Processes for specimen preparation

4.3.1. Pretreatment

4.3.1.1. Degrease

The surface conditioning of the samples before anodising is a very important step in the process, as a bad pretreatment may compromise the characteristics of the final anodic film.

The pretreatment comprises several steps. The first one is degreasing with a cotton cloth impregnated with butanone (methylethylketone or MEK). The main purpose of this is the removal of inks, oils and greases from the surface of the alloys. This step is not fundamental but prolongs the service life of the next bath.

The second step is immersion in an alkaline degrease bath. This bath is an aqueous solution of Turco® 4215 NC-LT manufactured by Henkel. The composition of this silicated, non-chromated, mildly cleaner is as follows:

Experimental procedure

45 Table 7. Chemical composition of the Turco® 4215 NC-LT

Component Percent stirring is enough to provide clean surfaces.

The pH of the bath is slightly alkaline (next to 9); the attack to the aluminium in the alloy is very slow. The bath removes inks, oils and greases.

The concentration of the product was controlled periodically by determination of the alkalinity of the bath an acid/base titration with 1 M HCl. New product was added when needed.

After the immersion in the alkaline cleaning bath, the samples were always kept wet until the end of the treatment, the unique exception is between anodisation and primer application.

Water rinse following the alkaline cleaning was undertaken for 5 min in distilled water. After the rinse, the work was then rinsed with showers for 1 min, paying especial attention to the complete removal of foam on the surface of the specimens.

4.3.1.2. Etching

The following step of the pretreatment is the immersion of the batch in an acid etching bath.

This bath is an aqueous dilution of Ardrox® 295 GD manufactured by Chemetall, a Cr-free liquid deoxidiser and desmutter for use on aluminium alloys following cleaning or alkaline etching. The composition is a mixture of H2SO4, HNO3, and Fe2O3.

Etching time and temperature conditions were adjusted to remove a determined superficial thickness. The attack factor of the bath was calculated by the gravimetric method. Table n indicates the amount of material removed in terms of μm surface^-1 for the four studied alloys under the selected conditions (10 min at 40 °C with stirring).

Table 8. Mass loss for the aluminium alloys after immersion in the etching bath for 10 min at 40 °C.

Alloy Mass loss

Experimental procedure

46

In order to maintain the etching properties of the bath, a concentration between 20-25% of Ardrox® 295 GD was kept according to manufacturer instructions. Apart from the attack factor, two periodical controls were established, the acidity of the bath and the oxidising power.

The acidity of the bath was controlled by determination of the acidity of the bath an acid/base titration with 1 M NaOH.

The oxidising power of the solution was controlled by redox titration with 0.1 M Na2S2O3 and starch solution as indicator.

The objective of the acid etching bath is the removal of intermetallics particles and the mechanically deformed layer produced during the rolling process of the aluminium alloy.

Water rinse was undertaken for 5 min followed by shower rinse for 1 min to ensure the removal of the remaining rests of the etching bath.

4.3.2. Anodising

The pilot plant has two different anodising baths which are following described

4.3.2.1. Tartaric/sulphuric acid anodising bath

The first anodising bath studied in this work is a tartaric acid/sulphuric acid (TSA) solution (fig.

22). The nominal composition of the bath is 0.46 M (40 g L^-1) H2SO4 and 0.53 M (80 g L^-1) C4H6O6. The anodising bath was be maintained within 40 ± 10 g L^-1 of H2SO4 and 80 ± 10 g L^-1 of C4H6O6.

Figure 22. TSA bath with two AISI 321 stainless steel cathodes.

This was controlled by acidity of the bath in organic media, and the total acidity. Besides, some parameters are important, such as the Al³⁺ and the Cu²⁺ concentration.

High concentrations of Al³⁺ can affect the bath. Although some efforts were directed towards a quick, inexpensive, precise determination of Al³⁺ in the bath by titration, the only methods for

Experimental procedure

47

determining was atomic absorption and ICP-MS. The last technique revealed an Al³⁺

accumulation about 1 g L^-1 in the TSA bath after 3 years of service life.

Copper is incorporated to the bath by dissolution of the alloys in acid media and outward migration of the Cu²⁺ ions during the anodising process. As a result, copper deposition occurs as a side cathodic reaction. A maintenance operation was periodical withdrawal of the AISI 316 cathodes from the bath and removal of the deposited copper by polishing. Cu was in the ppm range (last control 158 ppm of Cu).

