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Once more: Welding stainless to carbon steel

Once more: Welding stainless to carbon steel

Once more: Welding stainless to carbon steel

(OP)
A co-worker is designing a 150' tapered, stainless steel flagpole that will be welded to a carbon steel baseplate.

He intends to connect the pole to the plate using a full pen weld from the inside of the pole; on the exterior circumference he's calling for a carbon steel backer bar fillet welded to a carbon steel base plate. The first three feet of the pole will be buried.

I have several concerns:

1. Although stainless can be welded to carbon steel is galvanic corrosion of the base plate a possibility because it will be buried?

2. Similarly, is the carbon steel backer bar a potential cause of galvanic corrosion?

3. Since the weld between the pole and the base plate will be a chrome steel is there a fatigue cracking concern since the weld will not be as ductile as one between two stainless steel or two carbon steel plates?

Any thoughts, or am I over-thinking the problem?

RE: Once more: Welding stainless to carbon steel

1.
The buried part of the structure will have to be coated to prevent corrosion unless there's a convenient cathodic protection system to hook into.  The major problem will arise at pinholes in the coating on the carbon steel side should there be a significant amount of bare stainless steel exposed to the soil as well.  A good coating on both, including the welds, should protect for a while (several years)before maintenance becomes necessary. Of course, if the structure is buried in concrete that would serve to protect the carbon steel for a period as well.

2.
Yes, as above.

3.
A corectly made dissimilar weld should not be more prone to fatigue problems from a purely mechanical view point.  Correctly made = choice of joint design, consumable, fit up, correct welding parameters, attention to preheat and interpass temperature limitations, good workmanship and weld finishing.

Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/

RE: Once more: Welding stainless to carbon steel

(OP)
thanks Steve

RE: Once more: Welding stainless to carbon steel

Steve in #2 provided good advice.  I'll merely emphasize his reply #1:   DO NOT paint only the carbon steel portion of the structure,  as this can create a severe galvanic couple if/when small areas of paint damage occurs.  Therefore, be sure to paint the lower portion of the stainless steel as well (a few inches should be sufficient in this aplication).

RE: Once more: Welding stainless to carbon steel

The fab is easier, and safer as far as corrosion goes, if everything that you connect and bury is stainless.  A few pieces of SS plate are no big deal.

Even if it is all SS I would still paint the underground portion.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Once more: Welding stainless to carbon steel

(OP)
All,

Thanks for your assistance.

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