Hot (boiling) DI water seal after anodizing

A. Hi Jack. You may realize this but, just so that we don't mislead readers of this Q&A, I should note that cold D.I. is not going to seal your anodizing -- we're still talking about using boiling water, but D.I. rather than tap water. If you're not suffering any salt stains, pitting, or other problems mentioned on this page, keep calm and carry on :-)

Luck & Regards,

A. Hello Jack. Dyed work always uses nickel acetate. I don't pretend to understand in depth the exact chemistry of it all, but it acts as a catalyst shortening the required sealing time, often allowing slightly reduced temperature as well, causing less leaching of the dye, and also improving the corrosion resistance.

Some people feel D.I. is just as good for clear anodizing; others, including expert Robert Probert, feel that nickel acetate should be used even on clear anodizing for better corrosion resistance. We have numerous threads about sealing in general and nickel acetate specifically if you search the site.

If following threads leads to bouncing between stuff that's over your head vs. wasting time endlessly re-reading what you already know & understand, you may prefer to pay for a void-free tutorial approach: Robert's book offers that with 8 excellent large-print pages on sealing followed by 4 on chalking & smuts -- and no one has ever asked for a return/refund :-)

Luck & Regards,

A. Please note that sealing test with nitric is not possible for process of Dye Colored articles. First try to seal article in specially available sealing for Dye in market then try the test as suggested by supplier.

A. Satish,

I disagree... the 40% Nitric acid test is widely used to test seal quality on dyed parts; all you have to do is to leave it on for 2 minutes, rinse, and note if any discoloration/attack has taken place.

A. Please tell us more about your sealing tank, like the temperature, the pH of sealing bath, the time of holding the load in sealing tank. Principally for color seal you have to follow two step sealing:
1- using Nickel acetate plus nonionic surfactant
2- last step using hot water seal in separate tank. Finally, for seal quality test, follow specified standard.

A. Additives to the DI hot water seal do help prevent smut or bloom and give a better seal.

My question would be why bother with it. You will spend a lot of time, which is money, and may never equal the dedicated research that chemical companies have done to save possibly a dollar a day for an average size anodizer.

I would stick to a company that Aluminum is their main business.

The first two that come to my mind are Novamax and Clariant.

A. Sealing smut usually forms when the sealing tank becomes alkaline rather than slightly acidic. Unfortunately this is also an area which produces a very good hydrothermal seal due to the excess of OH- ions. Additives usually suppress the smut by dispersion of the precipitate.

Albright & Wilson has such an additive called "Speedseal" for conventional hot DI water sealing. The advantage is the excess hydroxyls speed up the seal time.

Better rinsing may help the problem since about 95% of the sealing smut problems come from sulfides from the anodize bath

A. Best answer for that question of what sealant to use after anodizing in hot D.I.water ...
Seal 2532 is best. Add 2 liters for 140 gal hot DI tank. It raises TDS, keep pH between 5.5 to 6.00, parts for 2 hours and see the difference.

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Ed. note: James is surely right that proprietary seals beat home-brew, but please don't recommend specific proprietaries (why?). It gives others carte blanche to run their ads too, turning the page into a spam fest. This site is only here for you courtesy of a small number of generous advertisers who make it possible. Some already dropped their support, unwilling to pay the cost of maintaining postings recommending their competitors who don't support the site. Non-advertisers suggesting their own products hasten the day when sites like this can no longer survive.

A. Hi Ted,

This additional step is completely unnecessary except as you pointed out for the CERTIFICATION. One would first ask the question: Is my current seal doing its job or not?

If the answer is YES, then why add an unnecessary step?

If the answer is NO, then you should rather think about solving your problem!

Now if you want to eliminate the staining after the Nickel Seal, you can call your last step a hot rinse rather than a DI WATER SEAL.

Regards,

A. There are tests such as a dye-stain resistance test which will determine if the part was sealed, but not how. I am not aware of a seal test that can determine what type of seal was used. Likewise I have never heard of a "thickness test" for seal, and cannot see why you would require one.

Your anodizer can provide you with a process certification and should be able to perform the seal test as required by Mil-Standards.

What are you experiencing that has you doubting the seal quality?

A. The seal has no thickness. To seal anodizing means to convert the aluminum oxide in the pores to hydrated aluminum oxide which is a larger molecule and literally "clogs" or fills up the pore with hydrated aluminum oxide.

ASTM lists sophisticated tests for sealing.

Several thickness testing instrument companies sell an impedance meter which determines the degree of seal. Some of these companies are listed by finishing.com.

Just dip a cotton ball in black dye and swab it on, then rinse it off. If any dye remains visible to the naked eye, then it ain't sealed !

A. The SEAL INTEGRITY TEST per ASTM B136 using a blue dye will work nicely. Basically it requires pretreating an area with weak nitric acid followed by application of a dye. Actually any standard stain (e.g., coffee, Kool Aid, etc.) will work as well. One easy test is to press your thumb against it and see if it feels sticky; the suction created is an indicator of open porosity that hasn't been sufficiently sealed.

