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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Home anodizing-Truths from the experts?
August 21, 2021
Q. Hello, my name is Chris and I am trying to de-smut, anodize, and dye a 6061-T6 aluminum part. Originally the part came to me bead blasted so it had a nice matte sheen. I electro etched my logo onto the part. Before etching I used alkyd paint as a resist and etched my design. To clean off the paint I used acetone
⇦ this on
eBay
or
Amazon
[affil links] Warning! highly Flammable!
, Brake Parts cleaner, and then soaked the piece in Oxyclean and very hot water to get to the hard to reach places. (this seems to have been a bad thing since the piece turned very dark grey) I have some aluminum cleaner that I used to clean up the piece but think I may still need to desmut before anodizing.
1) Is there a fairly simple home made desmutting solution that I can use?
2) I'm using a 12v battery charger that is selectable to either 2 or 15 amps. I tried to use the 720 rule calculator but am still not sure the best way to use it for my piece.
- Surface area= 96 sq inches or 0.67 sq ft
- I'm trying to reach 1 mil thickness
- 12 volts at either 2 or 15 amps.
- I'll attach a pic of the charger
What amps do I use (2 or 12) and for how long to reach 1 mil?
3) I have some scrap aluminum square tubing to practice with. Can I reuse the same battery acid ⇦ this on eBay or Amazon [affil links] solution for the practice pieces and the finished piece also?
Chris Contreras- Holly Ridge North Carolina
⇩ Related postings, oldest first ⇩
Q. I am attempting to anodize some small aluminum paintball parts at home. I have found several "how to" articles, but they seem to have conflicting information on certain issues. I have questions that I hope someone can help me with.
1. What is the correct amount of battery acid to distilled water for the bath? (I have read 50/50, 10%, and 2/5 ratio)
2. I have a 20 amp HAM radio 12v power supply, and a 2/12 amp battery charger
⇦ this on
eBay or
Amazon [affil links]
. Which would be a better power source?
3. What is a good paddle design for the negative lead? (I have read: A circle of aluminum wire covered with aluminum foil, wire mounted vertically in the tank, a flat piece of aluminum that runs the circumference of the bottom of the tank.
I am having problems sorting all this out, so I figured I would ask the experts!
Thanks,
Wayne Newberry- Clintwood, Virginia
2002
A. Hi Wayne.
1). The correct sulfuric acid concentration is about 15% by weight. I don't remember the standard concentration of "battery acid" as your starting point; but it's nowhere near 100%. So 10 percent battery acid would be way way low; probably 50/50 is more like it.
2). I was thinking of getting into paint balling myself. I have an old .22 caliber pistol and I also have one of those ping pong ball toy shotguns where you yank the stock towards you and then the air pressure blows the ping pong ball out. Which do you think would be the better one to use for introductory paint balling?
-- by which I'm trying to make the point that although a battery charger may be your better choice because you need to start with low voltage, neither a battery charger nor a HAM radio power supply is really a very decent anodizing rectifier :-)
3). The positive (anode) lead connects to the work, so by 'paddle' you must mean the cathode (negative lead)? I would suggest a 'flat piece of aluminum' as your cathode rather than aluminum foil I think. It needs to be able to carry the current.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002
2002
A. Hello Wayne,
I'll try to keep this as basic as possible for you. First of all, you'll want to make sure that your piece is free of any coatings, and free of all screws, and other non-aluminum hardware, and that the piece is as clean as possible. As far as your concentrations of acids go, if you are just doing this on your gun for decorative reasons, you'll want about 15%-20% solution of sulfuric acid to water (the cleaner the water, the better...tap water isn't the best). REMEMBER: ADD THE ACID TO THE WATER...NEVER THE OPPOSITE.
You'll want the bath temp to be around 70 °F. Your Ham radio power supply would be the better of the two, although it really doesn't meet the minimum requirements. Ideally you'll want to anodize at 12-15 amps per sq. ft of surface area of the piece you are trying to coat (your 12 volts may not be enough to accomplish this).
You'll want to slowly (2 min will do) ramp the power supply, not just jolt your piece with the full current. As far as the cathode (-) goes, a piece of aluminum (ideally 6063 alloy) sized to approximately 1/3 of the surface area of the piece you are trying to coat will suffice, you can just hang this piece over the side of your "tank".
