Before aluminium is anodised, it usually goes through a caustic etch. The etch removes the natural oxide layer, light scratches and contamination, and gives the surface a uniform matte finish. Done well, it is the foundation of a good anodised finish. Done badly, it causes pitting, uneven texture and wasted metal.

What the etch does

Caustic soda dissolves aluminium, forming sodium aluminate and releasing hydrogen gas. By controlling time, temperature and concentration, a thin, even layer of metal is removed from the surface.

Typical conditions

• Concentration: commonly around 5 to 10% NaOH (roughly 50 to 100 g per litre).
• Temperature: often around 50 to 60 °C.
• Time: from under a minute to several minutes, depending on the finish wanted.
• Additives: many shops use etch additives that keep aluminium in solution and give a smoother finish.

Follow your process specification; these are only typical ranges.

The full pre-treatment sequence

1. Degrease in a mild alkaline cleaner.
2. Rinse.
3. Caustic etch.
4. Rinse.
5. Desmut in an acid bath to remove the dark residue of alloying elements left by the etch.
6. Rinse, then anodise.

Controlling the etch bath

• Analyse free caustic and dissolved aluminium regularly. As aluminium builds up, etch rate changes.
• Top up with caustic soda to keep free NaOH within range.
• Partially dump and replace the bath when dissolved aluminium gets too high, to avoid hard scale on tanks and heaters.
• Use good quality caustic: low chloride and iron help avoid pitting and staining.

Safety

The etch releases hydrogen, so ventilation is essential and ignition sources must be kept away. The hot caustic bath causes severe burns; wear face shields, gloves and aprons, and add caustic to water slowly when making up or topping up.

Supply in Oman

For finishing shops in Oman, caustic soda flakes 99% are available from ready stock in Barka with next-day delivery within Oman.