CA2027703A1

CA2027703A1 - Process for the surface treatment of steel products by the action of a plasma

Info

Publication number: CA2027703A1
Application number: CA002027703A
Authority: CA
Inventors: Dorothee Nizery
Original assignee: Individual
Current assignee: Institut de Recherches de la Siderurgie Francaise IRSID
Priority date: 1989-10-17
Filing date: 1990-10-16
Publication date: 1991-04-18
Also published as: JPH03207872A; FR2653137B1; KR910008160A; FR2653137A1; EP0424211A1

Abstract

Process for the surface treatment of steel products by the action of a plasma ABSTRACT
Process in which the product is subjected to the action of a plasma generated in a rarefied gaseous atmosphere, the product to be treated being maintained at low temperature and subjected to a surface treatment with a low temperature plasma at a pressure of 1 to 10 3 Pa. This process is applicable in particular in the improvement of adhesion on the surface of the treated product, the gaseous atmosphere then comprising preferably a gas selected from hydrogen, nitrogen, chlorine and rare gases.

Description

i.~2~3 PROCESS FOR THE SURFACE TREATMENT OF STEEL
PRODUCTS BY THE ACTION OF A PLASMA
The present invention relate~ to a process for the surface treatment of steel products of the type in which said product is subjected to the action of a plasma generated in a rarefied gaseous atmosphere.
It is known to employ such prccesses for cleaning metal surfaces. In this case the atmosphere is formed by a rare gas, most often argon. The material, negatively polarized, attracts the gaseous ions of the plasma, and the ionic bombardment has a cleaning effect by the removal of material which results in a high reactivity of the surface with respect to the atmosphere and an increase in the roughness.
It is also known to effect surface treatments with plasma of the nitriding or cementation type. These treatments are carried out on materials heated to several hundred degrees Celsius.
Further, it is known to effect surface treatments of metal materials by conventional chemical reactions, such as oxidation, reduction, conversion treatment, etc., for the purpose of imparting to the surface of these materials particular properties such as for example the improvement of the resistance to corrosion, surface hardening, improvement in the adhesion of coverings, coatings or various protective layers.
It has now been found that surface treatment processes with a plasma could be employed instead of the chemical 7~

treatment processes for imparting to the treated surfaces the aforementioned particular properties.
The present invention therefore provides a process for the surface treatment with a plasma of the type indicated at the beginning of this specification, said process being characterized in that the product to be treated is maintained at low temperature, and the product is subjected to a surface treatment with a plasma at low temperature, at a pressure of 1 to 10~ Pa.

Plasma at low temperature, or "cold" plasma, generally designates a plasma obtained by a luminescent discharge in an atmosphere at low pressure (lower than 103 Pa). The discharge is obtained at a voltage of several hundred volts, preferably a dc voltage and moreover preferably 400 to 800 V, this voltage possibly being in particular applied between an anode and the negatively polarized metal product which acts as the cathode. It is also possible to superpose on the dc voltage a variable voltage at radio frequency. The current is preferably lower than 10 mA/cm The product to be treated is maintained "cold", i.e. its temperature is lower than about 300C. In practice, the temperature is generally maintained at around ambient temperature lower than 100C. This may be achieved by the use of a cathode which is cooled, for example by a circulation of water. In the case of a treatment applied to a cold metal sheet, the latter may be maintained at a temperature of around ambient temperature simply by means of 77~

sufficiently brief treatment sequences in the treatment enclosure, possibly completed by a cooling of the supports of the sheet, this being more particularly adapted to the treatment of a moving sheet. The required condition is that the rise in temperature only brought about by the treatment (in contrast to certain known processes in which a treatment with a plasma is achieved on a product which is intentionally heated and brought to temperatures of several hundred C) does not deteriorate the characteristics of the product.
Generally, the duration of the treatment is from one second to 10 minutes.
In the known treatment processes in which the treated material is heated either by specific heating means or by the very action of the plasma, the specific action of the plasma may be combined with chemical reactions in the treated material, of the nitriding type, owing to the relatively high temperature of this material. In some case~, such a treatment may favour the formation of oxides.
Contrary to this, the process according to the invention pexmits limiting the action of the treatment with a plasma to a surface zone of the material and, depending on the nature of the gas or gases in which the plasma is generated, permits for example improving the anticorrosion aptitudes of the treated material, or the adhesion to the surface of the latter, by specifically acting on the causes which may adversely affect the obtainment and the perennity of the~e ~ ~77~

