Finishing Technology logo

"Finishing Technology Hotline BBS" Postings by Topic

Ed. note: before the internet existed, there was a time when "Sysops" (system operators) ran publics forums via dial-up modems and "BBS" (Bulletin Board System) software. Here is one such discussion from 1994.


Msg. #4503 in ** Ask Plater B**
Posted on 01/04/94 at 21:41:54
To: ALL From: DENNIS WARNER - chromic acid anodising
Our shop overhauls aircraft propellers. As a part of the overhaul we must re-anodise the aluminum parts in a 7% by wt chromic acid/di water solution. the tank is approx. 60 gals. The problem is that we are getting a pitting of the aluminum parts if a gas bubble gets trapped. IE a screw hole (blind) or a sharp shoulder. It appears that the "mist" from the bubble is corrosive. I guess my question is 1) can some one recommend a lab to analyze the solution? local labs have no idea what it is that we do with this stuff and I have had quotes of 300-500 dollars to analyze the bath for sulphates, chlorides, and aluminum!. Is this price normal? second, I have had people suggest that the problem my be high sulphate. If this is so, can barium be used to lower the concentration as with hard chrome plating? I am not a chemist, nor a professional plater, and it seems hard to find anyone who still uses chrome anodising that is knowledgeable. The standard response seems to be to dump the tank and mix a new batch. I would like to avoid that if at all possible. thanks in advance.

Msg. #4504 in ** Ask Plater B**
Posted on 01/04/94 at 23:45:28
To: DENNIS WARNER From: TED MOONEY - Reply To 'chromic acid anodising'
Your analysis cost seems very high. I think $100 would be more typical. Chromic acid anodizing is becoming more rare for environmental reasons, and I know that some aircraft manufacturers are now allowing chromating (Alodine, Iridite) as an alternative--but it probably isn't approved for propellers which I assume suffer a lot of wear. Since this is rework and we therefore can't blame irregular alloying in the substrate, my first guess at the cause would be excessive chlorides, but maybe someone here has actual hands-on experience with this solution.

Msg. #4505 in ** Ask Plater B**
Posted on 01/05/94 at 04:08:57
To: DENNIS WARNER From: TOM PULLIZZI - Reply To 'chromic acid anodising'
ok, I'll tell you what I know, but don't blame me if there is no science to it. I worked for Bendix corp, and we used to get the same Horrible pitting. I mean, it would eat the whole part up for an inch around the blind hole. I tried and tried analyzing, found higher than normal chloride but that was about it. I thought it was the chloride, but never found out. then I did what I swore I would never do, I asked the plater. he said the problem was I was cleaning the parts according to the specification(show amazement). these parts were super cleaned in the machine shop, degreased, honed, all kinds of stuff. they looked beautiful. all the plater used to do was rack them, and put them in the anodize. -perfect parts. but no, I had to go and pull out the spec. alkaline clean, nitric acid dip, sulfuric acid dip. (what did I know). the Bendix shop is gone, but I believe it was some tiny residue of some acid and cleaner diluted in the hole which started the corrosion and, once started, would never end because of the tremendous surface area of the pitted surface. are you cleaning these parts with the normal cycle?

Msg. #4511 in ** Ask Plater B**
Posted on 01/05/94 at 17:14:36
To: TOM PULLIZZI From: DENNIS WARNER - Reply To 'chromic acid anodising'
Your message reads like you were using our anodize tank! The solution starts to produce a "ink like" ring with light corrosion, until, finally we scrap a part because half the threads in the hole are gone. We pre-clean the parts with solvents, usually mek or acetone, then they are etched in "mil etch" (caustic soda) and desmutted with Novamax's companion product 2405 desmutter (the number could be wrong, but I spoke to them and it is the product the recommend for use with mil-etch). they are rinsed and anodized. I suspected drag out from the desmut, but after watching the process, I can't see how.. They rinse the hell out of the parts even the holes.. I hope to find a good lab to send the sample to. As the pitting only happens in "old" solution It would seem it would have to be a contaminate of some kind causing this. I hope I can find the cause of this problem. Work has picked up, and the volume of anodizing as well. If I have to dump the tank every 8 months It will change our epa generator status. Thanks for the help. If I figure this mess out Ill let you know. thanks again, Dennis

Msg. #4512 in ** Ask Plater B**
Posted on 01/05/94 at 17:22:46
To: TED MOONEY From: DENNIS WARNER - Reply To 'chromic acid anodising'
Thanks, I think the labs in Tucson are getting outrageous in there prices. up till now, we would just dump the tank when things got out of hand, but this is becoming a nightmare. Alodine is approved as a substitute, but I personally believe that it is inferior to anodizing. I much prefer anodizing as it is also a great crack detector. The parts are penetrant inspected (several times), but we have found very tight almost microscopic cracks with the anodizing, that the level 3 penetrants didn't detect. (just a slight color variation in the coating.) I do however, understand why many shops have gone to conversion coatings.. Thanks again for the help Dennis

Msg. #4513 in ** Ask Plater B**
Posted on 01/05/94 at 19:00:47
To: DENNIS WARNER From: TED MOONEY - Reply To 'chromic acid anodising'
Are you familiar with "blue anodizing"? I recently ran across it at a jet engine plant, used on titanium parts, and claimed to be a big improvement as a crack detector. I don't know if there is a similar anodizing solution for aluminum, optimized for crack detection. To keep contaminants from getting into an anodizing bath, three techniques that I have at least heard of are hot & cold rinsing, neutralization with sodium bicarbonate, and a dip in nitric acid. One of them may be helpful in reducing drag-in contamination.

Msg. #4515 in ** Ask Plater B**
Posted on 01/05/94 at 20:42:08
To: DENNIS WARNER From: TOM PULLIZZI - Reply To 'chromic acid anodising'
just for fun, try some parts without the caustic and patent acid etch. the precleaning just isn't necessary, as I found out. I thought the problem was an old solution until I dumped the tank. I don't believe it is the alloy, the chromic anodize, or even the chlorides, it is some combination of the caustic and acid predip, we even made a dental tool-like thing to rinse each hole, it didn't work, because I think, the precleaning did something to the surface which you can't wash off. if you could do some honing of some sort, after degreasing, and never touch the part with NaOH or other acid, I think you will have the thing licked. otherwise I'm afraid you will have random trouble, but good luck anyhow.

Msg. #4516 in ** Ask Plater B**
Posted on 01/05/94 at 20:43:58
To: DENNIS WARNER From: TOM PULLIZZI - Reply To 'chromic acid anodising'
give me your address, I'll mail you a method for chlorides which will cost you a few bucks to set up and very accurate. titrate with mercuric nitrate to a cloud endpoint.

Msg. #4518 in ** Ask Plater B**
Posted on 01/06/94 at 07:53:33
To: DENNIS WARNER From: BERL STEIN - Reply To 'chromic acid anodising'
Dennis: I have to agree with what Tom said
- blind holes are notoriously difficult to rinse. If you are not using warm *agitated* rinses for sufficient times (at least 1min. after the acids, 2 after alkalis), you will be dragging in residues. As far as the cost for chemical analysis, prices will vary a lot, depending on what you want the lab to do and the method they'll be using. Ted's right, if a simple titration is involved. In order to find a decent local lab, you should look in the "Directory of Testing Laboratories", available from ASTM (1916 Race St. Philadelphia, 215-299-5400) for a nominal fee. My 1989 edition is somewhat outdated. Good luck, Plater B

Msg. #4525 in ** Ask Plater B**
Posted on 01/06/94 at 19:25:10
To: TOM PULLIZZI From: DENNIS WARNER - Reply To 'chromic acid anodising'
Tom, Thanks, that would be a great help. Address is Dennis Warner P.O. Box 11278 Tucson, AZ 85734 up till now the only check done on the tank was with a baumé stick. Thanks again Dennis

Msg. #4526 in ** Ask Plater B**
Posted on 01/06/94 at 19:32:24
To: TOM PULLIZZI From: DENNIS WARNER - Reply To 'chromic acid anodising'
I have been kicking the idea around about trying to anodize the without the etch, by glass blasting the parts, rinsing and then anodise. I just wonder if it would produce the same color and quality of anodize. I also lowered the temp of the bath a few degrees to 92 from 94 and that seems to help a little. I still need to check the contaminate levels tho. thanks Dennis

Msg. #4527 in ** Ask Plater B**
Posted on 01/06/94 at 19:41:23
To: TED MOONEY From: DENNIS WARNER - Reply To 'chromic acid anodising'
I am not familiar with blue anodizing. It would be interesting to see if anyone has developed an anodizing method for crack detection. Our shop at one time performed penetrant inspections on titanium turbine fans and discs. At the time, quite a few shops were complaining that the penetrants available, even the high sensitivity ones, would miss defects. That I know, we never had a problem missing defects, but the repeatability was. When you scrap 50 thou worth of parts, usually people get a second opinion. Anyway, insurance made it unprofitable for us to continue the turbine work and we quit accepting it. Thanks again for all the suggestions, I really appreciate it. Dennis

Msg. #4529 in ** Ask Plater B**
Posted on 01/06/94 at 20:57:02
To: DENNIS WARNER From: TOM PULLIZZI - Reply To 'chromic acid anodising analysis'
i'll send you all the tests we did for our chromic acid anodize tank, since I have them somewhere, along with formulations, it might be interesting.

