HCl corrosion resistance using Platinum group metal coatings

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thread338-315503

Although various SS alloys can work well with wet HCl gas and liquid HCl there are certain coatings that can also protect against HCl corrosion of some steels . Various Pt group alloys are highly resistant to HCl as is Ta.
The difficulty is the high melting point of these metals and in getting a continuous , defect free coating at a workable thickness . Some of these metals are also either too soft or too brittle when used as pure materials . Alloys seem to work best .

[tt]Does anyone have experience with Platinum group coatings for HCl resistance ?
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Coating must flex with a bending part such as a leaf contact or spring .
Concentration above 28% , temp. 250 ℃ .

 

[ehbadger]

Briefly used tantaline but not sure how it would work on a flexing par or how much hydrogen embrittlement (which you'd expect with tantalum at high HCl concentration and high temps like 250C) would matter. Had a valve with it but one small knick through the coating and it was basically game over.

 

[thickfilm1]

Tried Tantalize and had the same problem . Not durable enough for parts with contact to hard surfaces.
Thanks for the reply.

 

[ehbadger]

How big is this part? What is mechanically required from it? That is very hot for such a concentration, and adding in a mechanical abrasion mechanism I'm not sure you have much, if any, options. Too hot for long term fluoropolymer. Even a pure Ta or Ta alloy like Ta2.5%W if strong enough would struggle after some time I think from hydrogen embrittlement. Graphite is hardly flexible enough plus no resin will last at those temps. Maybe a silicon carbide can withstand the acid but again, not going to be flexible!

I will be watching this, I've got some applications with hot HCl (36%, 180C, 160psi) and our options are very limited even at these conditions, and I don't have nearly as difficult "flexibility" requirement. Please share if you do come up with something!

 

[thickfilm1]

Part is a torsion spring w. Max 180° rotation .
Must last 12,500 reps.
Made of super alloy w. High Cobalt & Moly.
My thought was either a sacrificial coating or pure barrier w. Platinum group metal alloy.
We are experimenting with various combinations now.
Corrosion occurs on outside dia. at about 150-200 reps Max .
Pt itself may work but chlorides ware present and seawater .
The only other HCl " impervious" metals would be Pt group and other nobles.
Perhaps Au ?
The HCl remains liquid due to high pressure .
Any info on high concentration HCl condituions is appreciated .