Safer Hard Anodizing of Aluminum

After experimenting with the anodization of aluminum using different acids, I wanted to see if any of them can be used for hard anodizing.

The NaHSO4 seemed to be a good candidate.

Most sources specify that hard anodizing gets done in a relatively cold bath.

For experiment 1, I cooled the bath before starting and dropped some ice cubes in. This was going to dilute the bath over time, but it seemed ok for a first try.

200 ml water at 0C

100 ml of water as ice

30g of NaHSO4

0.4A constant current setpoint

target time 45 mins.

15:34 : start 31V(max of supply); 0.29A rising

15:36 : 0.4A 29.3V falling fast

15:41 : 0.4A 28.6V

15:52 : 0.4A 27.0V

16:06 : 0.4A 25.5V; ice almost completely melted. Some of the NaHSO4 powder is still undissolved at the bottom

16:22 : 0.4A 23.5V; stop, rinse, submerse in dye

17:56 : remove from dye; bad absorption. Surface extremely hard & black where the undissolved powder pooled!

So did the pooling of NaHSO4 at the bottom cause extra high concentration and thereby assist with the hard anodizing?

Experiment 2. Higher concentration anodizing without cooling.

300 ml water

60g of NaHSO4. Leaving muc of it undissolved at bottom

0.4A constant current setpoint

19:20 : start. 0.4A V increasing rapidly

19:21 : 0.4A 14.9V

20:!7 : stop; 13.3V 0.4A rinse, submerge in dye. The area at the bottom where undissolved powder was present does look different than the rest of the sample. However the whole sample takes dye immediately upon submerging, indicating very porous anodization. Not very hard. Temperature seems key.

For experiment 3, I put the plating bath into an outer ice bath.

300 ml water cooled in fridge

30g of NaHSO4

1.0A constant current setpoint

08:41 : start. 31V 0.35A; current limit set to 1A

08:46 : 29.3V 1.0A

09:32 : stop, rinse, dye. Takes the dye ok, not great. Black stripe is at water level.

The result was a very very hard surface. The sample from experiment 1 had a black line at the bottom. That black area was very hard to scratch. The sample from experiment 3 had the black line towards the top of the bath, and that area was equally hard. But the whole sample was much, much harder to scratch than previously anodized and dyed samples.

So hard anodizing can definitely be done using NaHSO4. Much like with sulfuric acid baths, cooling is essential for good results.

I ordered a small aquarium pump and some tubing to try to get a better handle on bath temperature. More to come...


The pump and tubing came in, so I set up a cooling loop to try to keep the anodizing solution as close to 0C as I can.

Then, I used 300 ml of hot filtered water to dissolve 60 g of NaHSO4. To that I added another 300 ml of cold filtered water. I then waited for the anodizing bath to cool down.

11:14 : 23C; dropped one ice cube into bath to help things along.

11:26 : 16C

11:35 : 13C

11:46 : 10C

11:55 : 9C

12:00 : 8C; start 0.4A; 'fluffy cloud' builds up from the cathode. Was it a mistake to leave the samples in without current?

12:17 : 8C

12:29 : 8C

13:00 : stop; 8C; scratch test shows pretty hard coat, but no discoloration.

15:00 : restart cooling cycle.16C

15:15 : 9C

15:35 : 7C; put anode back in. continue at 0.4A; This time there is no 'fluffiness'

15:45 : 8C

16:15 : 8C

16:48 : 9C

17:12 : 8C

17:44 : 9C

18:48 : 9C

20:30 : 9C; stop; coating now dark brownish gold..

Next Day:

08:41 : restart cooling cycle. 21C

09:18 : 8C; put anode back; continue at 0.4A

10:17 : 8C

11:10 : 8C

11:55 : 8C

12:45 : 9C

12:55 : 9C stop; The sample sat a little higher this time and the level is visible. The sample continued to get darker in the bath, though a bit unevenly.

2 Days later

8:40 : 8C; put sample in bath; continue; hardly any current; 31.5V, 0.02A

9:40 : 6C; connected second power supply in series. Even at 62V only 0.003A stop.

New sample.

