Corrosion in low alloy steels 1

[metboss]

Good day !
We have procured wellhead equipment’s 5-6 years ago and they are under operational until recently.
Now, upon leak, during our inspection, severe corrosion and metal loss has been observed on internal parts and flange faces of these wellhead equipment’s (applicable standard API 6A , material grade low alloy steel AISI 4130/Class EE).
Based on equipment manufacturer’s advice, we replaced soft iron (Galvanized) gaskets of flange faces with SS 316 gaskets; but without any remedial work on flange internals. After installation of SS 316 gaskets on flange faces, now again, corrosion rate got accelerated.

My questions are:

What are the factors to be considered while selecting material class/material grade? Which material grade would be suitable for our service condition – (CO2 30 -50 psia, H2S: < 50 psia, pH: 7.5 Brine, Chlorine: < 2,00,000 ppm ). How to mitigate such corrosion issues in future?

What gasket material will be suitable for flange faces ? Since both soft iron and SS 316 gaskets corroded in above cases, we have no clue as which gasket material can withstand our operational condition.

What stringiest quality control measures can be proposed for new upcoming equipment’s for an anticipated service condition (CO2 30 -50 psia, H2S: < 50 psia, pH: 7.5 Brine, Chlorine: < 2,00,000 ppm )?

We are thinking of full cladding in future; however, it would be an expensive. Can we go for selective cladding and any internal corrosion resistance surface treatment/coating? If any, please recommend.

Any routine inspection can be recommended to assess equipment’s internals (like phased array UT or any other ) without dismantling flanges??

What remedial works can be proposed for corroded flange internals ??


Regards

 

[weldstan]

While the chloride content of the water is high, you don't state the temperature or quantity of water produced. If pitting is the problem, is it localized? How far downstream is it from the well head.

If localized pitting is the problem, digitl x-ray can be used to locate, followed by UT to determine pit depth.

 

[TomDOT]

I'm confused by your chloride levels. It appears you are saying they are less than 2 million parts per million - assuming the commas are correct, not the zeroes. Which is true of anything, even pure chloride (if you could create it) maxes out at 1 million parts per million.

Corrosion resistance of 316 "rule of thumb' is typically no more than 2,000 ppm chloride, without the additional variables of CO2, elevated temperatures, et cetera.

I would start reading here:

http://www.stainless-steel-world.net/pdf/10073.pdf

 

[OGMetEngr]

I would say iron gaskets are borderline at these CO2 levels, assuming enough water to enable corrosion and assuming you are referring to surfaces in contact with well fluid (along with other variables). Also sealing surfaces should be more corrosion resistant than low alloy steel. A nickel alloy selective cladding there would be the safest option. The stainless gasket on a low alloy steel flange will in many cases accelerate corrosion of the flange, hence the need for Ni-alloy cladding on that important sealing surface. Suitability of stainless gasket is dependent on chlorides, temperature and other chemistry of the well fluid. Also depends on how long you want this connection to last, along with risk level the customer is comfortable with.