Msg. #4530 in ** Ask Plater B**
Posted on 01/06/94 at 21:00:21
To: DENNIS WARNER From: TOM PULLIZZI - Reply To 'chromic acid anodising'
think about how the anodize is formed; from the inside out, what is on the outside of the part after anodizing is what was there when you started. I also have a mounted cross section of one of the eaten-away holes from this part we used to do. Please send it back to me when you are done, ill mail it to you with the formulae. maybe you can figure it out, but you can see that the corrosion is in the middle of the hole, not the bottom, a zone where mixing of chemicals has occurred?

Msg. #4539 in ** Ask Plater B**
Posted on 01/07/94 at 00:21:52
To: DENNIS WARNER From: KEN ROSENBLUM - Reply To 'chromic acid anodising'
I don't know a lot about chromic acid anodizing, but: 1) Are you really anodizing in a 60 gallon bath? If so, I'm sure the cheapest and easiest route would be to dump the bath. We have chromic acid (concentrated) waste treated for about $3.50/gallon. 2) If this is a 60 gallon solution, are these propellers for model airplanes? 3) We usually turn to Sandoz for anodize problems. They have 2 or 3 technical people who deal exclusively with anodize problems. If anyone knows the answer to your problems, I'm sure that they would. 4) Even if you spent a lot of money for analysis and pinpointed your problem (which is unlikely), the remediation of the problem will still probably be to dump your bath. 5) The problem may be in the material, in a cleaning step or a problem with your rinse water. Even if you dump your bath, I would talk with the people at Sandoz. If the problem is not in your bath, your problem will probably return again. 6) If you dump your bath, keep a sample for a while. Ken R.

Msg. #4541 in ** Ask Plater B**
Posted on 01/07/94 at 00:44:58
To: BERL STEIN From: KEN ROSENBLUM - Reply To 'chromic acid anodising'
Do yourself a favor, call the lab at Sandoz. If you can convince them that you might buy one of their products, they'll do the analysis for nothing.

Msg. #4577 in ** Ask Plater B**
Posted on 01/11/94 at 08:22:16
To: DENNIS WARNER From: BERL STEIN - Reply To 'chromic acid anodising'
Dennis: > this whole problem started when we converted to a closed loop system and recycled the rinse water back into the tank.< I think it just concentrates the contaminates in a very accelerated way. You are absolutely right as far as the accumulation of impurities goes. That's why no one should even attempt to close the loop until he/she thoroughly understands the chemistry of the process and what impurities will have to be dealt with. The fact that you pot helped solve the problem tells me that it was caused by the trivalent Cr ions, being generated through the reduction of hexavalent chromium. To reduce or eliminate the proble, several manuals are suggesting to maintain the cathode/anode surface area ratio at no higher than 5:1. In other words, don't anodize parts with a small surface area by themselves. My book also says that to convert the trivalent chromium back to the hexavalent you have to use a lead anode and a steel cathode with a ceramic diaphragm around the cathodes. I guess that is what you are doing now. Looks like you'll have to do it regularly to keep the Cr3+ in check. Good Luck, Plater B

Msg. #4590 in ** Ask Plater B**
Posted on 01/13/94 at 17:47:38
To: ALL From: DENNIS WARNER - chromic acid anodizing
well, I would like to thank everyone for the input, Tom Pullizzi for the chance to view the sample that he had problems with. so far everything is a ok again. lowering the temp of the bath and running a "pot" in the evenings has solved(?) the problem. also, we are paying closer attention to the racks used to suspend the parts, cleaning them more often. we are also checking the bus bars and cleaning them as well. someone suggested that a high resistance might be causing the part to heat up, and then if a bubble got trapped, the solution wasn't there to cool it off.. I still don't know what the problem was, or why it only happens in older solutions. but for now anyhow its ok. I am real impressed with the pot. It cleans all kinds of stuff out of the tank, we even noticed a change in the color of the solution.. thanks again Dennis

Msg. #4591 in ** Ask Plater B**
Posted on 01/13/94 at 18:04:10
To: DENNIS WARNER From: TED MOONEY - Reply To 'chromic acid anodizing'
Was this porous pot from Clarence Peger (Cleveland, OH)? He has been advocating them for chrome plating forever, and they apparently work. It has been nice hearing from you, Dennis; thanks for keeping us updated.

