nitric acid high in iron

for strong acids you shouldn't be pulling iron out of your tank wall in the time it takes to haul it. by the way that is several lbs. of iron /load. that is a lot. suspect that you are getting Fe contamination from storage.

you need to take samples at the loading station and have them tested.

it was loaded at a 2ppm and has a transit time of 25hrs. customer used to not test, but new accts desire Fe under 30ppm. The tank was entered and had no foreign debris. Just extremely burnt,(more than usual). This specific tank has successfully pulled these loads before with no problem. Problem happened within a 2 week span. I am just wondering after buffing do you believe I will stil have problems with the iron in this SS. By the way when the 101ppm load was rejected we returned it 38 hrs later the Fe content was 238ppm. Is a nitrating acid load going to pickle or passify this tank correctly. That is all this trailer hauls. 98% and concentrate nitric. How weak should the solution be for passifying. Thanks

316 should be fine with 98% acid. it should be fine with mixed acid too unless the temperature is excessive or it contains chlorides.

238 ppm is a lot of Fe. you are not going to get that from stainless. If you were, the tank would be leakinhg about now.

Iron contamination during the buffing procedure is a good possibility. You need to find out what was used. Have them 316 ss buff a coupon and send it to the testing lab. for a few bucks you'll find out whats going on.

Agree with hacksaw's comments. I would add that electropolishing will give the best surface for the job: smoothest & most passive. Have some test coupons, maybe buffed and non, electropolished.

Careful with dilute nitric- my understanding is that passivation with conc nitric is recommended before putting tanks into dilute nitric service.

Agree with electropolishing- it can be surprisingly cheap, and gives the best passivation IMHO. It's extremely effective at removing surface iron.

You said 316 is your stainless. Sedrik's "Corrosion of Stainless Steel" gives the corrosion rate of 316 in HNO3 98% as higher than your tank is seeing if there is any sensitization. The recommended grade for this service is 304L which performs 10x better than 316L and a lot better than 316.
Since tanks are welded you may have some contribution from a degree of sensitization. Nitric acid corrosion resistance is proportional to %chromium. This performance may not be other than what is expected, if the tank is 316.
Switch to 304L and you'll never have the problem.

You mentioned mixed acids (nitrating mixtures). What is the compsition of this mix. If its HNO3/H2SO4 you might have a little bigger problem.

agreed, the mixed acids present additional questions, but the concern is about a tank that has carried nitric previously showing signs of excessive Fe upon delivery.

the Sedrick graph only deals with boiling nitric, not low temperature storage.

304L is the material of choice not only because of cost, but because of the absence of moly.

316 is also good if that is what you have to work with, again at the same temperatures. The moly content of 316 does pose a potential for increased corrosion at higher temperatures, but should not be a big issue at room temperature. Both 304 and 316 have the same Cr content ~ 18%, so it would seem that corrosion of the Cr is a non-issue.

The details of the stainless steel does not explain the build up of Fe in the acid.

For a 50000 # load you are getting > 5# of Fe-compound in solution in less than 24 hrs. That is a lot, given the use of a ss tank.

For 304/316 you could have a corrosion rate of 20 mils per year, say, that is about 2# per year, or so.

The possibilities are improperly repaired tank, incorrect fittings, or contamination from incorrect buffing, etc.

I really appreciate all the replies I have received. The buffing is complete now and is going to be sent to penske in spart sc to go thru the passivation process. By the way it was buffed with scotch brite pads, no shavings were present, will be wiped down before passivation, their process involves misting nitric solution into tank and let it sit for a while. When this trailer is not running would it be any help to myself to keep it closed up after being flushed and steamed instead of open to the atmosphere? Hopefully this will take care of the problem. Thanks

Storage tanks for HNO3 only should last for years, our tanks(4) at the loading/unloading dock are 50 yrs old. They are 304L with .030 C. all butt welded. The store 67% acid @ 50°C- 60°C . The last inspection in 1996 showed no change from the previous inspection in 1986. A very light etch on some of the weld metal. Our big production tanks are 30 yr old 304lL .020 C with no corrosion, only what we call the acid stain. This maybe what “”is calling burnt.
Our corrosion rate on 304L .025% or less Carbon was less than 0.001 IPY. in tanks used for storing strong 67% HNO3.

Depending on the past service the tanks have seen will depend on how rapidly the HNO3 will corrode the 316 S/S. I would think that he has contaminated the trailer and then charge HNO3 into it. Every thing can and does go wrong in tank trucks. There is big problem with rinsing with high CL water and leaving a heel, then adding acid, big problem if not caught.

Depend on what this nitrating mixture is will tell a lot. One of the these mixtures is call S/S dissolving solution, we used this in the laboratory during the days of wet chemistry. If there is sulfuric in it the sulfuric will move a lot of iron and leave it somewhere.

The idea of some bad material in the process is very good one. We had a brand new taker truck fitted for HNO3 service according to the transport company. The only thing was that they used Monel valves. The tank was ½ full of nitric when all three let go.

Hacksaw
1.Sedrik does show rates vs temperature.
2.Commercial 304 always has 2% more chromium than 316.
3.316L has about ten times greater corroaion rate than 304l, let alone 316.
So, I disagree and respectfully suggest that 304L is the solution and that straight 316 will continue to present problems. Perhaps electropolish will help, but buff and passivate won't.
Joemack34, can you let us know how this turns out?

with corrosion your respect is appreciated, but not always warranted.

at least my copy of sedrik only shows the boiling point corrosion rate for 304 (and two grades of ferritic stainless) vs concentration. the room temperaqture data in the asm literature suggest that 304L and 316 are comparable for 99% nitric. the various grades of 316, and the limitations of bench top corrosion data are recognized

we agree at lesser concentrations, and i look forward to see how this all sorts out.

can someone expand a little on electropolishing?

50-50 sulfuric phosphoric at 90C. Make the tank the anode. It removes least corrosion resistant surface features and smoothes out peaks and crevices. It will greatly improve corrosion resistance of welds and abraded surfaces.

I think electroplishing is a bad idea at this scale. There are too many risks. It will take quite high current and give off H2 and O2. I don't think you will see any benefits by making this tank shinny.

If his corrosion is a high as he says it is polishing isn’t going to help. No matter how fine you polish S/S you still have grain boundaries.

In the real world if you put 316 S/S in pure concentrated HNO3 service its going to go.

If this tank trailer has performed well in the past, somthing has changed in the process.

Unclesyd and joemack34
Is it known that the tank performed well before and did the customer just start checking for iron?