FR2684393A1 - METHOD OF COLLAPTING AN ANODIZATION LAYER OBTAINED IN CHROMIC BATH. - Google Patents

Abstract

To simultaneously obtain good corrosion resistance and good paint adhesion, the sealing of the oxide layer, obtained by chromic anodization, is carried out under the following conditions: - Composition of the bath: base: demineralized water, potassium dichromate or sodium: 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%.

Description

The present invention relates to a process for sealing a layer

  anodizing obtained in a chromic bath, and more precisely of such an anodization layer obtained by a process of a type that makes it possible to obtain high performances with regard to resistance to corrosion, especially in saline medium. Corrosion resistance is a very important requirement for the aerospace industry Test standards and requirements vary somewhat by manufacturer, but still reflect a high level of performance, and there are many different processes.

  chromatic anodizing intended to achieve them.

  The problem to which the invention responds derives from the following observations: The anodising layers when they are not clogged offer a low resistance to corrosion and a good adhesion of the paint When they are clogged at the maximum hydration rate, they offer good resistance to corrosion but poorer

adhesion of the paint.

  The present invention therefore aims to reconcile two contradictory requirements: on the one hand, a very good resistance to corrosion in a salt medium, and a

  very good hanging of a painting.

  To achieve this result, research focused on controlling rate variation

hydration of the oxide layer.

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

opposite above.

  In order to obtain the desired result, the invention provides a process for clogging the oxide layer obtained by anodizing aluminum or an aluminum alloy in a chromic bath, this anodizing being of a type which makes it possible to obtain very high performance with respect to corrosion resistance, this method having the particularity that said clogging is carried out under the following conditions: Bath composition: base: demineralized water, potassium or sodium dichromate 8 to 12 grams per liter , p H: adjusted to: 4.5 to 6.5 by addition

suitable caustic soda.

  Bath temperature: 75 to 850 C. Immersion time: sufficient to ensure a level of hydration of the alumina coating between

8 and 15%.

  It has been found that the process according to the invention makes it possible to obtain a slow variation in the degree of hydration and therefore a high degree of precision in the determination of the optimum value. The obtaining of a hydration becomes

thus industrially feasible.

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

a test on a specimen.

  The process of such an experiment is described below: After anodization, the specimens are immediately rinsed (demineralized water), taken back to the laboratory and dried with compressed air. They are then weighed, ie P1 the weight of the test tube before clogging As the weighs are weighed, the specimens are placed back on the tools and stored in the laboratory. The duration of the weighed operation does not exceed 5 minutes.

  This is followed by a clogging as indicated below.

  The clogging time elapsed, the assemblies are removed, rinsed with demineralised water and dried with compressed air, the specimens are weighed again, or P 2 the weight of the specimen after clogging The specimens are weighed a third time after dissolving the oxide layer in a phosphochromic solution, P 3 is the weight of the specimen devoid of its oxide layer. All these manipulations of specimens are done in

  white gloves, holding test tubes by slices.

  The hydration rate T is then calculated: D P = P 2 P: denotes the weight of the water taken during the clogging, DP 2 = P 2 P 3 denotes the weight of the alumina with water, D P P 2 Pl

T% = - =%

DP 2 P 2 P 3

  In order to evaluate the performance of the process according to the present invention, aluminum alloys of the type 2024 T 3 and 7075 T 73, previously anodized according to the "BF 5" process (registered trademark), were treated according to the process described above. of the GALVANOPLASTY Society

INDUSTRIAL).

  This anodizing process is described in the document FR-A-2 278 790, which describes a process for anodic oxidation of aluminum, whether or not alloyed, in which a bath is used which contains, in the new state, between 20 and grams per liter of chromic anhydride Cr O 3 and an amount of chromium chromate (Cr O 4) 3 CR 2 of between 0.5 grams per liter and an upper limit which is, as the case, 15 grams per liter or the solubility limit of this compound at the temperature of use of the bath Preferably, the electrolysis is conducted at a voltage between 5 and 90 volts, at a temperature below 650 C. This method makes it possible to build a layer capable of ensuring good resistance to corrosion, especially in saline atmosphere, after clogging, for example in demineralized water to boiling Without clogging, it is specified in the patent that the layer obtained provides a very good base for glues,

varnishes and paints.

  With regard to salt corrosion resistance 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 substrate after 750 hours of exposure. Regarding the adhesion of the paint, tests were made according to a technique described by the INTERNATIONAL CELOMER company, which comprises the successive application of a layer of primer P 99, based on phenolic butyral, then the deposition of a PAC 33 anticorrosive primer followed by a polyurethane-based PU 66 topcoat, the above designations being registered trademarks of the INTERNATIONAL CELOMER Company, 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 test of adhesion "Q + S" according to the standard ISO 2409-1972 gave as a result, before and after immersion of 14 days in

  water, the O level (excellent) of this standard.

Claims (3)

  1 Process for clogging the oxide layer obtained by anodizing aluminum or an aluminum alloy in a chromic bath, this anodization being of a type allowing very high performances in terms of resistance corrosion, characterized in that said clogging is carried out under the following conditions: Bath composition: base: demineralized water, potassium or sodium dichromate 8 to 12 grams per liter, p H: adjusted to: 4.5 to 6 , 5 by an addition suitable caustic soda.   Bath temperature: 75 to 850 C. Immersion time: sufficient to ensure a level of hydration of the alumina coating between 8 and 15%.   2 Process according to claim 1, characterized in that the degree of hydration is defined by a prior test thus defined: After anodization, specimens are immediately rinsed (demineralized water), brought back to the laboratory and dried with compressed air They are then weighed, let P be the weight of the specimen before clogging. As the weighings proceed, the specimens are placed back on the fixtures and stored in the laboratory. The duration of the weighed operation does not exceed 5 minutes.   This is followed by a clogging as indicated below.   The clogging time elapsed, the tools are removed, rinsed with demineralised water and dried with compressed air, the specimens are weighed again, or P 2 the weight of the specimen after clogging The specimens are weighed a third time after dissolving the oxide layer in a phosphochromic solution, P 3 is the weight of the specimen devoid of its oxide layer. All these manipulations of specimens are done in   white gloves, holding test tubes by slices.   The hydration rate T is then calculated: D P1 = P 2 P1: denotes the weight of the water taken during the clogging, DP 2 = P 2 P 3 denotes the weight of the alumina with water, D P P 2 Pl T = - =%. DP 2 P 2 P 3   3 Process according to claim 1 or 2, characterized in that it is applied to an oxide layer obtained by anodizing aluminum or an aluminum alloy in a bath containing, in new condition, between and 100 grams per liter of chromic anhydride Cr O 3 and an amount of chromium chromate (Cr O 4) 3 CR 2 of between 0.5 grams per liter and an upper limit which is, as the case may be, 15 grams per liter, or the solubility limit of this compound at the temperature of use of the bath.