Use of Alloy-20 for acidic and basic chemical environment

[Normankhan]

There are 4 agitator tanks in which we add different type of solutions to give a required pigment/slurry Grade based upon PH values. The max. Temp in agitator can get close to 85-C(centigrade).
The input chemicals/solutions in a tank are:
-Fresh & Spent Sulphuric Acid
-Zirconyl orthosulphate
-32% sodium hydroxide
-sodium aluminate
-sodium silicate
All above chemicals are added in to raw pigment slurry to give different final grades.
For almost 5 grades we try to maintain final PH> 8.0 (Basic); for remaining 3 Grades PH<6.0 (acidic)
Problem:
we have a short spool/pipe around 2m long made of SS-316 going in to agitator tank gets corroded/eroded with in a week. I want to replace SS-316 with a better alloy. I found that alloy-20 is usely resistant to acidic environment specially Sulphuric acid (H2SO4), as it is a major constituent which is added to above chemical to reduce PH to a desire level required for a different grade.

The question is does alloy-20 a good choice as a MOC for basic environment as well or in other words for above chemicals other than H2SO4(acidic)???
Any other recommendations other than Alloy-20;kindly share

Thanks
Norm

 

[EdStainless]

Certainly a better choice than 316 for all of those chemicals.

You are mixing these additive first and then add then right? What concentration is the acid and caustic when you introduce it?
There are some other alloys that you might consider.
If there any Cl, Fl, or Br in your mixtures?

In general, you get resistance to caustics with higher Ni and resistance to acids with higher Cr.
Considering how small the piece is and what it will cost in 20 you might want to just use C22 (or 686 or 59) and be done with it.

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Plymouth Tube

 

[moltenmetal]

How about a 2m spool of teflon-lined pipe, immune to all those chemicals? Even the 32% base is no problem to 120 C.

Otherwise, I agree with EdStainless- don't bother with the half-step to A20- go to a Hastelloy C grade.

 

[strider6]

the problem is the temperature, as you acn see from the following isocorrosion data at that temperature material resist only at low-medium H2SO4 conc.. you should try to define the acid conc in order to select the correct materials.. even the alloy 20 at 85°C resist approx to a conc of 20%..

http://www.sulphuric-acid.com/TechManual/Materials/materials_metals_a20.htm

http://www.outokumpu.com/applications/corrosion/image_viewer.asp?image=sulphuric1.jpg

http://www.corrosionist.com/corrosion_protection_sulfuric_acid

S

Corrosion Prevention & Corrosion Control