Detecting iron in stainless steel

[amron]

Hi
I hope that my question is not too odd. I'm looking for a material that can indicate existence of carbon steel ("iron") in stainless steel. I have a tank made of ss and it has svere points of rust. I was told that when the tank was assembeld, they used 7018 electrodes (by mistake) to hold the parts instead of ss electrodes and after that the seams were welded with ss electrodes, so that points of carbon steel are embadded in the stainless steel, causing points of rust. I told to grind out those parts and I would like to know if there is any material that I can apply in order to know if any carbon steel is still exists.

 

[Shox Dr]

Salt, or any thing caustic I would have thought.

Regardless, if you had it built? I'd be taking it back

 

[gordon stephenson]

You have already found the first indicator (RUST)

 

[amron]

Thanx.
I haven't built it, I have to repair it. how will I detect it? it will "buble" coming it touch with the carbon steel?

 

[julieanncharters]

difficult one to sort out i would of thought,take it your tank is 304L grade
if the rust marks are very localized then a grind out and reweld might cure it . it only takes a tiny amount of contamination to cause the rust spots.
just scratching a bit of stainless on a steel bench can make a mess of the finish which is why i always top my table with ali plate when doin the shiney stuff .

as has been said get it re-done and leave it outside for a day or two .
it'll soon show its flaws if its there.
add a wipe of sumit caustic to help it along

 

[PhillipM]

Iron-X might get you out of trouble if it's small particles of contamination, but the rust will still have to be ground out.

 

[gavuk]

Sounds like a nightmare job,even if you grind the steel out you risk new contamination of the stainless from the sparks.no guarantee's unless you remake or reduce in size and cut the wells out?

 

[Drains]

You could use dilute sulphuric acid. Concentrated sulphuric won't affect carbon steel but under 70% dilute will. It will also affect types of stainless, but if you can put a drop on a sample of m/s and a drop on some s/s you should be able to see a difference in discolouration over time (minutes), which you could use as a guide.
The problem you may have though is dilution of the weld/parent metal. Even the stainless weld may behave differently to the stainless parent.

Hydrochloric may also work, but you'll have to do some fishing around on t'internet on Sulphuric and Hydro to get an idea.

There would also be nitric acid to consider, or even - and this will almost certainly work as an indicator after an hour or three - lemon juice. (citric acid)

There are loads of articles online about corrosion/acid/steel.

 

[TechnicAl]

a magnet

 

[langy]

Sounds like a nightmare job,even if you grind the steel out you risk new contamination of the stainless from the sparks.no guarantee's unless you remake or reduce in size and cut the wells out?

You could use a tungsten burr

 

[gavuk]

You could use a tungsten burr

You volunteering? Langy.......I'd be running in the other direction ....any pics?depends how many tacks etc is it your tank Amron? or are you quoting as a contractor? .

 

[hotrodder]

I'm confused. If i've read between the lines correctly someone had a stainless tank built, the origional fabricator tacked it up with mild steel and then welded over (god knows how many) tacks with stainless. Tank starts rusting and the orgional customer is paying you to fix it?

Not a job i'd fancy doing either.

What's being stored in the tank? Is rust an inconvenience (appearance/service life) or important due to hygienic reasons or contamination of the contents?

 

[dannyp]

id say run away

 

[Drains]

a magnet

Al, I'm shocked.

 

[Welderpaul]

Sounds like its a 'start again' job. If you take the job on you could do what you can to grind out the MS and reweld, then it rusts again (and it will), you will no doubt be held responsible as he will see that he has paid you money, and you still havnt sorted him problem.

I'd leave it well alone.

 

[TechnicAl]

Al, I'm shocked.

why

have i read it wrong...........hes got a SS tank welded with MS which has been welded over with SS

a magnet should pick that up

 

[Drains]

why

have i read it wrong...........hes got a SS tank welded with MS which has been welded over with SS

a magnet should pick that up

Well it might pick up the stainless weld too, as there's often a spot of ferrite even in a 316 weld.

A magnet would be a good place to start though, but i wouldn't assume you could avoid false positives.

 

[hotrodder]

I would have thought a lot will depend on the size of the tank... can typically detect a slight pull if waving a magnet at some cold worked austenitic stainless or a weld. Hold a bit mild steel behind a piece of thinnish ss and the same magnet will stick properly. OP hasn't told us how big this tank is, i would have thought it's not somthing small and simple or as said be better to start over?

 

[TechnicAl]

316L will only slightly pull a magnet. Id expect to tell the difference. As Hotrodder says it depends

Personally Id either scrap it or cut it out to a place where there is absolutely no chance of contamination. You will never do it by cutting out bits here and there.

Cut it out and weld some strips in

 

[Welderpaul]

Depends on the design, it's use, and what the client will accept.

Why not run the plasma down the welded corners, shear the sheets down (Cutting off the welded edges) and remake the box smaller, or use some stainless angle to form the corners (saves the plates) - either lap the plates onto the angle or butt the plates to the edge of the angle. Messy job either way mind. T'will pull to hell.