Corrosion Protection of Steel in Biogas Operations

[mechjason]

Hi guys,
I'm designing a lab-scale digester tank to produce biogas, and have initially worked on the assumption that I will make it all from 316 stainless steel to protect against H2S (up to 3,000ppm in somne instances). I'm now wondering if using mild steel and a coating could be an alternative option. The tank will operate at around 40C (104F) with constant agitation and contain waste and a volume of gas. I've read a bit about electroless nickel plating (possibly with hard chrome also), which looks like a good idea, or possibly nickel-PTFE, or some ceramic coatings. I cant find any quantitative data to determine the H2S resistance of different options, when compared to 316, so just looking for anyone's advice/experience with this sort of conditions. In terms of wear resistance, it also looks like EN would be a better option than 316.

 

[micalbrch]

3-5 mm rubber lining.

http://www.rema-tiptop.com.

Always my first choice for wear and corrosion resistant coating. How attractive that is (price) depends upon the size of tank. The bigger the tank the more expensive is it out of stainless steel. Your tanks seems to be rather small (lab-scale). Perhaps a plastic tank is an alternative?

 

[moltenmetal]

Lab-scale? It won't be in service long enough to need anything other than 316SS.

If you're considering coatings, rubber is one, but so is a hard (alumina filled) epoxy such as Placite.

 

[CoryPad]

moltenmetal,

Plasite?

http://www.carboline.com/products/plasite-linings/

 

[mechjason]

thanks for the feedback guys, I'm looking at having it in service for maybe a couple of years. I'd probably preffer to stay with steel for fabrication purposes, but is there any type of plastic coating in particular that you would recommend given that?

 

[moltenmetal]

OP: Yes. That's an option for you at your conditions.

Your conditions are so mild that you might consider carbon steel with a corrosion allowance. Under anaerobic (reducing) conditions, which are what you're getting if you're generating methane and CO2, there shouldn't be much corrosion even if there's some H2S. However, if the thing is being opened and closed constantly to add waste, which will also add oxygen to the mix, you may be in trouble with plain uncoated carbon steel.

Plasite is far easier to apply properly than rubber is, especially to small equipment. Both are erosion resistant but in different ways. Neither is any good if the part to be lined is designed improperly or if the lining is installed by someone incompetent.

Any piece of equipment to be lined for chemical resistance needs design, fabrication and surface prep suitable for lining. The critical things in that are having a GENEROUS radius at all sharp edges (transitions from shell to nozzles, nozzle necks to flange faces etc.). Other points are the elimination of threaded connections entirely unless those threaded bosses etc. are made of solid chemically resistant material (NPT threads have sharp edges which are impossible to reliably coat for long-term corrosion resistance by just about any means, and the second you thread a fitting into them the lining is destroyed anyway), making all connections of adequate size to permit access for lining, providing body flanges etc. to permit access for lining etc. If you're going with an epoxy like Plasite, a near-white grit blast profile on all surfaces to be coated is a must for a durable coating, so you need the part to be designed so you can do that.

If your container is too small to accomplish these design requirements in, then you should go with solid uncoated 316SS and just monitor it for corrosion. I assume it's not high pressure, so if you do have corrosion the consequences will be a leak, which your system should be designed to mitigate anyway because not only corrosion can cause leaks.

Best of luck.