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Up Topic Welding Industry / Technical Discussions / Welding method
- - By skww Date 01-06-2009 09:56
I am working in a shipyard as a mechanical engineer. Is there a specfic welding method to prevent stainless steel welds rusting in sea water? Because the previous boat only went out once, some of the welding parts already got rusted. Can anyone help me and give me some professional suggestion on corrosion resistance?

Thank you
Parent - - By 803056 (*****) Date 01-06-2009 13:11
Interesting question.

Now I have a  couple of questions for you. Are the two base metals being joined both stainless steel and if so, what alloy? Are you welding carbon steel to stainless steel? Do the welders use dedicated grinding disks and stainless steel brushes on the stainless steel exclusively? What are the ferrite numbers in the weld? Has the ferrite numbers been checked after the welding is complete. You answered the question about the stainless steel alloys being welded, now what is the alloy of the filler metal?

There is no simple answer to your question without more complete information.

NACE (National Association of Corrosion Engineers) would be the organization I would look to for this type of information. Corrosion is their specialty.

Best regards - Al
Parent - By Uncle Chuckles (*) Date 01-06-2009 14:44
Sorry this is more of a question then an answer.  Without knowing exactly what material or welding conditions you are working with, is it not standard practice to Pickle and Passivate sea going stainless?

Charles
Parent - - By skww Date 01-07-2009 06:39
Thank you Al. I am a new guy in boat industry, so I still have a lot to learn, and my manager wants me to find out a way to prevent rusting on the stainless steel.

I think the two base metals being joined are both stainless steel (316), and the welders told me they use dedicated grinding disks and stainless steel brushes on the stainless steel, I hope they really did that. And I am not quite sure about what the ferrite numbers are, is it the % of ferrite content in the weld? Is there a way to find that out? And I have to check it before and after the welding? I believe the alloy of the filler metal are also 316.

But is 316L good enough in harsh environments, for instance, if the boat needs to stay in the ocean everyday? Or I should use something like 317, 904 or 2205?

I have attached some pictures on the web, you may want to see it, so you will have a better understanding on the problem I am dealing with.

http://www.boatdesign.net/forums/attachments/boat-design/28234d1231308108-material-selection-.jpg
http://www.boatdesign.net/forums/attachments/boat-design/28235d1231308108-material-selection-b.jpg
http://www.boatdesign.net/forums/attachments/boat-design/28236d1231308108-material-selection-c.jpg
Parent - By tazmannusa (**) Date 01-08-2009 02:22
From what I know about the stainless for marine. 316/316L in the best for corosion resistance above the water line, use 316/316L stick, or wire for welding, use just enough heat to weld together, dont over do it . clean with stainless wire brush and use pickling paste last.
304/304L can be used if you mainly stay in fresh water inviroments.
Tom
Parent - By tazmannusa (**) Date 01-11-2009 14:32
  Food for thought, They make special floppy sanding disk and grinding wheels for stainless plus keep a stainless wire brush, keep them all cleen and only use on stainless. I have made marine parts out of 304 grade for sailboats "trailer sailers" I prep for welding with floppy or grinding wheel, weld up with mig using a tri-mix gas for stainless or stainless stick rod then I clean it up with fine git floppy if needed then polish it up with stainless wire cup wheel in grinder with light pressure. When I am satisfied with the appearance I take them outside in the shade and give them the pickling paste treatment "nasty stuff"  washing real good after because the picking paste will cause problems if not washed off good. So far I have not had any problems with the rusting, the finish stays the same. Not a high tec aproach but ot does work.
  Tom
Parent - - By defaced (**) Date 01-06-2009 15:47
Where is the rusting happening?  Is it in the weld or next to the weld, like wagon tracks?  If it's wagon tracks, you're likely experiencing sensitization of the base material because of the heat caused by welding.  The only way I'm familiar with getting around this is to either solution anneal the entire structure (likely not possible), or change to a stabilized grade of base material. 
Parent - - By skww Date 01-07-2009 07:39
The rusting is happening in the weld. I've attached a few pictures above, so you can take a look of the rusting. And regarding changing a more stabilize grade, I am thinking something like 316L, 317, 904, or 2205. Are you familiar with them?
Parent - - By GRoberts (***) Date 01-08-2009 01:42
The first picture (The one that looks like a U bolt welded to a plate) looks like general corrosion of the weld & HAZ.  You said you were using 316?  Some of that might be eliminated or at least reduced by using 316L which would reduce sensitization if that is the problem.  the 2nd picture that looks somewhat like a downrigger with a handle looks like the corrosion is starting near crevices, such as the nuts.  That is I would believe a problem with the grade of stainless.  316L can do ok in seawater as long as it isn't stagnant, isn't warm, and doesn't have any crevices.  In those cases you should look for a more corrosion resistant alloy.  317L is a step above 316L.  The top of the line would probably be super duplex like 2507/Zeron 100 or Super austenitic like AL6XN or 254SMO.  You probably want something in the middle though.  There is plenty of information available from stainless steel vendors that you can sort through in your quest for the most economical material that will meet your corrosion requirements, and a lot of it is on the internet.  See if you can contact someone from companies like Allegheny-Ludlum, Avesta, Sandvik, or Outokumpu to tap into thier expertise.
Parent - By 803056 (*****) Date 01-08-2009 04:49
Greg has some good suggestions.

