There are four types of sealing that are in common use for sulphuric anodising and hard anodising:
- Hot water sealing
- Dichromate sealing (sodium dichromate or potassium dichromate - the sodium/potassium ions don't have an impact on performance, the dichromate ion provides the functional benefit).
- Nickel acetate sealing (cobalt acetate sealing is very similar, but not in common use, at least in the UK)
- Cold sealing
The first three are in use for marine purposes and worth discussing. Coldsealing is best avoided for marine use. All of the first three seal by the same mechanism of action (although the nickel acetate sealing is dual action), which is by hydrating the aluminium oxide, causing it to expand and close the pores. The nickel acetate has a second sealing mechanism as well, it forms a nickel hydroxide plug in the pore. Dichromate sealing does not enhance the sealing itself, but the dichromate ions are a corrosion inhibitor and become sealed in the coating which improves corrosion resistance significantly - particularly on high copper aluminium alloys (2000 series, which can be problematic in terms of corrosion resistance).
Your choice is really between hot water sealing and nickel acetate sealing. All hexavalent chromium compounds, including dichromates are becoming increasingly restricted (the REACH and RoHS regulations particularly) and as such we wouldn't recommend them on new products outside the aerospace/defence industries and even in those cases we would discourage their use. Duplex seals with both sodium dichromate and nickel acetate provide the best corrosion resistance, but the same concerns above apply.
As it proceeds by two separate mechanisms, nickel acetate sealing should provide superior performance. However both nickel acetate sealing and boiling water sealing provide very good corrosion resistance. The industry standard benchmark neutral salt spray exposures of 336 hours (which we regularly test on 2024-T3 aluminium), do not show a difference between the two and they both reliably pass without issue. For longer exposures we would expect nickel acetate to provide superior performance, but this is not routinely tested.
Please note, the references in this article are to the nickel acetate seal that we use, which is a high temperature seal (96°C and above, like boiling water sealing). There are other nickel acetate seals on the market that operate at lower temperatures - these may not benefit from the dual sealing mechanisms.
There is no significant difference in cost, turnaround time or other quality for either the hot water or nickel acetate sealing.
Where appropriate, corrosion resistance of anodising can be further enhanced by applying a suitable paint scheme. If applying a paint scheme on top of the anodising is practical and economical, this will yield the longest product lifespans.
Disclaimer
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.