Corrosion, Failure Analysis and Materials Selection Specialists

 

 

 

 

 

Corrosion Testing / Failure Analysis

 

 

 

Corrosion Testing

Failure Analysis

Field Investigations

Litigation

Metallography

Technical Papers

CTL Profile

Pricing & Policies

Contact CTL

Quality Assurance

Return to Failure Analysis Case Histories

Technical Brief: Denickelification of Cupronickel Tubes 

Denickelification of cupronickel tubing is a common occurrence in the heat exchangers that use these tubes.

Robert Mifflin and Donald Bird1 studied the performance of various alloys in a heat exchanger using untreated, brackish Delaware River water as the coolant.  The primary cause of waterside failure of cupronickel (70/30) condenser tubes was identified as plug type denickelification.  Quoting from Mifflin:

This corrosion phenomenon involves the creation of an active corrosion cell that dissolves the alloy and then redeposits the copper back on the surface.  Low cooling water velocities and/or high heat flux across condenser tubes were found to be the prime causes of this type of attack in the Delaware River water environment.

Mifflin also tested cupronickel (90/10) tubes with the following results:

90/10 copper-nickel was first tested at 4 ft/sec cooling water velocity where it suffered both general (24 mpy) plus pitting (29 mpy) corrosion…. As the velocity was decreased further to 2 ft/sec, this trend changed and more severe denickelification resulted with 90/10 copper nickel than with the 70/30 copper nickel.”

Donald B. Bird and Kenneth L. Moore in a similar study discussed a possible mechanism for denickelification:2

One possible explanation of the cause of this denickelification is that local boiling or high temperature conditions exist on the wall of the tubes, causing patch scale formation and subsequent pitting from resultant oxygen concentration cells.  Once pitting has begun, it apparently continues, unabated by changes in operating conditions, due to the self-generating corrosion cell.  This corrosion appears under two conditions, whether high condensing heat loads or low cooling water velocities of which the latter has been the major cause.  Thick scale has been the major cause.” 

With regard to corrective action, Bird2 states:                           

The life of 70/30 cupronickels can be lengthened by maintaining adequate cooling water velocities above 5 fps.  Screening of the water to remove stones, grass and marine life prevents partial blocking of the tubes and subsequent stagnant water conditions.

In most applications, a cupronickel alloy obtains its corrosion resistance by forming an adherent oxide coating that is preferably formed by clean, oxygenated water, at least initially.3   Ferrous sulfate is sometimes recommended to passivate the surface and form this film.  This film can become unstable under stagnant conditions, such as would exist for the entire tube if the water flow is turned off, or for areas of the tube covered by debris.  In some studies, waters with pH’s around 7, were found to be more prone to denickelification than when the pH was around 8.

A final area of vulnerability of cupronickel is sulfides and ammonia, both common by-products of organic decay. 

 

References

 1. Robert C. Mifflin and Donald B. Bird, Alloy Performance in Brackish Delaware River Water, Getty Oil Company,  Delaware, in-house paper.

2. Donald B. Bird and Kenneth L. Moore, Brackish Cooling Water versus Refinery Heat Exchangers, Materials Protection,  Vol. 1, No. 10, pp. 70-77, October 1962.

3. C.A. Powell and H.T. Michels, Copper Nickel for Seawater Corrosion Resistance and Anti-fouling – A State of the Art Review, Marine Applications of Copper Nickel Alloys, Nickel Development Institute and Copper Development Association, undated.

Site Index

Site Copyright © 1995 - 2007, All Rights Reserved,

Corrosion Testing Laboratories, Inc.

60 Blue Hen Drive

Newark, Delaware USA 19713

Phone: 1-302-454-8200

Fax: 1-302-454-8204

web@corrosionlab.com