Mercury Attack

[Ein-no-stein]

Dear corrosion geeks,
Did anyone have experience on the CP design for mercury containing oil or water system?
The typical anode materials including Al, Zn, Mg are susceptible to mercury attack.
If the solubility of mercury in the oil or water is high enough that no precipitation is expected, do I still need to worry about the mercury attack on my anodes? Is it necessary that only precipitated mercury can attack anodes?
I can add more mass to count for the mercury attack, but do I need to worry about the recycling of mercury?

Thanks,

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[Chumpes]

Hello,
In my opinion, the so called "mercury attack" is a form of liquid metal embrittlement where mercury at liquid state dissolves the solid metal and passive scales, maybe preferentially at chemical discontinuities (grain boundaries). It is dissolution, amalgam etc. and it should have nothing to do with oxydation of metal or electrolytic corrosion. Thus, if you have identified that your system is subject to the mechanism of "mercury attack", cathodic protection will not help.
if mercury is dissolved in water or oil without phase separation, then i believe there will be no problem of liquid metal embrittlement.

Did you observed any damage or is it a theorical question ? are you totally sure about the mechanism ?

It is said that mercury attack is a problem for aluminium and zinc attack is a problem for stainless steels.

Regards

 

[TugboatEng]

Depending in the materials of your system, eg copper alloys, you can use iron anodes.

 

[Dhurjati Sen]

Dear Ein-no-stein,

The "mercury attack" that you are talking about is basically Liquid Metal Embrittlement. It occurs by the direct interaction of the liquid metal atoms and the highly strained atoms of the anode at the crack tip. Wetting is the first requirement for LME.

In your case, wetting is ruled out as solubility of mercury is high enough.

Secondly, the anode material is not having high strains.

So, you need not worry about LME of the anodes.

Regards.

DHURJATI SEN

 

[SJones]

The principal damage mechanism in an aqueous environment with elemental mercury will be amalgam corrosion which requires the contact of the elemental mercury with the metal surface. Some metals may also be corroded by mercury chloride and/or dimethyl mercury. In what chemical form does the mercury exist in the water?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Ein-no-stein]

All,
Thanks for your reply.
I am more concerned about the amalgam corrosion rather than LME.
The mercury I guess will be element "mercury" in the condensate or water phase.
My thinking process is since no mercury will precipitate, there will be no LME on the anode chunk. Therefore, the anode will not fall apart and de-attach from the tank surface. However, the dissolved mercury can still have chance to slowly amalgam react with anodes.
But I am not sure if the mercury will recycle its reaction or not based on the following chemical reaction equation.
2 Al + 3 Hg2+ + 6 H2O → 2 Al(OH)3 + 6 H+ + 3 Hg
Hg + Al → Hg·Al
2 Hg·Al + 6 H2O → 2 Al(OH)3 + 2 Hg + 3 H2

Thanks,

 

[EdStainless]

The problem is that the Hg solubility is very low, and some will inevitably be present as blobs of liquid Hg.
Some of the Hg will recycle, some will be consumed in the amalgam.
I remember a case of a Monel valve in the bottom of a tank. After a few years of operation they wanted to drain it during an outage.
The valve broke right off, and there was a tiny drop of Hg in it.

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

 

[Chumpes]

Mercury is more dense than water, and mercury attack may be a problem at low points only.