I would seek some advice from NACE. They are the corrosion experts. I'm sure they have some literature on the problems you are encountering.

Best regards - Al
Parent - - By DaveBoyer (*****) Date 01-08-2009 04:55 Edited 01-08-2009 04:57
Using the 316 L material and filler is a step in the right direction.

Electropolishing helps by removing iron from the surface of the material. This is worth doing on anything that You can fit into a tank.
Parent - - By js55 (*****) Date 01-08-2009 13:49 Edited 01-08-2009 13:54
Lets keep in mind that rust is an oxidation of iron. Even a sensitized 316 will form Cr oxide layer.
Also, sensitization would take place on a thin zone immediately adjacent to the weld. This is too wide spread.
I don't think a more expensive alloy is gonna help. What you need is better process control.
Parent - - By DaveBoyer (*****) Date 01-09-2009 04:52
By sensitized, do You mean that the chrome has formed chrome carbides within the material? When this happens the chrome is depleted around the carbides and the chrome oxide film is compromised in this area. I agree that this would be a problem in the weld and HAZ only.
Parent - - By Superflux (****) Date 01-09-2009 05:29
I was told by a Letourneau grad that Sensitization and Carbide Precipitation are synonomous.
Parent - - By js55 (*****) Date 01-09-2009 13:32
It might be if Cr carbides were the only carbides that precipitate. But many elements combine with carbon to form carbides. Thats why the convention of using M as a prefix for carbides (such as MC, M2C, M6C, M23C6, etc). Often the elements are even mixed. For example you may have Cr, Mo, or somethhing else in that M23C6.
And more precisely sensitization is related to the depletion of Cr that allows selective corrosive attack, not the precipitation per se.
Parent - By Superflux (****) Date 01-09-2009 15:06
js55,
Thanks for that elaboration. As always, one answer leads to many more questions. I'll be doing some homework...
Parent - By GRoberts (***) Date 01-10-2009 01:39
Another way to say it could be that all sensitization is carbide precipitation, but not all carbide precipitation is sensitization.  I agree that weld sensitization would only be adjacent to the weld, but we don't know the whole fabrication story of these parts.  Did they heat them to get them straight after welding?  Sensitization can happen lots of ways besides a weld bead.
Parent - By jarcher (**) Date 01-09-2009 11:59
You might also want to consider nitronic 50 and 60. They are very close to 316 mechanically, but salt water corrosion resistance is much higher
Parent - - By vittorio (*) Date 01-09-2009 10:48
As a general rule stainless steel 316 can be used for marine atmosphere and also short time immersion . Looking at your photos is looks like the material is not a 316 but a 304 probably (no Moly and lower corrosion ressitance to Cl-) I'll check the materials first and then the welding procedures, risk of sensitization. one important factors that has an influece on corrosion resistace of SS 316 is also the surface roughness.

here there are two publication that can give u good advice:

Selecting Stainless Steel
http://www.imoa.info/downloads/stainless_steel_series/stainless_steel_selection.html

Stainless Steel for Coastal and Salt Corrosion Applications
http://www.ssina.com/publications/salt_corrosion.html

hope this help

Vitt

Corrosion Prevention & Control
Parent - - By skww Date 01-10-2009 01:33
But I found that most of the rusting are happening at the welded joint. Is it because the filler material or the polishing method? For 316L, what is the best polish method to prevent it from rusting?
Parent - By DaveBoyer (*****) Date 01-10-2009 03:04
Mechanical polishing is OK, but when followed by electropolishing You will have the best chance of sucess.

Crevis corosion is a problem any place the surface is not exposed to air, be it mating surfaces or even from accumulation of dirt. When You fasten the stainless item bed it in a selant that actually seals out the water, or rust will bleed out of the joint. If not exposed to the oxygen in the air the stainless will lose it's passive surface.
Parent - By MBSims (****) Date 01-10-2009 23:37
It looks like the rust particles are embedded iron, most likely from iron oxide grinding wheels or sanding material.  The suggestion to pickle and passivate is a good one.  Use one of the commercially available pickling pastes or solutions after all grinding is complete.  Here's a link to a good resource on pickling:

http://www.outokumpu.com/applications/upload/pubs_113142858.pdf

And another good resource on post-fabrication cleaning:

http://www.corrosionsource.com/technicallibrary/nidi/literature/pdf/10004.pdf
Parent - By welderbrent (*****) Date 01-10-2009 19:11
By the way, welcome to the forum.

So far I tend to agree mostly with the concerns over process control.  Also as was stated, maybe in different words though, even stainless will rust especially after welding and depending upon the processes used and how treated afterward.  Al has rightly suggested seeking advice from another group that are really into the exact situation you are in.  That does not mean there is no one here with good advice and we would love to know what you find out.  Most here thrive on new knowledge and application info.

Have a Great Day,  Brent
Up Topic Welding Industry / Technical Discussions / Welding method

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