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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.

 

Figure 1. Close-up of the bottom side of the tube.

Figure 2. Denickelification of the cupronickel tube. (63X original magnification).

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