The basic anodising cycle is shown in fig. 23. All the anodised specimens described in this work were anodised following these parameters, unless other conditions are indicated.

Figure 23. Anodising program

The anodising program consist of a potential ramp from open circuit potential (OCP) to 14 V for 5 min, this ramp is for avoid current peaks, then there is a plateau of 14 V for 20 min and finally, a 30 s descendent ramp from 14 V to OCP’.

The rectifier controls the anodising voltage, it is designed for controlling a maximum current of 30 A, therefore for a 14 V anodising cycle, obtaining current densities between 0.5-1 A dm^-2 depending on the alloy, the maximum work load is 30 dm^-2 per anodising cycle.

4.3.2.2. Molybdenum modified Tartaric/sulphuric acid anodising bath

The second bath was developed during this work, such bath consists of a TSA bath with 0.25 M Na2MoO4·2H2O, named as MoTSA. This bath has been patented by Airbus [79]. Detailed description of the optimisation of the Na2MoO4 concentration will be described in chapter 5.4.

The methods of control previously described for the TSA bath were also valid for the determination of H2SO4, C4H6O6, Al and Cu in the bath. Besides, Mo determination was difficult due to the complex chemistry of Mo in acidic solutions and therefore, Mo concentration was controlled by ICP MS. After one year of, the concentrations of Al and Cu in the MoTSA bath were 54 and 14 ppm, respectively.

Experimental procedure

48 4.3.3. Postreatments

The racks with each specimen batch were withdrawn from the anodising bath immediately after the end of the anodising cycle and immersed in a deionised water rinse bath for 5 min followed by shower rinse for 1 min. This operation is crucial as the anodic film can be attacked by the electrolyte in the absence of imposed current.

The anodised samples not followed by painting or any other postreatment were dried in an air furnace at 60 °C for 5 min and stored in a controlled environment. In other case, one of the following postreatments was applied without drying the anodised samples.

4.3.3.1. Chemical conversion coatings

The chemical conversion coatings (CCC) are usually applied on the bare alloys without anodising. However, in this work, chemical conversion coatings were applied by immersion of the anodised samples in the CCC solutions at RT for 1 min to evaluate the possible use as anodising postreatments.

Two different CCC were used. The first one is a commercial Cr (VI)-containing chemical Alodine 1200 (pH=1.8) manufactured by Henkel. Composition is given in table n.

Table 9. Alodine 1200 composition

The other CCC was a Cr (VI)-free product described in the patent of Navair [51]. According to the manufacturer, such product is based on trivalent chromium salts.

After application of CCC, the specimens were water rinsed for 3 min and dried at 60 °C.

4.3.3.2. Sealing

Sealing postreatment was undertaken in one of the following sealing baths.

The first sealing bath is a 10 L boiling distilled water (min 95°C) beaker. The sealing was performed by immersion in the sealing bath immediately after water rinse of the anodised pieces.

The second sealing bath is a 10 L boiling aqueous 30 mM K2Cr2O7 solution. This sealing is undertaken at a minimal temperature of 95 °C for 20 min.

Experimental procedure

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After the sealing postreatment, the specimen were rinsed in distilled water for 3 min and dried at 60 °C.

4.3.3.3. Primers

Different organic primers were applied on the treated aluminium alloys. In general, such organic primers were applied by conventional air spray gun, passing as many crossed hands as needed for obtaining the desired coating thickness.

A primer Cr (VI)-loaded epoxy organic solvent currently in use by Airbus was utilised on anodised specimens to test the compatibility of this postreatment with the anodic films generated in the TSA bath.

Besides, some new water based primers were tested on variously treated AA2024 specimens.

Such water based primers were of two kinds, a Cr loaded water based primer and a Cr (VI)-free water based primer with inhibitors.

All the primed specimens were dried in an oven at 60 °C for 2 h. Some of these samples were subsequently coated with a polyurethane topcoat impermeable to fluids that provides mechanical strength. When this topcoat was applied, a second dry process (60 °C for 2 h) was necessary.

In all cases the minimal curing time after the application of the last coating layer was 7 days prior to any test. and the coating system thickness in the case of primed specimens.

Methodology

Anodic layer film thicknesses were determined employing a Fischerscope MMR with an ETA 3.3 sensor specific for non ferric materials. Prior to measuring, the equipment was calibrated on several locations of a bare piece of the corresponding aluminium alloy. The measures were taken at a minimum of 10 locations on the surface of the anodised or primed specimens.

4.4.1.2. Scanning electron microscopy

Objective