Ammonium chloride/rust

[Chipper]

Probably one for Chunko but anyone else more'n welcome to chime in of course..
Used this a few times, always worked very well. My young lad made a big metal spoon as a prop for a film project at skool (he did the hammerforming and welding ) - wanted it rusted and manky looking. So we duly dunked it in a solution of ammonium chloride.

Shortly it was thickly covered in rust - but it all fell off! It was like a furry formation sat on top of the thing and not actually part of it - the spoon was bright & clean... And after a longer while it took on a silvery-grey etched look as if a coating had formed (looked nice actually).

Tried a weaker solution - same deal... Any ideas? The powder is a bit old, is it unstable?

We hung it up and misted salt water on it instead, which worked fine.

No big deal, just curious.

 

[chunkolini]

It does need immersing in the solution.
Make up a strong solution, pop a bit of washing liquid in it to break up any grease and break the surface tension (avoids droplets forming).
Give the part a scrub with it using a scouring pad, then leave it outside for a couple of days. Worth misting it with water every now and again.
The maths.
NH4Cl. The Ammonia is given off, NH3 leaving behind a weak Hydrochloric Acid HCl which does the rusting bit.

 

[Chipper]

Cheers. It was degreased, sanded, and the strong mix just did as above - really weird - disembodied rust... like fluffy loose rust but completely separate just sort of resting on the piece..
I might just bin the powder. It always worked fine and fast before.

O well, salt spray worked quickly. Piccy here, not bad for an 13 yr old guided by an idiot -

 

[chunkolini]

I have seen that effect, it looks like badly applied paint and flakes off or even wipes off when touched.
Weird.

 

[strada]

Where do you buy ammonium chloride from ?

 

[Chipper]

Fleabay, very cheap too.

 

[Chipper]

If hydrochloric acid is what's really doing the job, sulphuric must work too - seems to do a job on battery trays.. Got loads of that from drain cleaner and old batteries so will try that next time.

 

[MrFluffy]

Brick acid (muriatic acid) is the king for causing corrosion, just dont leave it in your workshop to get into the air...

 

[Chipper]

Muriatic = hydrochloric, which is the product of amm.chloride solution which is nicer to have around. Normally works a treat but blowed if I know why not this time, weak or strong made no odds. Still, the job got done with salt.

 

[strada]

When you apply ammonium chloride,do you rinse with water when the desired colour is achieved. To stop the reaction.

 

[Chipper]

I chuck things into plastic crates with hot water, plywood as a "lid". Stays hot for ages even outdoors, and works pretty quick. Then next day rinse off, I usually linseed oil things, makes the colour dark and the gloss dies down after a bit. It's fresh on the pic up there ^.

 

[MattF]

Normally works a treat but blowed if I know why not this time, weak or strong made no odds.

Was there any type of coating or crap on the metal before you tried rusting it?

 

[Chipper]

No, it was all degreased and sanded, or brass wire brush to descale the hammered bit because it was a bit dark from annealing. it made rust like billy-o, but it was all detached.. most odd! Then rusted PDQ strung up and salt-sprayed.

 

[MattF]

What metal was it made from? Have you tried either dunking the part and then letting it air dry, else just fuming by placing the part and the solution in an enclosed box?

 

[Chipper]

All mild steel, I've done this before using the same bottle of crystals and same dunking method (not tried other ways), so I'm foxed. Admittedly a bit haphazard on the quantities but it's been good before. Strong or weak this time, same strange detached rust.
Next time I'll buy new powder, it's so cheap. Just to see if it makes any odds. And the acid; he wants to know how to make flowers so we'll have some stuff to play with.

 

[MattF]

Must admit, I've no idea as to why the specific one didn't work. However, the fact that ammonium chloride can be used in a welding flux does suggest that it has qualities which could have, for wont of a better term, cleaned the part as it actually rusted it due to it being immersed, hence why I'd guess one of the other methods may work more reliably.