Stainless rusting...

[davek0974]

Ok, so I have been making plaques from stainless for a few years now, no complaints so this question may be needless but i like to learn

I read that if you work cut stainless with tools - wire brushes, files, saws etc that been used to cut carbon steel, the stainless will rust.

What i currently do is plasma cut the plaque from brushed stainless 1.2mm sheet, clean up edges on the belt sander (used for carbon steel too), hit the back side (unseen when fitted) with a cup brush (used on carbon steel) which gets rid of any dross, then hit the front with a rotary flap drum (not used for steel) and finish with a final working with scotchbrite - the front working is always done in line with the brushing.

When working the front i work on a clean wood pad so no crud is dragged up from the steel bench.

What i am slightly concerned about is rusting - people tend to view stainless as rustless so although i have not had a complaint yet, i also don't want any in the future.

I have also read that if the stainless is brushed up clean it will re-passivate and not rust or i could soak in Nitric but i really don't want to go there if possible.

Any thoughts???

[exapprentice]

Hi Dave

Hope all is well

I believe the only grade of St/Steel that will not show sign of rusting (due to the metelergy) is 316 which is truly austenetic whereas you can can some st/steels that are martensitic and will eventually show signs of rusting?

Maybe I am wrong but I am sure that some fellow members will add in?

"The austenitic/higher chromium stainless steels, usually required in very high or very low temperatures, are generally more corrosion resistant than the lower chromium ferritic or martensitic stainlesses"

try gogging "will st/st rust"?

[_Ogre]

I read that if you work cut stainless with tools - wire brushes, files, saws etc that been used to cut carbon steel, the stainless will rust.

this is true, though it probably doesn't pertain to your usage

when i did pharmaceutical grade stainless piping we had to be concerned about cross contamination with carbon or ss
particles of carbon steel can stick to grinders and get embedded in the ss, only the carbon particle stuck in the ss will rust
we had grinders, files and tools marked for ss only

now i work with hotrodeutical grade stainless exhaust systems and we don't worry about cross contamination
there are different types of stainless with varying amounts of nickle, chromium and other metals in the mix
304 and 316 are pretty much rust proof, 409 is what most oem exhaust systems are made with, 409 will surface rust and seal itself from further rusting


from the internet:
304 contains 18% chromium and 8% nickel
316 contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum is added to help resist corrosion to chlorides (salt)
T409 has a chromium content of 10.50% to 11.75% while its nickel content is only 0.50%

[davek0974]

Thanks guys,

The stuff i currently have is 304 grade, it seemed the best all-round back when i got it.

Good to know it should be ok, my main concern having read the data again is those customers by the coast but as i said, i have had no issues yet in over 5 years so fingers crossed...

Maybe on the next order i'll go to 316 grade if they do it in brushed finish.

Also, incase anyone's interested i have found a good cut recipe:-

Hypertherm PM45, 30A consumables at 30A, whatever cut speed suits your machine, 3mm perpendicular lead-in, no lead-out, add 0.5mm over-burn and a path rule to kill the torch 1mm before the end - works lovely.

[DXF]

Here are 2 pics of a front tag I cut from 304 stainless steel a few years ago. I could not figure out why it was rusting until reading on this forum that using the same grinding discs on carbon steel and stainless steel will penetrate the stainless steel and cause this. I have since stopped using the same grinding discs on both steels and no problem. Also, if you cut a hanging stainless steel piece and use a galvanized "s" hook to hang it from.....when the s hook wears thru the galvanizing it will cause the stainless steel to rust where it rubbed against it. Anyone know where to buy stainless steel "s" hooks?

Dave Hanks

[davek0974]

Yes that matches the front door number I made for our own house (test piece) in fact ours is a bit worse than that. I'll have to get it down and buff it up again.

I have since tried to separate the tools used where possible so hopefully that will help.

[SeanP]

Also, incase anyone's interested i have found a good cut recipe:-

Hypertherm PM45, 30A consumables at 30A, whatever cut speed suits your machine, 3mm perpendicular lead-in, no lead-out, add 0.5mm over-burn and a path rule to kill the torch 1mm before the end - works lovely.

Will remember that one Dave thanks, are you on genuine 30a parts or have you found any decent alternatives? the ones I got from plasmatec are horrendous, slipped up there, no probs with 45a ones though.

[davek0974]

Yeah i've gone back to genuine all round, too many other possibilities for trouble so i thought it best to stick with known parts, never tried the plasmatic 30A as i don't use many but the 45A ones didn't seem to last quite as long as genuine.

[bhamer62]

Mr Hanks, McMaster Carr has SS S hooks https://www.mcmaster.com/#stainless-ste ... s/=183i73p
Also, when you weld or cut SS the heat can locally burn the nickel content out of the steel in the area of direct contact allowing it to rust....Pharmaceutical and food grade items are sometimes passivated as a secondary operation after cutting or machining to eliminate this, but it is costly.....

[DXF]

Mr Hanks, McMaster Carr has SS S hooks https://www.mcmaster.com/#stainless-ste ... s/=183i73p
Also, when you weld or cut SS the heat can locally burn the nickel content out of the steel in the area of direct contact allowing it to rust....Pharmaceutical and food grade items are sometimes passivated as a secondary operation after cutting or machining to eliminate this, but it is costly.....

Thanks,those will work.