Good luck.....

A. General rule of thumb is 1 hour for every mil.

A. Hi Derek,

Generally, sealing time should be 2.5 minutes to 3 minutes per micron of film thickness in D.I.water at 96 °C (minimum). But you are targeting to 2 mils thickness, i.e.,
2 X 25.4 = 50.8 micron ... I don't think normal sulfuric acid anodising will give you that thickness without having cracks in the film! Please ensure that first.

Good luck.

A. The industry standard is 6 minutes per 0.1 mil thickness at 212 °F and pH 6.0; however, 0.002" seems to meet the controversial definition of "hard coat", and you should not seal hard coat because the hydrated aluminum oxide is softer than the non-hydrated as anodized aluminum oxide - you make the coating softer.

A. Yes, this is true Robert but there are many applications where the type III is sealed.

Electrical resistance, color retention, and corrosion resistance are all enhanced by sealing.

A. Seal the part at least 2 Hours

A. Boiling water is too hot for ion exchange resins, particularly for anion exchange resins. If the water is cooled to < 120 °F, then it can be deionized without damage to the resins.

A. Hi. Hot water sealing is pretty energy intensive. A near boiling seal tank evaporates a lot of water, and it may be that just demineralizing the required make-up water is sufficient to hold an acceptable PPM?

If not, pending feedback from those with operating experience, I'd probably propose using an evaporative condensor to cool the water to the 120 °F or 170° point for the circulation through the ion exchanger.

Regards,

A. Hi Sumit. Low pressure steam (<15psig) in stainless steel heating coils is probably the most common approach. It is inexpensive and relatively energy-efficient.

Regards,

A. If you start with DI water, the solids you are trying to remove are mostly what was dragged into the tank on parts.

If the parts are efficiently rinsed before the boiling DI there will be very little drag in and hence little build up of solids.

I would always use a soak rinse before sealing and preferably a spray rinse with DI (use a small garden spray) over the rinse tank before the boiling DI. Measure the TDS and only treat (or replace the DI) when necessary. You may be surprised how long it lasts.

Unless you have a 24-hour operation, the tank will usually cool enough to treat overnight. With good rinsing a weekend treatment may be enough. Even running two tanks with one cool and treating while the other is in service would be cheaper than using a heat exchanger to remove the very expensive energy you put into boiling.

If you are ever forced to use a cooling coil, consider using the output for a hot water rinse.

Save heat energy in your boiling DI by insulating the tank and using chroffles - floating plastic balls - to cover the surface.

A. I agree with Geoff. I've always approached stuff like this, not by treating the problem, but more by prevention. He's spot on with insuring that you have adequate rinsing techniques. Albeit I go a step further in that I employ a tank with a manifold that has rows of conical spray heads that flood the part with DI water prior to sealing in order to insure that all areas of the parts are well rinsed, as some areas could easily be missed with a manual hand sprayer.

Tack your TDS daily from the start of a new bath until you approach your max desired TDS, and schedule your regular tank change-outs based on those results. You may find that you can just schedule a weekly, or bi-weekly change out on a regular basis, which simplifies things.

A. Good day Josh.

What is the TDS of the hot DI seal?
I am targeting 15 PPM max., pH 4-7.
How long are you sealing?
What is the pH of the dye, and are you using a nickel acetate seal, either cold or hot? pH?
What is your spec?
I have mistakenly forgot the nickel seal, and used only hot DI,= smut.
Our spec calls for cold nickel seal and we use the hot DI as a quick rinse to assist in drying.
Hope this helps.

Regards,

A. Hello Mike
Try to look at ASTM D1193 "Standard Specification for Reagent Water". Sometimes AMS specs reference this standard for water types and limits.

A. Hello Mike,
The process spec telling you to use a Boiling water seal should tell you the maximum TDS, either in mg per litre, or as a conductivity reading. You can buy a simple conductivity meter quite cheaply; it gives readings in Seimens(S), milliSeimens(mS), microSeimens(µS).
Most specs require a conductivity below 100 µS.
This is a good way to control seal water TDS, as you just dump the old and make up with fresh de-ionised water, generally below 20 µS.
Hope this helps
Best regards
Mark

This question is complicated by the fact many folks (including myself) run an additive (which i assume is there as a surfactant/buffer) in the hot water seal. So going in with water <10 ppm tds as sodium chloride equivalency, the additive would add a bit of TDS. Not sure how much, as I've never checked after addition. Yikes. Might need to start!

If you wanted to track it, you can check your water in, then the fresh bath with additive, note how much the additive adds, and use that to correct readings you are taking to track buildup from drag-in. Of course, if you're running strictly to a spec, you've got a hard ceiling.

If not (arguably, you should do this anyway), there are two other things you should monitor:
- is pH suddenly drifting or unstable?
- is the bath visibly discolored?

On the up side, it's the easiest bath to dump and remake!