If you follow these parameters, it should take approx. 20 min to build .0005" of anodizing. You didn't mention anything about dyeing your piece, so I'm assuming you are looking for clear anodizing. Your part should come out of the tank with a slight grey color (this depends on the alloy of the piece you are coating). Once you are done coating, the part should be thoroughly rinsed in clean water, then sealed. To accomplish the sealing you can boil some very clean water (once again, not tap water), and immerse your piece in the boiling water for a period of 25-30 min. Remember to use rubber rubber gloves ⇦ this on eBay or Amazon [affil links] and eye protection when working around the acid bath.
Hope this helped you out! There are a lot more fine details that apply to anodizing than what I've mentioned here. Keep in mind that for probably less than $50, you could have this professionally done.
See what you get when you ask "experts"? You get 2 different opinions! I respectfully disagree with Ted's power supply choice :)
Marc Green
anodizer - Boise, Idaho
2002
Your anodizing experience greatly exceeds mine, Marc, and I'll go along with whichever power supply you think is the least bad of the two. My point was just that when a proper power supply costs $40, asking which of two improper power supply choices is the better one can take us down the rabbit hole.
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
I hear ya Ted!
Marc Green
anodizer - Boise, Idaho
2002
Q. I have to anodize some 7075 Aluminum receivers. I have an ancient rectifier that has a voltmeter that goes to 15 volts, and an ammeter that goes to 25 amps. It has a single large dial that goes from 0 - 100.
It is all I have and finances are very scarce right now. I'm going to be using a 5 gallon bucket for a tank with sheet lead anode.
Is this going to be a lost cause or will I be able to make this old rectifier work.
Thanks
- Port Lions, Alaska
September 17, 2013
A. Hi Ron. I think 3 things are important about an anodizing power supply.
1). That you have sufficient voltage. 15 volts should be enough, although it's borderline for alloys like 7075.
2). That you have sufficient amperage. 12-18 ASF should be enough, so 25 Amps should handle parts up to about 2 square foot in surface area.
3). That you can either control the volts or limit the amps. When you start anodizing, you are working with highly conductive aluminum. When you finish anodizing you are working with a surface that has a thick, insulating anodized film on it. So you must start at low voltage (or limit the current) to keep the parts from burning, but increase the voltage (or maintain the current) as the insulating anodizing film builds up and adds resistance.
If you don't yet have experience with 1xxx aluminum or 6xxx aluminum, you might want to start there rather than starting with quite difficult 7xxx. Presumably this is a hobby, not a small business which should have various hazardous waste management permits.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 23, 2013
Anodizing for school science project
Q. Here's a video that we made for school about the process; you can also see the finished result against the original
Jose Estremadoyro
- Mexico City mexico
April 30, 2015
May 2015
Hi Jose. Great work!! (Readers: please click through to view it and like it on YouTube).
But we with experience out in the world are frightened for your health when we view this. Please help yourself and your lab partners stay young looking and handsome/beautiful by always wearing goggles and rubber gloves when working with chemicals! Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
! Did I hear right?
"A thin layer of lead will form on the aluminium"
Please tell me I heard wrong.
Geoff Smith
Hampshire, England
May 3, 2015
Hi Geoff. You didn't hear wrong. They were confused (at one point at least), thinking the lead cathode plates out onto the aluminum anode. Taking it as a whole, I think they eventually learned otherwise but didn't do another "take" to fix their first "chapter" of filming ... and they did get a lot right.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
How to anodize 7075 using a welder as a power source
December 3, 2015Q. I am a graduating mechanical engineering student and have been playing with anodizing aluminum. I have recently taken to anodizing aluminum for 2 of my hobbies. I am involved in downhill longboarding and my truck hangars (parts that hold the axles and wheels) are made from 7075 aluminum as I have found. The other hobby that I am involved in is firework manufacturing. The vast majority of firework tools are machined from 6061 aluminum and due to their nature have exceptional benefit from having a type III anodize with the teflon additive. Most of the tools are essentially pistons and cylinders with specialized pistons for different purposes, or one tool in particular is called a star plate. This tool is essentially a plate with a square matrix of pins sticking up with a matching female plate. It is essentially a group of piston and cylinders. The purpose of these devices is to compact damp powder into pellets. The damp powder has a water activated glue in it so it tends to stick to bare aluminum. It doesn't stick to the hard teflon anodize though.
