304, 304L versus 316 in corrosion resistance against Nitric Acid 4

[estaban]

We have a situation where several engineers are in disagreement regarding using 316 stainless steel in a nitric acid pumping scenario. A couple of guys say that under no conditions should 316 be used, only 304 or 304L. Who is right?

 

[Surin]

Nitric Acid is a specific environment for intergranular attack (IGA) on sensitized austenitic grades. As such the low carbon and stabilized grades are used i.e.,304L ( instead of 304), 321 and 347. A nitric acid grade (NAG) of very low carbon, silicon and phosphorus to improve resistance to IGA is available.<br>
<br>
Molybdenum-bearing grades (316) are usually not employed in the nitric acid producing plant due to the susceptibility of molybdenum to oxidation and the adverse effects of certain intermetallic constituents. (The nitric acid production process consists of air oxidation of ammonia under pressure to produce nitrogen dioxide which is absorbed in in water to produce 60% nitric acid.) Again, if 316 grade is to be used, a NAG 316L is preferred .<br>
<br>
Surin

 

[jmw]

for information on improved 304 resistance see this article:

http://www.mext.go.jp/english/news/2000/09/000961.htm

JMW

http://www.viscoanalyser.com

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[mcguire]

Surin is right on target.

 

[EdStainless]

There are low residual grades of 310S that are also used in acid service (as I recall low C, P, S and B). The Mo does not help you, Cr and Ni do.
Corrosion rates is nitric are very temperature sensitive. Higher temps will lead to IGA in materials that look like they have a clean structure. The grain boundaries must be very good for hot acid service.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[tarojm]

We have a lot of experince with respect to use and corrosion of s/s 316 in our nitric acid plant.I advise to use s/s 304L and in severe conditions nitric acid grade (with low c,p,s).

 

[unclesyd]

A look at the following website concerning the use of Zirconium in HNO3 service. Zr has become a very important material for use in HNO3 plants. Our HNO3 plant 1500+ tons/day has a very large ZR HE that replaced a 304L SS. The 304L HE was lasting less than 1 year, the Zr HE has been in service for more than 12 years with no problems.

Our spec for 304L requires 0.015C or less with the lowest S and P obtainable with this carbon content. We do allow 304SS flanged valves from certain manufacturers. There is no 316SS of any persuasion in the liquid end of the system. HNO3 in the pure state is transported in 304ELC lines (seamless pipe) and stored in 321SS and 304ELC SS tanks. All the 321SS tanks are skirted while the 304ELC tanks are on grillage bars. All tank seams are double vee with 100% PT overall with 100% RT on the bottom and 50% (progressive) on the walls and roof.
We tried several of the high purity alloys (as actual tank sheets) with mixed results and the economics weren't there. Some of the secrets of using 304ELC is the choice and care of the welding electrodes along with a very good welding program.

http://www.wahchang.com/wahchang/pages/products/data/zirconium/nitric.htm

 

[YBG58]

To Unclesyd

I understand that your temp of NA could be well within limits (say<90~100 Deg C, I do not recall exactly Pl pardon. Can be confirmed with wahchang!), or else you stand danger of pyrophoric reaction when Zr tubes r there for HE!! One has to be careful whne Zr tubes r there.

If not so could you pl elaborate yr service parameters.

This could be additional info from the basic disc'n of 304/304L /316L. We have NA 60% & 98.5% plants and have 304L as piping MOC and stoarage tanks from 304LN for 60% NA & AL tanks for 98.5% NA.

YBG58

 

[unclesyd]

YBG58[/],

I will try to get the exact parameters for the Zr HE the first of the week.

The acid plant is a 1500+ ton/day 65% bleached acid for onsite use in a HNO3 oxidation process. It is one of three that were built at about the same time. The two others converted to Zr about 3 years prior to our conversion.

You might have heard about the prime mover. The air compressor, high pressure expander, and steam turbine are all on the same shaft in a single housing. It is the only one of the three that still has this configuration.

 

[unclesyd]

YNG58,

The Zr exchanger is a Low Pressure Weak Acid Cooler.
Inlet Temperature is 392°F
Outlet temperature is 114°F
The shell side is CTW with summertime temperatures approaching 85°F.