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Recovery Boiler Tube Failure by Mechanisms of Stress-Assisted Corrosion and Phosphate Hideout

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This paper presents an unusual case example of recovery boiler waterside tube deterioration by a combination of SAC and phosphate hideout corrosion mechanisms. The case shows boiler tubes that exhibited through-wall corrosion and cracking.

Product Number: 51315-6059-SG
ISBN: 6059 2015 CP
Author: Max Moskal
Publication Date: 2015
Industry: Process Industries
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Tube damage due to stress-assisted corrosion (SAC or Corrosion Fatigue) is relatively common in recovery boilers. While some recovery boiler water leak incidents have been attributed to SAC thedamage mechanism by itself rarely results in through-wall leaks. Another damage mechanism known as “phosphate hideout” has been attributed to under deposit corrosion and failures in high-pressure drum-type boilers. Phosphate hide-out is characterized by the retention of phosphate in the boiler during conditions of high pressure and the subsequent release of phosphate when the pressure is reduced. The condition is most often reported in high-pressure units e.g. 2500 psig. Paper mill recovery boilers typically operate at pressures below 1500 psig but may be frequently taken off- load or swing due to shut down of a paper machine or to “chill and blow” an operation procedure used to release upper boiler fireside deposits. There have been few if any cases of tube failures reportedin recovery boilers due to phosphate hideout corrosion.This paper presents an unusual case example of recovery boiler waterside tube deterioration by a combination of SAC and phosphate hideout corrosion mechanisms. The case shows boiler tubes that exhibited through-wall corrosion and cracking. The phosphate corrosion damage appeared secondary and cracking was present under deep SAC deposits; the secondary cracking was characteristic of intergranular stress corrosion cracking (SCC) mechanism. The combined SAC and SCC damage zones exhibited deposits mainly of sodium-iron-phosphate maricite a chemical that is the reaction product between mono- and di-sodium phosphate and the boiler tube protective oxide layer magnetite. In the case example the SAC damage was attributed to marginal water treatment and high residual stresses whereas the phosphate corrosion damage appeared to be from problems in controlling phosphate equilibrium in the boiler feedwater.

Key words: conference papers, conference papers 2015,  stress corrosion cracking, SCC, stress-assisted corrosion, SAC, tube failure, boiler, phosphate corrosion, phosphate hideout, phosphate treatment, water treatment

Tube damage due to stress-assisted corrosion (SAC or Corrosion Fatigue) is relatively common in recovery boilers. While some recovery boiler water leak incidents have been attributed to SAC thedamage mechanism by itself rarely results in through-wall leaks. Another damage mechanism known as “phosphate hideout” has been attributed to under deposit corrosion and failures in high-pressure drum-type boilers. Phosphate hide-out is characterized by the retention of phosphate in the boiler during conditions of high pressure and the subsequent release of phosphate when the pressure is reduced. The condition is most often reported in high-pressure units e.g. 2500 psig. Paper mill recovery boilers typically operate at pressures below 1500 psig but may be frequently taken off- load or swing due to shut down of a paper machine or to “chill and blow” an operation procedure used to release upper boiler fireside deposits. There have been few if any cases of tube failures reportedin recovery boilers due to phosphate hideout corrosion.This paper presents an unusual case example of recovery boiler waterside tube deterioration by a combination of SAC and phosphate hideout corrosion mechanisms. The case shows boiler tubes that exhibited through-wall corrosion and cracking. The phosphate corrosion damage appeared secondary and cracking was present under deep SAC deposits; the secondary cracking was characteristic of intergranular stress corrosion cracking (SCC) mechanism. The combined SAC and SCC damage zones exhibited deposits mainly of sodium-iron-phosphate maricite a chemical that is the reaction product between mono- and di-sodium phosphate and the boiler tube protective oxide layer magnetite. In the case example the SAC damage was attributed to marginal water treatment and high residual stresses whereas the phosphate corrosion damage appeared to be from problems in controlling phosphate equilibrium in the boiler feedwater.

Key words: conference papers, conference papers 2015,  stress corrosion cracking, SCC, stress-assisted corrosion, SAC, tube failure, boiler, phosphate corrosion, phosphate hideout, phosphate treatment, water treatment

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