EP0545785B1 - Process for sealing an anodic coating obtained from a chromic acid bath - Google Patents

Description

The present invention relates to a method of sealing an anodization layer obtained in a chromic bath, and more precisely of such an anodization layer obtained according to a method of a type allowing high performance to be obtained in which relates to corrosion resistance, in particular in a saline environment.

Corrosion resistance is a very important requirement for the aeronautical industry. Test standards and requirements vary somewhat between manufacturers, but still correspond to a high level of performance, and there are several chromic anodizing processes intended to achieve them.

The problem to which the invention responds derives from the following observations:

Anodizing layers when not clogged offer low corrosion resistance and good adhesion of the paint. When clogged at the maximum hydration rate, they offer good corrosion resistance but less good adhesion of the paint.

The present invention therefore aims to reconcile two contradictory requirements: on the one hand, very good resistance to corrosion in a saline environment, and very good adhesion of a paint.

To achieve this result, research focused mainly on controlling the variation in the hydration rate of the oxide layer.

During conventional sealing operations, this rate varies very quickly in the intermediate values, which must be achieved in a manner compatible with industrial requirements to reconcile the opposite requirements above.

• Composition du bain :
     base : eau déminéralisée,
     bichromate de potassium ou de sodium :
     8 à 12 grammes par litre,
     pH : ajusté à : 4,5 à 6,5 par une addition convenable de soude caustique.
• Température du bain : 75 à 85°C.
• Temps d'immersion : suffisant pour assurer un taux d'hydratation du revêtement d'alumine compris entre 8 et 15%.

To obtain the desired result, the invention provides a method of sealing the oxide layer obtained by anodizing aluminum or an aluminum alloy in a chromic bath, this anodization being of a type allowing very high performance to be obtained with regard to corrosion resistance, this process having the particularity that said clogging is carried out under the following conditions:

• Composition of the bath:
  base: demineralized water,
  potassium or sodium dichromate:
  8 to 12 grams per liter,
  pH: adjusted to: 4.5 to 6.5 by a suitable addition of caustic soda.
• Bath temperature: 75 to 85 ° C.
• Immersion time: sufficient to ensure a hydration rate of the alumina coating of between 8 and 15%.

It has been found that the method according to the invention makes it possible to obtain a slow variation in the rate of hydration and therefore great precision in its determination. Obtaining an optimal hydration value thus becomes industrially feasible.

The hydration rate referred to here represents, by mass, the amount of water fixed during clogging, relative to the total mass of the coating (alumina + water). This hydration rate is determined by a test on a test tube.

The process of such a test is described below:

After anodization, the test pieces are immediately rinsed (demineralized water), brought back to the laboratory and dried with compressed air. They are then weighed, ie P1 the weight of the test piece before clogging. As the weighings progress, the test pieces are replaced on the tools and stored in the laboratory. The duration of the weighing operation does not exceed 5 minutes.

Then a clogging is carried out as indicated above. Once the sealing time has elapsed, the assemblies are removed, rinsed with demineralized water and dried with compressed air. The test pieces are weighed again, ie P2 the weight of the test piece after clogging. The test pieces are weighed a third time after dissolving the oxide layer in a phosphochromic solution, ie P3 the weight of the test piece devoid of its oxide layer.

All these manipulations of test pieces are done in white gloves, holding the test pieces by the edges.

DP1 = P2 - P1 :

désigne le poids de l'eau pris lors du colmatage,

DP2 = P2 - P3

désigne le poids de l'alumine avec eau,

$$\text{T =}\frac{\text{DP1}}{\text{DP2}}\text{=}\frac{\text{P2 - P1}}{\text{P2 - P3}}\text{.}$$

We then proceed to calculate the hydration rate T:

DP1 = P2 - P1:

indicates the weight of the water taken during the clogging,

DP2 = P2 - P3

denotes the weight of alumina with water,

$$\text{T =}\frac{\text{DP1}}{\text{DP2}}\text{=}\frac{\text{P2 - P1}}{\text{P2 - P3}}\text{.}$$

To evaluate the performance of the process according to the present invention, aluminum alloys of the type 2024 T3 and 7075 T73, previously anodized according to the "BF5" process (registered trademark of the Company), were treated according to the process described above. INDUSTRIAL GALVANOPLASTY).

