Wastewater application - H2S Corrosion 1

[sadochemeng]

Hi all,

I am just trying to understand the mechanism and chemistry behind the situation happening in our application.

Wastewater comes in a pipeline, and low DO in water, bacteria, organics, H2S occurs. Does it occur in gas form or dissolved in water? If both, which would be dominant? My research so far says H2S is not very soluble in water. But it can be up to 4-6 g/l or 6000ppm.

Then it enters our process. There is some turbulence in the air but it is enclosed area and so air cant really change or go anywhere, so we had readings up to 800ppm. We also had pH readings on metals as low as 4.5. I read H2S is a weak acid so, I dont think it causes this low pH levels. Can it become H2SO4 with some kind of reaction?

And how does it attack to steel components? In gas form or liquid form? What reactions happen? How does it work?

I dont know if this subject is covered, did a search but couldnt find anything. If it is, if you can direct me, I'd appreciate it.

Thanks.

 

[rconner]

See my post a few minutes ago to another inquiry at

http://eng-tips.com/viewthread.cfm?qid=290886&page=1

, and particularly the references and linked articles that get into these issues in much more detail. While I don't know your specific conditions, I guess it is possible sulfuric acid is being biogenically created in your airspace, and that might in turn create a lower pH (than you expect with initially generated sulfurous?) Also, I believe turbulence might enhance the release of hydrogen sulfide, as well as moisture, into any airspace (and I believe these things are known to exacerbate the conditions described). Have a good weekend.

 

[SJones]

A couple of items for you to work through.

http://www.corrosioncenter.ohiou.edu/freecorp/

Slightly oil & gas orientated but you will get the mechanisms and the chemistry.

Steve Jones
Materials & Corrosion Engineer

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

 

[EdStainless]

There are a couple of different mechanisms. You probably do have some H2SO4 forming. Some of the attack is from that. Some is from the formation of metal-sulfur compounds on surfaces. Many of these compounds are soluble in water, hence the metal dissolves.

Interactions with other species in the water can make this a very complicated set of reactions.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[bimr]

H2S in sewers has been extensively studied and it is hard to believe that you have not found information.

Sulfate (SO4) in sewer water is reduced to Sulfide (S) by naturally occuring anaerobic bacteria.

Additional bacteria convert the sulfide and water in the sewer into hydrogen sulfide.

Additional bacteria convert hydrogen sulfide and oxygen into sulfuric acid (H2SO4), a strong acid.

The pH influences dissociation of the sulfid ion species in the sewer. At a pH of 6, more than 90% of the dissolved sulfide is hydrogen sulfide. At a pH of 8, less than 10% is in the form of hydrogen sulfide.

If the pH is low, the corrosion will be more severe because it will be caused by sulfuric acid.

http://www.clihouston.com/news/mechanisms-of-mic-in-wastewater-applications.html

http://nepis.epa.gov/Exe/ZyNET.exe/...ge&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL