White spotting on black anodising

A. David, first look for pits in the incoming metal. Next look for chlorides in the dye tank. Then measure the voltage between the titanium rack and the stainless steel coil, then impress a reversal of that voltage with a lab rectifier. Then take two aspirin and call me in the morning.

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A. Fairly common with cast and some extruded aluminum. exactly what material and shape are you black anodizing? The less information you provide the less likely you will get a quality answer. James Watts - Navarre, Florida A. My guess would be galvanic pitting caused by either running aluminum parts on titanium racks or other galvanically dissimilar aluminum alloy (that is from the parts being anodized). Other potential causes might be entrapped sulfuric acid electrolyte in pits (if parts are castings). Or it could be excessive chlorides in the sulfuric electrolyte. Or perhaps embedded abrasive particle that is preventing the underneath aluminum from anodizing. There are plenty of possibilities as others will build on these! Pictures are easily posted and in cases such as failure analysis are worth 10,000 words! Check these out, be on the lookout for others and good luck (we've all been there!). Milt Stevenson, Jr. Syracuse, New York A. 1. What is free acid and dissolved aluminum? Aluminum sulphate is White. Put a 5% nitric acid between the anodizing and dye. 2. Chlorides in the line? 3. Galvanic in the dye tank: titanium versus stainless steel 4. 7075? All zinc compounds are white. Heat treated 7075, zinc migrates to the grain boundaries falls out, calcium catches in the pits. 5. Calcium in chloride pits, aggravated by galvanic in the dye All calcium compounds are white, oxalate, citrate, acetate (from the dye). gluconate (from the anodizing additive, metasilicate from the cleaner. White spots are very common, can come from many different sources. Robert H Probert Robert H Probert Technical Services Garner, North Carolina A. Danny, You haven't given us a lot to go on so I will have to make a few assumptions. The problem sounds to me as if you have porous castings and sulfuric acid is getting trapped in the pores. After sealing and drying the sulfuric acid leaches out of the pores and bleaches the dye in the area of the pore. How to solve the problem? You could start by getting the castings impregnated, this will seal the pores. Secondly, after anodising and before dyeing alternately rinse the parts in warm (up to 60C) and cold water several times. This will help "draw out" the sulfuric acid from the pores and greatly reduce the appearance of white spots. Brian Terry Aerospace - Yeovil, Somerset, UK A. I would guess that a few things are possible here: 1.) Improper cleaning, resulting in grease / oil contamination of the surface, thereby causing voids in the dyed or anodized finish. 2.) A galvanic difference. I was told by U.S. Specialty at one time that having dyes in a stainless steel tank and anodizing on titanium racks / hooks equated to a mess sometimes. 3.) Possible cross contamination, I won't say where I think it might be possible here. I'm just trying to spur though here. Maybe help lead you in a direction outside of this. Matthew Stiltner plating company - Toledo, Ohio

A. We have also faced similar problem during Black anodising. If it is happening only on 7075 alloys, then the problem is in base material wherein the Zinc is precipitated, and I agree with one of the answers given.
This could be avoided by dipping in mixture of Nitric acid 30% and HF 10% for less than a minute and then rinsing thoroughly (important, otherwise you will get into bigger problems) and anodise. By increasing the conc. of sulfuric acid by about 20 gram per litre more than you are already working will also help in avoiding pits in 7075 alloys.
If this problem is with all the alloys of aluminium then the obvious reason is Chloride contamination and high conc. of dissolved aluminium.

A. Hello Andrew, It looks to me as if you have a airborne problem with the spots. By the looks of it, I would guess you have a scrubber spraying mist on the high pressure side. Try tenting over the dye area with plastic and don't let the anodized parts be exposed to the plant atmosphere. Plastic is a quick fix, but you need to locate the real problem. Isolate the problem by walking a dyed, non-sealed part through the affected areas. The worse the condition, the closer the source will be. Good luck. Bruce Holt Sample Manager - Newark, Ohio, USA October 30, 2009 A. Alloy? Nitric Dip between anodize and dye? Titanium Rack and stainless dye tank? Free acid and Dissolved aluminum? First seen: after dye, after seal? Calcium/Magnesium content of the seal? Pits in the metal to start as seen with a 10X loupe? Robert H Probert Robert H Probert Technical Services Garner, North Carolina October 30, 2009 A. Hi The white spots can appear from many places, so please tell a little more about the shape of the pits and if you have done any microscopic investigation. Just a few places to start searching, galvanic contact tank and workbar anywhere in the process, inadequate rinsing after anodizing, splash from acid or alkali in racking area before starting the process, chloride content in anodizing tank, acid fume the wrong way in the production line. Well, you see many places where you can start the problem, so a microscope pic could give a little more information. Sunny regards Anne Anne Deacon Juhl - - Trustrup Denmark

A. Check for anodize solution contamination, electrical contact, rectifier electrical output, change rinse. If still problem, change the black solution. If problem still, change desmut solution and check the caustic solution (it may be contaminated). Good luck.

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A. Hi Leonardo.

There are different types of white spots, so fastest progress will probably be made if you send a quality photo of the spots to mooney@finishing.com for posting here. Thanks.

Regards,

A. The most likely, but not only cause is entrapment of anodizing solution in porosity. It looks like you have a good rinsing cycle, so you'll need a good microscopic examination of the aluminum to look for porosity in the raw material, or as the result of machining operations which may smear slivers into the surface.

Do other parts processed through the same process show the same problem?

