Deaerators SICC

SICC Resistance of Deaerators

To make clear that carbon steels applied in deaerators can differ very much in corrosion resistance under practical conditions because of minor differences in the chemical composition of the steels.
In the electricity generating industry, ‘mild’ (i.e. ‘plain carbon’) and low-alloyed steels are used in huge quantities, for example in the construction of deaerators, boilers, steam generators, heat recovery boilers and waste incineration boilers. To prevent cracking failures in deaerators and boilers, much attention is given to control both the chemical and the physical operating conditions. In the past, however, the corrosion resistance of carbon steel often was not even considered worth of discussion. Designers frequently do not realise that within the group of these steel types, corrosion resistance can vary considerably.

Convential research projects

Pressure vessels, such as deaerators and wet steam lines, have been susceptible to SICC (Pastoors 1986, 1989 and 1990). Extensive research on stress relieving of welds and the influence of water chemistry was undertaken but the influence of steel composition usually was neglected.
Only one published paper, (by Lenz, 1986), considered to any extent the influence of steel composition as a variable that could affect corrosion and cracking behaviour.

Database of the tested steels

The resistance to Strain Induced Corrosion Cracking (SICC) was determined by measuring the “repassivation” behaviour of the steels at freshly ground surfaces with an electrochemical technique. The corrosion current measured with time was used to calculate the cracking rates of a CT specimen.

A correlation was found between chemical composition, corrosion resistance to SICC and experiences under practical conditions. The results of early published papers on boiler corrosion (testing in FeCl2 solutions), erosion corrosion (testing in wet steam at 20 bar), Nitrate Stress Corrosion Cracking (testing in NH4NO3 solutions ) and Strain Induced Corrosion Cracking (SICC), together with those originating from in-service failures, were compiled into a reference database. Paper nr 58 is a compilation and review of that work.

The database and formulae presented make clear there is often a direct correlation between chemical composition of ordinary “C-steel” and these specific types of corrosion failures. Steels of inservice failures had a C-content higher than 0.12% and a Mo content less than 0.3%.

More research can become a topic for an University project.

 

See for more information papers nr: 40,  58 influence of steel composition and 64.

Home » SCC » Deaerators SICC » Deaerators
4000.jpg
4000.jpg
Environmental assisted Corrosion Cracking in a tube to a deaerator. Pitting and cracks were noticed. (ref 40, 58, 64)
4001.jpg
4001.jpg
`Black and wight picture of the same tube of Environmental Assisted Corrosion Cracking in a tube to a deaerator. Pitting and cracks were noticed. (ref 40, 58, 64)
4002.jpg
4002.jpg
Tube was asymmetrical bowed. Environmental assisted Corrosion Cracking in a tube to a deaerator. Pitting and cracks were noticed. (ref 40, 58, 64)
4003.jpg
4003.jpg
Optical microscopic image of the pitting location. On top of the pitting a oxide cap has been formed. (ref 40, 58, 64)
4004.jpg
4004.jpg
Example 1 of the EACC. (ref 40, 58, 64)
4005.jpg
4005.jpg
Example 2 of the EACC. (ref 40, 58, 64)
4006.jpg
4006.jpg
Example 3 of the EACC.(ref 40, 58, 64)
4020.jpg
4020.jpg
Autoclaves and the water refreshing system for the Low Cycle Corrosion Fatyique experiments. (ref 40, 58, 64)
4021.jpg
4021.jpg
Autoclave and the elctric monitoring system. (ref 40, 58, 64)
4022.jpg
4022.jpg
Top of the autoclave with the load measuring cell and the linear transpolation system. (ref 40, 58, 64) (foto’s KEMA).
4023.jpg
4023.jpg
Three bend specimen mounted in the autoclave. (ref 40, 58, 64) (foto’s KEMA).
4024.jpg
4024.jpg
4025.jpg
4025.jpg
Crack length was measured by means of compliance measurements. (ref 40, 58, 64) (foto’s KEMA).
4030.jpg
4030.jpg
Low Cycle Corrosion Fatique experiment nr 12. (ref 40, 58, 64) (foto’s KEMA).
4031.jpg
4031.jpg
Experiment nr 21, in which crack growth was tried to stop by dosing extra NH3 (Exp. 21, 22 and 23). The moment of change of waterconditioning is indicated with A. (ref 40, 58, 64) (foto’s KEMA).
4032.jpg
4032.jpg
Experiment nr 24, in which crack growth was tried to stop by dosing extra Oxygen (Exp nr 24). The moment of change of waterconditioning is indicated with A. (ref 40, 58, 64) (foto’s KEMA).
4040.jpg
4040.jpg
Cross section of one of the cracked specimen. (ref 40, 58, 64) (foto’s KEMA).
4041.jpg
4041.jpg
Cross sections of specimens nr 23 (A) and 24 (B). Near the crack tip the many small cracks indicate on Hydrogen SCC. The straigth crack is the result of Low Cycle Corrosion Fatique. Near MnS inclusions corrosion pits are found. (ref 40, 58, 64) (foto’s KEMA).
4050.jpg
4050.jpg
SEM picture of a cracked specimen. F: Low Cycle Corrosion Fatique crack. H: Hydrogen Cracking. (ref 40, 58, 64).(foto’s KEMA).
4051.jpg
4051.jpg
4052.jpg
4052.jpg
SEM pictures of the oxide in one of the cracked specimen. (ref 40, 58, 64) (foto’s KEMA).
4054.jpg
4054.jpg
SEM picture of hematite oxide in the crack. (ref 40, 58, 64) (foto’s KEMA).
4056.jpg
4056.jpg
SEM picture of various oxides in the crevice of a cracked specimen. In the crevice the solved iron from the LCCF cracking is oxidised to coarse crystalline magnetite (Fe3O4) and hematite (Fe2O3). See pictures A, B and C. At high Oxygen contents in the water hematite plates are formed. Aside these plates and octaeders dendrites of hematite crystals have been found. On the flanks of the crack stalagmites of magnetite crystals are formed. See Picture D. (ref 40, 58, 64) (foto’s KEMA).
4057.jpg
4057.jpg
SEM picture of magnetite grown on the flanks of a cracked specimen. (ref 40, 58, 64) (foto’s KEMA).
4060.jpg
4060.jpg
"Results of the Low Cycle Corrosion Fatique experiments in Water (0 to 0.2 mg/L NH3) with Oxygen (250 μg /L). (ref 40, 58, 64) "
4061.jpg
4061.jpg
Results of the Low Cycle Corrosion Fatique experiments, where crack growth was tried to stop by means of changing water quality (extra NH3 or lower Oxygen). (ref 40, 58, 64)