Magnetite Layer Formation in Waste Heat Boiler

[sahsanb]

Since I could not a response in Corrosion Engineering forum I am writing the same question in this forum in hope the expert metallurgists here may help me in my query. The query goes like this:

We are trying to explore an NDE technique to detect pitting on tube OD of our Waste Heat Boiler. With BFW at shell side and Syn Gas at tube side we understand there will be a thin magnetite layer on the tubes OD. Due to some operational upsets in early years of equipment history we strongly suspect there must be some pitting on the tubes OD, however owing to good operational control since then magnetite layer must be covering them now.

We are unable to find an Inspection technique that can guarantee minor pit detection in presence of strongly bonded magnetite layer covering them. Mentioned below are brief equipment details:

Tube Material:L 13CrMo4-5
Tube OD : 38 mm
Tube thickness: 4 mm
Shell Side: BFW (324oC)
Tube Side: Syn Gas (999oC)

Request your expert opinion on following queries:

What is the typical thickness of magnetite layer?
Does magnetite layer continue to grow over the years if satisfactory operational control is exercised or at some point it stops growing
How strongly bonded is magnetite layer with the base metal

Regards,

 

[metengr]

Why don't you remove one or more actual tube samples that capture the above and have a nondestructive testing firm fine tune the remote field eddy current test method using the tube samples to find the OD pits, and size them. Going about the way you are doing will be a wasted effort.