layer ( paint, varnish or photosensitive material) which can be baked at a relatively high temperature; obviously, this solution involves certain difficulties in the handling and treatment of the layers and therefore proves to be relatively expensive.

However, we know that, when using a matt or semi-matt chromium layer, it is much easier to hang on said layer a layer of pe-inture, varnish or photosensitive material without the absolute necessity of have to cook this surface layer.

The present invention relates to the preparation of non-glossy layers (matt or semi-matt9) of chromium on a ferrous surface.

It is known that the shiny or matt character of a metal surface can be appreciated, according to the standards by observing by reflection on the surface to be contrasted characters written in black on white and by appreciating the readability of these characters according to their distance from said surface.
It has been known for some years that the protection by chromium plating of a metal surface (such as a ferrous surface) can be carried out by the spite of chromium metal, chromium oxide or a mixed deposit of chromium and oxide. chrome plating. The chromed layers produced according to the present invention are of one or other of these types, namely layers of chromium, chromium oxide or mixed layers. varnish or offset plates), the thickness of the layers produced according to the invention is low.

In all cases, this thickness is less than 1 micrometer, preferably less than 0.5 micrometer; but it can also, in the case of materials for cans in particular, be preferably less than 0.1 micrometer.

It is also known that in the (shiny or matt) chromium, the chromium can be either of hexagonal crystalline nature, or of centered cubic crystalline nature. The deposits produced according to the present invention are preferably, ultimately, of centered cubic crystalline nature. . It is possible that at the time of deposition a certain proportion of this will be hexagonal crystalline in nature, but in this case it will generally be desirable to subject the layer to an appropriate heat treatment to transform the hexagonal chromium into a crystalline structure. cubic centered. This means that the process according to the invention will preferably be carried out under experimental conditions giving rise directly to a chromium of centered cubic structure.

The present invention therefore relates to a process for obtaining a matt or semi-matt chrome deposit on a ferrous metal, characterized in that - immediately before carrying out the chrome plating, the iron surface
reuse is subjected to an anodic treatment, - and that the chromium deposition is carried out at a temperature above
greater than about 300C, preferably greater than 400C and with
a current intensity greater than 20, preferably 30, A / dm2
surface area to be treated.

We call back
- that the chromium deposit may consist of chromium or oxide of
chromium or of a chromium-chromium oxide mixture, which the chromium deposit aims, as regards chromium, to obtain
tion of a metal having a central cubic crystal, - whether the chrome spite is matt or semi-matt.

It is also specified - that by ferrous metal is meant either steel or black rer, - that the ferrous metal used may have, prior to setting
implementation of the process according to the invention, is subject to a degreasing
wise and stripping by known methods; that by anodic treatment is meant an electrochemical treatment
mique in which the ferrous metal is at the anode; conditions
of this electrochemical treatment may vary depending on the metal
ferrous treated and according to prior degreasing treatments
and pickling to which said metal has been subjected; however, these
conditions can be summarized as follows
processing temperature: at least about 20 C
duration of treatment -: at least about 1/3 s
electrical intensity: at least approximately 2 A / dm2
it also seems that this anodic treatment can be carried out
preferably using either a basic medium (for example a
medium containing at least approximately 20 g / l of sodium hydroxide), i.e. a medium
vox acid has tested more specifically a medium containing at least
20 g / l of Cr203 and a little sulfuric acid); that said anodic treatment must be carried out immediately
before carrying out the chromium deposit; that means that between
said anodic treatment and said chromium deposit should be
carefully avoid acid pickling treatments, such as
than used to date between degreasing operations
(which could also include anodic treatment of the metal
ferrous) and the chrome plating operation It is possible on the other hand
(without modifying the surface of the metal as obtained after said
anodic treatment) to rinse the ferrous metal and possibly
to subject if necessary to said metal a short treatment
cathodic, this one seems to allow obtaining a layer
more regular; ; - that the deposition of chromium and / or chromium oxide can take place in
using currently known electrochemical baths. he is
curious to note that the operating conditions in which
we use said baths (temperature-intensity) are very general
only those that the producers of said electrochemical baths
recommend for the realization of deposits of shiny chrome; he
seems that thanks to the anodic treatment carried out according to the invention
tion said chromium portions have a matt or semi-matt appearance from
the beginning of the deposit, that is to say even for very thin deposits.

In addition, the deposits produced according to the invention have a certain number of properties which can prove to be very interesting - the covering power of these deposits is very good, that is to say that with
a small amount of chromium and / or chromium oxide on the surface
underlying is well protected; - the hardness of chromium and / or chromium oxide is relatively low,
namely weaker than that which corresponds to the qualities of
shiny chrome; this property can be particularly interesting
when using offset plates.

