Digester stainless steel corrosion

[sjt01]

This is a reducer flange from the digester roof that had to be replaced after 8 years due to gas leakage. Its about 4 mm thick stainless, but only 304 rather than 316. Its quite frightening the amount of corrosion on this and another flange. I had thought a stainless tank would be a good idea, but no longer. Ours is vitreous enamel Permastore.

Had to get a team in with breathing apparatus and working at height gear to swap this flange and the overfill hatch which had even more corrosion.

Just a reminder to everyone to keep a sharp eye out. It was only a gentle hiss but we were losing about 40 m3/hr of biogas.

 

[kill]

This is a reducer flange from the digester roof that had to be replaced after 8 years due to gas leakage. Its about 4 mm thick stainless, but only 304 rather than 316. Its quite frightening the amount of corrosion on this and another flange. I had thought a stainless tank would be a good idea, but no longer. Ours is vitreous enamel Permastore.

Had to get a team in with breathing apparatus and working at height gear to swap this flange and the overfill hatch which had even more corrosion.

Just a reminder to everyone to keep a sharp eye out. It was only a gentle hiss but we were losing about 40 m3/hr of biogas.
View attachment 630992

Hope you didn't find that while smoking?

 

[Digester Man]

Bloody hell Stephen that is shocking! Who cut the corner with the 304?

I was advised by Hochreiter, that high o2 leads to accelerated corrosion, I think they said it leads to formation of sulphuric acid. I try to keep below 0.5% and ideally around 0.3% in normal circumstances. Not sure if it would cause the above though!

 

[ollie989898]

Stainless steel will corrode where oxygen is unavailable to form it's passivation layer. In this instance, we are describing a crevice which I suspect prevented any oxygen from reaching it. Throw in the ubiquitous hydrogen sulphide/sulphuric acid (and possibly chloride ions?) and you are bound to get some corrosion over time. Particularly where dissimilar metals may be joined?

 

[Digester Man]

Stainless steel will corrode where oxygen is unavailable to form it's passivation layer. In this instance, we are describing a crevice which I suspect prevented any oxygen from reaching it. Throw in the ubiquitous hydrogen sulphide/sulphuric acid (and possibly chloride ions?) and you are bound to get some corrosion over time. Particularly where dissimilar metals may be joined?

There is almost no oxygen in a digester, hence anaerobic

Stainless is the preferred material for all pipework and some even construct the tank itself from it

 

[sjt01]

What surprised me is it appears to have started in the middle of a piece of flat plate, away from folds or welds. Its been replaced by 316 of twice the thickness. We normally run at 0.5% oxygen 100-200 ppm of hydrogen sulphide. The gunk that can be seen in the tube is elemental sulphur.

 

[ollie989898]

There is almost no oxygen in a digester, hence anaerobic

Stainless is the preferred material for all pipework and some even construct the tank itself from it

There is no free oxygen in a digestor (or rather very tiny levels) but there is bound to be some in compound with something.

There has to be some oxygen present as it is this that forms the passivation layer that in turn protects the steel.

A lot of environments will cause corrosion to stainless steel of course a digester is a veritable soup of chemistry so id be guessing at what may occur in a typical system.

In the example above it has corroded in a mating surface which suggests it had a localised bit of chemistry in that area.

My cousin is a materials engineer who could explain it a lot better. He did explain to me once that corrosion usually occurs but the rate it happens is the important bit.

 

[TurboCon]

Nasty one, looks like some serious compatibility issues there...would it be possible to install PTFE lined pipe/flanges?

 

[sjt01]

Nasty one, looks like some serious compatibility issues there...would it be possible to install PTFE lined pipe/flanges?

How do you get PTFE to stick to it? PVC attached by Sikaflex TS might be an option

 

[TurboCon]

Special pipework...the liquid will only come in contact with the PTFE ....used in industry as a cheaper alternative to Hastelloy or other specialty metals. If its eating 304 it may well eat 316.

https://www.google.co.uk/search?q=p...1pjZAhVCKlAKHW-1BfIQ_AUICygC&biw=1289&bih=665

 

[ADMaestro]

This is a reducer flange from the digester roof that had to be replaced after 8 years due to gas leakage. Its about 4 mm thick stainless, but only 304 rather than 316. Its quite frightening the amount of corrosion on this and another flange. I had thought a stainless tank would be a good idea, but no longer. Ours is vitreous enamel Permastore.

Had to get a team in with breathing apparatus and working at height gear to swap this flange and the overfill hatch which had even more corrosion.

Just a reminder to everyone to keep a sharp eye out. It was only a gentle hiss but we were losing about 40 m3/hr of biogas.
View attachment 630992

We had a company called Eriks on site to do a leak detection survey - they used some fancy bit of gear and identified a few leaks. Unsure how much it ended up costing but might be worthwhile for anyone to look in to if they suspect any leaks but can't verify them!