304L coated pipe buried in tidal water zone

[rothrock]

We are installing 14" diameter fuel distribution pipe with a PE coating for an air base. The trenches are dug deep enough that prior to back fill tide influenced brackish groundwater can submerge the pipe.

The owner has told us that allowing the brackish water to come in to contact with the 304L will "potentially lead to corrosion cells". They also say that once a corrosion cell has started that "a simple wash by clean water will not be sufficient to repair the natural protective layer".

The only place the water directly contacts the 304L is at exposed field weld joints prior to the application of heat shrink type coating closures.

If the exposed pipe is welded, and coated in a week or less,(welds must remain exposed until a hydrotest is completed), is that sufficient time for brackish water to start a corrosion cell that cannot be stopped?

Would like some help understanding why cleaning the pipe, drying it and applying the heat shrink will not stop the corrosion cell. And what procedure is needed to first know if a damaging corrosion cell is present, and second what can we do to stop the corrosion cell and restore the protective layer?

Is the protective layer necessary under pipe coating?

Thanks for any help

 

[Metalguy]

Water/moisture is a requirement for Cl attack on SS at ambient temps. *IF* you have confidence that the weld areas will remain dry because of the coating closures, there is no realistic problem. It is much better to install either a sacrificial anode or impressed current CP system which can protect any defects/breaks in the coating. Your current requirements would be very low with a decent coating.

If it's possible/practical, you can treat the weld areas with a 50% nitric acid (in low Cl water) for an hour or so at ambient temp. to restore a good passivated layer. Be sure to thoroughly rinse the acid away.

 

[stanweld]

I have had some unfavorable experiences with X-Tru-Coated 316 S.S buried pipe. Chlorides concentrated at breaks in the coating and resultant crevice corrosion rapidly led to perforation of the pipe and subsequent leakage of product. Similar crevice corrosion was seen in uncoated, non-cathodically protected 316L pipe that had to be replaced at costs approaching 15 million dollars after less than a year in service. Replacement with cathodically protected, coated 316 SS pipe effectively mitigated corrosion.

You may be in for an expensive learning experience!

 

[rothrock]

Thanks Metalguy and Stanweld,

The system will be cathodically protected. We are waiting for some analysis of the chloride content of the water. It appears the water is mostly freshwater from the surface. For what its worth the field guys did a taste test and could not taste any salt.

I'll post additional information as it develops.

 

[rothrock]

Water test results:

Chlorides per EPA300.0 = 605 mg/L
pH = 7.0
Conductivity = 0.31S/m

If the exterior, i.e. anodes outide of pipe, CP system has brought the pipe potential up to the protective criteria of -850 mV relative to Cu/CuS04, does that also mean that the interior surface of the pipe is being protected as well? I am assuming that the 304L metal will all be at that potential not just the surface. I know current cannot flow into the pipe, so is that what prevents CP working inside?

Thanks for any help