Initiation and Propagation of Stress-Assisted Corrosion (SAC) Cracks in Carbon Steel Boiler Tubes

[+] Author and Article Information
Dong Yang

The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405

Preet M. Singh1

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245preet.singh@mse.gatech.edu

Richard W. Neu

The George W. Woodruff School of Mechanical Engineering, and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245


Corresponding author.

J. Eng. Mater. Technol 129(4), 559-566 (Jul 05, 2007) (8 pages) doi:10.1115/1.2772330 History: Received December 20, 2006; Revised July 05, 2007

Industrial boilers experience bulbous cracks in carbon steel water-wall tubes and other water-touched surfaces. Because these cracks are blunt and different from sharp fatigue cracks, they are generally referred to as stress-assisted corrosion (SAC) cracks. The performance of carbon steels in industrial boilers strongly depends on the formation and stability of the magnetite film on the waterside surface. To understand the mechanism for SAC crack initiation and propagation, slow strain rate tests were conducted in a recirculation autoclave under industrial boiler water conditions. The dissolved oxygen in the water was maintained from a negligible amount (5ppb) to 3ppm. The SAC crack initiation and propagation mechanism involves magnetite film damage and requires the presence of dissolved oxygen in the water. Increasing the test temperature accelerates the process. A mechanism for SAC cracking is proposed, and interrupted slow strain rate tests were carried out to validate this mechanism. Temperature and dissolved oxygen in boiler water are important factors in initiation and propagation of stress assisted corrosion cracks. SAC in boilers can be controlled by controlling the dissolved oxygen levels around 5ppb.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

(a) Sharp corrosion-fatigue (CF) cracks, typically found on carbon steel tubes in high-pressure utility boilers and (b) blunt SAC cracks with bulbous appearance, typically found in industrial boilers

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Figure 2

Schematic of the recirculation loop and autoclave system used to simulate boiler environments

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Figure 3

Effect of SSRT temperature on (a) crack density and (b) crack velocity for SA-210 carbon steel samples in pure water with dissolved oxygen either 3ppm (denoted “With O2”) or only ∼5ppb (denoted “Without O2”)

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Figure 4

Surface cracks on a SSRT sample tested in 3ppm oxygenated water at 300°C

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Figure 5

Effect of test temperature on the ductility of SA-210 carbon steel samples

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Figure 6

Effect of dissolved oxygen on the ductility of SA-210 carbon steel samples

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Figure 7

The temperature dependence on crack velocity of SA-210 carbon steel with dissolved oxygen of 5ppm

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Figure 8

ISSRT to simulate boiler shutdown/start-up cycles. Micrographs show crack morphology at different times during the test.

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Figure 9

Schematic showing proposed mechanism of initiation and propagation of SAC cracks

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Figure 10

Validation tests conducted in oxygenated water (5ppm) at 300°C: (a) test without interruption showing sharp crack and (b) ISSRT showing bulbous crack formation under simulated boiler operation with shutdown/start-up cycles



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