GB2205584A

GB2205584A - Electrolytic treatment

Info

Publication number: GB2205584A
Application number: GB08808675A
Authority: GB
Inventors: Dr Andrew Derek Turner; Anthony Richard Junkison
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1987-04-14
Filing date: 1988-04-13
Publication date: 1988-12-14
Anticipated expiration: 2008-04-13
Also published as: GB8808676D0; GB8708945D0; GB2205584B; GB8808675D0; EP0289168A1; JPS63318214A; GB2203756B; GB2203756A

Description

i C) 4- 2405584 Electrolytic Treatment This invention relates to a method

for electro- lytically treating a metal surface.

Electrolytic treatment to modify surfaces is a known process involving establishing an electrolytic cell where an object having a surface to be treated constitutes an electrode thereof, and passing an electric current through the cell under conditions such that the surface is modified by removal of material therefrom, e.g. to improve its appearance and/or structure. Electrolytic polishing (also known as electropolishing) is a form of such treatment and is described in GB-A-530,041, for example. GB-A-530,041 describes the process of electrolytically polishing objects of Fe, Co, Ni, Cr, and their alloys, comprising the step of making the object the anode in an acid aqueous electrolytic bath containing ions having a position in the lyotropic series according to Cooper following after the sulphate ion and capable of forming easily soluble salts with said metals, using a current density sufficient to cause a removal from the surface of the object of solid anodic oxidation products primarily formed in the treatment. An example of an acid aqueous electrolytic bath described in the above-mentioned patent is a nitric acid bath.

However, a problem when using a nitric acid electrolyte as described above is that a high current may be required to achieve satisfactory removal of material.

The high current may then create excessive local heating giving rise to a poor surface finish. The present invention attempts to meet this problem, in the treatment of specific metal surfaces, by incorporating chromium(VI) oxide into the electrolyte.

2 Thus, the invention includes a method of electrolytically treating a surface of a chromium containing metal which comprises the steps of (i) establishing an electrochemical cell including the metal surface as an electrode of the cell and an aqueous electrolyte in contact with the metal surface, the aqueous electrolyte comprising aqueous nitric acid and chromium(VI) oxide; and (ii) passing an electric current through the cell under conditions such that the surface is modified by removal of material therefrom.

The presence of chromium(VI) oxide (Cr03) is found to significantly reduce the magnitude of the current required to electropolish the metal surface. For example, provision of 20% by weight of Cr03 in 10M HN03 is found to reduce the current density required to electropolish stainless steel from 1 Acm-2 to 0.5 Acm-2. This is believed to occur because the electropolishing process is controlled through the solubility of Cr03 at the dissolving surface of the metal. Where the metal surface is of stainless steel, the role of the HN03 is to dissolve components thereof such as Fe and Ni. The HN03 concentration may, for example, be in the range from 1M to 14M.

The ability to carry out the present invention at low current densities and achieve electropolishing is determined principally by the temperature, the nitric acid concentration and the Cr03 concentration. Experiments may therefore have to be carried out to determine the optimum combination of these parameters in particular cases.

3 The metal surface is preferably that of a Cr-containing alloy such as a steel which may also contain Ni. Such steels are usually known as stainless steels.

In addition to being operable at low current densitiez, the method of the invention has the advantages that it does not produce hydrogen gas and that it can be carried out at relatively low temperatures. Moreovery it is capable of electropolishing the surfaces of metal bodies of complex shape, i.e. it has good "throwing power".

The invention will now be particularly described in the following examples thereof. Reference will be made to the accompanying drawing, the sole figure of which is a graph showing the relationship between current density and Cr03 concentration to achieve electropolishing at various temperatures.

EXAMPLES

An elecrochemical cell was prepared in which the anode was a stainless steel specimen to be electropolished (total surface area 20 cm2), the cathode was a titanium cathode and both anode and cathode were immersed in an electrolyte comprising 12M aqueous HN03 with either no r03 dissolved therein or with different concentrations of Cr03 dissolved therein. The cell was connected to a source of electromotive force (4.5 V) and a current passed.

The current at which the stainless steel was electropolished, as estimated visually, was measured The experiment was repeated at different concentrations of Cr03 in the electrolyte and at different temperatures. Referring to the figure of the accompanying drawing, the results are summarised in three curves# each representing values obtained at different 4 temperatures (200C, 12'C and 5'C), as marked on the figure.

Also marked on the figure (by crosses) are values obtained at OIC and -40C for a particular concentration of Cr03 in the electrolyte.

It will be seen from the figure that, as Cr03 concentration is increased, there is a general fall in the required current density which eventually reaches a minimum at about 250 g/1 of Cr03.

Also, the required current density falls as temperature falls; this is further confirmed by the specific values obtained at OOC and -40C.

is Similar experiments were carried out using 10M HN03 and 14M HN03 When using 10M HN03, the required current density decreased with decreasing temperature and with increasing Cr03 concentration beyond 450 g/1. When using 14M HN03. the required current density decreased with decreasing temperature and reached a minimum at a CrC)3 concentration of between 20 and 30 g/1.

Experiments were also carried out to compare the throwing power of an electrolyte used in the present invention with that of a conventional H3P04 electrolyte and of an aqueous HN03 electrolyte. Throwing power is a measure of the ability of a process to treat complex surfaces uniformly. Experiments were carried out to measure the length of polished surface produced down a tube bore, normalised to the square root of the orifice radius, by electropolishing using, as electrolyte, the conventional H3P04 system (20-60 mA cm-2), 12MHNO3 with 260 g/1 Cr03 at 5OC (250 mA cm-2), and 14M HN03 at 50C (480 mA cm-2). The required current densities are indicated in parentheses.

The results obtained (in CM2) were as follows:

H3P04 0.3 to 0.7 12M HN03/260 g/1 Cr03 2.3 14M HN03 2.12 The process of the present invention is seen to give the best results.

Claims

1. A method of electrolytically treating a surface of a chromium-containing metal which comprises the steps of (i) establishing an electrochemical cell including the metal surface as an electrode of the cell and an aqueous electrolyte in contact with the metal surface, the aqueous electrolyte comprising aqueous nitric acid and chromium(VI) oxide; and (ii) passing an electric current through the cell under conditions such that the surface is modified by removal of material therefrom.

2. A method according to claim 1 wherein the metal surface is of stainless steel.

3. A method according to either of the preceding claims wherein the concentration of nitric acid in the electrolyte is in the range from 1M to 14M.

4. A method according to claim 1 substantially as described herein with reference to any of the examples.

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