Chromic anodizing
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
Chromium anodizing is usually carried out in a bath containing 30 to 50 g/1 of chromic anhydride (CrO3), at a temperature of 40 to 50°C. The process is carried out according to stress cycles such as the Bengough-Stuart cycle. Chromic anodizing forms very thin layers, 2 to 5 μm, with a swelling equivalent to 33% of the film thickness.
This treatment is therefore suitable for parts with very tight dimensional tolerances. It is widely used in the aerospace industry for its intrinsic anti-corrosion properties, its ability to adhere to paint films, its moderate influence on fatigue characteristics and the revelation of the metallurgical structure of the underlying alloy: grain, roughness, heterogeneity related to heat treatment (heterogeneous quenching) or to machining (burning). With an appropriate range, it can detect resistive defects. It also has the advantage of not attacking the metal in case of product retention on parts with complex geometry, assembled or molded. The oxide can be sealed to increase corrosion resistance. Without sealing, it provides a good base for paint and adhesive adhesives, but has a low intrinsic corrosion resistance. An "optimized sealing" process is available to achieve good corrosion resistance and adhesion of paint systems. Coloring of the oxide layer is possible but is usually limited to black.
ASPECT: opaque, more or less dark depending on the alloy
THICKNESS: 2 to 7 μm
MICRODURITY: 200/250 Hv0.05
SURFACE CONDITION: no degradation
ELECTRICAL INSULATION: 300 to 500 volts depending on the alloy COEF OF COLDING: good under low load
FATIGUE DECLINE: 20 to 30% in alternating tension depending on the alloy grade
CORROSION RESISTANCE: (depending on the alloy grade):
- without clogging: ~ 100 h (ISO 9227 NSS)
- nickel salt clogging: ~ 300 h
- chromium VI salt clogging: ~ 750 h
SUBSTRATE:
All grades of alloys and for all processing methods.
However, the alloy has a strong influence on the performance level obtained and the appearance.
VARIANTS:
Use of baths consisting of a mixture of chromic anhydride and organic acids.
Variations of tension cycles.
APPLICATIONS :
Corrosion resistance.
Bonding base for organic coatings (paints, glues...)
Detection of defects such as cracks or folds in the metal.
Applications : aeronautics, mechanical applications...
SELECTION CRITERIA - LIMITATIONS :
Chromic coatings have a better coefficient of friction than sulfuric coatings, but less abrasion resistance.
ENVIRONMENTAL IMPACT:
The use of hexavalent chromium, toxic and carcinogenic, in surface treatments is tending to disappear. New regulations require the development of new anodizing processes.
REFERENCE:
EN 2101 : aeronautical sector
EN 12 373-2 /17 : methods for measuring the anodic layer
ISO 9227-NSS : neutral salt spray test
EN 12 373-18 : Pitting corrosion rating system
Implementation
If you are a company interested in being listed on this page, please contact us: a3ts@a3ts.org