Dave Hanks

[hsolve]

Good morning gents.
hate to be the bearer of bad news, but even 316 S/S will 'rust', develop a 'Tea' stain. If the job is close to the ocean, the salts will over time cause corrosion of the surface. I have pickled 316 S/S parts made for fishing vessels and even they get a tea stain over time. I can be removed with 'Oxailc acid' mixed with water, or it can be removed with electro pickling. Depending on the conditions the slats which dry onto the S/S surface, then absorb humidity from the atmosphere are the worst. It actually forms a brine solution in micro locations on the S/S and if not removed will burrow into the S/S and I mean 316S/S. If you don't want any corrosion or spotting you have to get duplex S/S. Sorry

[johno down under]

Hi gents, I have just been reading the above comments about the s/steel.
I am still very new to this whole process, but I have a very good supply of s/steel in two grades 304 and 316.
The guys that I get it off make commercial kitchens and anything under 400mm sq is rubbish and is thrown in the scrap bin!!
Not no more it isn't! at the price of $0 it is all mine. ( I just did him a couple of cut outs for his misses)
Now, my question to you knowledgeable fellows is....... is there a difference in cutting these two steels???
One I can cut the 316 and the pieces (free pieces) just drop out and the other one 304 my main cuts are not cutting right at all, they sort of weld themselves back together??
I think (please correct me if I am wrong) the 316 that is sort of polished is the food grade and the other one looks like it has lines (brushed) is the 304 marine grade.
Should there be different settings for the two types ???
Thanks for any advice.
Cheers Johno

[Gamelord]

The composition of the two metals are different, 304 and 316. You will have to adjust your cut depending on the one you are cutting.

[davek0974]

Hi all,

Still struggling with this rust issue, the segregated tooling helps with the surface staining but not all my problems.

Had a complaint from a customer today, large Xmas gift plaque was put out at end of December and is already rust stained, they're not happy, i'm in discussion now with them.

It's the plasma-cut edges that rust up, there must be some way to repair the stainless so it stops this surely? If not i'll probably have to give up on SS work as I don't want disgruntled buyers and people generally expect stainless not to rust.

The stuff is 3mm thick so a flap wheel does not get in there, must be some option here??

[weldguy]

That's odd. What grade of stainless are you using? What has touched the edges that are rusting or is it just a straight up plasma cut edge?

[davek0974]

Its 304, just the plasma cut sides causing problems.

[davek0974]

So far it seems the only chance i have is pickling and then passivating it.

Trouble is these parts are pretty big, 500-800mm dia so would need a big tray or dish maybe and would have to be done cold.

Not sure i can win this battle, sending it out would break the price, clear-coat is a possibility but sort of defeats the object of using stainless.

This was interesting...
https://app.aws.org/forum/topic_show.pl?tid=3270

[weldguy]

I wonder if just finished the plasma cut edges back to a silver stainless shine would be all that you needed. A quick test on a piece of scrap would be interesting if the edges on one side were refinished and the egdes on the other side just bare plasma cut

[davek0974]

They probably would, as long as all of the depth of the HAZ were removed, so a serious amount of work and I could not do that the fiddly arty plaques i cut. A chemical method is about all that could get in there I think.

[davek0974]

Just had this from a customer and i'm gutted really.

This is only 6 weeks old, 304 stainless, plasma cut, I use segregated tools for finishing and work on a wood sheet not the steel bench.

I might as well have cut it from mild steel.

Have offered a refund or to have it reworked and returned etc.

20200204_121632_resized.jpeg

20200204_121639_resized.jpeg

Looks like if i cant get on top of stainless i'll have to give it up and stick to mild steel. I cant afford this sort of damage to reputation etc.

I could possibly re-polish and clear-coat it ??

Any ideas, any do extensive stainless plasma cutting??

[robertspark]

I suspect that it is your finishing process that is creating "pitting"

pitting will lead to corrosion on stainless steel.

try to mirror polish / buff it and that should remove the pitting, then dip in nitric acid to pickle.

https://www.bssa.org.uk/topics.php?article=31

https://www.finishing.com/241/60.shtml

I suspect that the nitric acid part will be the one that leads you to avoid SS work maybe.

[davek0974]

Thanks, it was finished with a fine flap disc.

I have citric acid here which popped up as a more DIY suitable pickle but even that is a difficulty as it needs heating a bit and holding at temp - just cant do it, especially in winter as i can only do this in the garden. These parts are pretty large too, up to 800-900mm dia so needs a very big tank/tray.

I have said i'll rework it and clear-coat as an experiment, customer is happy to see what happens, decent guy. If clear-coat fails then I'll have no option but to drop stainless from my listings.

[ben de lappe]

Hi Dave, sorry to see the above pic. I'd feel gutted as well and I'm glad you've got a reasonable customer. In my CMC days I processed tons of 304 #3 polished into commercial/residential kitchen vent hoods and installed them all over the south eastern U.S. but have never seen 304 rust like that. I remember at times receiving 5x10 sheets of 18 ga. 304 that a magnet had no issues sticking to. I chalked that up to cheap ss with too much carbon that seemed to be flooding the market but the ONLY signs of rust I ever saw were from cross contamination where welds had been buffed down by employees that never cared anyway. My scrap trailer behind the shop was loaded until it could hold no more, maybe take a month and never have I seen the pitting/flash rust like you've got.

Wishing you all the best in this endeavor,

[lockeyone]

If you used a flap disk or plan too never ever use one that saw any other metal first. Dedicate your tools for the metal you are working with.

[davek0974]

If you used a flap disk or plan too never ever use one the saw any other metal first. Dedicate your tools for the metal you are working with.

Yep, already do, i even work off a wood sheet not the metal bench.