This anodization process is described in document FR-A-2,278,790, which describes a process for the anodic oxidation of alloyed or unalloyed aluminum, according to which a bath is used which contains, in new condition, between 20 and 100 grams per liter of chromic anhydride CrO₃ and an amount of chromium chromate (CrO₄) ₃ Cr₂ between 0.5 gram per liter and an upper limit which is, depending on the case, 15 grams per liter, or the limit of solubility of this compound at the temperature of use of the bath. Preferably, the electrolysis is carried out at a voltage of between 5 and 90 volts, at a temperature below 65 ° C. This process makes it possible to build a layer capable of ensuring good resistance to corrosion, in particular in a saline atmosphere, after clogging, for example, in demineralized water at boiling point. Without clogging, it is specified in the patent that the layer obtained provides a very good base for glues, varnishes and paints.

• aucune détérioration du substrat après 750 heures d'exposition.

With regard to resistance to salt corrosion in the absence of paint, a series of neutral salt spray tests, according to ISO 9227 - 1990 "NSS" (neutral salt spray), gave the following result:

• no deterioration of the substrate after 750 hours of exposure.

With regard to the adhesion of the paint, tests were carried out according to a technique described by the company INTERNATIONAL CELOMER, which comprises the successive application of a primer coat of primer P99, based on phenolic butyral, then the deposition of a PAC33 anti-corrosion primer, then a PU66 finishing layer, based on polyurethane, the above designations being registered trademarks of INTERNATIONAL CELOMER, the first layer having a thickness of 4 to 11 micrometers, the total of the first two layers a thickness of 14 to 29 micrometers, and the total of the three layers a thickness of 39 to 64 micrometers. A "Q + S" adhesion test according to ISO standard 2409-1972 gave the result, before and after immersion of 14 days in water, level 0 (excellent) of this standard.

Claims (3)

1. Process for sealing the oxide layer obtained by an anodisation of aluminium or of an aluminium alloy in a chromic bath, this anodisation being of a type which makes it possible to obtain very high performance in respect of the corrosion behaviour, characterised in that the said sealing is carried out under the following conditions:

   - Composition of the bath:
      base : demineralised water,
      potassium or sodium dichromate:
      8 to 12 grams per litre,
      pH : adjusted to : 4.5 to 6.5 by an appropriate addition of caustic soda.

   - Bath temperature : 75 to 85°C.

   - Immersion time : sufficient to ensure a degree of hydration of the alumina coating of between 8 and 15 %.
2. Process according to Claim 1, characterised in that the degree of hydration is defined by a preliminary test thus defined:
   After anodisation, the test specimens are immediately rinsed (demineralised water), returned to the laboratory and dried with compressed air. They are then weighed, the weight of the test specimen before sealing being taken as W1. As the weighings proceed, the test specimens are replaced on the equipment and are stored in the laboratory. The duration of the weighing operation does not exceed 5 minutes.
   Sealing is then carried out as indicated below. When the sealing period has elapsed, the tool assemblies are withdrawn, rinsed with demineralised water and dried with compressed air. The test specimens are reweighed, the weight of the test specimen after sealing being taken as W2. The test specimens are weighed a third time after dissolution of the oxide layer in a phosphochromic solution, the weight of the test specimen devoid of its oxide layer being taken as W3.
   All this handling of test specimens is performed with white gloves, the test specimens being held by the edges.
   A calculation of the hydration ratio R is then carried out:

   WD1 = W2 - W1 :   denotes the weight of water taken up during the sealing,

   WD2 = W2 - W3   denotes the weight of the alumina with water,

   $$\text{R = (WD1/WD2) = (W2 - W1)/(W2 - W3).}$$

   
3. Process according to Claim 1 or 2, characterised in that it is applied to an oxide layer obtained by anodisation of aluminium or of an aluminium alloy in a bath containing, in the fresh state, between 20 and 100 grams per litre of chromic anhydride CrO₃ and a quantity of chromium chromate Cr₂(CrO₄)₃ of between 0.5 grams per litre and an upper limit which is, depending on circumstances, 15 grams per litre or the solubility limit of this compound at the temperature of use of the bath.