GB2134545A - Electroplating chromium - Google Patents

Abstract

In the molten salt bath electroplating of chromium onto a substrate, such as stainless steel, the substrate, a reference electrode and a chromium anode are put into a molten salt bath (e.g. a lithium chloride/ potassium chloride mixture) containing Cr<2+> ions and one or more short- duration over-potential pulses (typically 200 mV to 1V with respect to ECr<2+>/Cr<o>) are applied to the substrate with respect to the reference electrode to form a plurality of small nuclei on the substrate and, subsequently, a much smaller over-potential, conveniently a continuous potential, is applied for deposition of the coating. <IMAGE>

Description

SPECIFICATION Electrolytic production of chromium surface coatings This invention relates to the electrolytic production ofchromium surface coatings.

Such coatings are widely used for protection against corrosion, forwear resistance and for decorative purposes. Many commercial techniques are known for forming such coatings. The present invention is concerned more particularly with a molten salt electroplating technique. It has been proposed to deposit chromium and other refractory metals on substrates from eutectic melts of alkali fluoride mixtures. In such techniques, the operating temperatures were typically around 900-1000,0 and the coating produced was keyed to the substrate by an inter-diffusion zone. The very high temperatures were necessary because ofthefluoride mixture used.

The present invention is directed to improvements in the electrolytic production of chromium coatings from molten salts and is directed in particularto a technique giving improved control of parameters affecting the coherence of the coating and adherence to the substrate.

According to the present invention, a method of depositing a chromium coating on an electricallyconductive substrate electrolytically using a molten salt bath containing Cr2+ ions comprises the steps of putting the substrate and a chromium anode into the bath with a reference electrode, applying one or more short duration over-potential pulses to the substrate with respect to the reference electrode to form a plurality of small nuclei on the substrate and subse quentlyapplying a much smaller over-potential as a coating potential to deposit chromium over the substrate. The continuous potential may be maintained until the desired thickness of deposit is obtained.

The initial pulse or pulses of over-potential preferably have a magnitude of at least three times and typically a magnitude often times the over-potential subsequently applied for deposition ofthe coating.

Typically the initial over-potential might be in the range of 200 mV to lv with respect to E cr2+Cr and moretypically in the range of 250to 500 mV.The maximum value ofthis over-potential is limited by the formation of unwanted deposits from the molten salt material at very negative potentials. For example if the molten salt is a lithium chloride-potassium chloride eutectic,the potential would be limited by the formation of lithium deposits at very negative potentials. The object of this initial over-potential is to form small nuclei on the substrate. It is desirable to have as many as possible nuclei which are small and close together.The duration ofthe pulses is not critical but is kept short to prevent growth of the nuclei. Typically a pulse might be applied for 5 secs. More than one pulse may be applied but in practice it has been found that one pulse is enough. If successive pulses are applied preferably there is a restingtimebetweenthepulses of longer duration than the pulses, for example 2 or 3 minutes. The continuous potential subsequently ap plied causes deposition of chromium overthe nuclei.

Afairly rapid deposition is not only possible but is desirable in order to form small crystals in the main coating layer.

With this technique, it is possible to avoid the formation of large crystals and dendrites which may occurwhen deposits are grown by diffusion processes. The coating is coherent and shows good adherence to the substrate. This electrolyte has good throwing power and it is possible therefore to coat complex shapes with a uniform coating. It is found thatthe deposits have a small grain structure; typical grain sizes are up to 7 microns. The pulsing producing a large number of small nuclei on the surface helps to prevent voids occurring on the interface between the crystals of the coating. The formation of large crystals and dendrites on the outer surface ofthe coherent layer in the latter stages of the deposition can be minimised by appropriate control of the deposition parameters.However if dendrites are formed, they can be removed by mechanically brushing without damaging the initial layer.

It is generally convenient to operate at as low a temperature as possible and, for this reason, a convenient molten salt bath to use is a lithium chloride potassium chloride mixture, preferably substantially the eutectic which permits the coating to be carried out atabout4500C. The Cr2+ ions may be added to the melt as anyhdrous CrCI2 or by the anodic dissolution of pure chromium. If any CrCI3 is present, this can be reducedtoCr2+ ions in the presenceofchromium metal.