Msg. #4634 in ** Ask Plater B**
Posted on 01/19/94 at 11:43:54
To: TED MOONEY From: DENNIS WARNER - Reply To 'chromic acid anodizing'
I just purchased a pot from Clarence Peger. Hard chrome plating consultants. For the price, it is a real nice looking item. As for my limited experience with them, they do work. I have no idea as to how they work in a hard chrome bath, but at least one hard chrome shop in my area swears by them. (that is how I found clarence.) He is real opinionated in the two bus bar system boy! tried to convince me I NEEDED to be a hard chromer. As far as the claims of removing chrome from rinse water with the pot I tried it, it would work, but I think it would really cost in electricity to run the rectifier the required length of time. The trouble is that there is no real hard proof of what the pot is cleaning that I can find, but again, for the price, its worth the gamble, and for whatever reason, it seems to help.

Msg. #5072 in ** Ask Plater B**
Posted on 04/21/94 at 11:33:48
To: ALL From: MIKE SALATER - chromic
How do you control the total acid and alumina in you're chromic anodizing bath? Do you ever have to dump? If so, how do you dispose of the old bath Mike

Msg. #5076 in ** Ask Plater B**
Posted on 04/22/94 at 19:45:16
To: MIKE SALATER From: BERL STEIN - Reply To 'chromic anodizing'
Mike: The chromic acid can be titrated with thiosulphate (Na2S2O3), you will find the procedure in the Metal Finishing Guidebook and Directory'93, p.456. I'm sure a local lab will do an Al test for you, but, if memory serves me well, a simple viscosity test can be used instead. You'll have to go to old articles in MF or P&SF; to find the description of that test. Good luck, Plater B

Msg. #5078 in ** Ask Plater B**
Posted on 04/26/94 at 08:08:05
To: BERL STEIN From: STEVE RUDY - Reply To 'chromic anodizing'
Knowing the chromic acid titration, could one titrate to a total acidity, then calculate aluminum content, based on the titration difference? This is commonly done to analyze for aluminum and sulfuric acid in this type of anodizing bath.SFR

Msg. #5084 in ** Ask Plater B**
Posted on 04/29/94 at 16:51:56


To: DENNIS WARNER From: TED MOONEY - Reply To 'chromic acid anodizing'
The following reply came in over the Internet, with a request that the responder's name be kept anonymous, although it is obvious he is from Boeing: I just finished reading the exchange concerning chromic acid anodizing with the resultant pitting in aircraft propellers as shown in the May 94 "Heard on the Hotline" article of Metal Finishing. It sounds as if the problems are located primarily at areas of high current density (blind screw holes-presumably with threads--and sharp shoulders). "Anonymous" does not state what his voltage ramp-up rate is, and I am wondering if this could be the cause of his problem. Normally, the initial current is high and drops off as the oxide coating (insulator) builds up. In the Boeing specification for CAA (BAC 5019), the ramp-up rate is 7 volts per minute, so it takes three minutes to reach the anodize voltage of 22 volts. BAC 5019 also has chloride and sulphate limits as follows: Chloride: 0.2 g / liter maximum as NaCL sulphate: 0.5 g / liter maximum as SO4 The caller does not state what levels of chloride and sulphate that he has found in his 60 gallon tank (he apparently has not tested for these yet). Also, we found that our new Boric-Sulfuric Acid Anodizing solution causes somewhat less IGA/EGP than the Chromic Acid bath. Perhaps this may be due to the fact that hexavalent chrome is a much stronger oxidizing agent than sulphate ion? "Anonymous" may want to investigate the Boeing patented Boric-Sulfuric Acid Anodize process, which is currently used as a replacement for CAA including all fatigue critical parts such as wing skins, spars, chords, etc. It produces a coating weight of approx. 330-370 mg/sqr. ft. on 2024-T3 and about 460-530 mg/sqr. ft. on 7075-T6. The proper Boeing contact for a license to use this process is Bob Ames at Boeing Associated Products. His phone number is 206-865-6950.

