Hot dip galvanized (high temp.) tubing

[XL83NL]

Im not that familiar with hot dip galvanizing applications, most work we do is in stainless. Hence hoped someone could help me with this.

A reknowned supplier for HEX devices is providing us with a design of an air cooled HEX. The tubing in the HEX is finned and has a design temperature of 350 °C. The drawing shows the tubing is hot dip galvanized steel on the outside. Based on some research (same as found in this topic) I found the HDG process is applied around 450 °C. hence using it at 350 °C (which is of course the theoretical max) seems too hot. Other literature show a typical max temperature of 200 °C for HDG steels.

The supplier has now provided us with documentation showing HDG finned tube is suited for use @ max 360 °C. It doesnt make sense to me, but before further commenting on their design I hoped some input could give me advice.

 

[moltenmetal]

Above about 200 C, zinc is no longer meaningfully protective as a corrosion resistance coating.

If your concern is about liquid metal embrittlement, the temperature (350 C) is too low to be a worry.

You're worried about the interior metal temperature no doubt, assuming the tubing is immersed during hot-dip galvanizing and hence completely coated on the interior. Obviously if the interior of the tubes is at 350 C, the exterior will be at a temperature quite a bit lower than that given that the tubing is finned.

 

[metengr]

Absolutely agree with the 200 deg C maximum service temperature limit stated by moltenmetal.

https://galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/in-extreme-temperatures

 

[moltenmetal]

I don't have a problem with the Zn being present at temperatures above 200 C but significantly below 450 C though. It just doesn't serve a useful purpose in service. Once it is all oxidized to ZnO on the outside, it won't provide any corrosion protection either.

 

[metengr]

The problem is the peeling effect of the HDP coating. I would stay away from this application. You have other barrier coatings like diffusion to deal with corrosion issues in HEX tubing.

 

[XL83NL]

Thanks all. This is sufficient input for us to proceed.

 

[XL83NL]

A small update.

The MFR has now switched to 'annealed galvanized' as the surface treatment. Does this ring a bell? Not to me, so after a short Google search I ended up with what I think he means should be galvanneal. Furthermore, the tubes have finnes and are only treated on the outside, the steel tubes inside are left untreated.

Any comments?

 

[EdStainless]

The galv process isn't the limit here. Any use of a Zn (or ZnAl) surface coating will see rapid degradation above 200C.
There is no risk in using it as long as you are under the upper limit, but it will not last long.
It will really only provide protection until the plant begins operating, then they will just have bare tubes.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[XL83NL]

Well, the design temp. of 350°C of course is a theoretical value. Operating will be lower. And, these temperatures apply to the inside, whereas the coating is applied externally, on the fines and tube OD. Thanks for the advice. Hopefully my colleagues will use it to their benefit.

 

[metengr]

XL83NL;
I don't like this approach because the fins will be plugged with the coating and impede further heat transfer. I would stay away.

 

[XL83NL]

I know metengr, but sometimes people believe a manufacturers proposal just like that. If it fails it’s their problem, the thought then is.
Ive rang the bell more than once on this

 

This sounds like a case of a designer overthinking things...
Finned tubes (never mind coated) are an immediate red flag to a failure analyst.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."