Although your process looks good, you could try additional rinsing after anodize to see whether it will help. Alternate warm/cold rinsing might help.

Also try a few parts without the etching step to see whether you might be over etching.

A. 1. Try a 5% Nitric Acid dip between the anodize and dye, as in anodize-rinse-5% nitric-rinse-dye.
2. If the dye tank is stainless steel and the rack is titanium, you have a galvanic problem. Takes some parts off the rack and dye on stainless steel hooks, if that works, come back and we will tell you 4 alternatives to fix the problem.

A. Hi Leonardo,

First of all, check your DI water conductivity (if you use it in rinsing after anodizing). The max reading should be 10 micro siemens.

2nd: After cleaning, you could try to dip the part in Ammonium Bifluoride tank (conc.: 50 g/L, RT). If not confident, you could try in small/lab scale first.maybe 50 pieces

3rd: you should control your Hot water sealing quality in term of pH. the ideal pH is >4.5 & above

Aluminum 1,100 (small parts for perfumery)
Clean parts
rinsing
Ammonium Bifluoride dip
rinsing
matting
Washing (3 times)
neutralization
Wash (2 times)
Anodizing (electrical current density 1.2 for 30 minutes)
Washing (4 times)
Color (mixture of two dyes)
Wash (1 time)
Sealing (8 minutes hot sealing)
Washing (3 times)
drying

Thank you.

A. Hello Goran.

While your problem is not uncommon, the solution might be depending upon the source of the white spots. Past experience indicates to me that the spots are resultant from one or two sources. The first would be poor rinsing after the etch process. You are saying that the spots appear after the dye process but generally unless black dye is contaminated, the spots are already present, just not visible until the parts are dyed ... any color ... as you indicate. Returning the parts to the anodize tank with current previously used, rinse, and dye again. Many many times this condition will take care of itself.
The second source is exactly as you indicated ... machined surfaces. This is typical of castings. The as-cast surface will generally appear nice and black but the machined surface displays the white spots. The short explanation is because the machined surface has now exposed the pores and the pores hold chemicals which the parts have previously came into contact with in your process. Fortunately, the fix is the same. Take the parts back to anodize for 5-8 minutes with the same current used previously and then rinse and dye again. The appearance will usually improve greatly and many times the white spots do disappear. I would not make the situation any worse by introducing a nitric acid rinse to these parts after the anodize process. For my liking it is just one more chemical you will try to rid yourself of prior to the rinse.
Oh, and welcome to the world of anodizing. This will not be the first head scratcher that you will encounter.

A. Hi again Goran

I am sure there is some type of theory behind the additional time but for my part it has been experience that has shown us it is possible to correct anodizing defects using this procedure. You could most assuredly experiment yourself and see if the additional tank time prior to dye would work. (Nothing would be lost except a few minutes time). Also you are indeed correct: by taking the parts back to anodize tank for that few minutes, the parts will indeed shed some color and that color will linger in the tank. (I can always tell when somebody had what we call bleed out issues). Sometimes I see the anodize tank is blue, other times it is red ... I am sure you understand what I am saying. The good news is the tank does not stay discolored for weeks but maybe a day or so max. It does not affect parts being anodized, meaning parts anodized for clear will not come out with a red tint on them. (I have never seen this happen). The degree of color loss on parts being brought back to the tanks should be directly related mostly to the dye you are using. (A less durable dye might surrender more color). When I take black back to tanks, the parts will usually come out a steel grey after the extra time.

A. Hello Goran,
Frankly,I suppose source of this problem out of anodizing line. Most likely it's related to metallurgical problem such as alloy composition or homogenizing. Please check alloy composition and share with us.Some element(s) may be out of tolerance and coarse MgSi layer during homogenizing.

Despite my guess out of anodizing line you would try these in your anodizing line if you want:

1-skip etching bath for one bar and check out the pits after anodizing

2-polish some profiles and anodize the profiles as polished black and check out

3-(most important) Try ACID ETCH BATH instead of traditional caustic (alkaline) etch. I am sure you won't see these pits because acid etch mechanism runs differently from alkaline etch.

Good luck

A. Hey Goran

I see a familiar problem there. White Spots, Black Anodizing on Titanium Racks. No obvious cause and yet there it is. Over the past 2 years, we've been dealing with a very similar issue, the main difference seems to be temperature related between the process you run and the one we run at our plant.

We tried everything we could think of. Aluminum racks, extra rinse post anodizing, acidic rinse post anodizing, checking for contaminants (Including Chloride) and nothing gave any indication that we were on the right, or wrong, track.

Well, you have to try SOMETHING. We ordered a small sheet of 5 mm magnesium plate, attached some to an aluminum rod and fastened it to the loadbar after anodizing. Letting it run all the way through colour, rinse, seal and watched our problem disappear over night.

Even if you can't measure a current, the titanium racks work a bit like a capacitor and can be a reason for a discharge over time. This can cause galvanic corrosion which looks like white spots. The Magnesium being a more reactive metal will help avoid this.

Bear in mind, this is my experience, in our plant, but it worked and still works, every day. All I can hope is that it does so for you, should you try it. There is much more theory behind this, that I'd need to brush up on, if I were to give a more in-depth explanation of the science behind it. Also, it's not like we invented the wheel, someone else did, years ago.

Best of Luck!

Thanks, M! There IS something new under the sun :-)

A fuller discussion of magnesium sacrificial anodes for aluminum anodizing can be found in thread 1594 but it never occurred to me that white spots could be "rectified" this way.

Regards,