characteristics.
Thus, in the case of the application of the process according to the invention for improving the corrosion resistance of a steel product, such as a sheet of steel, the treatment is carried out by the action of a plasma at a low temperature in an atmosphere comprising at least one molecular gas selected from among oxygen, ozone, nitrogen, hydrogen, air, carbon dioxide, carbon monoxide, nitrogen oxides, water, gases of combustion or mixtures thereof with a neutral gas, the product being maintained a low temperature.
Under the effect of the electric field, the molecules of the gas are dissociated, excited or ionized; in the electric discharge thus created, a low energy plasma sweeps across the surface of the material and the various gaseous species react with the surface atoms in accordance with their chemical affinity. By the combination of the chemical effect of the gas (oxidizing or reducing for example) and the sputtering effects, a large number of elements disappear from the treated surface. After treatment, the surface is generally passive with respect to the atmosphere, i.e. the conventional pollution elements C, S, P, O...
One of the most interesting characteristics of a treatment with a molecular plasma according to the invention is that it practically does not change the surface roughness of thP material, even on layers having a low melting point.
In contrast, with rare gases, the erosion is greater and may '; t~

lead to a very high reactivity with respect to the contaminants of the atmosphere.
In the case of the application of the process according to the invention for improving the adhesion to the surfaces thus treated, the gaseous atmosphere preferably comprises at least one gas selected from among hydrogen, nitrogen, chlorinated compounds and rare gases.
Preferably, the gaseous atmosphere does not include oxygenated compounds.
10By the application of this process, the inventors were able to find a suppression of segregated elements on the ~ face which are h~ l to ~ adhesion i such as aluminium, lead, calcium and magnesium oxides, silicon, manganese...
The surface treatments most generally employed up to the present time for improving the adhesion to the surface of ferrous products are carried out under wet conditions by putting the surfaces to be treated in contact with acid or alkaline chemical reagents. However, such treatments have several drawbacks:
20most of the reagents employed are corrosive and their use presents problems of safety and pollution, the solution of the compounds to be eliminated from the surface is not always selective and one is unable to avoid a surface solution of the metal matrix resulting in a modification in the state of the surface and a higher reactivity with respect to the atmosphere.
The process according to the invention permits avoiding / ' ' i ~ ~ 3 these drawbacks and is advantageously substituted for chemical treatments in the preparation of products such as metal sheets, in particular galvanized sheets intended to be 8ubsequently subjected to phosphating or chromating treatments, to be coated with lacquers, to be assembled by a hot pressing with a sheet of polymer, for example for manufacturing "sandwich" sheets, or to be assembled by adhesion.
The results obtained in the improvement of adhesion by means of the process according to the invention can be explained by the cleaning effect and the passivation resulting from the combination of the mechanical action of the ions of the plasma on the treated surface with the chemical action of the gas, for example reduction by the hydrogen or formation of volatile compounds, eliminated by the pumping employed for maintaining the required low pressure, with chlorinated gases.
As illustrative examples of the application of the process for improving adhesion on the surface of the treated products, characteristics and results of different tests are mentioned hereinafter.
Example 1: Surface treatment carried out on bare mild steel sheets.

-The treatment was carried out at a dc voltage of 400 V

and a current of 200 mA, the distance between the anode andthe product (cathode) being 4 mm, the test specimen having a dimension of 70 x 120 mm.

