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Neutralization of acid residues (post brazing/chromate conversion of Aluminium)

 

We manufacture aluminium assemblies which are typically flame or dip brazed. The assemblies then receive a chromate conversion coating. As such the assemblies are exposed to acids through the production process. This may result in acid entrapments within any porosity in joints or cast components.

Is there an aqueous rinse or similar on the market which could be used to neutralize trapped acids? Is this a problem other people have encountered ?

Rich Mosley
- UK

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It is possible to vacuum impregnate as a guard against porosity, and to seal joints with an anerobic sealant. But one of the beauties of chromate conversion coatings is that the chromic acid is usually considered protective rather than corrosive.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

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Dear Mr Mooney,

Thank you for your reply.

To give a little more background to my previous letter / enquiry.

We manufacture microwave and rf systems for a range of industries, i.e., telecommunications, defence, space etc etc. As such the assys can be complex and contain a number of brazed joints and cast components. In many cases these assys are pressure impregnated in order to give a seal for both porosity and for pressurised systems. However at times, whilst a pressure seal is achieved acid may still be trapped within certain areas of what has been termed micro porosity. Additionally, in some cases it is impossible to impreg the assy due to customer or service requirements. When referring to acid I am meaning acid from the pre braze cleaning opps and the pre chromate conversion opps rather than the chromic acid from the conversion.

The process currently used in order to attempt to neutralise any trapped acid is to run the assys through a neutralising bath of Turco's T4215 NC. This is a non silicated, non chromated mildly alkaline solution. This process was introduced many years ago and since I have taken over I wish to review its effectiveness, hence my enquiry,

a) Is this a problem other people have encountered ?

b) If so is there a product on the market specifically for this purpose ?

Apologies for being long winded. Please feel free to edit and publish any part of this mail at your discretion should you feel it warranted. Any further comment or advice would be appreciated.

Best Regards,

R.W.Mosley
-UK

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I don't know the best way to analytically quantify the residue, but it can be done with laboratory instrumentation, and perhaps someone who works hands-on with similar analyses will chime in here.

As for how to minimize it, it addition to employing ultrasonics in the rinses, two effective measures are alternate hold and cold rinses, and employing a neutralizing agent (as you are doing). A third method which I have heard of many times from anodizers, but I don't truly understand the dynamics of, is rinsing in dilute nitric acid to displace the pickling acid before rinsing and neutralizing.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey