Problem with new set up.

[GramSlam]

Hello all,

I am having some problems with pitting and dissolution.

I have:
-25 gal tank (15"x12"x30") with air agitation.
-sulfuric concentration 4.8% by volume.
-cathodes are two lead 8"x8" plates on the ends of the tank.
-temp steady at 64°F
-power is sorensen DCS40-25 (40VDC @ 25A)

The parts are hung from a rack using caswell's titanium hanging wire
It seems as though the connections are holding well.

The parts are always more than 4" away from a cathode. They can be as far as 14" away if they are in the center of the tank.

For cleaning I use several stages:
1, hot water and dish detergent
2, aluminum rim cleaner in ultrasonic
rinse
3, SP degreaser
rinse
4, sodium hydroxide 15-30 sec
rinse
5, caswells desmut
rinse
7, Ultrasonic RO water.
rinse

Anodizing tank approx 90 min based on 4.5 amp density.

Parts total area is 100 in² and the current has been set at 3.1A


At this point, the only thing I can think of is agitation problems.
The air bars seem to move the fluid around plenty, but perhaps it is not enough?

Thanks!

[M_D]

The process looks ok. I wouldn't immediately suspect that agitation is the culprit. What is the part appearance that makes you believe it is dissolution? I don’t doubt that may be the problem, but am curious about the symptoms.

My first suspicion is the connections between the Ti wire and the parts are not as good as they might seem. How are you ensuring a tight connection? Also, if it is a single part that large (.694 square feet), and there is only one small Ti hanging wire and connection, that may be too small to carry enough current even with a good connection. (This is just a guess on my part, Fibergeek can probably say with relative certainty if that is correct or not). Also, the length of the wire would be a factor. It’s not a hard and conclusive way, but if any of the wires get noticeably warm it’s probably a good indication the wire capacity and/or connection(s) are inadequate.

Also, the connections to the cathodes need to be equally as good, and in my opinion the cathode connection should be made well above the fluid and checked often.

Do you know what the ending voltage is? If a connection is bad, and the power supply is a CC type it will compensate by raising the voltage. A higher than normal voltage is an indication of a bad circuit.

The tank temperature may be a few degrees too low to obtain the best dying qualities, but I don’t think that is a factor if your problem is dissolution, it should actually help fight dissolution.

[GramSlam]

Thanks for the reply M_D

There were 9 parts total. One of them was approx. 60in² and the rest are very small.

The one that was 60in² seems to have no problems, but some of the others have a very slight discoloration, and small white pits

The voltage at time 0:00:00 was 13.00 and 14.28 at PAR
Now, looking at those numbers I realize that I probably had the density at closer to 6amp/foot²

Anyhow, the small parts did take the dye, but there were several of them with the pitting, and slight blotching of the dye. The blotching of the dye is only slightly visible.

Thanks!

[GramSlam]

What I will try is anodizing at a current density of 3.5A ft²
And I will assume that the surface area is less than what I thought.
I will do my calculations for a surface area of 65 in² and run the current at 2.03 Amp's.

I will post the results in a few hours.

G

[Fibergeek]

Good choice with the DCS40-25, I am quite familiar with the Sorensen DCs series, excellent units.

I agree with M_D about an anode connection issue; pure titanium is 16X less conductive than copper, which makes it about 12X less conductive than aluminum, and this assumes the same wire size. Read the sticky The Importance of Good Connections at the top of this page.

I'll bet that the pieces that are pitted are the ones that had the best connections (not the worst). By Ohm's Law; getting the current to divide proportionally to surface area requires almost no voltage drops in the connections. This means you anodized the small pieces at much too high a current density, and the large piece at a lower current density than you think. If you were to monitor the current on the large piece and any of the small ones you will see this. This will totally defeat any PAR measurement all by itself.

You should be using the 720 Rule, M_D (or someone else) will show you how to use it.

Lose the titanium wire, use SOFT aluminum wire, and don't anodize the large piece with any of the small ones. Its OK to do the small pieces together. You can not just hang the parts from wire hooks and expect any sort of satisfactory results, this can work work in in electroplating but not in anodizing.

This applies to both titanium aluminum wire; if you can detect any temperature increase in the wire above room temperature, the wire is too small. If the problem is a bad connection the wire may not get hot at all, but the connection point to the work will. This is the most common cause of "burning".

M_D,
I'm going to step out of this thread, unless called back for another electrical issue.

[M_D]

I’m not an electrical engineer or mathematician, so I’m explaining all of this in layman’s terms. If there are any errors I would appreciate someone bringing them out.

The 720-rule is a way to determine the required current density and anodizing time to reach a certain anodize layer thickness. Since I don’t have a thick tester to verify how accurate it (the 720-rule) is, I just go by the fact that it works well for me. Fibergeek has a gauge and has reported the 720-rule seems to hold pretty true (within the range of what certain types of anodizing is capable of).