~J~ 77~3~

Several tests were carried out with different gases:
a) treatment for 5 minutes under an N2 ~ H2 plasma b) treatment for 5 minutes under an N2 ~ 2 plasma c) treatment for 5 minutes under an N2 ~ H2 plasma followed by a treatment for 5 minutes under an N2 ~ 2 plasma.
It was found, from a subsequent analysis of the surfaces by luminescent discharge spectrometry ~LDS), that all the treatments eliminate the contaminants of the extreme surface such as sulphur, phosphorus, aluminium and boron. The calcium only disappears with a reducing treatment under an N2 ~ H2 p~asma.
Sheet specimens treated in this way were then phosphated.
The tests for phosphating by means of a trication bath effected without a prior alkaline degreasing gave very good results for the treatments N 2 ~ H2 and N2 ~ H2 followed by N2 ~ 2 : the phosphating is fine, homogeneous and in the form of small cubic blocks.
On the other hand, after an N 2 ~ 2 treatment alone, the crystals are blunted, irregular and zones are non-phosphated, which confirms the advantage of a treatment with a plasma in a gaseous atmosphere which does not include oxygenated compounds.
Furthermore, adhesion tests were carried out on specimens of bare mild steel sheets.
The reference specimen is simply degreased with 9 ~

chlorotene. The specimens treated in accordance with the invention had undergone a treatment with an N 2 ~ H2 plasma for 4 min. at a voltage 400 V and a current 200 mA.
The adhesion was effected with a bicomponent epoxy 5 adhesive which polymerizes at ambient temperature (sold by the firm CI13A GEIGY under the reference AW134).
The adhered assemblies were then aged by exposure for 48 hours in a hot and humid atmosphere (65C with 1009 relative humidity).

The results obtained by the 3 point bending test ~French standards NFT 76143 and NFT 30010) are indicated in the following table where Fmax is the maximum fracture force.
Non-aged Aged Fmax Fmax Reference 67 43 N2 ~ H2 Treatment 81 55 There is observed for the adhesions of the treated products an increase in the maximum fracture force of 20% in the non-aged state and 28% in the aged state.

Example 2: Surface treatment carried out on sheets coated with zinc (Galvanized IFS steel sheets of the"Monogal type"treated on the zinc-coated side) The treatment was carried out at 400 V and 200 mA, the specimen having the same dimensions as before and the gases employed being respectively N2 ~ H2 and N2 ~ 2 .
The LDS analysis of the surfaces thus treated showed ~th 77~

that the treatment carried out under N - H2 permits the rapid elimination of the extreme surface pollutants such as P and S, the elimination of Ca, Al and Mg requiring a longer period of about 5 minutes. Similar results were observed with a treatment with an N2 ~ 2 plasma, but a substantially double treatment time is required.
The analysis of the surface of the sheets thus treated and then phosphated show that the treatment with an N2 ~ H
plasma results in a homogeneous and fine phosphating without a prior alkaline degreasing.
After treatment under N2 ~ 2 ~ the phosphating is slightly homogeneous.
Adhesion tests were carried out on these galvanized sheets under the same conditions as for the previously-mentioned bare sheets.
The results of these tests are shown in the followingtable where dmax is the maximum deformation before fracture at the interface of the adhered assembly in the standard 3 point bending test.
Non-aged Aged dmax dmax Reference 0.29 0.5 N 2- 2 treatment for 5 min. 0.32 0.42 N ~ - H2 treatment for 4 min. 0.32 0.68 N 2- H2 treatment for 9 min. 0.45 1.08 These results confirm the increase in the adhesion after treatment with an N2 -H2 plasma with respect to the merely ti 1 o -degreased sheet (reference). This improvement is all the better because the plasma cleaning is complete, which requires a sufficient treating time depending on the initial substrate.
The maximum deformation is increased by 55% after 9 min.
of treatment in the non-aged state and by 116~ in the aged state.
Treatment with a gas of the N - O type has been found to be but little effective.
Example 3: Surface treatment carried out on qalvanized steel sheets (of the tvPe currentlv desiqnated bv the name 'Galvadur") Four treatments were carried out on specimens of the same product having dimensions 70 x 120 mm.:
T1: sheets subjected to an alkaline cleaning and then to chromating, these specimens acting as comparative references.
T2: sheet treated with an N2 ~ 2 plasma having 20%
oxygen for 5 min. at 400 V and 200 mA, then chromated.
T3: sheet treated with an N2 - H2 plasma having 10%
hydrogen for 4 min. at 400 V and 200 mA, then chromated.
T4: sheet treated with an N2 ~ 2 plasma for 5 min., then for 4 min. with an N2 ~ H2 plasma at 400 V and 200 mA, then chromated.
The sheets thus treated were then all covered with a lacquer of polyester type in two coats: 5 ~um thickness of the first coat and 15 ~um thickness of the finishing coat.

