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Ferous Contamination of Super Duplex and 6MO SS Tubing

Ferous Contamination of Super Duplex and 6MO SS Tubing

Ferous Contamination of Super Duplex and 6MO SS Tubing

I am currently on assignment in a shipyard, commissioning an oil and gas platform. I am responsible for the topside equipment which will provide electrical and hydraulic power to equipment which will be installed subsea.

I am concerned about SS 2507 and 6MO hydraulic tubing, rated at 15k psi, which has been run, and is now accumulating dust from the ongoing fabrication around the platform. You can already see that the ferous-based dust has settled and oxidized in the saliferous coastal environment as the surface of the tubing has, in many places, gone brown.

Would I be right that this ferrous-based oxide layer will be blocking the passivation of the stainless steel tubing and making it susceptible to pitting corrosion?

What would be the most efficient way of removing this surface contamination? I assume a picking operation would be ideal, but is sadly not practicable with the 10's of kilometers of tubing run across multiple layers.

Any insight or experience would be hugely appreciated.

RE: Ferous Contamination of Super Duplex and 6MO SS Tubing

Whilst the discoloration is disconcerting, work by Allegheny-Ludlum, reported in NACE Corrosion 2000, Paper 00457, demonstrated that, for the stated alloys, there was no detrimental effect on marine atmosphere corrosion resistance even after high energy contamination methods such as direct shot blasting.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.

RE: Ferous Contamination of Super Duplex and 6MO SS Tubing

The material was well passivated before the surface contamination, so no worries.
I have seen many 6Mo systems installed that 'blushed' with orange after installation, and the next rain washed it off.
The only location that I would have any concern about would be where the tubing is splashed by seawater, but protected from rain. Here the construction grime can combine with salt, and when it drys out you have a bad mixture. With these alloys that could result in a lot of surface rust, but they won't pit.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Ferous Contamination of Super Duplex and 6MO SS Tubing

Super duplex and 6 moly alloys generally resist well to pitting (I'm echoing 'EdStainless' post); however I would suggest to perform ASTM A380/ 967 tests or perhaps just ferroxyl test as per 380 should be enough to determine the cause of free iron on the surface - is it truly mild rust staining? Nickel development institute has published enough articles on (chemical) cleaning methods on off-shore surfaces though it's bit tedious to apply those to tubing. Few vendors had given some demo on such cleaning practices though one needs to gain access to full surface in those cases. I would inspect tubing under support and clamps - just to be more cautious on crevice than pitting corrosion.

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