4.1.2 Other copper-alloy tube failure mechanisms - part 2
(c) Stress corrosion cracking
Failure of condenser tubes by cracking under the combined action of tensile stress and corrosion has occurred in the past owing to high residual tensile stresses inadvertently remaining from the tube making process. However, with modern condenser tube production practice, such failures are now uncommon (Fig 4.11 (b)).
(a) Local high temperatures on the outside of tubes can lead to abnormally rapid corrosion at corresponding positions on the inside (cooling water side) of the tubes. Hot spot corrosion takes the form of a highly localised pitting attack, a characteristic feature being that the pits often contain metallic copper. The illustration shows a section through a pit filled with metallic copper produced by a hot spot attack in a copper/30% nickel alloy tube.
(b) Stress corrosion cracking can arise from the imposition of longitudinal stresses when certain tubes are rolled into ic tubeplates. It is also important to avoid over-rolling the tubes to prevent excessive expansion and the consequent failure by cracking.
(c) Failure of tubes by cracking can occur as a result of the combined action stress and corrosion. In corrosion fatigue, the stresses are cyclic or alternating and are applied in-service, for example, as a result of cyclic temperature changes or externally-transmitted vibration.
FIG. 4.11 Photomicrographs of hot spot corrosion, stress corrosion cracking and corrosion fatigue cracking in tubes
Cracking can result from stress corrosion at the tube-to-tubeplate expansion joints. The cracks initiate at the edges of the expanded regions on the CW side of the tube and propagate into the tube wall under the influence of residual stresses remaining from the 'rolling-in' operation. The corrosive medium is suggested to be ammonia, formed from the decomposition of organic slime, and concentrated by evaporation during outages. Effective media control such as washing with clean water before prolonged outages or fitting protective sleeves, appears to provide a solution to this problem.
(d) Corrosion fatigue
Corrosion fatigue can occur in any tube material and is caused by steam buffeting, or structure-borne vibrations, in association with inadequate tube support. Generally, failure occurs at midspan where tube thinning occurs due to excessive movement and contact with adjacent tubes (Fig 4.11 (c)). Additional support plates or the fitting of antivibration damping equipment solves this problem.
(e) Steamside ammonia corrosion
Ammonia corrosion usually attacks brass alloy air cooler tubes in the form of pits on the external tube surface, with grooving at points adjacent to tubeplates and support plates. Titanium and the copper-nickel alloy tubes are highly resistant to this form of attack and the problem can be solved by retubing in an appropriate material.