Finned boiler tubes with nitrate induced SA-IGA in the bents. Finned pipes are used to increase the surface area of the heat exchangers.

Cross section of tube material reveals Stress Corrosion Cracking.

A -SEM image of the microstructure of a 15Mo3 steel that showed nitrate induced SCC.

(B) - Carbon WDX mapping of the same area shows the distribution of carbon (white is carbon, black is no carbon).The pearlite islands are normal in this material. The large amount of ternary cementite that has deposited on the grain boundaries is not normal, however.

Nitrate Stress Corrosion Cracking in Waste Heat Recovery Boilers

R.G.I. Leferink, W.M.M. Huijbregts. (paper 53)

Anti-Corrosion Methods and Materials, Vol 49 (2002), No 2, p 118-126

(Highly Commended Award 2003 of Emeraldi Literati Club)

pdf available

Summary

Waste Heat Recovery Boilers (WHRB's) enhance the thermal efficiency of gas turbine power generating plants by capturing the heat from the exhaust gas and utilising it to raise steam for increased electricity production or district heating. Under normal conditions the exhaust gas from the turbine is cooled from around 550°C to 80°C. Care is taken that condensation will not occur in the cooler parts of the boiler.

Nevertheless, during normal operation, dry ammonium nitrate deposits can be formed in the WHRB. These deposits will become wet when the unit is started or shut down, when the temperature falls below the dew point. The deposits may also 'sweat' (i.e. absorb atmospheric moisture and become liquid) during extended plant outage periods. In consequence, Intergranular Corrosion Attack (IGA) and Stress Assisted Intergranular Corrosion Attack (SA-IGA) (in general called stress corrosion cracking) can occur in carbon steel construction materials. the sensitivity to IGA for a total of 86 steels, of known compositions, was tested. The steels were subjected to aerated ammonium nitrate solutions of 90°C with concentrations ranging between 2 and 35 per cent. After an exposure of 65 hours, cross sections of the steels were studied.

If IGA occurred, the steel was considered sensitive to ammonium nitrate at that (critical) percentage. During the investigations it was observed that alloying elements in several types of steel influenced its resistance to IGA in ammonium nitrate. Elements such as molybdenum, manganese and chromium have a positive effect on the resistance to nitrate induced IGA, SA-IGA and stress corrosion cracking, whereas carbon and copper are detrimental. The microstructure of the steel also appeared to be important. In many specimens taken from in-service failures, strings of carbide precipitates were found to be present at grain boundaries. The precipitates were identified to be ternary carbides. It seems, therefore, that the presence of carbides at grain boundaries increases the susceptibility of a steel to intergranular corrosion.

Conclusions

• Ammonium nitrate was found in waste heat recovery boilers and is most probably formed by the combination of NOx from the exhaust of the gas turbine with aerosols in the combustion air. Crystals of nitrate tend to deposit in the cooler regions of a WHRB.
• During start-up and shut-down the metal temperatures in the boilers tend to fall below the dew point. Condensing water (with some sulphuric acid) will dissolve the precipitated ammonium nitrate, resulting in the formation of a concentrated nitrate solution on the steel surface.
• Carbon steel and low alloy steel are sensitive to IGA in ammonium nitrate solutions if the nitrate concentration is above a critical composition, and the corrosion potential of the material in the environment is more anodic than a critical potential.
• The critical concentration for nitrate induced IGA can be calculated from the chemical composition. The elements Mn, Mo and Cr have a positive effect on the corrosion resistance while Cu and C have a negative effect.
• Ternary cementite precipitated as a continuous ribbon on the grain boundaries of carbon and low alloy steels will increase the sensitivity to nitrate induced IGA.

Pourbaix diagram of iron. Shaded areas indicate danger zones where SCC caused by nitrates, phosphates, carbonate/bicarbonate and hydroxide respectively can occur.

Correlation between measured and calculated critical nitrate concentrations for C-steels, 15Mo3, 13CrMo4.4, 10CrMo9.10, 9% Cr and 12% Cr steels.