Corrosion

[schmidth]

Hi! I'm new to the forum and I have a question for you guys. I encountered corrosion in a metal that I'm working with. I have attached three different pictures of the corroded piece. Does anyone know what type of corrosion this is? And if so, how did you determine that?

 

[racookpe1978]

Hmmmn.

What "metal" corroded?
Under what conditions did it corrode?
How long was the metal exposed to those conditions?
(Pressure, chemicals, temperatures, longevity/continuous/intermittent/spray/liquid/solid all matter.)
Have you built with this "metal" before, and if so, what changed in this installation?

 

[racookpe1978]

By the way, the "pipe" (bellows??) in two of the three pictures is filled with a solid mass of ?????? Start by figuring out why your pipe is filled with a dry solid mass.

 

[tbuelna]

Is this a picture of a metal bellows or a screw thread? It almost looks like an external screw thread that had some type of surface plating. And after the plating failed, whatever fluid the base material was exposed to aggressively attacked and corroded it. The big black thing to the right of the corroded area appears to be an o-ring. The bright metallic surface on the un-corroded areas looks like it could be chrome, zinc or nickel plating.

 

[weldstan]

More info is needed. What is the material? What is the fluid service - chemicals, temperature, pressure exposed to?

 

[schmidth]

The metal was supposed to be 416 SS, but when I checked the composition it was far from 416 and didn't have an exact match. It was actually composed of mostly Nickel. It was exposed to chilled water for a service of two years. Other than that I don't have too much other information.
The piece that you are looking at is a pivot bolt of the valve disk . The third picture is a cross section of part of the corrosion. It wasn't supposed to be plated in any type of material, but it is becoming obvious that it was not the anticipated material in the least bit. Nickel plating would make sense with the composition reading. And yes, that is an O-ring.

 

[TVP]

I suggest that you have the composition analysis performed again. It sounds like the initial reading was determined by using something like XRF directed at the surface. If the part was nickel plated, then the analysis will be off significantly due to the presence of pure nickel on the surface. Next, you need to determine what was in the chilled water. Chloride ions is one significant contributor to pitting corrosion and environmentally assisted cracking (EAC). Having a proper metallurgical evaluation performed by a reputable lab is advised.

 

[schmidth]

The first test was done using a Niton Gun, and I don't think it was very accurate. It was used on a cut face to try to avoid taking a reading on solely the plating (if there is any). The next step I am going to take is to send it to a lab for AES/OES testing to determine what type of alloy I am looking at. I think it is true that I definitely need more information on the application of the part.

 

[tbuelna]

From the pictures you can see that much of the outer surface is unaffected, while the basis material just beyond where the unaffected outer surface ends is severely corroded. This seems to indicate that the outer surface has some sort of plating or coating applied.