r~

The adhesion of the assembly thus formed is tested by means of the 3 point bending test before and after aging.
The adhesive employed for the 3 point bending test is a bicomponent epoxydic adhesive polymerized for 60 min. at 80C.
The results of this test are indicated in the following table.
Non-aged state Aged state TREATMENT Fmax (N) Fmax (N) T1 134.9 77 T2 138.2 119.2 T3 129.9 109.1 T4 147.6 107.6 All the treatments result, before aging, in maximum fracture forces (Fmax) which are close to one another.
On the other hand, it was observed that, for the treatments T3 and T4 (with an N2 + H2 plasma), the fracture occurs exclusively at the interface between the first coat and the finishing coat, whereas for the treatments T1 and T2 (alkaline cleaning with an N2 + 2 plasma), certain fracture zones are observed at the metal sheet - lacquer interface.
Thus it can be observed that, in the case of a treatment with an N + H plasma, the adhesion at the metal sheet lac~uer interface is very good.
After accelerated aging, the maximum fracture force (Fmax) decreases relative to the non-aged state, by only about 20% for the sheets which had undergone a treatment ~iJ~7~3 with a plasma, as against 50% for those which had undergone the conventional chromating cycle including an alkaline cleaning.
Fracture zones at the lacquer - metal sheet interface are then observed.
Microscopic observation and X-ray analysis show that, for a sheet which had undergone alkaline cleaning (T1), the delamination starts in the oxidized zones.
On the other hand, with the treatment with an N2 + H2 plasma (T3), no presence of oxides is noted, which may explain the small decrease in the Fmax relative to the non-aged state.
These different tests show that the treatment according to the invention generally permits improving the adhesion on the surface of the sheets thus treated relative to the conventional treatments employing an alkaline cleaning in the case of phosphating or subsequent chromating, or relative to the simple cleanings and degreasing employed in the case of the adhesion. Even in the case where this relative improvement is less distinct, this process for surface treatment with a plasma permits replacing treatments with corrosive chemical products and thereby eliminating the risks related to the use of such products.
The process according to the invention is also applicable, as already stressed, for improving the corrosion resistance of the products thus treated, in particular stainless steel sheets.

Claims

1. Process for the surface treatment of a steel product comprising subjecting said product to the action of a plasma generated in a rarefied gaseous atmosphere, said product being maintained at low temperature and subjected to a surfa-ce treatment with a low temperature plasma at a pressure of 1 to 10 Pa.

2. Process according to claim 1, wherein the plasma is generated by a dc voltage.

3. Process according to claim 2, wherein the voltage is between about 400 and 800 V.

4. Process according to claim 2, wherein the current is lower than 10 mA/sq.cm.

5. Process according to claim 2, comprising negatively polarizing said product and generating said plasma by a voltage of several hundred volts applied between an anode and said product which constitutes a cathode.

6. Process according to claim 1, comprising effecting said treatment for a period between 1 second and 10 minutes.

7. Process according to claim 1, applied to the improve-ment of the adhesion on a surface of said product, said gaseous atmosphere comprising at least one gas selected from the group consisting of hydrogen, nitrogen, chlorinated com-pounds and rare gases.

8. Process according to claim 7, wherein said gaseous atmosphere does not comprise oxygenated compounds.

9. Process according to claim 7, wherein the treated product is composed of steel coated with zinc.

10. Process according to claim 1, applied to the impro-vement of the corrosion resistance of said product, said gaseous atmosphere comprising at least one molecular gas selected from the group consisting of oxygen, ozone, nitrogen, hydrogen, air, carbon dioxide, carbon monoxide, nitrogen oxides, water, gases of combustion, and mixtures of the later with a neutral gas.

11. Process according to claim 10, wherein said product is composed of stainless steel.