Copper melted into 316 SST Tube_.094 OD and .078 ID. Galvanic/Stress Corrosion issue or not? 3

[TVill]

Hello,
I have a galvanic/stress corrosion question for some of you material guru's if you don't mind.

The situation is that we have two 316 SST tubes are to be brazed together into another subassembly. Our vendor used a spot weld technique to keep the tubes together during the braze process. The wall thickness of the tubes is a minimum of .0065" and a maximum of .0085".

The first issue that QA wrote up was that there is material removed from the stainless steel tubes due to the process. I am guessing that in a worst case, an estimated .003" was removed or "flashed" by the spot weld process.

The second issue was found on closer observation under a microscope that there was another metal that was fused into the stainless steel. I am waiting for a response from the vendor as to the material but would assume it is a copper alloy.

The part is a sensor that is used in a jet fuel or polyalphoolephin (PAO) application. At times, there could be small amounts of water in the fuel that range from a temperature of -40°F to 135°F.

In looking at the Anodic index for copper (0.35 Volt) and that for 18% chromium type corrosion resistant steels which is 316 (0.50 Volt) the difference is 0.15 Volt which from what I am reading, is acceptable for harsh environments which state the there should not be more than .015 Volt difference. Sited from

http://www.corrosion-doctors.org/Definitions/galvanic-series.htm

The problem is that due to the spot welding process, not only is material removed making the tube wall thinner but also the fused material has gone through the passivation layer.

My concern is that although theoretically the copper alloy and the 316 stainless are compatable from a numbers and chart look up point of view, that due to the spot weld fusing the copper into the 316 pass the passivation layer that further problems such as a galvanic or stress corrosion issue will develop over time. Most undesireable in a fuel situation.

My question:
1. Is there a galvanic/stress corrosion concern?

Your response in helping me figure this out would be most appreciated.

Trey

 

[TVill]

Typo:
...harsh environments which state the there should not be more than 0.15 Volt difference. Sited from

http://www.corrosion-doctors.org/Definitions/galva...

 

[EdStainless]

The copper may have caused liquid metal embrittlement of the material.
Reject it and start over.
The grain boundaries in that area may only be together mechanically and not truly bonded.

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Plymouth Tube

 

[TVill]

Thank you for your reply EdStainless. Most appreciated.

Regards,

Trey

 

[TVill]

Hi EdStainless,
I have one more question if you would.

Is it possible for the spotweld itself to cause liquid metal embrittlement as well of the material? If so, I will need to let our vendor know and probably place a note on the drawing stating that spot welding is prohibited.

Thanks,

Trey

 

[cloa]

The tiny amount of the braze won't cause galvanic corrosion- its the ratio of anode area (copper) to cathode area (316) that is the issue. The lower the better. Yours (depending on the tube length) could be close to zero depending on how big the water bubble is (and the issue is how it long it sits right on the join) .

 

[metengr]

If the copper contains any zinc by alloy, this will cause LME of austenitic stainless steel.

 

[OGMetEngr]

Both zinc and copper cause liquid metal embrittlement of stainless steels. If either shows up in the analysis, scrap and start over if cracking cannot be tolerated.

 

[EdStainless]

Cu on SS will cause localized corrosion also, not really galvanic but it allows for a local change in the repassivation that will result in corrosion.

I sure don't like spot welding on SS with Cu (or Cu bearing) tools. It is just too much of a risk.
I have seen SS tubes that hit the head of a brass bolt develop LME when they were annealed.

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Plymouth Tube