Sulphuric Acid Anodising
Sulphuric acid anodising is the most common protective treatment
for aluminium and leaves a clear (“natural”) silver
coloured finish, which can be subsequently dyed. The anodising
process involves the oxidation of the aluminium surface to
form a thin layer of aluminium oxide under controlled chemical
conditions – this process occurs naturally at a much
slower rate for aluminium exposed to oxygen in the air. As
anodising does not involve the coating of the aluminium with
a different metal (or other man made coating), the process
will not hide any surface imperfections or blemishes in the
aluminium substrate. In fact, the vigorous nature of the process
will generally degrade the surface finish of the aluminium
– this should be taken into account when considering
the quality of the surface material to be treated if a highly
decorative finish is required.
Anodising is an electrolytic process which requires an electrical
contact point at all times during the treatment – this
means that a very small set of jig marks will always appear
after treatment. It is important that we are made aware of
any specific jigging requirements where certain surfaces must
be free from jig marks for decorative purposes or for critical
bores and sealing surfaces.
Sulphuric
acid anodising is generally used where a decorative and corrosion
resistant finish is required. The resulting anodic film is
usually between 5 and 20 microns thick and is electrically
insulating. We typically treat components in all industry
areas with special emphasis in the electronics, motorsport,
aerospace, medical and general engineering sectors.
Materials
Sulphuric acid anodising is available across all commercially
available alloys and castings. The most common alloys treated
by Metro are 2000, 6000 and 7000 series. Due to the differing
levels of metallic elements used in the various alloys, the
anodised finish can vary significantly and it is advisable
to discuss the choice of alloy with us at the design stage
for new components. We have particular expertise in the handling
of 2000 series (copper rich) alloys such as 2014A and 2024
where particular care is needed to avoid burning or melting
of the alloy during the anodising process.
Castings
We can carry out sulphuric acid anodising but recommend that
suitability should be discussed beforehand.
Maximum
Size Criteria
The table below outlines the maximum size criteria for sulphuric
acid anodising:
Dimension |
Clear |
Black* |
Length |
2100mm |
2100mm |
Width |
750mm |
750mm |
Depth |
800mm |
800mm |
*Dimensions for all other colours should
be discussed specifically as our dye baths vary in size.
Coating
Thickness
Sulphuric acid anodising provides a coating of between 5-25µm
with a thicker coating giving a greater corrosion resistance.
Where colour dyeing is required a surface coating of 15-25µm
is recommended.
Available
Standards
We predominantly work to 3 specific standards as detailed
below although we are happy to work to any specification provided.
PTFE Coating
Anodised parts can be treated by immersion into a highly concentrated PTFE dispersion. This process will produce an abrasion resistant coating with a high degree of lubricity.
Sealing
Five seals are available for application after Sulphuric acid
anodising:
- Hot
Nickel Acetate Seal - for greater corrosion protection
- Hot
De-mineralised Water Seal
- Dichromate
Seal (yellow-green finish) - for high corrosion protection
in salt water environments
- Cold
Seal
- 3
Stage Sealing for outdoor use
Surface
Conditioning
We offer a range of surface conditioning options including:
- Pre-treatment/Etching
- to reduce surface scratches
- Satin
Finish - achieved through increased etching
- Masking
- available to customer requirements
- Bead Blasting - to cover machining lines and for aesthetic purposes
- Chemically Brightening - Increase the gloss level of the part, often used in unison with bead blasting
Jigs
and Component Preparation
Jigs are provided for mounting and dipping
components. It is recommended that good jigging features are
added to components at the design phase as contact marks will
not receive a surface coating and can result in visible blemishes.
It is also recommended that welding on components is minimised
and that all non-aluminium parts, such as steel inserts, are
removed before anodising. Masking can be carried out for holes
and threaded sections.
Colour
Dyes
Colour options are available with sulphuric acid anodising
for an additional charge. We use the selected colours below
from the Clariant dye range although other colours are available
for large orders:
- Gold
- Yellow
- Aluminium Orange
- Aluminium Bronze
- Pink
- Aluminium Fiery Red
- Aluminium Bordeaux
- Aluminium Violet
- Blue
- Aluminium Green
- Aluminium Grey
- Deep Black
- Olive Drab (for military applications)
