[Boston Doug]

Hey everybody,

I'm hoping to get some info/opinions on an issue I recently discovered on some sanitary welds that were done several months ago... it seems the corrosion resistance of the stainless has been compromised pretty severely and I'm trying to understand the cause to prevent it in the future.

I did this weld, so I'm hoping to determine - is this corrosion due to a procedural issue with how it was welded, or is it more likely due to lack of proper passivation after the welding was completed?

Or something else?

Here's what I found:

IMG_5815.JPG (29.52 KiB) Viewed 1530 times

IMG_5816.JPG (29.74 KiB) Viewed 1530 times

This is a 304 ss ferule fitting welded to 304 ss tubing, 1.5" .065 wall. I don't vary my settings much I'm generally

-42 amps (2t button with upslope / downslope) walking the cup
-3/32 tungsten
-#10 jumbo gas lens with 25-30 ish argon flow
-full argon purge, right about 20 cfh purge pressure

I'm honestly not sure they bothered to passivate this at all when they put it in service, and doing some research it looks a lot to me like the passivation problems called out in the images in this article: https://www.thefabricator.com/tubepipej ... -stainless (which I found in another thread on here, thanks all).

Any other help or advice on the cause of this, or even ideas on potential causes, would be very helpful.

Happy to provide any other info as well.

Thanks very much in advance,

Doug

[Lightning]

I'm far from an expert, but my understanding is that much (most? all?) of the problem is due to carbide precipitation, and the way to reduce carbide precipitation is to get in and get back out as fast as possible, because the longer the metal is hot, the more carbide precipitation you get.

https://vacaero.com/information-resourc ... ssues.html

[Poland308]

Also might have been a lack of purge. It looks sugared. Also might have been from a contaminated grinding wheel or a brush.

[tweake]

judging by the heat tint probably purge problem.
check where your vents are.

[Olivero]

If they didn't passivate it and the chrome got burned out, that would do it. I doubt it's a purge problem or it would be more consistent or at least everywhere.

[tweake]

just looking at that again, that rust is back away from the weld. bit of steel contamination like it was sitting on a hook.

[Boston Doug]

Thanks a lot for the responses.

I guess what confuses me about it being a purge problem is that the issue isn't manifesting right at the weld. When I've screwed up purging stuff in the past it always seems like it's extremely evident, immediately, in the finished weld. The pics are kind of bad but there was really no sugaring here at the bead... the corrosion showed up later and is further out (maybe like 3/8 to 1/2 inch) into the heat affected zone.

So how much heat tint is too much heat tint? Anybody have pics of the inside of a "perfect" sanitary bead?

I appreciated that article on carbide precipitation... I've read / watched some of Jody's stuff on that too. I guess the follow-up question there is what to do about it. Can't imagine any body actually flash cools every weld in a run as they go along. Hotter / faster?

And if the issue does have to do with carbide precipitation in the heat affected zone, would proper passivation help mitigate that at all? Or is it just trashed?

[tweake]

inside of a "perfect" sanitary bead i think would be simple, its completely silver. no discolor at all.
i done that with crude gear and i'm certainly no pro.
i have had some gear welded by other people thats rusted later on all around the weld aera. no doubt it wasn't pickled.

one thing that puzzles me is that i would expect rust to be more uniform over the heat tint area.
its almost like something was in there, like a purge plug or inspection tool, that was contaminated some how.

[Poland308]

It’s possible that someone just brushed it with a wire brush that had been used on carbon.

[Boston Doug]

Well thanks again for the responses... I guess it remains a little bit of a mystery to me.

In this case, I'm pretty confident it wasn't a case of contamination by a foreign object... I'll sometimes cut tubing with a portaband and deburr with a file, but I'm extremely meticulous about keeping the stuff I use on sanitary stainless dedicated to sanitary stainless.

What's more, the particular welds in these pictures were actually "out of the box" sanitary fittings, in this case ferules onto Ts, so no cutting or deburring was done.

I dug way back into the archives of my phone and actually was able to find a couple process pictures... so for a little context, here's a picture of my purge set up and another one of the part welded up (after polishing):

IMG_5499.JPG (88.97 KiB) Viewed 1167 times

IMG_5500.JPG (79.43 KiB) Viewed 1167 times

Unfortunately I don't have a picture of the insides, but obviously they would have been pretty visible so anything out of the ordinary would have been a red flag.

I guess I'm still hopeful that the root cause is failure to passivate, rather than a procedural welding issue, but I'll be continuing to do my homework.

Again, really appreciate the responses thus far and any other suggestions.

[LtBadd]

Doug, I've done a lot of that kind of work, per our customer it had to be electropolished (passivated) before we delivered. Since that has seen service, I would say it wasn't passivated. Your welds look good on the ID.

[tweake]

i'm thinking its not a welding issue but rather a part supply issue as most of the rust is outside of the weld and heat zone.
probably manufactures poor finishing. i'm guessing tool marks left behind.

tho i think you could do better with your purge setup. seal up the ends your not welding on a whole lot better. you can loose to much argon out out the other ports and don't get enough flow going past your weld.
tin foil is fine for hacks like me but some decent purge plugs would be better.

[VA-Sawyer]

That looks like a carbon steel vise. I can't see protection keeping it from touching the tube. If I was doing critical SS welding, I would look hard at a SS vise, or at least an aluminum one.

[Poland308]

Did you poke holes in your tape to vent the purge? Also Argon is heavier than air so purge from a low point and vent at the high side.

Edit
I usually only use painters masking tape for my purge.