NOx, put my mind at ease, smell PPM?

[anachronism]

Dave, steel, stainless steel and ferrous derivatives such as Kovar.

Edit: For example look at how reverse AR works.

 

[FrugalRefiner]

Dave, steel, stainless steel and ferrous derivatives such as Kovar.

Edit: For example look at how reverse AR works.

Those are far different animals. Reverse AR works on stainless steel because the nitric acid passivates the chromium in the stainless. Without the chromium, the steel will not survive. There is a reason nitric acid is shipped in stainless steel containers, not mild steel or carbon steel.

Dave

 

[snoman701]

If the oxygen dissasociates from the NOx and recombines with the iron, it in turn would be creating iron oxide, which as we all know is rust.

If chlorine attacks iron, it would be creating rust too, but (this is a guess at best) I think it would be creating ferrous chloride, which would then oxidize via atmosphere into iron oxide.

One of our chemistry guru's will certainly set us straight soon.

Chlorine as a gas does attack iron, very violently. Pretty cool...look it up on youtube!

However, I think you speak of hypochlorous acid, not chlorine gas. If chloride + iron = iron chloride, all the cars in Michigan would be rusting to iron chloride, not iron oxide. (due to our use of salt on roads)

Well, there *is* a reason why vehicles get (more) rust damage when they are closer to the oceans, or in colder climates (that's why -salt).

I think you overlooked one line of my post. (underlined)
Where the ferrous chloride (ferric?) would then disassociate with the chlorine and combine with the oxygen in the air to become iron oxide.

I saw it.

The chloride ions increase the conductivity of the electrolyte on the surface of the iron. This increase in conductivity increases the speed of the reaction between water and iron.

 

[g_axelsson]

Chloride is way worse on rusting stuff, the chloride ions work almost as a catalyst and attacks iron. Similar to how copper chloride etch works according to some sources.

It seems like the chloride ion is able to break down the protective surface of oxides and
Just google "chloride-induced corrosion" and you will find a lot of information.

One of the areas that deals with chlorides and their effect on iron objects are marine archeologists. Objects found in the sea is infused with chloride ions and one step in the conservation of iron is to remove the chloride ions.

The process where HCl is accelerating the corrosion of iron is basically :
1. Fe+2HCl -> FeCl₂+H₂
2. 4 FeCl₂ + 10 H₂O + O₂ = 4 Fe(OH)₃ + 8 HCl
The HCl is free to go back to dissolve more iron. It doesn't improve the situation that the iron chloride salts are hygroscopic and sucks moisture from the air.
More about marine archeological treatment of iron objects here
http://nautarch.tamu.edu/CRL/conservationmanual/File9.htm

I once saw a spectroscopic lab on the university where a 500g cylinder with HCl gas had sprung a leak over the weekend. The only metal thing they managed to save was a wrench and that was all brown from rust. The five HP IR-spectrometers in the lab were all scrap.

Göran

 

[snoman701]

Chloride is especially responsible for the pitting and undermining of the surface.

When I age steel, I spray it in HCl, then I sprinkle table salt on it. Wherever the table salt crystals settle, you get a really nice pit.

Chemistry is fun!

 

[goldsilverpro]

Yes, nitric will dissolve steel and other iron alloys, except for SS.

I once spent quite a bit of time working on a process using nitric to dissolve the very heavy silver layer from steel aircraft bearings. If I remember right, there was about 1/2 oz Ag on each half bearing. We had many, many drums of these bearings. I found that, if I used straight concentrated 70 percent nitric, it would dissolve the silver without attacking the steel. With even a 2 or 3 percent addition of water, the steel would react vigorously. The fly in the ointment was that, as the silver dissolved, it would heat up the solution slightly and then, after a few minutes, this would make the steel would start dissolving. The bearing halves were about 3" dia. X 4" and I was experimenting in a 4 liter beaker. I was sitting there watching it and, after a few minutes, there seemed to a simultaneous flash over the entire steel surface and then it would all start dissolving. Failure - I never could tame it. We ended up chucking 2 bearing halves together in a lathe jig and cutting the silver off.

Another time we had many drums of silver plated kovar electronic lead frame scrap. The silver ran 7 percent, or about 1 troy oz Ag/pound - good stuff. The kovar was 54 iron, 29 Nickel, and 17 Cobalt. Here again, the same thing - concentrated Nitric would dissolve the silver but not the kovar. This wasn't quite as sensitive as the bearing steel, but you couldn't run too much at one time or the solution would get too hot and attack the kovar. We also let it cool between batches. If I were to run these or the bearings again, I would use a heat exchanger and keep the solution cool.

I have always promoted the use of about 40 percenthot nitric to dissolve the kovar on TO5's and ceramic CPU packages.

 

[Lou]

Not to derail the thread much further, but that phenomenon is passivation and that's what's related to the concentration of the acid.

and when de-passivation happens, then you have the big time NOx issue.

So when GSP is talking about the fly in the ointment being the heat, I somewhat agree as that can shift the potential curve. What really is the true fly in the ointment is that as silver dissolves in concentrated nitric acid water is produced:

Ag + 2 HNO3 = AgNO3 + H2O + NO2

and therein lies the issue.

 

[snoman701]

Thanks for the response!

Had he simply used excess nitric, he would have likely been fine.

Although with that much silver, mechanical removal does certainly seem better. Then final removal of intermediate layer can be refined in a more controlled manor.

 

[kurtak]

Snowman

Something I do in my lab to monitor/check if I am having a problem with acidic fume in my lab is leave 3 or 4 pieces of pH paper sitting out at different locations in the lab

Due to the fact that I keep/let my fume hood run 24/7 I have little or no problem with fumes building up in the lab (according to the pH paper) also I have tools (iron) & copper (used for cementing) that has been in my lab since the day I built the lab (about 4 1/2 - 5 years) that show no sign of corrosion

I actually have more of a problem with tools & other iron corroding outside the lab (in the shop &/or storage area) during the summer when humidity often runs high then in my lab --- so apparently not only does keeping my hood running 24/7 keep fumes out &/or from building up in the lab - but it also helps to keep the air dry

Therefore IMO whether iron is corroding or not is not really a good indicator of fumes or no fumes (especially if you live in a high humidity area) because it will certainly corrode even if there are no acid fumes

pH paper sitting out on the other hand will tell you if you have a fume problem - even if the fumes are low level you will see a color change in the pH paper in a relatively short time - depending on fume level the pH paper will change color within a few minutes to within a few hours

Example #1 put a piece of pH paper on the work bench - open a jug of acid next to it for 10 - 15 seconds - the pH paper will change color in a few minutes (or less)

Example #2 put a piece of pH paper on the bench - take the cover off a waste bucket 5 - 10 feet away for about an hour & put it back on - at that point you wont really see a change in the pH paper - come back 4 - 5 hours later & you will see "at least" a slight change in color if not a full color change

IMO pH paper is a great indicator for telling you if your chem work/storage area is within safe parameters & of course a "good" fume hood is a must for keeping it safe - never turn the hood off

Kurt

 

[snoman701]

Thank you Kurt...that is an excellent idea!

 

[anachronism]

Kurt's full of excellent ideas. He's one of the guys who has worked hard at the "coal face" so the speak and his posts come from hard earned experience.