What do I really need to know about handling the higher alloy materials? I always hear about proper handling procedures, but is there anything to that?
There are a few things you need to be aware of. The duplex, austenitic and nickel based alloys are resistant to atmospheric corrosion in most industrial environments. But precautions do need to be taken to avoid contamination of these alloys with non-corrosion resistant materials or corrosive substances. Improper handling of these alloys could cause a negative impact on material performance. Special care should be taken to avoid these alloys coming in contact with other non-corrosion resistant materials like contaminated metal inspection tables, metal forks, chains and cables. Duplex, austenitic and nickel based alloys may be stored in direct contact with wood or on uncontaminated plastic, while alloys such as 13 Chrome may experience pitting corrosion if stored in direct contact with wood.
What happens when these higher alloys are contaminated?
Well, rust can form. There are generally two types of rust that can form on the surface of the pipe due to contamination. You might have heard the terms superficial rust and embedded rust thrown around. Superficial rust is iron oxide deposited on the original surface of the pipe. With superficial rust, the surface of the pipe has not been disturbed and therefore the corrosion resistance has not been compromised.
Embedded rust, on the other hand, is rust that lies below the original surface of the pipe. The process of embedding it has created a pit or gouge in the pipe. This pit or gouge can act as an initiation site for localized corrosion of the pipe and as a stress riser where stress corrosion might initiate.
Okay, so superficial rust doesn’t sound that bad.
You’re right, it’s not that bad. Superficial contamination with non-corrosion resistant materials has no detrimental effect on CRA materials. Superficial rust is not bonded to the surface of the pipe. So although not a necessity, if this type of contamination occurs during transportation, handling or storage, it can be removed with Scotch Brite pads without removing any of the base metal of the pipe. You should utilize careful judgment, however, to differentiate between superficial and embedded contamination.
So from the sound of it, embedded rust is a bigger deal.
Yes, embedded rust is a bigger deal. Since it has affected the original surface of the pipe. It needs to be remediated. We recommend the removal of embedded contamination be done with a 36 or finer flapper wheel that does not contain iron or iron oxides. The ground area should blend with a material without edges or contours. The operator must use caution to prevent excess material removal and heat buildup. The remaining wall thickness and od should be verified with a UT gauge to ensure it does not fall below the minimum specification requirement. Then a copper sulfate solution can be applied to any area where contamination was present to ensure it has been removed completely. The mechanical remediation can be followed by a grit blast to ensure contamination has been removed and a uniform surface is visible.
The grit blasting level should be in accordance with ISO 8501-1:2007.