9:42 : 6C; set for 1A; 0.2A at 31.5V; Add second power supply. Set 1.0A CC; needs about 36V

9:46 : 8C; 32.6V, 1A; a black coat seems to be growing down from the water line

09:53 : 11C

10:00 : 12C; 31.5V

10:35 : 13C; 29.4V

11:19 : 14C; 28.0V; black deposit seems to slowly grow

12:11 : 13C, 28.1V; then added 1 ice cube to solution to replenish evaporated water and help with temp.

12:46 : off. leave sample in bath.

16:47 : continue. cooled bath before powering on. 5C,32.2V

17:28 : 7C Looks like the black spots are all gold now.

19:10 : 10C, 29.1V

19:45 : 8C, 34.2V

20:37 : 11C, 46.7V

20:42 : 11C, 35.9V

21:00 : 11C, 41.6V

21:05 : stop

higher voltages tend to correspond to the times I dumped more ice in the cooling bath. The thermocouple is in the center of the plating bath, so there will be a lag at least...

After washing off the sample, bubbles kept coming off of it at the separation of two of the gray areas.

The new sample is all gray to black. The darker black areas do seem very hard, but the gray areas are not any harder than just bare aluminum. In any case the gold sample is FAR harder to scratch than the gray sample

Part of the coating is flaking off. It looks like the bubbles came out from between the layers.

I suspect temperature to be the overriding issue here, with the higher current density mainly serving to heat the sample. After removing a kink from the tubing, it looks like I can maintain the temp at 6C without current, so I will put the current setting to 0.3A and see what happens.

New experiment

09:13 : start. 6C 0.3A. Voltage rises to 36V over a few seconds, then starts dropping very slowly to 33.5V.

09:28 : 6C 31.6V

09:45 : 5C 31.5V

09:48 : stop electric power.

11:15 : 4C. continue. 0.3A 31.8V

11:40 : 6C 30.9V

12:06 : 4C 30.8V

12:49 : 4C 29.6V

13:20 : 6C 28.2V

14:15 : 6C 27.3V

15:37 : 6C 26.5V

20:02 : 6C 25.1V

21:15 : 7C 24.6V; stop

The color is not uniform, but the sample is uniformly very hard. Still, again, the gold area is far harder to scratch.

I think that the water bath was not temperature controlled very effectively. Also, with a hull-cell like configuration, the current density part of the results would be more obvious.

So, I am taking a step back and perform NaHSO4 anodizing with a hull-cell approach to see if I can get some feedback for current density with a room temperature bath. The next cooling experiment will use a different cooling setup. I am not very fond of the plastic tube coil in the previous setup. I don't think it was a very effective heat exchanger.

Reusing the solution from the hard anodizing experiments.

09:21 : start. 22C. Set to 0.4A

09:39 : 25C 28.1V

09:44 : 25C 27.7V ; a black line has formed along the water line.

10:09 : 27C 25.2V

10:25 : 28C 24.0V. stop. sample is gray, just like one would expect from non-hard anodizing. Dye by agitating in dye for a few seconds.

There is no obvious gradient. The process seems to be self-regulating because of the insulating effect of the oxide to a large extent...

The black line around the water level is not hard anodization. I scribed a scratch across it and looked at the result with a microscope. That line is 'fluffy', probably containing air bubbles. In some areas, it can be made to come off...

Result: when 6061-T6 alloy can be hard anodized with NaHSO4. The color is a matte gold, darkening as the layer grows.

To get a better handle on the bath temperature. I obtained a stainless steel coil to replace the rubber hose. I give it a thin coat of clear lacquer to make it non-conductive. I then placed the entire bath on a stir plate, so that I can agitate the solution as well.

I used my last 56g of NaHSO4 together with 1000 ml of filtered water.

09:08 : start cooling cycle T=18C

09:17 : 10C

09:45 : 6C

10:19 : 6C moved pump.

10:42 : 6C. power on. 0.6A

10:48 : 31.6V 0.17A

11:04 : 31.6V 0.24A 7C

12:24 : 9C 0.51A

12:50 : 8C 0.57A

14:16 : 11C; added ice; 0.58A 28.5V

15:37 : 11C; added ice; 0.57A 27.9V

17:35 : 12C; stop.

The sample had started to darken from the top down:

Regardless of color, the sample is extremely hard all over. Looks like with just a little more patience, this would have resulted in a dark gray hard anodizing...