nitric acid high in iron
nitric acid high in iron
(OP)
nitric acid was starting to raise in iron. noticed extra char on 412 316ss acid trailer that has handled many loads before. In the process of getting tank buffed. Will it need to be pickled or passifyed after the buff job if we intend on hauling more nitric acid and nitrating acid mixtures? Will this buffing take care of the high iron that seemed to be getting worse every load. first first high load was 68 ppm and next load was 101ppm which was out of tolerance for customer. Help will be appreciated





RE: nitric acid high in iron
you need to take samples at the loading station and have them tested.
RE: nitric acid high in iron
RE: nitric acid high in iron
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.
RE: nitric acid high in iron
RE: nitric acid high in iron
Agree with electropolishing- it can be surprisingly cheap, and gives the best passivation IMHO. It's extremely effective at removing surface iron.
RE: nitric acid high in iron
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.
RE: nitric acid high in iron
RE: nitric acid high in iron
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.
RE: nitric acid high in iron
RE: nitric acid high in iron
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.
RE: nitric acid high in iron
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?
RE: nitric acid high in iron
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.
RE: nitric acid high in iron
RE: nitric acid high in iron
RE: nitric acid high in iron
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.
RE: nitric acid high in iron
Is it known that the tank performed well before and did the customer just start checking for iron?