Msg. #5315 in ** Ask Plater B**
Posted on 06/20/94 at 21:44:20
To: TED MOONEY From: VALENTINUS BRINKMANN - Reply To 'chromic acid anodizing'
Though I can't say where, I've installed a boric-sulfuric system as described and can testify as to it's effectiveness. And a lot less hassle.

Msg. #5843 in ** Ask Plater B**
Posted on 12/08/94 at 17:12:38
To: ALL From: GORDY SEPPANEN - Anodize Sealing
Is there a method to detect if an aluminum has had the anodize sealed? I have a product which is 7075 and is to be hard coated to about 2 mil followed by treatment with black dye and duplex sealing. The duplex is used because it has been found that without it the product has excessive pump down time when used in a vacuum. Part of the assembly is an epoxy bond to the anodize and we are observing delamination. Nothing seems to be out of order (cure time, pressure, lack of adhesive) so I would like to be complete and verify that the anodize was sealed properly. A single sample was examined using SEM/EDX and no Ni or Cr was found. The anodize vendor does not offer hot water sealing. Could the lack of Ni or Cr indicate that it was not nickel acetate nor dichromate sealed?

Msg. #5845 in ** Ask Plater B**
Posted on 12/08/94 at 20:29:18
To: GORDY SEPPANEN From: TED MOONEY - Reply To 'Anodize Sealing'
The tests have restrictions and limited applicabilities, but ASTM B680-80, "Seal Quality of Anodic Coatings on Aluminum by Acid Dissolution" and B457-67 "Measurement of Impedance of Anodic Coatings on Aluminum" may help you. It sounds like a surface contaminant may be interfering with the adhesive cure such that improved final rinsing or a hot seal may be the real requirement.

Msg. #5850 in ** Ask Plater B**
Posted on 12/09/94 at 22:48:32
To: GORDY SEPPANEN From: BERL STEIN - Reply To 'Anodize Sealing'
I am searching for a coating to apply to black chromated zinc to prevent dehydration of the chromate during baking. My customer has a part which requires at least one bake at 200 degrees F and a vacuum bake at 160 degrees which discolors the chromate. so I would like to be complete and verify that the anodize was sealed properly. A single sample was examined using SEM/EDX and no Ni or Cr was found. The anodize Well, it is a couple of toughies, Gordy. The thing is, chromates simply undergo dehydration at the temperatures you've listed, and nothing will stop that. I'm afraid there is no seal that would allow to bake a chromated Zn coating at 200F. There is a standard test method for Seal quality of anodic coatings (ASTM B 680-80), but it sais in the description that the method is not applicable to hard anodic coatings or the ones that were only sealed in dichromate solutions. Not much help, ha? Well, maybe you could still use the method to compare different coatings. Cheers, PlaterB

Msg. #5867 in ** Ask Plater B**
Posted on 12/11/94 at 21:18:14
To: GORDY SEPPANEN From: KEN ROSENBLUM - Reply To 'Anodize Sealing'
I would suggest the following test: Under the observation of someone from your organization, have three parts hard anodized. Dichromate seal one part, nickel acetate seal one part and leave one part with no seal. Now, test each part for Ni and Cr with SEM. Hopefully, this will tell you whether SEM has the potential to be a test for anodize sealing.

Msg. #5998 in ** Ask Plater B**
Posted on 01/04/95 at 23:01:28
To: ALL From: MIKE UETZ - Galvanic anodize of magnesium
Welcome back to the real world everyone! Are there any experts (or at least knowledgable individuals) with respect to getting a black galvanic anodize finish on magnesium die casting? We have a customer who wants us to do this process for them (DOW 9) and have provided us with some general info about solution make-ups, temperatures and immersion times. What I don't have are things such as: what are the critical control parameters and what are the test procedures?, what process qualification tests should be performed and how often? What construction materials are best for the various tanks require? Ventilation? Safety concerns? How black can we expect the parts to be and how much depends on the casting alloy and or casting quality? Are their any other things I need to know before I jump into this with both feet? Please let me know. Thanks Mike Uetz

Msg. #6111 in ** Ask Plater B**
Posted on 02/02/95 at 02:20:56

Msg. #6114 in ** Ask Plater B**
Posted on 02/03/95 at 21:36:33
What was wrong with the two methods you mentioned? (Control with ion exchange or decanting by analysis)


Back to Home Page Finishing Technology small logo