Corrosion of 316L in less than .5% Sulfuric acid 2

[dolphinue]

I'm designing a biodiesel plant and will be using sulfuric acid as the catalyst. I would like to use 316L for the tank and piping but find mixed information on corrosion rates. The other problem is that all the information I do find is sulfuric acid in water and our system will be sulfuric acid in what is basically animal fat. The mixture will typically be 95% animal fat, 4.5% water and .5% sulfuric acid at 90C and 100 psig. The flow rate will be around 4.5 fps in the piping. Going to alloy 20 or Hastalloy could sink the project due to cost. I've also looked at teflon lined piping and tanks and again costs are high.

 

[ApC2Kp]

dolphinue,
The 90 deg C may be too hot. The acid corrosion depends on both concentration, temperature, and other ions like chlorides, fluorides, beneficial traces of copper, iron, molybdenum, etc. The 316 SS might be able to sustain weak acid to maybe 50 C. The most common materials for weak acid service are CPVC plastic and vinyl ester fiberglass. These would have small margin for temperature excursions above 90 C. The water's chloride content probably needs to be controlled ( maybe less than 25 ppm?) Some lab bench tests of material coupons would be useful before trying a pilot plant of your process. Other metals to consider for a corrosion testing program are 317, 904, AL6XN, 254SMo....
KYNAR fluoropolymer would have 150 C. limit at somewhat less cost than teflon.

 

[rolledalloy]

Looking at the isocorrosion charts for sulfuric acid 90C is just above the borderline for less than 0.004 inches per year corrosion in 316L. You might look at a duplex stainless like 2205 (UNS S32205/S31803) it is 22Cr, 5Ni, 3 Mo.

In dilute sulfuric, especially if some chlorides are present 2205 will provide improved corrosion resistance with much less of a cost adder than an alloy 20 or Hastelloy C-276. Expect it will run maybe 10-20% more per pound than 316L stainless.

 

[AndyCarlson12]

Hey

Just a thought.....

You could switch to a different catalyst such as sodium methylate which is compatiable with 316L stainless steel.

 

[EdStainless]

If yout total mixture is 4.5% water and 0.5% acid, isn't that saying that you have oil with 5% of a 10% acid solution in it? You are not working with a 0.5% acid.

Wherever water will settle of collect in your system you will have localized problems. Consider making different sections (bottom vs top) out of different materials. It is more complicated, but can save a lot of money.
The 2205 is good thought, but testing is even better.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

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