In the following description of specific examples of the invention, reference will be made to the accompanying drawings in which: Figures 1 a and 1 bare graphical diagrams showing a potential pulse applied to the substrate and the resultant current profile that is obtained; Figure 2 is a diagram illustrating the different processes which may occur during the formation of an electrodeposit in carrying outthe invention; and Figure 3 is a diagram illustrating the apparatus for effecting the electrodeposition.

In the following examples, the melt employed, that is to say the electrolyte is a LiCi-KCI eutectic (58.5 : 41.5 mol % ratio). Before use, the salt mixture was desiccated under high vacuum in a slowly increasing temperature until the molten statewas reached. The last traces of waterwere removed by bubbling Cl2 or HCI through it for 2 hours at450 C. Residual gases were then removed by bubbling argon through the melt for 3 to 4 hours.The argon was a high purity grade (99.999%) which was purified further by passage through a molecular sieve column for extra desiccation and through copper wool at 400"C to eliminatetracesofoxygen.Themeltwasfurther purified by electrolysis under vacuum at 2.5 V between a graphite anode and a stainless steel cathode (or aluminium ortungsten), until the residual cu rrentfalls near zero. The melt was finally filtered, cooled and stored in a dry atmosphere e.g. over P205 in adryboxuntil requiredforuse.

The substrate to be coated must be cleaned. In the examples, the material was 20/25 Nb stabilised stainless steel. Cleaning was effected by firstly treat ing the substrate anodically at room temperature in a solution of H2SO4 (sp. gr. 1.53) and applying 6 V with respect to lead cathode for one minute. The steel substrate was then washed in distilied water, and acetone, and dried.

The C+ ions were added to the melt as anhydrous CrC12. They might however alternatively be added by anodic dissolution of pure chromium. The CrC12 may contain some CrC13 but this is reduced to Cr2+ in the presenceofchromium metal.

The deposition apparatus is shown diagrammatically in Figure 3. The electrolytic cell 10 is a three electrode arrangement having a working electrode 11 which is the substrate to becoated,a reference electrode 12 which in this case was a Ag/AgCI (0.5M electrode), and a counter electrode 13 (the anode) consisting of pure chromium pieces. The eutectic mixture with the added Cr2+ ionswas put in the cell as shown at 14 and the Cr2+ concentration in the melt was maintained at 1 M.

The chromium electrodeposition was carried out under controlled potential conditions by using a potentiostat and a programme generator unit 15 so that different current and potential functions could be applied to the working electrode. Initially a very large over-potential was applied to the working electrode (300 mV and 425 mV with respect to ECr2 +/Cr= in Examples 1 and 2 respectively) for 5 seconds. This pulse in Example 2, was repeated once more after 2-3 minutes resting time. Then the overpotential was reduced to the growing potential which was -2C mV and subsequently -100 mV in Example 1 and was -35 to -40 mV in Example 2. The growing potential was maintained atthis value until the desired thickness of deposit was obtained.On 20/25 Nb steel typical layer thicknesses were 30-35 pm.The poter vial function applied and the currents obtained are of the form shown in Figure la and Figure 1b respectively. In Figure 1 a, tN and NN are the time and over-potential for nucleation respectively, and tg and NG atthe time and overpotential for growth respectively. In Figure 1 b, G is the current density obtained during the growth of the deposit. Some typical working conditions used in the experiments are given in Table 1.

Table 1 Example 1 Example 2 Sdemperature, c 450 450 Cr+2 concentration, mol /dm3 0.35 0.77 EOCr/C,+2 vs Ag/O.SMAgCl ref. -700 -675 Nucleation overpotential |nN), mV -300 -425 Nucleation litre (tN),o sec. 28-48 5 No. of nucleation pulses 1 2 Growing potential (rlG), mV -20, -100 -35, -40 Growing current density (IG) mA.Qn2 3.4 - 64 10-60 Final deposit thickness, en 20-60 30-35 The final stageofthetreatmentwasthe removal of any residual frozen melt from the surface ofthe coated specimen. This is readily achieved by washing in water after removal from the electrochemical cell.

