EP0527062B1 - Development bath after phosphoric anodization of titanium alloy - Google Patents

Description

• - 1 - Dégraissage classique ou trempé dans un bain alcalin.
• - 2 - Rinçage à l'eau froide dans un bac d'eau courante ou par aspersion.
• - 3 - Enlèvement éventuel de la couche écrouies, environ 5 µm par attaque fluo-nitrique.
• - 4 - Rinçage à l'eau froide dans un bac d'eau courante.
• - 5 - Activation chimique au trempé dans un bain sel acide pour attaque avec effet macrographique.
• - 6 - Rinçage à l'eau froide dans un bac d'eau courante
• - 7 - oxydation anodique dans un bain de phosphate trisodique, la pièce à contrôler étant en position d'anode.
• - 8 - Rinçage à l'eau froide dans un bac d'eau courante.
• - 9 - Révélation par attaque partielle dans un bain nitrofluorhydrique.
• -10 - Rinçage à l'eau froide le plus rapidement possible et le plus complètement possible et séchage de la pièce.
• -11 - Lecture des défauts qui apparaissent selon des formes et des couleurs (blanc, bleu, gris-bleu) qui leur sont propres.

The operating conditions of turbomachinery, in particular aeronautics, lead to the use of numerous parts made of titanium or titanium alloy. It is important that these parts undergo a non-destructive test capable of highlighting the various defects that they may contain. In particular, it is necessary to look for any production defects - segregations, inclusions, porosities etc ... - transformation - cracks, encrustations, heterogeneities, contaminations etc ... - machining or polishing - hardening, overheating local, etc ...
To this end, there is an electrochemical attack process well known in professional circles under the name of "BLUE-ETCH".
This electrochemical etching process generally consists of carrying out the following operations on the part to be checked:

• - 1 - Classic degreasing or soaking in an alkaline bath.
• - 2 - Rinse with cold water in a running water tank or by sprinkling.
• - 3 - Possible removal of the work hardened layer, approximately 5 µm by fluo-nitric attack.
• - 4 - Rinse with cold water in a running water tank.
• - 5 - Chemical activation by soaking in an acid salt bath for attack with macrographic effect.
• - 6 - Rinse with cold water in a running water tank
• - 7 - anodic oxidation in a trisodium phosphate bath, the part to be checked being in the anode position.
• - 8 - Rinse with cold water in a running water tank.
• - 9 - Revelation by partial attack in a hydrofluoric acid bath.
• -10 - Rinse with cold water as quickly as possible and as completely as possible and dry the part.
• -11 - Reading of faults that appear according to their own shapes and colors (white, blue, gray-blue).

This known method has, however, some drawbacks.
In particular, the operation of revealing by attack in a hydrofluoric acid bath must be carried out for a time of 2 to 10 s. The handling means impose a transfer time, between the revelation operation -9- and the rinsing operation of between 2 and 5 s.
Exceeding one of these limits results in complete discoloration of the part and makes it impossible to detect faults. In addition, industrial handling means do not allow time limits to be respected for large parts and complex geometries, such as, for example, certain turbomachine disks.

The object of the present invention is precisely to remedy this drawback by reducing the activity of the bath, that is to say the reaction kinetics.

To this end, and according to the essential characteristic of the invention, titanium is dissolved in the development bath at a rate of 4 to 7 g / l. Advantageously, and according to the invention, the revelation operation is carried out in a bath whose temperature is between 20 ° and 30 ° C for an immersion time between 25 and 50s.

The invention will be better understood and other characteristics will appear on reading the details and comments which follow.

• Acide nitrique (HNO₃) 320 g/l
• Acide fluorhydrique (HF) 13 à 22 g/l
• Eau (H20) en quantité suffisante pour 1 l.

The known method uses, for the revelation operation, a hydrofluoric acid bath, the composition of which is as follows:

• Nitric acid (HNO₃) 320 g / l
• Hydrofluoric acid (HF) 13 to 22 g / l
• Water (H20) in sufficient quantity for 1 l.

We have seen above that these baths lead to revelation times of 2 to 10 s. We understand that this is perfectly satisfactory for small and simple parts that can be handled and rinsed quickly. It is no longer the same for large and complex shaped parts, some parts being subjected to the developer bath for much longer times than others. It was therefore necessary, for these parts, to decrease the activity of the revealing bath in order to allow a duration of immersion compatible with an industrial process and an increased transfer time.

A first idea to reduce the reaction kinetics comes naturally to mind, it consists in reducing the concentration of hydrofluoric acid in the bath.

Unfortunately, this solution leads to a bath which runs out very quickly, which therefore has a very short lifespan and which does not allow reliable results to be obtained.

The solution proposed by the invention and which consists in enriching the dissolved titanium bath at a rate of 4 to 7 g / d gave very satisfactory results, in particular with a bath comprising 22 g / l of hydrofluoric acid, that is to say having an HNO₃ / HF ratio = 14.5.

This solution leads, with a bath whose temperature is between 20 ° and 30 ° C to immersion times close to 30s followed by 15s of transfer into the rinsing bath, which is perfectly compatible with an industrial process, even for large parts.

Claims (3)

1. Bath for development after phosphoric anodization on titanium alloy for an electrochemical etching process comprising, in succession, degreasing, activation by acid etch, anodic oxidation in a trisodium phosphate bath, and development by etching in a nitrohydrofluoric bath, the composition of which is:

   - nitric acid, 320 g/l,

   - hydrofluoric acid, 13 to 22 g/l

   - water in quantity sufficient for 1 litre

   all these steps being separated by rinsing operations, characterized in that it comprises, in addition to the above composition, dissolution of from 4 to 7 g/l of titanium.
2. Development bath according to Claim 1, characterized in that the concentration of hydrofluoric acid is 22 g/l.
3. Development bath according to either of Claims 1 and 2, characterized in that the use temperature is between 20° and 30°C.