Why is a weld often the first area to rust?

[AndyStobbs]

In the course of my job, I'm often on farms, working on farm machinery.
On a machine that has been completely painted / powder coated, the first areas to blister are the welds, with many actually rusting under the paint.

Is it because,
a) the welded area is subject to greatest movement forces, as a weld typically joins pieces of the machine that would tend to pull apart through the forces on them

b) The finished weld has not been dressed, or in some other way is more likely to hold moisture

c) The ultimate metal composition is altered, making blistering more likely

d) Other...

 

[Paul.]

In the course of my job, I'm often on farms, working on farm machinery.
On a machine that has been completely painted / powder coated, the first areas to blister are the welds, with many actually rusting under the paint.

Is it because,
a) the welded area is subject to greatest movement forces, as a weld typically joins pieces of the machine that would tend to pull apart through the forces on them

b) The finished weld has not been dressed, or in some other way is more likely to hold moisture

c) The ultimate metal composition is altered, making blistering more likely

d) Other...

d), Hydrogen escaping from the weld metal and combining with oxygen in the atmosphere to create H20, (water), so b) is on the right track

 

[AndyStobbs]

Ah, makes sense. Thanks

 

[dan.taylor.1]

Ive noticed this too, any way to prevent it?

Dan

 

[TechnicAl]

I think that, if true, its coincidental. Prior to powder coating the steel has to be treated, usually phosphated. This will react differently with the weld metal than it will with the parent plate (mild steel I assume)
But there could be lots of other reasons, thickness of coating, corners, proud welds, surface contours etc etc.
But first of all are welds more susceptible? Im not sure thats a fact

 

[PhillipM]

More surface area on a weld from the ripples, etc, unless it's been ground smooth, probably makes it more likely to hold moisture too.

 

[FranV8]

http://www.google.co.uk/url?sa=t&rc...=-S_VVnmHs7QRPdRouJ8Dnw&bvm=bv.42261806,d.d2k

Long PDF but it does explain it!

 

[Splatterer]

http://www.google.co.uk/url?sa=t&rc...=-S_VVnmHs7QRPdRouJ8Dnw&bvm=bv.42261806,d.d2k

Long PDF but it does explain it!

I don't suppose there's a dictionary section on here to explain what all the long and complicated worms in that document mean?

 

[AndyStobbs]

http://www.google.co.uk/url?sa=t&rc...=-S_VVnmHs7QRPdRouJ8Dnw&bvm=bv.42261806,d.d2k

Long PDF but it does explain it!

I do intend to read that, given you took the time to post the link. Just not had chance yet.

Andy

 

[chunkolini]

Aaah
Big Al Him speak, me listen.
Chunko'.

 

[the snooper]

could this be the reason that cars 99% of the time fail at the seams ? although it does take a while there is usually a welded seam involved, so is more research in to this required ? does the weld start to coorode under the seam sealer until it expands enough to push the sealant from the surface at which point we see the damage

its an interesting but complex topic

 

[FranV8]

I thought the simple answer was that energy had been put into the weld area and thus, like any chemical reaction, it was sped up.

Reading that pdf it seems there are many mechanisms for weld corrosion. I think with seams it's design of the weldment. There are two surfaces in close but not fused contact. Moisture collecting in surrounding areas tends to drain toward the seam, where capillary action pulls it into and along the seam, keeping it there with access to air to ensure the rusting process has an ongoing field day.

 

[TechnicAl]

could this be the reason that cars 99% of the time fail at the seams ? although it does take a while there is usually a welded seam involved, so is more research in to this required ? does the weld start to coorode under the seam sealer until it expands enough to push the sealant from the surface at which point we see the damage

its an interesting but complex topic

or is it because the galvanized surface has been compromised

Its only a suggestion

To be honest Im convinced the original statement is actually correct. Do welds rust first?
Ive not seen any evidence that they do..........but neither have I seen anything that says they dont

On a car I suspect they corrode first where muck can gather (wet muck plus salt is highly corrosive). Do these places coincide with welded joints?