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 and 64.