Carbon Contamination on A240-316L Plates

[sahsanb]

We have an Acetic Acid Above Surface Storage Tank made of 6.5 mm thick plates of A240-316L. We recently observed corrosion and pitting up to 1 mm deep on tank plates at what appears to be grinding marks used to remove weld spatter at the time of commissioning. Apparently manufacturer used grinder suitable for CS plates on tank plates hence resulting in carbon contamination.

We are planning to prevent further corrosion on these corrosion sites by either passivation or appropriate paint system application. Based on your experience please share which is the more suitable option of the two and is there any other option available to us to prevent further corrosion.

Regards

 

[EdStainless]

You mean carbon steel contamination.
If you have pits you need to grind them out and blend the areas.
If you use clean abrasives and then pickle/passivate the areas you should be in good shape.

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Plymouth Tube

 

[mrfailure]

Have you confirmed carbon steel really is present and causing such pitting under galvanic conditions? Might be worth confirming through a metallurgical failure analysis.

 

[sahsanb]

No we have not carried out a metallurgical analysis, but we did scrape off some corrosion product in that region and it was found to be magnetic, indicating cs contamination.

 

[EdStainless]

Not so fast, the corrosion products of 316 will be predominately iron oxide and will be magnetic also.
You need a more detailed analysis before you jump to that conclusion.

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Plymouth Tube

 

[sahsanb]

EdStainless

Many thanks for highlighting that. While carbon steel contamination seems the most probable cause to us, I agree we need to further investigate to identify exact cause of this pitting. I would like to share this AST is located in chemical offloading area of our plant where other utilities such as Paraxylene, Caustic and Fuel Oil are also present and yes these service fumes are also observed from time to time. While the cause of this pitting needs to be concluded I think we need to act on this pitting and pickling and passivation is the most suitable solution. Any comments on anodizing is it applicable on this situation?

Regards