The fourth stage of anodisation involves dyeing the newly formed oxide layer which can be done using an electrolytic or adsorption process.
In a nutshell:
• Anodised aluminium can be coloured using electrolytic or adsorption methods.• Electrolytic colouring uses metal salts for a durable, UV-stable finish but offers limited metallic colours (bronze, gold, silver).
• Electrolytic deposits pigments deep in the oxide layer; adsorption focuses on the surface/middle.
• Adsorption uses organic dyes for wide colour options but less UV stability.
Following anodisation, aluminium develops a porous and absorbent oxide layer that can be dyed using various methods, such as electrolytic colouring or dye adsorption. Each option uses a different process that yields different results, both of which possess pros and cons.
Electrolytic
Electrolytic colouring is achieved with non-organic pigments such as metal salts or metal oxide. This process involves placing the anodised aluminium in an acidic solution containing metal salts and an alternating current. The metal ions are deposited deep into the porous oxide layer, providing a durable finish with excellent resistance. However, using metal pigments means that the colour range is limited to metallic colours such as bronze, gold and silver.
Adsorption
The adsorption process is relatively simple as it involves submerging the anodised aluminium into a dye solution for a short period. The dye is distributed across the oxide layer however remains predominately in the middle and surface area of the anodic pores.
The use of organic pigments such as chemical dyes for adsorption provides greater colour flexibility than non-organic pigments. Although, despite its simple process and wide colour availability, the downside of dyeing with organic pigments is that the anodic layer has reduced UV stability compared to non-organic dyes.