CHEMICAL PRE-TREATMENT
CHROMALISING
The term chromalising describes the application of a protective layer in order to protect aluminium, magnesium and zinc surfaces.
When is it necessary to chromalise?
After the degreasing and the degreasing and pickling of component parts respectively, a chromate coating should be applied to aluminium, magnesium and zinc surfaces in order to achieve the optimal protection against corrosion and sub-surface migration for the component part.
Which ways of chromalising are there?
There are various ways of chromalising. Here the two most common ways of chromalising are presented:
- Yellow chromalising is certainly the most commonly used kind of chromalising. The protective layer is created from an acid and chrome VI mixture.
- Green chromalising is the food-compatible kind of chromalising. Here the chrome layer is created from chrome III ions.
What is the advantage of chromalising?
Chromalising is an independent protection against corrosion. No further surface protection on top of this layer is required. Just think of the fittings of windows, which to a large part are only chromalised. A further layer such as the powder lacquering increases the resistance of the surface protection system even further. Here resistances of far more than 1,000 hours of salt spray tests are achieved easily.
What is the disadvantage of chromalising?
Chromate coatings contain chromic acid. In yellow chromate coatings chrome is present as reactive substance in hexavalent form. This chrome VI is poisonous for man and the environment.
What does the scrap car directive mean in relation to chromalising?
Due to the fact that chrome VI is poisonous, this substance was added to the EU scrap car directive 2000/53/EC. This directive was amended on the 27th of June 2002 with the directive 2002/525/EC. Therefore the directive now regulates that chrome VI may no longer be used in vehicles from the 1st of July 2007.
Therefore replacement substances must be found for chrome VI. Currently, nearly every chemical supplier provides a replacement for the chemical pre-treatment of some kind, however, as long as there is no uniform standard regarding the chromate replacement, as it is the case with the chromalising (chromalising according to DIN 50939), wage processors will not be able to provide an equivalent replacement for all customers.
Chrome VI is poisonous, but handling chrome VI is technically completely controllable. Thus, it is in the applicator's and supervisory authorities' hands to observe and enforce the applicable laws regarding handling chrome VI.
Various replacement substances for chrome VI, however, are still at the early stages of being researched regarding their potential risks for health and environment. However, a missing threshold does not mean that the substance is harmless.
Chromalising in the company Leistner
We are specialists of chromalising aluminium, magnesium and zinc-plated surfaces. To this end we have got three chromalising baths, which may be used simultaneously, if required. The average chromalising time is roughly 150 seconds. After chromalising the rinsing is in a triple cascade with desalinated water. Now the component parts are prepared for further surface treatments.
To the chapters
degreasing
pickling
zinc phosphating
CHEMICAL PRE-TREATMENT
