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Old 04-19-2010, 02:14 AM   #1
PSIstolic
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Default Rust prevention - Passivation treatment after welding/grinding 304 SS

Hello fellow Subie fabricators!

This is my first thread post, if I get this wrong, please let me know!
I did a quick site-wide search for "stainless" and "rusting" and ended up with a few posts that basically just advised sanding off and painting, so I wanted to probe the welding and fabricating group here...

Short take (to avoid reading the paragraph below!): Stainless steel (304 grade) seems to rust at the weld spots, and surfaces that have been ground down (using a hand grinder disk). Passivation seems to involve an acid bath and heat baking. Is there an alternative to post-weld surface treatment besides the non-practical acid bath? I read that passivation can be accomplished using citric acid, am I stupid to believe that i can pick lemons from my backyard and smear the juice on the SS surfaces, then blast it with a heat gun? Your input is appreciated! Thanks!

Long Story (Background):

Alright, I fabbed up a roof-rack "basket", similar to the popular Yakima one, since I didn't want to pay the ridiculous Yakima price for it! I decided to use stainless steel (304 grade - square tubing) welded up using DC Tig, SS filler metal, and Argon shielding gas.

I figured that stainless wont rust, and I can abuse the basket by throwing stuff up there and not worrying about marring the surface and creating rust spots, but to my surprise, after a week of exposure to the environment, the thing started to get rust spots, expecially around the joints that were welded and at the bead areas where I grounded down flat. I did a google search about this and found that there's a passivation step involved to treat SS, and to create the barrier on the SS surface to give SS it's anti-rust properties, I was totally not aware of this!!!

Industrial passivation technique involves an acid bath (nitric acid, I think), and I was not about to order any acids to use in my garage (probably not legal anyway!). I then found that an alternative is to use citric acid, and since I know citric acid can be found in lemons, I was wondering if it would make any sense to batch up some "lemonade" and soak my SS in it + some heat with a heat gun to create the same passivation effect? Before I even tested this on my own, I got impatient and just painted the sucker with truck bed lining, since I wanted to start using it!

So, my project's over, but I'm still left with this mystery, and wanted to see what you guys think?

Thanks in advance!
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Old 04-19-2010, 07:03 PM   #2
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Because of the heat of the welding you reduce the stainless properties of stainless steel (you remove nickel from the metal)

There is a product called pickling paste (if i remember correctly, I only read the Dutch instructions on it)that is thick like a gel, that U brush on, let sit for 30 minutes, and then rinse with lots of water.
This somehow removes the impurities from the weld, and keeps those impurities from starting the rusting proces.

You can grind Stainless, without it rusting, but you need to use a fresh grinding/cutting disk.If you use one that has been used already on iron/normal steel it will indeed cause rusting.
Also, dont grind or cut iron/steel near stainless, as the sparks can cause rusting as well.

Here is a website with the paste on it, and explains how it works
http://www.azom.com/Details.asp?Arti...Pickling_Paste
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Old 04-19-2010, 11:11 PM   #3
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Thanks for the input, I believe you're correct about the contamination since I started out by using an old grinding disk without changing to a new disk.

I'll keep that in mind on my next job.

But I'm still wondering if you or anyone else has had experience with using citric acid (even something as simple as fresh lemons) to do the passivation? One of my friends pointed me to this product:
http://www.citrisurf.com/

But I think the other junk in a lemon (non citric acid elements) may actually cause more rusting by contaminating the SS surface.

When I get some time I'll test it out on some scrap SS pieces by applying a weld, watching it rust, then remove the rust, and add some lemon juice and heat... if anything interesting happens I'll post back.
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Old 04-20-2010, 07:49 AM   #4
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The paste is not very expensive, if you but a small quantity, so why bother with lemons, when there is a thing specially made for this, which is not too expensve, readily availasble and actually made for it??
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Old 04-20-2010, 07:29 PM   #5
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If your finish-welded stainless is rusting, you are either using the wrong filler rod or you are welding it at too high of an amperage. I build dairy machines DAILY using 304 stainless. When welded properly, the joints unfinished or ground will not rust when exposed to the elements. Something's amiss in your equation. Not baggin' on you, just trying to help a welding brotha out.
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Old 04-20-2010, 11:38 PM   #6
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Quote:
Originally Posted by Dutch Scooby lover View Post
The paste is not very expensive, if you but a small quantity, so why bother with lemons, when there is a thing specially made for this, which is not too expensve, readily availasble and actually made for it??
True, true, I'll consider ordering some on my next larger project, especially on critical welds, thanks for your info!

