Hi there and any tips for anodising cast Al

[Flex]

Hi there all,
Greetings from West Australia. Been playing around with anodising and recently discovered Caswell. I'm a keen Windsurfer and one aspect of windsurfing is about speed. To get speed you need flat water and to get that you either need a custom or specific landmass that allows the wind but no chop..or, you need weed. i.e. seaweed. Trouble is with seaweed it is highly abrasive and in some locations can destroy a $400+ carbon or fancy composite fin in hrs. Enter the aluminium windsurf fin. A fellow windsurfer has spent years making cast Al fins to some success. I anodised one of his fins in random fashion and it appeared to have much better wear resistance relative to its non anodised brothers. Anodising cast Al is of course problematic because of the porosity and other factors. Just putting it out there if anyone else has tried anodising cast Al. The cast is A356/7. Talking to Caswell Oz they reckon to polish as much as possible prior to anodise but experience seems to point to a 100 grit finish and LONG anodise....24hrs at 4.5 amps/sq ft density. This seems to fill defects. The ultimate aim is wear resistance but a secondary consideration is the dye. Not so much to look good but as indicator if the wear resistant layer still exists. I've taken a different approach to solve the problem and CNC the fin but would be very interesting if anyone in the community has had experience with cast Al anodising and if if extra long anodise time is beneficial or any other tips. Photos are of non dyed anodised fin and RIT dyed first attempt. Seems can only post 2 photos but have better result with turmeric dye.

[kcoffield]

No anodizing tips but for abrasion resistance, have you considered cast urethanes or urethane coatings? Urethanes can be very abrasion resistant and are used for core box tooling in the foundry industry and lining tubs in vibratory metal finishing tumblers the latter of which is an abrasion test in progress with the tumbler media. They are two-part thermosets and can be cast solid or applied as a coating. For the fin, I'd think a very high durometer (like the heal of a shoe) elastomeric urethane would be stiff enough but if not, it could be cast in silicone mold with a metal stiffening rib in place. Or, you could just coat your existing aluminum fin as they tend to be brushable and self leveling. Being elastomeric, it will have great impact resistance so resist chipping. Foundry supply companies can be a good source for small quantities. Here in the US, Freeman Manufacturing and Supply are such a source.

Best,
Kelly

[Flex]

thanks Kelly, Sorry for slow reply but thought no one was looking here....ya, will check that out to be sure but oddly my first CNC Al Fin just came off the assembly line and is just about ready for dunking. Because of the odd shape on fin I've had to make 5 custom tanks to dunk it in the Caswell chems. (Most of the fins don't fit into 20 litre buckets)

[KevinB]

Long anodize times can lead to thick coatings, but a thick coating does not necessarily make for good wear resistance. You can chew through 3 mils of a soft coating in a tenth of the time it takes to get though 0.5 mil of a good hard coating.

The longer you are in the bath, and the warming the bath is, the softer will the coating be. The only ways to reduce the immersion time is to reduce the coating thickness requirement (not an option) or increase you current density. Commercial shops typically run hard coat at 18-24 ASF. Running in this range needs be done between 28-40F to not burn. The lower temperature also slows the chemical process that attacks the coating making it soft.

Those numbers are just a rule of thumb. Castings take a bit more care because of the high silica content(assuming they are sand casting).
Never tried wind surfing, but I'll bet it's super fun.
GL
Kevin