Tartaric sulphuric acid anodising coatings normally average at 5-7µm and aim to provide a porous film, which can be used for a variety of applications:
- Unsealed porous coating used for adhesive bonding purposes
- Unsealed porous coating used as base for adhesion of paint systems
- Sealed coatings, for corrosion resistance while minimising fatigue risk.
- Low thickness coating for close dimensioned parts.
TSA is one of the alternatives to chromic anodising, which is increasingly restricted due to the hazards associated with chromium (VI). The TSA coating and process is safer for workers and the environment - it is free from REACH SVHCs. TSA coatings have properties comparable to that of chromic anodising which has helped the industry make the switch to this cleaner alternative. Another general purpose alternative to chromic anodising is thin-film sulphuric anodising.
Specifications we can currently offer
- Commercial basis
Other specifications may be available on request.
Painting and Bonding
TSA may be favoured in certain applications over standard sulphuric anodising due to the pore structure of the coating and low thickness, making it ideal for applications requiring adhesion - such as painting or bonding.
We can provide a “one stop shop” for TSA followed by priming/painting.
Best results for corrosion resistance on TSA, according to our testing, can be found on coatings sealed using a boiling water seal system. Sealing or overcoating with paint is essential for corrosion resistance. A boiling water sealed tartaric-sulphuric coating, can be expected to achieve a minimum of 336 hours of neutral salt spray resistance to ASTM B117 angled at 6°.
Where corrosion resistance is the main design goal, standard sulphuric anodising should be considered instead.
As with all anodising, the composition of the alloy impacts corrosion resistance. Corrosion resistance is tested on 2000 series (aluminium alloyed primarily with copper) as these are most susceptible to corrosion.
Information for Designers
Seals available for this coating:
- Boiling water seal (recommended)
- Surtec 650 seal
- Nickel acetate seal
- Duplex seal
Parts can be masked in the tartaric sulphuric anodising process for selective finishing. Our process has been successfully tested with tape and lacquer maskants.
In our internal trials, we have not encountered any particularly problematic aluminium alloys due to the low coating thickness. For best results, see our article about alloy selection for anodising.
It is still important as always to observe best practice in protecting bare surfaces prior to anodising.
TSA is generally not used for dyed coatings. Standard, sulphuric acid anodising should be preferred for dyed coatings.
Replacing Chromic Anodising
TSA is one of the alternatives to chromic anodising. It is not a “drop in” replacement for chromic anodising, but in many circumstances could be used without re-designing parts. Its properties are comparable in terms of corrosion resistance, adhesion promoting and fatigue behaviour. While the coating is thin, it is not the same as chromic anodising, for close tolerance parts this may need to be taken into account.
The industry considers it the closest alternative to chromic anodising currently available, however designers should recognise that for decades chromic anodising was the default choice in some industries and the specification of chromic anodising on a drawing may simply be inertia rather than a considered choice - strongly consider adapting designs to make them suitable for standard sulphuric anodising instead and benefit from a process that is very well characterised/understood, low cost and widely available.
Performance may not be comparable on castings, especially where there is material porosity or rough surfaces.
Parts that may entrap processing solutions should be re-designed, the TSA process is less tolerant to this than chromic anodising and processing residues may impact performance.
RoHS and REACH Compliance
All of our tartaric sulphuric acid anodising is RoHS compliant. No SVHCs are used in the process or present above the threshold in the final article.