Quote:
Originally Posted by syndromeofthedown View Post
If your finish-welded stainless is rusting, you are either using the wrong filler rod or you are welding it at too high of an amperage. I build dairy machines DAILY using 304 stainless. When welded properly, the joints unfinished or ground will not rust when exposed to the elements. Something's amiss in your equation. Not baggin' on you, just trying to help a welding brotha out.
No worries, I didn't take your comment negatively, I'm a newb at welding, and I appreciate any comments! I wouldn't be surprised if parts of my setup were incorrect, as what DutchScooby mentioned above, my post-work probably introduced a bunch of contaminates into the SS surface. and to take from your comment, I checked my setup just now and my filler rod was "ER308L" where the label showed it as suitable for SS 304L - Does the "L" parameter change anything from 304 SS (without the L)? I think I may have been using too high of amperage too (running at ~40A on 1/16" thick square tubing), since I had some trouble with overall warping on the longer tube lengths.

Thanks everyone for your input! One of the reasons why I love being a Subaru owner and joining this forum!
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Old 04-21-2010, 06:39 PM   #7
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Quote:
Originally Posted by PSIstolic View Post
True, true, I'll consider ordering some on my next larger project, especially on critical welds, thanks for your info!



No worries, I didn't take your comment negatively, I'm a newb at welding, and I appreciate any comments! I wouldn't be surprised if parts of my setup were incorrect, as what DutchScooby mentioned above, my post-work probably introduced a bunch of contaminates into the SS surface. and to take from your comment, I checked my setup just now and my filler rod was "ER308L" where the label showed it as suitable for SS 304L - Does the "L" parameter change anything from 304 SS (without the L)? I think I may have been using too high of amperage too (running at ~40A on 1/16" thick square tubing), since I had some trouble with overall warping on the longer tube lengths.

Thanks everyone for your input! One of the reasons why I love being a Subaru owner and joining this forum!
You are correct in using 308L filler rod on 304. The 308 is the proper rod for any stainless starting in 30x. I doubt that you introduced ANY contaminants into said welds in your post-work. Grinding on the metal is not going to give you any contaminants UNLESS you ground on carbon steel and then ground your stainless welds with the same sanding disk. This will in a sense "inject" your stainless with isty bitsy pieces of iron thus allowing your stainless to rust. Is your shielding gas set properly? I usually leave mine at 25cfm unless I need to stick the tungsten out further, then I will jack the cfm up. Your filler rod could have some **** on it which may contaminate your weld? However, when welding stainless or carbon for that matter, all the impurities will float to the top and usually not get trapped in the weld. The impurities are the little "fish eye" you see swimming around on top of your weld puddle. Just FYI, your amperage is based on each .001 of an inch you're welding on. So, on 1/16 which is .0625 or 63 thou, you'd want to set your welder at roughly 65 amps. Believe it or not, I do it everyday, the hotter you weld and faster you travel, the smaller your heat affected zone will be. 40 amps may be too cold to the point you are overheating the base metal. It can happen.
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Old 04-21-2010, 07:13 PM   #8
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Quote:
Originally Posted by syndromeofthedown View Post
If your finish-welded stainless is rusting, you are either using the wrong filler rod or you are welding it at too high of an amperage. I build dairy machines DAILY using 304 stainless. When welded properly, the joints unfinished or ground will not rust when exposed to the elements. Something's amiss in your equation. Not baggin' on you, just trying to help a welding brotha out.
this would be the Truth, something is not ideal in your equation
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Old 04-21-2010, 07:15 PM   #9
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looks like i should have read the full post some excellent advice there!
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Old 04-21-2010, 10:07 PM   #10
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Your choice of gas lens and cup size plays a large role in the "total outcome" of your weld as well. Just a lil appendage to my previous post.
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Old 04-25-2010, 01:39 PM   #11
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Take some scrap stainless and play around to see how low of an amperage you can work at. One thing that helped me a lot learning stainless was learning that its not going to puddle like steel, you have to start it along.
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