Heard on the hotline

Heard on the Hotline Ted Mooney, P.E.

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e’ve selecteda number of questionsabout anodizing and finishing of aluminum for this month’s column. The last responsethat you see printed here is not necessarily the ultimate disposition to each problem, though; rather, we’re merely resting at a way stop. It’s not over until our readers,by their silence, say it’s over. EFFECT OF ANODIZING STRENGTH

ON ALUMINUM

FATIGUE

I’m building a home-built airplane with an aluminum wing. I’ve been advised that anodizing the structural aluminum parts for corrosion protection will causea decrease in fatigue strength. Is this true? Is the decreasecignificant. or will it be more than compensatedfor by the lowered chancesof corrosion? What is the mechanismof the decreasedfatigue strength’?If it’s too complex of a subject to answer here. can you point me towards a good reference text? Bob Thomasson.Reno. Nev.. [email protected] Sulfuric acid anodizing doeshave the effect you mention, becausea fair amount of aluminum is actually dissolvedand converted to a porous aluminum oxide. Look into chromic acid anodizing, which has been a standard approach for aircraft parts for decades:the coating is much thinner, and if any residue is trapped it is noncorrosive. A responseto the following letter describes another approach; perhaps Mr. Corier can fill you in further on that. Ted Mooney. Finishing Technology, mooney@fini\hing.com VOLTAGE USED IN ANODIZING

PROCESS

I’m thinking about starting an anodizing operation and at this moment I would like to know how much voltage is usually applied in the process for both large and small materials, Can someonehelp me out on this subject? Fridrich Grundinger, Lager’s lnversionea C.A., [email protected] I work at a shopthat anodizesaluminum partsfor aircraft. Our solution is a boric (7.5 g/L)-sulfuric (45 g/L) mixture. Our tanks are48 ft long so we can run large and small parts. The voltage we useis 15 + I V. Since we do aircraft WC‘are rigid about our voltage staying within this range: f’ailure to

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adherecausesdecreasedfatigue life, burns, etc. Our voltage range doesn’t change regardlessof the size of the part. Voltage hasnever beena problem for us; rather. the problem is with the amount of current we draw, i.e., the workload. We’ve gone to a 10,000-A rectifier to remedy thi5. I guess it dependson what type of anodizing you are trying to do. With other techniques, such as hard anodizing or chromic. the voltage rangesare different. JamesA. Corier, KidBluelSl @aol.com It has been my experience that if you can figure your workload you are better off running by A/ft2. Voltage can vary because of many different reasons-loose contacts, tank temperature, chemical concentration, size of work load, etc. Voltage will change depending on what kind of anodizing you are doing. Darrell Barker, [email protected]

HARDCOAT ANODIZING TEMPERATURE

AT ROOM

I am a subscriber to Metul Finishing and follow your column quite closely. I am very interestedin the hardcoating processthat is done at room temperature (%I--70°F). What are the organic additives neededand their amounts(g/L) in a sulfuric acid solution‘?Is this a proprietary product, and what will be my current density and voltage needs’?Several of our customershave all said “think big” becauseof the large items they are currently having hardcoated. Can you provide me with some guidance or point me in the right direction? Any suggestionsor pointers? Kenny Bryant, [email protected] Anodizing additive packagesare available from a number of metal-finishing chemical suppliers as are proprietary anodizing “systems,” which include bath chemistries. The literature details a number of beneficial organic additives, which may be lessexpensive to purchase.There is no such thing as an industry standard. Most anodizers have their own favorite. Many refer to hot tank hardcoating as pseudo. or Type II- l/2. hardcoating. It may meet the minimum requirements for hardcoating under certain conditions but may not be

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METAL FINISHING . MAY 1998

equivalent to traditional cold tank processing. In general, with all factors being equal, colder is harder. Chris Jurey, Luke Engineering and Manufacturing Co. Inc., [email protected]

CHLORIDES

IN CHROMIC

ACID ANODIZING

I am looking for a procedure to test for chlorides in a Type I chromic acid anodize bath. I have tried a traditional conversion with silver nitrate. The content 1 determined was approximately 8 g/L. After further review 1 can hypothesize that some chromium nitrate was formed. 1 am currently testing that hypothesis. I am looking at further digestion with nitric acid to dissolve the chromic nitrate and leave the silver chloride behind. Any further help or experience would be appreciated. lra Donovan, Treffers Precision, [email protected] The following

should work:

1. Take a .50-m] sample and dilute with DI water to 250 ml. 2. Add 20 ml 42”BaumC nitric acid and boil. 3. Add 5 ml of 3% silver nitrate and allow precipitate to fall out. 4. Wash filtered precipitate with 2% nitric acid and dry. 5. Weigh as silver chloride, calculate as follows: grams of silver chloride X 8.2 = g/L of sodium chloride. It is important to note that, as in all gravimetric procedures, quantitative techniques are critical to results found. This is a variation of the basic procedure found in “The Surface Treatment of Aluminum and its Alloys” by Wernick, Pinner, and Sheasby. Ward Barcafer. [email protected]