The 720-rule considers current density per square foot multiplied by the minutes in the anodizing tank as the two variables, which will determine the probable coating thickness. Under most conditions, 720 amp-minutes should yield about a .001” thick coating. That is, if you anodize at 6 amps for 120 minutes, the resulting coating should be about .001” thick. One thing I should note is all alloys do not build the thickness at the same rate, 2xxx and 7xxxx series for example do not achieve the build rate of 6xxxx series. I’m not sure is the 720 rule is specifically tailored for 6061, but I believe it is close.

If you desire a .0005” thick coating, you would reduce either the amps or minutes in the tank by ½, so you would anodize for 310 amp-minutes, and assuming a 6-amp current density you would anodize for 51.666 minutes. You can round the minutes off and never notice the difference; it’s not that critical for success.

To find the minutes needed to get .001” thickness when anodizing at a certain current density, such as 6 amps per square foot,:
720 multiplied by 1 = 720
720 divided by 6 (amps CD) = 120 (minutes anodize time)

To find the answer .00075” finished thickness:
720 multiplied by .75 = 540
540 divided by 6 (amps CD) = 90 (minutes anodize time)

To find the answer for .0005” finished thickness
720 multiplied by .5 = 310
310 divided by 6 (amps CD) = 51.666 (minutes anodize time)

If you use 4.5 amps current density:
To find the answer for .0005” finished thickness
720 multiplied by .5 = 310
310 divided by 4.5 (amps CD) = 68.888 (minutes anodize time)

If you use 10 amps current density:
To find the answer for .0005” finished thickness
720 multiplied by .5 = 310
310 divided by 10 (amps CD) = 31 (minutes anodize time)

Hopefully I didn’t confuse anyone badly. Just so nobody gets the wrong idea, you need to keep the current density (amps per square foot) in the range of what actually works with common anodizing setups, in this case LCD. In other words, to save time you can’t just use 720 amps current density for 1 minute and expect success.
Where you state you may have had the surface area wrong because the voltage figured out “too” high, that may or may not be correct. High resistance in the circuit could also make it look that way. It’s better to know reasonably close what the surface area is, so you can know what the actual current density is. That way there are fewer variables to account for, and it will make anodizing various sized parts fairly predictable.

[GramSlam]

Thanks M_D, Thanks Fibergeek for the helpful replies!\

I tested out the 720 rule and it worked great.
I did three peices 50in² each and had great results.

The only problem I had was one part. It was being dyed red, and the red did not seem to have the intensity that the other colors do.
That part was a red to black fade. The fade turned out well, but I would have liked the red a bit brighter...

I will keep testing the 720 rule, and insure I use aluminum hanging wire.
One thing I would like to know though. What guage of wire works best?
I am using aluminum grounding wire from radioshack. I think it's about 2.5mm in thickness

I was thinking about using aluminum "Electric fence" wire. They sell it in 10 pound spools for about $20.00
It's about 1mm in thickness. I think thats 1/32 - 1/16 in

The three parts I did were the yellow one, the red-black fade, and the blue.





The blue turned out the best.

[GramSlam]

sorry about the GIANT images...

[GramSlam]

Just one quick question/observation regarding the 720 rule.

I noticed that if you want to anodize 1ft² using the 720 rule, and want a thickness of 0.00075”
And a current density of 4.5 Amp/ft²

It would be this:
720 x 0.75 = 540
540 ÷ 4.5 = 120 min.
So, 120 minutes @ 4.5 Amp/ft²

That is 30 minutes longer than what the PAR method suggests. (4.5 Amp/ft² for 90 min)

Is that right? It would seem that you would go past par, and suffer dissolution.

I am starting to become a little bit confused at this point…

[M_D]

The grounding wire that Radio Shack sells is what I have used, until I started using commercial Titanium racking. I think that is adequate, and the smaller gauge wire probably is for parts up to a certain size. I just couldn’t tell you specifically what size though, Fibergeek or someone else with a more intimate knowledge of electrical subjects would be better prepared to outline the capacity. I feel pretty safe to say that if the wire can carry the current without a voltage drop, and has a very good connection, it should be ok. You can get aluminum wire from McMaster-Carr too.

The parts look good in the photo. Red has it’s own issues; it gave us the most trouble at first, now it’s no big deal. I had to adjust red the dye ph before having the best success. I’m just quoting from memory, but 5-6 ph is what I think Caswell said the dye manufacturer said the red ph should be. We use 5.8 as a target ph.

About the 720-rule versus par, you do not need a full .001” usually for decorative anodizing. And anodizing to par will not necessarily indicate a .001” thickness. So the fact that the times do not match is expected, although at times they might coincide.

When you push the anodize time to get a thicker coating, such as .001” or even .0075”, you may or may not have problems with excessive dissolution. It is possible to reach par, and beyond that the coating may or may not deteriorate for a while. Having a cooler tank temperature with good connections is one way to help ensure the thicker coatings will be ok. But you will only be able to get them so thick, without special methods and experience. That’s why everyone doesn’t do hard anodizing. It’s all about getting an acceptable balance for what you are trying to achieve, such as a good surface for dying and toughness.