This will be a lengthy post, I apologize in advance, but I figure that a clear description will best suit a proper answer to my questions. Here is my current process
I have been experimenting in anodizing different alloys using a 3 amp/ 30 volt power supply under constant current. I "rack" my components with either aluminum welding rod, or 16 gauge aluminum wire. I dip the part in NaOH drain cleaner for a bit to etch/clean the part. Using a clean gloved hand I remove the part, spray it with a degreaser and rinse it in the tap. I then 'electropolish' the part by 'anodizing' it in the green phosphoric acid solution sold in home depot as 'Phosphoric prep and etch.' I notice that when doing this, the voltage is extremely high and as the electrolyte warms up, voltage drops and the current rises to my desired 3 amps max output. So far my parts have been small scrap pieces of bar or tube with a surface area of 11 square inches or so. I notice that this amount of current won't actually electropolish the aluminum, but gets it silvery and clear. In the lab at school I have used a 10 amp source with pure phosphoric and it mirror polishes my parts, but I do not have access to those at home. Sorry, I digress. My next step is rinsing the part by dunking it multiple times in distilled water. I then anodize the part at 3.16 amps and go with the time spec'd in the 720 calculator for 1 mil of thickness. This is usually 30 mins or so to an hour depending on the piece. The solution is 50/50 battery acid/distilled water at room temperature. I have placed the solution in the freezer overnight and anodized in it and my anodize layer was yellow/olive in color, indicating a possible hard anodize? My containers have just been tupperware that hold about 2 cups up to about a gallon. My cathode has been another piece of scrap aluminum about the same size. After anodizing, I rinse the part in distilled water. I heat my dye bath in the microwave until it starts to steam a bit. Not boil or cavitate, but steam a bit. I drop the part in for a few minutes until its the color I want. I dunk the part in boiling distilled water for a few moments then place it in a strainer and put it over a steaming pot for 20-30 minutes.
This method has worked to anodize, dye, and seal some parts using both Rit dye, and the commercial dye from Kingscote chemicals. Sometimes the color will still fade a bit when handling after its been 'sealed.' Here is where my troubles have begun. Thanks for sticking with me this long. I am forever grateful to whoever is still reading this.
My 7075 parts are a nightmare. It will not electropolish to a perfect mirror on my smaller power source. In fact it will still clean and brighten the part, but now it reveals the grain boundaries in the aluminum. It has somewhat of a 'hunter's camoflage' pattern to it. If I use a 10 amp power source, it will polish to a perfect mirror. I can get the 7075 to take dye, but it will rub completely off even after steaming for 30 minutes. I mean completely rub down to colorless again. The dye comes off in a powdery mess. It is very odd. I believe I read somewhere that 7075 takes more power, so I have begun to attempt to use a TIG welder in stick mode. This gives me the option to control the current that flows through. I am using a nice Miller welder and set the amperage to 10%. This equates to roughly 26 amps or so. My part (longboard truck) has a surface area according to my best guess of about 44 square inches. My problem here is that my racking material keeps getting cut in two right where it enters the solution. I figured this was from using too much current for the aluminum wire or welding rod. Interestingly enough, this happened when using my small power source also. I figured I would need to increase the diameter of the rack material to allow more current. I then bought a 3/8-16 stainless bolt and screwed it into my truck very tightly. I then clamp the positive to the bolt. I went through the whole process and noticed that the steel rod electropolished incredibly well. Absolutely beautiful, but now my aluminum is not doing anything. After further reading, I saw that stainless has low current carrying ability. I then cut threads into a 3/8" rod of aluminum and screwed it in to my truck. Same problem. The aluminum rod is now polishing and getting quite warm, but the truck is not. I do notice however this time, that my truck will instantly be covered in bubbles when I turn on the power, but it will not polish. It seems now that it is being etched, not polished. Even when anodizing, the truck will no longer anodize. It will just get hot where the threads are and only anodize right at the point where the aluminum thread screws into the truck. It is the strangest thing.
I have found that I was still seeing the melting/cutting in the stainless bolt. I find it odd that even with my small power source, my hanging wire would be cut in two when anodizing. I ultimately want to type III anodize all my parts, but want to get familiar with type II first. So now finally to my questions.
After seeing my process, what am I doing incorrectly?
How should I properly rack my parts?
Why is my current 'rack' only polishing, but my part isn't, even after being screwed in tightly.
Why are my Al parts no longer polishing but etching in phosphoric?
This is an exhausting, yet fun challenge. I am very eager to overcome this obstacle.
