Title:
METHOD OF ETCHING A TITANIUM OR TITANIUM ALLOY PART
United States Patent 3850712


Abstract:
The invention relates to a method of etching a titanium or titanium alloy part in which the part is immersed in a mixture comprising nitric acid, hydrofluorosilicic acid and water.



Inventors:
Broughton, Trevor (Etwall, EN)
Forman, Brian William (Burton-On-Trent, EN)
Application Number:
05/261815
Publication Date:
11/26/1974
Filing Date:
06/12/1972
Assignee:
Rolls-Royce Limited (London, EN)
Primary Class:
Other Classes:
216/108, 252/79.3
International Classes:
C23G1/10; (IPC1-7): C23F1/00
Field of Search:
156/18,20 252
View Patent Images:



Primary Examiner:
Powell, William A.
Attorney, Agent or Firm:
Cushman, Darby & Cushman
Claims:
We claim

1. A method of etching a titanium or titanium alloy part comprising contacting the part with a mixture consisting essentially of nitric acid, hydrofluorosilicic acid and water, and continuing contact with said mixture until a matt surface is produced on said part adapted for improved visual detection of flaws therein.

2. A method as claimed in claim 1 and in which the part is subsequently washed to remove remaining traces of the mixture.

3. A method as claimed in claim 1 and in which the part is immersed in the mixture for between 4 minutes and 20 minutes.

4. A method as claimed in claim 3 and in which the mixture temperature is between 16° and 22°C.

5. A method as claimed in claim 1 and in which the part is pre-treated before said immersion to pre-condition scale deposits.

6. A method as claimed in claim 5 wherein said scale deposits are removed.

Description:
This invention relates to a method of etching a titanium or titanium alloy part and to an etchant for use in this method.

It is sometimes necessary that titanium parts should be etched, and inspected for instance to disclose fine flaws in the surface of the part; this is particularly the case in penetrant inspection, where unetched parts give very poor results. Previous etching methods have suffered from two major disadvantages; firstly they have been rapid in action so that when a complicated part was treated by immersion there was a danger that pools of etchant which were left would cause rapid unwanted etching, and secondly this produced a bright grain etched appearance on which it was difficult to detect very fine cracks and other defects by binocular techniques.

The present invention provides a method of etching which is relatively slow and which produces a matt surface condition on which the grain etched surface is less pronounced and which is suitable for both binocular and subsequent penetrant inspection.

According to the present invention a method of etching a titanium or titanium alloy part comprises immersing the part in a mixture comprising nitric acid, hydrofluorosilicic acid and water.

The necessary time of immersion of the part in the mixture will vary in accordance with the amount of metal to be removed, the temperature of the mixture and various other factors, but we have found that for a metal removal of approximately a tenth of a thousandth of an inch, at a mixture temperature of about 20°C, an immersion time of from 4 to 20 minutes may be used.

The invention also comprises a mixture suitable for etching titanium or alloys thereof and comprising nitric acid, hydrofluorosilicic acid, and water.

Preferably the mixture comprises between 160 and 180 grams per litre of nitric acid, between 8 and 12 grams per litre of the hydrofluorosilicic acid, the remainder comprising water.

If the titanium or alloy part has been heated it may be necessary to pre-condition the oxide film so as to avoid uneven etching. Typically this would use an alkaline permanganate solution which would comprise 50-100 grams per litre of potassium permanganate, 100-125 grams per litre of sodium hydroxide and 100-125 grams per litre of sodium carbonate, the remainder comprising water. Alternatively a proprietary solution could be used.

If the oxide deposits are heavy then it may be necessary to clean these off either mechanically or chemically.

The invention is particularly useful when applied to parts such as compressor drums of gas turbine engines.

In an example of the method according to the invention a solution was made up comprising 170 grams per litre of nitric acid and 10 grams per litre of hydrofluorosilicic acid, the remainder comprising water. A tank was filled with the solution at a temperature of approximately 20°C and the compressor drum of a gas turbine engine was immersed in the solution. The drum comprised a titanium alloy known as IMI 684 which is basically titanium with the addition of 6 percent Al, Zr 5 percent, W 1 percent. The drum was left in the solution for some 20 minutes and was then removed and washed out with water. The drum was then inspected through binocular microscopes and using a penetrant technique to determine the location of any flaws such as cracks or the like.

It was found that some 11/2 tenths of a thousandth of an inch had been removed from the drum, and that the etchant had removed the surface without preferentially attacking grain boundaries. It had therefore made the cracks and other flaws more clearly visible and a particularly marked improvement was found in the detection using fluoroescent penetrants such as Ardrox 970 P16 water washable. In fact the detection rate of cracks with penetrant was improved from some 25 percent without the method of the invention to some 90 percent using the present method.

Thanks to its relative slowness of etching, it was easy to remove the drum from the solution and to wash the etchant from the various cavities of the drum without any danger of pooling of the etchant producing heavily etched areas which could have been sources of weakness.

In further examples of the invention, it was found that the time of immersion of similar titanium parts in the etchant mixture could sometimes be reduced to 4 minutes for a metal removal of about a tenth of a thousandth of an inch. However the time of immersion may be reduced still further if the temperature of the etchant mixture, which is normally room temperature, is increased above room temperature or if the ambient temperature happens to be high. We would expect the times of immersion not to be altered to a large extent within the range of 16°-22°C mixture temperature, and within this range we find that an immersion time between 4 and 20 minutes will produce metal removal of about a tenth of a thousandth of an inch, which is normally an optimum amount to minimise metal removal while producing the necessary surface finish to allow surface inspection.

In other examples of the method of the invention, drums were etched and inspected after they had been run on a gas turbine engine and were therefore coated with oxides. In these cases we found it preferable to pre-condition the oxides by treating them in alkaline permanganate solution as described above. This pre-conditioning modified the oxide layer on the part and enabled more even etching to be achieved.

In cases where the oxide deposit was heavy we prefer to use mechanical cleaning to reduce their thickness before treatment.

In addition to the alloy mentioned above, the method of the invention has been found useful with IMI 685 titanium alloy and we believe that it is applicable to all titanium based alloys.