Nitrohydrofluoric development bath for titanium alloy components

A nitrohydrofluoric development bath for use in an electro-chemical etching rocess, known as the "blue etch process", for the non-destructive inspection of titanium or titanium alloy components such as turbo-machine blades and discs, the bath comprising, per liter, 320 g of nitric acid, from 13 to 22 g of hydrofluoric acid, from 4 to 7 g of dissolved titanium, and water as the balance.

FIELD OF THE INVENTION 
The invention relates to the nitrohydrofluoric development bath in an 
electro-chemical etching process for titanium alloy components comprising, 
in succession, the steps of degreasing, rinsing, activation by acid 
etching, rinsing, anodic oxidation in a trisodium phosphate bath, rinsing, 
and development by etching in a nitrohydrofluoric bath. 
BACKGROUND OF THE INVENTION 
The operating conditions of turbo-engines, especially aircraft engines, 
have led to the utilization of numerous titanium or titanium alloy 
components in such engines. It is important that these components should 
be subjected to a non-destructive checking capable revealing the various 
defects from which they may suffer. In particular, they should be examined 
for possible manufacturing defects such as segregations, inclusions, 
porosity, etc., transformation defects such as cracks, incrustations, 
heterogeneity, contaminations, etc., and machining or polishing defects 
such as work-hardening, local overheating, etc. For this purpose, there is 
in existence an electro-chemical etching process which is well known in 
the art as the "blue-etch process". 
This electro-chemical etching process consists, generally, in carrying out 
the following operations on the component to be checked: 
1. Conventional degreasing by immersion in an alkaline bath; 
2. Rinsing with cold water in a tank of running water, or by sprinkling; 
3. Possible removal of a work-hardened layer, about 5 microns, by 
fluo-nitric etching; 
4. Rinsing with cold water in a tank of running water; 
5. Chemical activation by immersion in an acid salt bath for etching with a 
macrographic effect; 
6. Rinsing with cold water in a tank of running water; 
7. Anodic oxidation in a trisodium phosphate bath, with the component to be 
checked being in the anode position; 
8. Rinsing with cold water in a tank of running water; 
9. Development by partial etching in a nitrohydrofluoric bath; 
10. Rinsing with cold water as quickly and thoroughly as possible, followed 
by drying of the component; and 
11. Reading the defects revealed, on the basis of shapes and colours 
(white, blue, grey-blue) which are peculiar to them. 
However, this process does have some drawbacks. In particular, the 
nitrohydrofluoric development bath used in step 9 generally has a 
composition comprising, per liter, 320 g nitric acid (HNO.sub.3), from 13 
to 22 g hydrofluoric acid (HF), and water the balance, and this requires 
the development to be carried out within a period of from 2 to 10 seconds, 
and the transfer time between the development bath and the rinsing of step 
10 to be between 2 and 5 seconds. Exceeding one of these limits brings 
about complete discoloration of the component, making any detection of 
defects impossible. 
As will be appreciated, it is not a problem to keep within these limits 
when treating components which are of relatively small size and simple 
shape, since these can be quickly handled and rinsed. However, this is not 
the case for relatively large components of complex shape, such as 
turbo-engine discs for example. It is therefore necessary, for such 
components, to reduce the activity of the developer bath so that the 
immersion and transfer times can be increased to be compatible with the 
process and handling equipment required for the components. 
One way of reducing the reaction kinetics of the development bath is to 
reduce the concentration of the hydrofluoric acid in the bath. 
Unfortunately this solution results in a bath which becomes exhausted very 
quickly, and which therefore has a very short life and does not permit 
reliable results to be obtained.

DESCRIPTION OF THE INVENTION 
The invention provides an alternative and more acceptable way of reducing 
the activity of the development bath, that is to say its reaction 
kinetics, by including in the bath from 4 to 7 g/l of dissolved titanium. 
More precisely, according to the invention there is provided a 
nitrohydrofluoric development bath for use in an electro chemical etching 
process for titanium alloy components comprising, in succession, the steps 
of degreasing, rinsing, activation by acid etching, rinsing, anodic 
oxidation in a trisodium phosphate bath, rinsing, and development by 
etching in a nitrohydrofluoric bath, said development bath comprising, per 
liter, 320 g of nitric acid, from 13 to 22 g of hydrofluoric acid, from 4 
to 7 g of dissolved titanium, and water as the balance. 
Preferably the development step is carried out with the bath at a 
temperature between 20.degree. C. and 30.degree. C., and with the duration 
of immersion between 25 and 50 seconds. 
The use of the development bath in accordance with the invention, i.e. with 
the bath including dissolved titanium in the proportion of from 4 to 7 
g/l, has given very satisfactory results, particularly when the bath 
contains 22 g/l hydrofluoric acid and the HNO.sub.3 /HF ratio is 14.5. 
With the bath at a temperature of between 20.degree. C. and 30.degree. C., 
the invention enables the development step to be operated with an 
immersion time close to 30 seconds followed by 15 seconds for the transfer 
to the rinsing bath, which is perfectly compatible with an industrial 
process, even for large components.