selective weld seam corrosion 1

[AGVAS]

What is the mechanism behind selective weld seam corrosion? In a low-carbon steel, why is a ferrite-enriched zone more susceptible to corrosion? Is acicular ferrite more susceptible than ferrite, if so, why?

 

[Metalguy]

What is the application? Are you a student?

 

[AGVAS]

No I am not a student. I have recently encountered a problem with weld seam corrosion. I read that the ferrite enriched-zone is more susceptible to corrosion but I really don't understand why and I would like to. Does it have something to do with the low carbon content in ferrite?

 

[Metalguy]

Are you referring to aust. SS (304/316 etc)?

 

[AGVAS]

No it is not a SS. It is a low-carbon steel. The weld seam appears to be acicular ferrite. Extensive corrosion took place almost exclusively at the seam. I read this happens at times and is due to the acicular ferrite. I am new to this field and I don't really understand what makes the ferrite more susceptible. Is ferrite the weak link because of it's low carbon content? Can anyone shed some light on the situation?

Thanks

 

[Metalguy]

First, be advised that I don't think you're supposed to post the same question in more than one area (Corrosion).

Now, what I suspect you have is a heat-affected-zone (HAZ) problem rather than a weld metal one. You appear to have martensite or bainite in tne HAZ (acicular/needle-like structure). Such areas are anodic to the normal pearlitic microstructure, meaning they will preferrentially corrode.

If you know or can determine the base metal actual composition, it will help identify how likely these anodic areas would form while being welded (preheat-related). Just because it's "low-carbon steel" doesn't mean you can weld it with no preheat, assuming you want high quality welds and HAZ'z.

 

[AGVAS]

Metalguy,
I am sure the corrosion is in the seam. The vicinity surrounding the seam has some pitting and general corrosion but it is outside the HAZ. Metallographic examination revealed a weld seam that appears to be almost all ferrite. The ferrite in the seam is needle shaped and, no, it does not appear to be martensite or bainite. This is a semi-killed, low-carbon steel manufactured in the 1950's.

However, you raise an interesting point. What is it about the bainite and martensite that makes it anodic to the pearlitic structure?

 

[stanweld]

I take it that you are refering to the longitudinal weld seam. If you are seeing ferrite, I assume that the pipe was Electric Resistance Welded. A ferrite line at the center of the seam was always present on the ERW pipe produced by the steel manufacturer that I worked for in the '60s. Metalguy is correct in that a galvanic cell is created between the seam and the pipe base steel and this is the cause of corrosion.

Many ERW and continuous forge welded pipe steels in the '50s had superior corrosion resistance in buried service than subsequent "clean" BOF melted products. However, the galvanic mismatch at the ERW seam in these steels was sometimes severe enough to cause rapid pitting along the seam.