A particular advantage ofthe molten chloride system compared to the use offluoride meits is the ease of removal of residual material at the end of the treatment.

The conditions described above, and shown in Table 1, representsometypical operating parameters that have led to successful coatings. However, there is considerable flexibility in va rious parts of the process. For example, alternative steps in the puri- fication procedure are permissible. Alternative anodic cleaning processes for preparation ofthe test piece are acceptable. Awide range of Cr2+ concentrations and working temperatures can be used. For example concentrations between 0.3M and 45 wt.% CrC12 are permissible, and an operational temperature rangeforthe deposition of 400-650"C is possible.

The initial over-potential pulse can reach as high as 1.0 V with respect to ECr2+,Cp being limited by the formation of Li deposits at very negative potentials.

The length oftha cathode pulse, the length of the resting time and the frequency of application of this cycle can be varied according to the desired structure of the deposit. The value of the final potential forthe growth ofthe deposit can also be changed.

Figure 2 illustrates diagrammatically different processes which may occur during the formation ofthe deposit. This diagram illustrates from top to bottom successive states of the surface layer showing deposits being formed. Initially, as shown at 20 the surface is cleaned. The high over-potential (uni) pulse or pulses will produce small nuclei as shown at 21. If this pulse is prolonged, these nuclei will tend to grow as shown at 22 and 23 and a non-coherent deposit will be formed. If on the other hand, this pulse is followed buy a lowoverpotential (rl2) electrolysisthe nuclei will tend to grow as shown at24and eventuallytheywill join up to form a coherent deposit. The technique of the present invention enables control of the initial nuclei formation and growth to be effected independently of the rate of build up of the coherent layer in the later stages of the deposition. It is readily possible therefore to produce deposits having a smail grain structure and typical grain sizes are up to 7 microns.

Reduction of the over-voltage during the later stages of deposition enables the formation of large crystals and dendrites on the outer surface of the coherent layerto be minimised. It is preferable to form a large number of nuclei in the first step and it is undesirable to allowthese nuclei to grow excessively. Also, it is possible to apply high over-potential pulses during the build-upofthe main depositto minimise still further the formation of voids in the coating. After the coating has been formed, high temperature annealing undervacuum can assist in improving the adhesion to the substrate by forming a diffusion bond with the substrate. This annealing will also improve the levelling and coherence ofthe chromium layer.

Claims (12)

CLAIMS:

1. A method of depositing a chromium coating on an electrically-conductive substrate electrolytically using a molten salt bath containing Cr2+ ions * comprising the steps of putting the substrate and a chromium anode into the bath with a reference electrode, applying one or more shprt-duration over-potential pulses to the substrate with respect to the reference electrode to form a plurality of small nuclei on the substrate and subsequently applying a much smaller over-potential as a coating potential to deposit chromium over the substrate.

2. A method as claimed in claim 1 wherein said smaller potential is maintained as a continuous potential until the desired thickness of deposit is obtained.

3. A method as claimed in either claim 1 or claim 2 wherein the initial pulse or pulses of over-potential hasa magnitude at least three times the potential subsequently applied for deposition of the coating.

4. A method as claimed in any of the preceding claims wherein the initial over-potential is in the range of 200 mV to lv with respect to Ecr2+tc.

5. A method as claimed in any of claims 1 to 3 wherein the initial over-potential is in the range of 250 to 500 mV with respect to ECr2+/Cr .

6. A method as claimed in any of the preceding claims wherein the molten salt is a lithium chloride potassium chloride mixture.

7. A method as claimed in claim 6wherein the salt mixture is substantially the eutectic.

8. A method as claimed in eitherclaim 6 orclaim 7 wherein C+ ions are added to the melt as anyhdrous CrCI2.

9. A method as claimed in either claim 6 or claim 7 wherein C+ ions are added to the melt by anodic dissolution of chromium.

10. A method as claimed in any of the preceding claims wherein two or more of said over-potential pulses are applied with a resting period between pulses of longer duration than the pulses.

11. A method of depositing a chromium coating electrolytically from a molten salt bath substantially as hereinbefore described with reference to the foregoing examples.

12. An article chromium plated by the method of any of the preceding claims.