The following nonlimiting examples illustrate the invention EXAMPLE 1
A steel plate from the company USINOR, thickness 40/100 is used and, on this plate, the following operations are carried out successively - degreasing in soda baths (100 g / l)
cathodic degreasing: temperature 50 C
intensity 20 A / dm
duration 10 s
anodic degreasing: temperature 500C
intensity 20 A / dm
duration 5 s - rinsing, - stripping
stripping is carried out by immersing the degreased plate in a
bath containing 10 of sulfuric acid, at 40 C, for 10 s; - rinsing)
The operations described above are conventional, that is to say normally used for the preparation of ferrous metal surfaces which must be subsequently treated by electroplating, that is to say that the treatment conditions for degreasing and pickling are known. and may vary according to the starting products used. But this also means that these operations are considered by all specialists as essential to obtain a starting product having a clean and reproducible surface and that they are always carried out even when, in some publications concerning electroplating, they are not expressly mentioned.

- treatment according to inventions
The plate obtained is subjected to an alkaline anodic treatment; this plate is immersed in a bath containing 80 g / l of sodium hydroxide and having a temperature of 40 ° C., the plate forming an anode, the intensity of the current being 10 A / dm2. The duration of treatment is 5 s. The plate thus obtained is then immersed in an alkaline bath where it is placed in the cathode; the alkaline bath contains 100 g / l of soda, the intensity of the current is 10 A / dm2, the temperature is 40 C, the duration of the treatment is 2 s.

rinsing, @ chrome plating,
The plate obtained is immersed, under current, in a chromium-plating bath containing
250 g / l CrO3
2.5 g / l of SO4H2 the temperature of the bath is 45 C, the intensity is 45 A / dm2.

I, the duration of treatment is such that the chromium deposit has a thickness of the order of 0.25 to 0.4 micrometers. The chromium obtained, very covering, is perfectly uniform, matt, with a milky appearance.

By way of comparison, the above test was repeated by modifying various stages of the process.
EXAMPLE 1a: The "treatment according to
the invention. "The chromium obtained is brilliant.

EXAMPLE Iter: The conditions used in the operation are modified.
chromium plating; for this operation, the intention
current is reduced to 15 A / dm and the
duration of the chrome plating is extended so that the deposit
final has a substantially similar thickness
(0.3 micrometer) to the thickness of the deposit
example 1; the chrome obtained is shiny.

EXAMPLE 1c: In the treatment according to the invention, one sup
premium cathodic treatment; the chromium obtained
is dull but it seems that the deposit is less
regular;
EXAMPLE 1 quinter: In the treatment according to the invention, we reverse
the order of anodic and cathodic treatments;
the chrome obtained is matt but the deposit appears
less regular.

On the plates thus prepared, the same layer of ink-loving photosensitive polymer was deposited (reference PR 12 of the
PCAS Company). After drying, exposure and development, the surface of the plates is subjected to known tests making it possible to assess the adhesion of the ink-absorbing layer on the plate (these tests consist on the one hand in bending the plate at 1200. and in rubbing the layer. at the place of the fold and on the other hand in applying an adhesive tape to the surface of the plate and abruptly peeling off said tape); the results obtained are as follows
Plate example 1 very good
Sample plates 1 quater and 1 quinter good
Very bad example plates 1 bis and 1 ter.

EXAMPLE 2
Example 1 was reproduced by depositing on the treated plates only about 0.1 micrometer thickness of chromium.

The same results as in Example 1 were observed for the five plates thus produced, namely plates 2, 2 quater and 2 quinter matt chrome covering well and adherent photosensitive layer, plates 2 bis and 2 ter glossy chrome and low photosensitive layer. adherent.

EXAMPLE 3
Example 1 was reproduced using, to carry out the chromium deposit, chromium-plating baths with fluorinated catalysts.

The deposit conditions were
- temperature 53 C
- 3, 3 ter, 3 quater, 3 quinter, intensity 60 A / dm
-3 bis intensity 10 A / dm2 thickness of the deposit approximately 0.3 micrometers.

Results were obtained for the various plates which were quite similar to those obtained for the corresponding plates of Example 1.

EXAMPLE 4
A chromed steel plate called SOLCHROME from the Sollac company is used as starting product.This 28/100 type TFS thickener plate is, according to the manufacturers, covered with 0.060 g / m2 of chromium metal, 0.015 g / m2 d chromium oxide and 0.003 g / m2 (at the surface) of an oil protecting the chromium layer.

The following operations were carried out on this plate
- coarse oil removal
- rinsing
- treatment (similar to previous treatments
degreasing and stripping) to remove chrome
and chromium oxide;
- treatment according to the anodic invention in a bath
containing 120 g / l of NaOH at a temperature of 400C
with a current density of 10 A / dm2 and during
a duration of 10 s.

After rinsing, the plates are chromed using on the one hand the chroming baths of Example 1, and on the other hand the chroming baths of Example 3. This gives the plates (10 in total) of which the properties are quite comparable (with regard to the appearance and the properties of the chromium optionally coated with a photosensitive layer) to those of the corresponding plates of Examples 1 and 3.

