Sulfuric Acid Anodizing Type IIB procedure & problems

A. Sorry I can't help with the thickness change, I knew 2B coatings to be dyed black.

A. Just to clarify and prevent confusion, note the difference between Type IIB "thin film sulfuric" and Type II Class 2 "dyed sulfuric". A Type IIB Class 2 would be thin and dyed (say, black, for example) but a Type IIB Class 1 would be thin and undyed.

I don't recall parameters, but I believe that Type IIB cannot be performed in a Type II process tank - it's not a matter of just varying the time or voltage, which might give you the desired coating weight but compromise coating performance. Concentration, temperature, time and voltage, as well as second order interactions will have to be worked out to get an adequate coating.

? Hi James,

I must admit, I have never heard or read the term "thin film anodize". To which specification are you referring?

Thank you,

A. Hey Tom,

I was referring to MIL-A-8625 / MIL-PRF-8625 [on DLA]F. Paragraph 1.2.1 lists "Type IIB - Thin sulfuric acid anodizing, for use as a non-chromate alternative for Type I and IB coatings..."

I notice it doesn't actually use the term "thin film" - I guess that's just something I've picked up along the way. (I have seen the term in industry specs)

Thank you James,

I was looking at revision E, which does not mention the thin coating. What kind of coating weight or thickness are we talking about here? The E revision mentions 70 millionths of an inch, is that what you call thin? I don't see why the sulfuric bath can't provide a thin coating, or at least, I can't find a reference that says it can't be used.

A. Hey Tom,

As to coating weight, the MIL spec. calls out 200 to 1000 mg/sq.ft. Some might want to raise the minimum slightly if corrosion resistance is a big issue, and lower the maximum significantly if fatigue is a concern. For thickness; its been quite a while, but seems to me something close to, but under, a tenth of a mil was desired. A Type II sulfuric bath could provide a coating of that weight/thickness, but the bath/process is designed for the thicker coating. The coating characteristics (e.g. pore density and individual pore dimensions) are not optimized for the thinner coating and performance of the coating (esp. corrosion resistance) will be compromised. Of course, if one's customer acceptance criteria are looser than those in the MIL spec, a coating produced in a standard Type II tank might be just the ticket.

A. Hi Hammad. I don't follow your arithmetic (you've only shown us a piece of it, without any numbers plugged in). What are you using as the density of the alumina, and what is the meaning of the "10" term?

But remember that the coating thickness is measured per side, whereas ^ ^some coating weight specs include both sides. Maybe you just omitted to divide the coating weight by two?

Regards,

A. Hi again, Hammad. For a formula or calculation to work, the units must balance. So let's check ...

If you want thickness, T, on the left side of your equation to be expressed in µm, [i.e., m/1,000,000] then the units on the right hand side of the equation must evaluate to m/1,000,000 ...

You have already said that the weight is expressed in mg [i.e., g*1000]. You have given us the density of alumina in g/cm^3 [i.e., g/(cm x cm x cm ), i.e., g/(m/100 x m/100 x m/100)]. So the only way the formula makes sense is if the area in the formula is expressed in cm^2 [i.e., cm x cm, i.e. m/100 x m/100]. Checking ...

T_[m/1000000] = weight_[g*1000] x 10 / (area_{[m/100 x m/100]} x density_{[g/(m/100 x m/100 x m/100)]})

So, indeed The Canning Handbook formula is:
T_[µm] = weight_[mg] x 10 / (area_[cm^2] x density_[g/cm^3])

If you want to use a similar formula but with the area expressed in square feet, you'll need to insert a conversion factor of 929 cm^2/ft^2:

T_[µm] = weight_[mg] x 10 / (area_[ft^2]*929_[cm^2/ft^2] x density_[g/cm^3])

The second paragraph of my previous response turns out to not be applicable now that I understand what you are trying to calculate. And your factor of .128 ft^2 (the area of the test panel) isn't applicable either because the 1000 mg weight requirement is per square foot, not per test panel.

Regards,

A. Hi Hammad. Sorry, I am not going to answer that one. It is a 20-page specification, with links to other specifications, and it says what it says ... I won't attempt to condense it and miss something. Further, there are frequent revisions. You'll have to read the spec and decide for yourself what you think it says with respect to your personal situation. Sorry.

Regards,

A. Hi. No you can't skip necessary conversion factors like that. A meter is 100 cm and a micrometer is 1/1,000,000 of a meter, and a milligram is 1/1000 of a gram -- and you must always insert correction factors for these conversions as necessary. As you start canceling out these strings of conversion factors, we learn that if you express everything on the right hand side of the equation in centimeters and milligrams, multiplying by 10 gives you the thickness in micrometers.

Regards,

A. Hi Ander,

Poor adhesion like you are seeing tends to be more to do with surface cleanliness rather than compatibility issues. That is not to say that the particular combination of thin film sulfuric anodizing and primer is not a problem. If you have been using this process for a while and this is the first time you have seen the problem then I would suspect cleanliness, if you have seen it frequently it could be a compatibility issue, but just as easily be a cleanliness one, problems with mixing of the primer, application issues, drying issues ... unfortunately the list goes ever on.

If you haven't done it yet then I would suggest you "walk the process", from anodizing through to final testing of the primer, that way you can pick out anything that is obviously wrong, put it right and see if it solves the problem.

Thank you very much Brian

As you recommended we have followed the entire process from the beginning. We visited our suppliers to ensure the conditions of the anodizing, the cleaning and painting. Some dust and excessive humidity might be the reason.
Anyway, we are changing to a new Chrome-Free primer. We have also performed the process in some trial specimens with both primers, and the adhesion is much better with Cr-Free one.

To sum up, the waterborne primer quality plus the non-strict conditions of the process generate those irregularities.

Thanks again for your help. I will keep you informed about our progress.

Kind Regards
Ander

A. You didn't mention what your required thickness is. There are a couple of things an anodizer can do with just plain sulfuric acid anodizing to influence the color of the coating without dyeing (most seals do not impart any color to the coating).

1. Lowering the temperature of the anodizing bath leads to darker coatings

2. Increasing the current density will also lead to darker coatings.

3. Increasing the film thickness will lead to darker coatings.

Alloy selection also plays a role. 5000, and 3000 series Al tend to have lighter natural coatings, while 6000, and 7000 tend to be darker, as well as 4000 series.

A. Hi Darren. I personally haven't seen Type IIB anodizing done. But I'd suspect that the reason for it absorbing no color or insufficient color from the dichromate is the same as for Type II anodizing: no anodizing or insufficient anodizing in those areas of those pieces. Try to do an anodizing thickness test.

Luck & Regards,

A. Boric Acid is insoluble in mid-pH rinse water. It just will not rinse off by dipping. A 5% nitric acid "post dip" will remove the trace boric acid (but the Lockheed school boy engineers will not allow), otherwise, use high force spray hose rinsing. The dichromate "takes" better after the boric film is removed.

Thank you
There is no conductivity in the area but I will check the coating thickness for uniformity.

"Could this be from heat or other contamination. I attach a photo."
Yes, heat or work hardening from the stamping.