CAN ANODIZED AND DYED ALUMINUM WITHSTAND STERILIZATION? We’re color anodizing Al-Mg-Si parts that have to withstand at least 150 sterilization cycles. The processes used for sterilization are 275°F water steam and a 122°F hydrogen peroxide plasma. Unfortunately, the color comes completely off after a few sterilization cycles. Our anodizer says an anodized surface will never withstand sterilization. Thus my questions concerning the subject are: Is that true’? What alternatives do we have? Is there a more resistant coating for royal blue aluminum parts? Robert Mosch. Student. [email protected] I seriously doubt if any dyed anodize will stand up to 1SO cycles of sterilization and most certainly will not when you add a peroxide. Find an anodizer who does two-step anodizing since the color is from a metal salt ‘.plated” into the pores rather than a dye. This is architectural quality. I read about

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a possibility that this might work. Black would probably be a better color. JamesWatts, JamesWatts Inc., [email protected] I would believe that Mr. Watts is correct in stating that organic dyes will not stand up to peroxide. But it might be that an inorganic dye would be okay if you could find one in an acceptable color. In responseto a totally different question, the following reply was received from Paul Vernon, but it is a coincidence that might be worth looking into. Ted Mooney. Finishing Technology, [email protected]

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Titanium 6-4 surgical instrumentsare routinely anodized for color coding. The first blue color appearsaround 28 V. The exact relationship between color and voltage will vary with each setup. You will have to experiment. If you are using titanium clips to hold the work, the anodized layer on the clip will needto be removed mechanically or in a dilute (5%) hydrofluoric acid solution (handle with care) between each run. Paul Vernon, Designed Materials Research. [email protected]

ADDING A CENTER ANODE TO AN ANODIZING TANK We anodize aluminum extrusions in a sulfuric acid bath. We are planning to add a central cathode to our bath so that we can anodize two loads simultaneously. Is this a good idea? Will this work for our electrocoloring tank too? Sanjay A. Bulchandani. Ajit India. san,[email protected] It should work okay with a central cathode. Be sure your rectifier can handle the load. You probably should anodize the loadsat the sametime and be sure the surface areasof the two racks are about the sameor you could get coating thickness differences. Remember it’s an electrical circuit and the electricity will take the easiestpath. Lee Branch, Albright Bi Wilson. [email protected] As Lee has pointed out, this should be okay; however. you will be halving the current density and this may cause a very different anodic film to be deposited. This may or may not be acceptable,but for surethe parameter5for color anodizing afterwards will alter. 1 think you need to experiment. Personally, 1 addedcenter cathodesnot to increasethe load, but to give a more even film to improve coloring afterwards. Ian Peart, GrangesBuilding SystemsLtd.. [email protected]

ZINCATE ADHESION TO 6101 SERIES ALUMINUM We are having problemswith our zincate layer sticking to 6101 seriesaluminum. We run a single zincate processwith 58

METAL FINISHING . MAY 1998

a tri-acid desmut. We recently bumped up the phosphoric dip and it seemed to help. Is there anything else we can do‘? Brian Dutz, [email protected]

what can we do to get theseto plate better? When we run the softer material. the zincate layer is great. Brian Dutz, [email protected]

The main problem with zincating aluminum for preplating purposes is that aluminum forms an oxide extremely quickly, which hampers plating. Typically. I have never had good results with a single zincate cycle. I would suggest a double zincate. which allows for a finer deposited zincate and more sites for subsequent adhesion. Also to be noted is that you do not. repeat do not. allow the parts to become dry at any time. As you can gather, any staging in rinses throughout the preplating cycle, or any delay in the preplating cycle, can allow aluminum oxide formation. Further, any delay in the plating cycle up to whatever your strike is will cause you problems. The other i.ssuewith aluminum finishing of any kind is the cleaning cycle, which is undoubtedly the most neglected part of any aluminum finishing, especially if one is a plater and not an anodizer. An excellent resourcewould be to contact the manufacturersof zincate products asthey will have recommendedzincate and cleaning cycles for aluminum. Be sure to let them know what your Lincate formulation is. as somezincate formulations are to be used for different types of plating. such as electrolytic nickel versus copper. etc. Ward Barcafer, [email protected]

There should not be a noticeable difference between the tempers: however, your problem is most likely due to the alloying constituents. Alloys that are more difficult to zincate, such as higher silicon-containing alloys, definitely require a double zincate. Also, depending upon the alloy types, one needs to use different acid etching or pickling solutions.A good one for high silicon-containing alloys is a 7.5%nitric/25% hydrofluoric acid etch. This is a dangerous solution to work with and requires that special first aid treatments be available. Another solution that is not as dangerousis an ammoniumbifluoride/nitric acid solution as described in the Metul Firlislziq Guidebook. That solution is used prior to your first zincate. Use a 50% nitric acid to remove the first zincate and then rezincate. This should fix your problem. My suggestionwould be to accessthe available literature and get a hold of your zincate solution supplier. If you have questions. which you can not get answeredin depth from your resources,feel free to E-mail me your phone number or a list of questions. Ward Barcafer, [email protected]

We use a phosphoric dip between the desmut and the Lincate to eliminate oxidation. We have changed our dip time in this tank from 30 to 90 secondsand this hashelped. One of our biggest problelns is plating the harder 6101 series.We use 3 or 4 different hardnesses(T-61. T-63 , T-65, and so on). la there a major difference between them, and

Try going into the tank “live.” It took care of my problems. Gary Witherspoon, [email protected] Ted Mooney is an independentconsultingengineerbasedin Kinnelon.N.J. MF

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