EXAMPLE 5
Example 1 was repeated but replacing the cathodic treatment described in the treatment according to the invention by immersing the plate, without current, in a bath containing approximately 100 g / l of chromium oxide and 2 g / l. sulzuric acid; the duration of this treatment is 2 s; it appears that this treatment also has, like the cathodic treatment in an alkaline bath described in Example 1, the consequence of obtaining subsequent deposits of more regular chromium.

EXPLANES 6 and 7
The tests of Example 1 were repeated, but the chrome plating operations were carried out under the following conditions.

EXAMPLE 6
temperature 35 C
intensity (6, 6. bis, 6 quater and 6 quinter) 35 A / dm2
current 6 ter 10 A / dm EXEMPE 7
temperature 300C
intensity (7, 7 bis, 7 quater and 7 quinter) 30 A / dm2
intensity 7 ter 10 A / dm2.

All the results obtained are identical to those described for the corresponding plates of Example 1.

In these two tests, it was noted that the chromium deposited contained non-negligible proportions of hexagonal chromium; it is possible to modify the crystal structure of this chromium and, consequently, to stabilize the deposits by subjecting the plates, for about 10 min, to a temperature of the order of 200-220 C.

EXAMPLE 8
Examples 1 and 3 were taken again by carrying out the deposit: of chromium in several stages. It is in fact known that one way of modifying the properties (mattness or efficiency of protection of the underlying surface) of the chromium layers is to produce said layers in several stages with interruption of the current between each stage; on the other hand, it may be desirable, in particular if it is desired to carry out the chrome plating of a continuous strip moving at a certain speed, to carry out the chrome plating in several successive vats, which amounts to chrome plating in several stages.

It has been found that i'on can easily carry out the chromium deposits in two or three successive stages, the baths being, in each stage, identical or different.

EXAMPLE 9
A 40/100 USINOR plate similar to that of Example 1 is used.

This plate is degreased, rinsed, pickled and rinsed as described in Example 1, then the plate is subjected to the treatment according to the invention described below - the plate being in the anode is immersed in a bath containing 250 g
of CrO3 and 2.5 g of SO4H2; the bath temperature is 25 C, ltin-
current intensity is 5A / dm2, treatment time is
3 s; - the plate is then2 after rinsing, soaked in a fair bath
nant 100 g / 1 of soda; the plate is cathode, the temperature of the
bath is 40 C, the current intensity is 5 A / dm2, the duration of
processing is 3 s.

The plate is rinsed and then chromed as described in Example 1; a very uniform, milky-colored chromium deposit (approximately 0.2 micrometers thick) of excellent quality was thus obtained.

EXAMPLE 10
Example 7 was repeated with plates obtained by
dechromining of chromed plates based on black iron.

Deposits of chromium St of chromium oxide were carried out on the plates obtained using known electroplating techniques (namely simultaneous deposition of chromium and chromium oxide or successive deposition of chromium 2 then of chromium oxide).

The following chrome plating conditions were used:
- simultaneous deposit
composition of the bath 30 g / l Cr203
1.5 g / l ammonium fluoride temperature 30 C
intensity 30 A / dm2 it has been observed under these conditions that deposits as thin as 0.05 microns already presented a milky (matt) appearance and allowed the adhesion of varnish;
- successive deposits
1st step: composition of the bath 250 g / l Cr2O3
2.5 g / l SO4H2
temperature 40 C
intensity 40 A / dm2
2nd step: composition of the bath 30 g / i Cr2O3
0.2 g / l SO4H2
sodium fluoride
temperature 300C
intensity 25 A / dm2 it has been observed under these conditions that even for very thin deposits (0.02 micrometers) a layer of matt appearance is obtained which holds the varnishes well.

In the above tests, a reduction in the current intensity to 10 or 15 A / dm2 leads to the production of shiny deposits on which the varnishes are difficult to stick.

Claims (4)

1 - Process for obtaining a matt or semi-matt chrome deposit on a ferrous metal, characterized in that the surface of said netal, after having optionally undergone the known degreasing and pickling treatments is subjected successively - to an anodic treatment , preferably alkaline, - then, immediately after said anodic treatment, to chrome plating carried out at a temperature above about 30 C, preferably greater than 40 C, with a current greater than approx. ron 20 A / dm2 and preferably greater than 30 A / dm2. 2 - Process according to claim 1, characterized in that the only operation carried out between said anodic treatment and said chromium plating is, in addition to the rinses, a short cathodic treatment. 3 - Method according to one of claims 1 and 2, characterized in that the ferrous metal is steel or black iron. 4 - Method according to one of claims 1 to 3> characterized in that the anodic treatment is carried out at a temperature above 20 C, for a period greater than 1/3 s and with an electrical intensity greater than 2 A / dm2 ,