Once more: Welding stainless to carbon steel

[bridgebuster]

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?

 

[SJones]

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/

 

[bridgebuster]

thanks Steve

 

[mshimko]

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).

 

[EdStainless]

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

 

[bridgebuster]

All,

Thanks for your assistance.