Inhibition of Zn-rich surfaces

[Neil00001]

I'm currently deep in argument with a colleague over whether it's possible to inhibit a Zinc-rich (7072) aluminium internal clad on a heat exchanger tube with conventional ethylene-glycol coolants. I've always believed this internal clad to be a waste of time, as the frankly crap mechanical properties of this clad compared to the relatively high-strength modified 3005 reduce overall component strength. Could anyone please give me a copper-bottomed justification of my beliefs!?!

 

[kenvlach]

Are you talking about Al 7075 clad with Al 7072?
I checked out Al 7072 at MatWeb.com and find it hard to believe that anyone would use such a low strength alloy (YS = 9.9 to 11.6 ksi), with a little bit of zinc (0.8-1.3%) thrown in for corrosion vulnerability, for its own properties.
Its sole purpose seems to be as a cladding for the high-strength 7000 series alloys, for which “a cladding of pure aluminum would not provide the required sacrificial protection. For these alloys a cladding of a 1.25 per cent zinc alloy on a 99.7 per cent base is used.” – The Technology of Aluminum and Its Alloys, P.C. Varley, CRC Press (1970).

Some properties to consider:
Al 7075-T6 sheet (unclad): YS = 63-69 ksi
Al 7075-T6 sheet, Alclad: YS = 58-65 ksi
Al 5052-H38 YS = 37 ksi
Al 3005-H18 YS = 32.6 ksi
Al 3003-H18 YS = 26.8 ksi
Al 5005-H38 [99.2% Al] YS = 26.8 ksi
Al 7072-H14 YS = 11.6 ksi

In terms of strength, clad 7075-T6 is far better than 3005. However, note the galvanic series in MIL-STD-889B, Notice 1, on which 7072-clad 7075 is the most anodic of all Al alloys shown. Al 5052-H16 is the best of the Al alloys listed.
However, the reference above notes for refrigerator heat exchangers, normally 99.5% Al is used, or else the 1.25% Mn alloy [Al 3003].

But, the real proof is in the field:
“3000 series aluminum welded tubing with 4000 series cladding is used as the base material in the manifold design.”

http://www.thermasys.com/heat.html

You can also download the Alpema Standards, of the
Brazed Aluminum Plate-Fin Heat Exchanger Manufacturers' Association from

http://www.chart-ind.com/p_he_literature.html

Page 45 gives typical materials of construction. Heat transfer fins are often 3003 or 3004, and header alloys include 3003, 5052, 5454 and 6061.

In summary, I would suggest that 7072 cladding was used in your case to protect a higher strength 7xxx alloy, there may be a strength-to-weight requirement, since the combination of 7072 clad 7075(?) is rarely used outside of aerospace application, and the fluid should include anti-corrosives specific for aluminum alloys.

Please give details of the cladding, is it 7072-clad 7075?
And, is there a strength-to-weight requirement?
Somone once went to the trouble to engineer this, I presume they had reasons.

 

[Neil00001]

Thanks for your reply,

The 7072 is used as an internal clad on 3003/5 radiator tubes.As hard as this is to believe (remember that I too think it's a crap idea) up to 10% of gauge is sacrificed for this clad. It's a japanese principal, that's based on the idea of some idiot filling his rad with water from the Okoyama river (pH 3.5) and obviously not practising good coolant management.

My argument is that in trying to pass this extremely unlikely scenario we are detrimentally affecting real-world performance (all that zinc diffusion causes intergranular corrosion in our higher copper tubes). I would rather simply rely on good coolant management, but my colleague's argument is that the 7072 can be considered a 'belt and braces' solution. Is this the case? Is humble pie in order?

 

[kenvlach]

So, instead of using a pretty corrosion resistant alloy 3003 or 3005 as-is, which is normally the case for HXers, someone decided on a sacrificial anodic cladding of 7072 a la a galvanizing layer of zinc on steel.
I agree with you, this is a poor design (although I haven't worked out the possible ramifications of zinc ions in solution -- what other metals are in your system?).

Rather than using the 3003/3005 in its original thickness, increasing the 3003/3005 thickness by an amount equivalent to the cladding, or cladding with high-purity Al (>99.7%) to form a more corrosion-resistant layer would have both been more appropriate means to give the desired 'belt and suspenders' extra security.