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Failure Analysis Case Histories
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Technical Brief: Denickelification of Cupronickel Tubes
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Denickelification of
cupronickel tubing is a common occurrence in the heat exchangers that use these
tubes.
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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:
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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.
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Mifflin also tested
cupronickel (90/10) tubes with the following results:
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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.”
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Donald B. Bird and
Kenneth L. Moore in a similar study discussed a possible mechanism for
denickelification:2
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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.”
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With regard to corrective
action, Bird2 states:
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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.
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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.
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A final area of vulnerability of cupronickel is
sulfides and ammonia, both common by-products of organic decay.
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References |
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1. Robert C. Mifflin and
Donald B. Bird, Alloy Performance in Brackish Delaware River Water, Getty Oil
Company, Delaware, in-house paper.
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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.
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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.
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