Hobbyist/student - Terrell, Texas USA
A. Hi Jarrod. First, you can't put steel or stainless steel into the anodizing bath. All the current will flow to it, and no anodizing will occur.
I'm confident that you can buy much heavier aluminum wire, such that it will not heat up or burn through. It should be good for about 600 A / sq. in. But the problem remains that the wire will tend to suck up the electricity that you are trying to direct to your 7075 parts.
A racking approach that I've seen used on architectural extrusions, which might or might not work for your truck hangers is aluminum screws that would go through your trucks and into threaded holes in a permanent contact block. The heads of the screws press against the truck, and the screws carry the current from the tapped holes in your contact block to the heads of the screws and the trucks. The tapped holes tend to not anodize, both because they are tapped holes and because there is a screw filling them during use. You can replace the screws or strip them in the NaOH each cycle.
When all the dye rubs right off, it probably means the surface is not anodized (anodizing makes pores or drill holes that absorb the dye, and you can't rub the dye out of those pores). But failure to properly seal will mean the dye rubs off too easily too.
If you've been able to successfully anodize with the right equipment in school, you're doing pretty good! ... and should probably focus on how you can use the right equipment rather than making other adjustments to try to compensate for using inadequate equipment :-)
Luck and Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2016
Question about amp draw on my anodizing tank
Q. I've been anodizing RC car parts for about 4 years now and have pretty good success. Recently I've noticed that to achieve some colors I've had to anodize for longer periods. As of now for 7075 my anodizing time is 60 minutes and for 6061 it's 80 minutes. Now I didn't think much about it until talking to a fellow anodizing shop close by and he told me that his 6061 time is 30 minutes and 7075 obviously less. So that got me thinking: where am I losing voltage, amps or whatever I'm losing that's costing me 30 minutes per run.
Our setups are nearly identical, we both use constant voltage @ 15v, both have 10 gallon tanks, both use titanium racks bought from same store, both use same acid ratio of 16% by weight. The only difference that I can tell is that I use an aluminum grid anode that I bolt my part racks to and his anode is a copper bar that he hangs his racks on.
Just before typing this I checked the voltage going through my anode and with rectifier set on 15 volts I was only getting 13.2 volts from cathode to anode. To get 15v through the grid I had to raise voltage to 17.1. With the lambda power supply if just check leads I get 14.8 with digital readout set on 15.0, So does that mean I'm losing the 2 volts in my grid, and if so would that double my times?
Sorry for long winded post just trying to be thorough. Thanks in advance.
Small shop - Hays, North Carolina USA
February 12, 2016
A. Hi Jason. If you search this site for "720 Rule" you'll learn that the thickness should be directly proportional to the current, such that if you are losing 15% of your current you should be losing 15% of your speed, not 50% of it. Additionally, I don't understand your issue of 6061 taking 33% more anodizing time as 7075; it should actually take slightly less time.
Do you have any idea what anodizing thickness you are shooting for? Maybe 0.0002" is enough for lighter colors, but 0.0003-0.0004" is needed for darker colors? Probably even a little more would be required for a robust black.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2016
Q. Ted, when I was researching this several years ago, I thought I read several times that under the same conditions that 7075 and 2000 series aluminum took less time at same voltage to reach same anodic layer. I have since been told that by my tech guy at the chemical supplier I use.
As far as the size of the layer, I haven't really been that technical as long as I was able to get the desired color. I have noticed that black does require more time.
The times I'm using now produces great colors; I was just hoping to figure out why my times are so much longer than industry standard.
- Hays, North Carolina USA
February 15, 2016
A. Hi Jason. I'm not a hands-on anodizer, but I've never heard of that claim about 7075 anodizing twice as fast.
The 720 Rule will tell you what the "industry standard time" is to get to any given thickness, but it's impossible to say what the "industry standard time" is to get to an unknown thickness. The case may be that your competitor is doing better dyeing than you, and thus doesn't need as much anodizing thickness to get to the saturation level that you are looking for.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2016
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- Hays, North Carolina USA February 19, 2016 February 20, 2016 A. Ted, I think the confusion here is that you're talking about the 720 rule (using constant current), while Jason is anodizing by constant voltage. I could easily see the 2000, and 7000 series aluminum anodizing quicker using constant voltage, hence the reason anodizers prefer to use constant current, as it's a repeatable and much more predictable process when anodizing to a desired coating thickness.  Marc Green anodizer - Boise, Idaho |
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