Anodising

The anodising of metals is an electrolytic surface oxidation process which enhances the natural aptitude for the metal to oxidise; coatings can be 1000 times thicker than the natural layer Aluminium is the most important material to be anodised, with alumina (Al₂O₃) formed at the surface: magnesium, titanium, tantalum and niobium are also anodised in smaller quantities [3, CETS, 2002, 45, France, 2003] [118, ESTAL, 2003].

Aluminium is normally (90 % of cases) anodised in sulphuric acid electrolyte. For special applications, aluminium may be anodised in many different types of process solution:

phosphoric acid, sulphuric/oxalic acids, sulphuric/salicylic acids and chromic acid electrolytes.

[73, BSTSA, ].

A wide variety of current forms and wave shapes may be used for the process, depending on the solution selected and the purpose of the anodic film. Direct current (DC), alternating current (AC), and DC with superimposed AC. are all used in various processes.

The workpiece or substrate to be treated is made anodic. During the anodising process the negatively charged anion migrates to the anode where it is discharged with a loss of one or more electrons. The metal reacts with the oxygen of the anion and a layer of oxide forms on the surface.

Figure 2.9 shows examples of decorative anodising options and Figure 2.10 shows a typical anodising line layout.

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PT/EIPPCB/STM_BREF_FINAL September 2005 63 Figure 2.9: Example of decorative anodising process options

[45, France, 2003] Note: Rd means rinsing ratio

The alumina coating is sealed to improve corrosion resistance and retain any surface colouring (see Section 2.5.14). Sealing is described in Section 2.5.15.

Overall environmental considerations for all anodising processes

Anodised aluminium (coloured or not) is very easily recycled for metal recovery, i.e. anodising processes have no significant effect on the recovery of aluminium metal, and other metals are not added.

Typical emission levels after typical waste water treatment are given in Section 3.3.1

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64 September 2005 PT/EIPPCB/STM_BREF_FINAL 2.5.13.1 Sulphuric acid anodising of aluminium

On most aluminium alloys a colourless, transparent aluminium oxide is formed, but alloys containing high quantities of elements such as iron, manganese, silicium and others [118, ESTAL, 2003] tend to give greyish or brownish coloured layers. The anodising voltage is in the range of 17 – 22 V and the temperature of the electrolyte. is generally 20 °C ± 5 °C and electrolyte concentration is generally 190 g/l ± 40 g/l H₂SO₄. A film thickness of 5 - 30 µm is used for decorative and protective sulphuric acid anodising. The working parameters may vary according to the application, especially in architectural anodising. Sulphuric acid anodised coatings are often coloured by special colouring processes. (See Figure 2.10). The oxide coating is sealed to obtain improved corrosion resistance (see Section 2.5.15).

In hard anodising, a film thickness of 25 – 250 µm is obtained. The electrolyte is operated at -below 5 °C [73, BSTSA, ]. Processes with oxalic acid and sulphuric acid with organic additions can also be used for hard anodising. The hard anodising layer is normally not sealed. In this state it can be impregnated with lubricants.

Environmental considerations

Acid concentrations and operating temperatures are low. If necessary, aerosols can be minimised by a layer of foam. [118, ESTAL, 2003]. However, the suppressants may be based on PFOS, which is toxic and persistent see Annex 8.2, [109, DEFRA, 2004], although alternatives to PFOS exist and are used successfully [159, TWG, 2004].

Effluent may be treated in typical waste water treatment plants.

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PT/EIPPCB/STM_BREF_FINAL September 2005 65 Figure 2.10: Typical anodising plant layout

2.5.13.2 Chromic acid anodising of aluminium

This forms an aluminium oxide film on most aluminium alloys ranging from a light to dark grey. The process is mainly used for aerospace and military applications. Parts can be treated after joining since residual chromic acid in the joins does not cause corrosion and the process gives very limited degradation of fatigue properties of the materials [73, BSTSA, ].

The electrolyte contains 30 – 100 g of chromic acid per litre. The temperature is kept within the range of 38 - 42 °C for most aluminium alloys. The voltage cycle used depends on the alloy treated and must be closely followed to avoid etching. The coating is thin with an average of 2 - 5 µm and is relatively soft. Its corrosion resistance is good and the coating is normally not sealed.

The regeneration of anodising baths using a retardation process extends the electrolyte life by reducing its aluminium content.

Environmental considerations

Effluent may be treated in typical waste water plants.

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66 September 2005 PT/EIPPCB/STM_BREF_FINAL Process tanks may to be equipped with fume extraction to remove generated acid aerosols and

gases. Section 1.4.4.1includes a discussion of the health effects of chromium and Section 2.5.3 for environmental considerations for hexavalent chromium solutions. [118, ESTAL, 2003],.

[116, Czech-Republic, 2003]

2.5.13.3 Anodising of magnesium, titanium, tantalum and niobium

[73, BSTSA, ] Anodising of magnesium gives the best protection for abrasive wear. The coatings are relatively porous and contain crystalline phases like Mg(OH)₂ and MgO. The oxide layer can be 25 µm or more.

Anodising of the DOW-17 type is dark grey or green and has very good adhesion. Alternating current is preferable. The coating has an attractive appearance and is well suited for being coated by for example a transparent paint. The oxide layer is soluble in the acid electrolyte meaning that the deposition rate decreases until a steady state condition is reached. Thorough rinsing is required and if the parts are not to be painted a sealing in 50 g/l Na₂Si₄O₉ at 93 - 100 °C for 15 minutes can be performed.

Anodising of the HAE type gives the hardest oxide and is well suited for protecting against abrasion. It has very good adhesion and gives good protection against corrosion. The coating is from light to dark brown, depending on alloy and thickness and the best properties are achieved by using alternating current. Dipping in a Na₂Cr₂O₇/NH₄HF₂ solution at 20 - 30 °C for 40 - 60 seconds can seal the coating.

Environmental considerations

High temperature, toxic and aggressive electrolyte (for DOW-17) require good ventilation.

2.5.13.4 Phosphoric acid anodising

Phosphoric acid anodising is being increasingly used as a preparative treatment for subsequent applications of organic or plated coatings. Phosphoric acid anodising produces a relatively thin, porous oxide that is used for bonding, painting, plating and printing applications. [132, Sheasby and Pinner, 2002].

This process has been originally developed by the aerospace industry and is presently used mainly as a pretreatment for preparing adhesively bonding aluminium structures in the aerospace industry, for painting aluminium in architectural applications and for the printing industry.

Phosphoric acid anodising produces a porous, hydration-resistant oxide and increases fracture toughness. The anodic coatings resulting from phosphoric acid anodising have a high porosity compared to those produced with other processes.