|
Return to
Failure Analysis Case Histories
|
Denickelification of Cupronickel Tubing in an Oil Refinery
|
|
ENVIRONMENT:
|
Oil Refinery-Crude Unit Vacuum Column |
|
EQUIPMENT:
|
70 / 30 Cupronickel tubing |
|
MATERIAL: |
Decades |
|
FAILURE MODE:
|
Denickelification |
| |
|
A crude unit vacuum column had several tube failures in
its condenser. Reportedly, the condenser used 70/30 cupronickel tubes installed
several decades ago. One tube segment from a failed tube was sent to CTL for
failure analysis.
|
|
The condenser had a mixture of hydrocarbons and steam on
the shell side. The hydrocarbons contained H2S, NH3, H2O, H2, chlorides, and
naphtha. The ID (inner diameter) of the tubes contained once through cooling
water from the brackish river water.
|
|
Deposits on the ID were heavier on one side; therefore,
we deduced that this was the bottom side of the tube in service. The bottom
side of the tube under close examination was found to contain many small
through-wall pits, Figure 1. The fracture face consisted of two portions: a
straight wall portion that failed primarily from corrosion, although some
tensile forces were involved, and a portion that was primarily ductile tearing,
although some corrosion was involved.
|
|
A metallographic mount of the tube cross-section revealed
extensive pitting in a finely grained microstructure. This pitting was caused
by denickelification of the cupronickel structure, Figure 2, commonly referred
to as plug-type denickelification.
|
|
As discussed in the
Technical Brief: Denickelification of Cupronickel Tubes, cupronickel tubes
get their corrosion resistance by the formation of a passive oxide film on the
exposed surface. The stability of this passive film requires that oxygenated
water maintain access to the surface. Heavy deposits on the bottom of this tube
resulted in stagnant, non-oxygenated water in contact with the cupronickel. Once
the denickelification is started, it can be self-propagating because access of
oxygen to the pit environment is limited.
|
|
|
