Alkali resistance

Enhanced protection for anodised aluminium through combination process

Combined process of Alfiseal 982/5 and Alfiseal 961


The proportion of aluminium implemented in car manufacturing is increasing rapidly. Besides applications in chassis and component construction, this sought-after metal is now also being more frequently implemented in the field of decorative car design.

The appropriate protection is ensured by forming an artificial, porous oxide layer by means of an anodising (electrolytic oxidation of aluminium – Eloxal) process. Once the final sealing of the porous oxide layer has been carried out, the material is highly resistant to natural environmental influences such as rain, sunshine, dirt, etc. However, where conditions prevail that attack the oxide layer, this protective effect is eliminated and signs of corrosion will appear. The consequence is an irreversible dulling of the once bright aluminium surface. This is due to the amphoteric character of aluminium oxide which causes solubility both in acids and in alkaline solutions. This effect can also occur, e.g. in car wash facilities, where alkaline cleaning agents (with a pH-value > 10) are in use. Car wash installations of this type are common, e.g. in the USA.    

Requirements of the automotive industry

The automotive industry has responded to these influences by specifying in their standards that anodised and sealed components must feature a certain resistance to alkalis.

Achieving the goal by simple means?

For suppliers to the automotive industry, this requirement entails considerable effort because the specified resistance to alkalis cannot be achieved by way of simple hot-sealing during the anodising process.

Alufinish has set itself the goal of fulfilling the demands for alkali resistance in the anodising process by optimising the sealing process itself. The product combination Alfiseal 982/5 (cold sealing) with Alfiseal 961 (hot sealing) is tailored to this requirement.


Convenient handling

Long retention time (cold sealing) 

High process reliability thanks to a constant and high quality level

No separate addition of fluoride to maintain bath activity

High degree of protection by a simple combined sealing process

Can be analysed completely

Avoid visible immersion limits on trims after alkali resistance test

The following bath sequence and parameters are recommended as the ideal pretreatment:

combination of Alfiseal 982-5 + Alfiseal 961


Alfiseal 982/5

Alfiseal 961

fluoride content

1.5 - 1.8 g/l nickel
28 - 32°C
1.5 minutes/µm
6.3 - 6.6
500 - 600 mg/l

8 - 12 ml/l
at least 96°C
3 minutes/µm
6.3 - 6.5

Alfiseal 982/5 is a cold sealing product containing nickel-fluoride which is specifically intended for large throughputs for long retention times while keeping continuous quality and enabling high process reliability. After cold sealing, rinsing is carried out in fully demineralised water prior to after-treatment using Alfiseal 961, a special hot-sealing product containing nickel. 

In practical testing, this combination met the requirements specified by VW TL 212 (10 min., pH 12.5) and Volvo test (16 min., pH 12.5 + ACT test), without leaving any visible immersion marks on the test material. Further improved alkali resistance is also possible. On this occasion umpteen additional demands like corrosion tests, varying climates and others requested by the automotive industry must be considered.