Latest advances in the understanding of acid dewpoint corrosion: corrosion and stress corrosion cracking in combustion gas condensates

W. M. M. Huijbregts, R. G. I. Leferink (paper 56)

Anti Corrosion Methods and Materials Volume 51, No 3, (2004), 173-188

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Summary

Corrosion failures due to condensing flue gases containing H2O, SO3, NOx and HCl still occur more often than might be expected.
The corrosion failures can be of several types:general corrosion, pitting attack and stress corrosion cracking (SCC).
The chemistry of condensing gases is discussed, and some examples of corrosion in large scale installations are presented, including blast stoves for steel production, heat recovery steam generators (HRSG's), and waste incineration boilers.

The use of thermal insulation inside boiler casings can result in nitrate stress corrosion cracking when the flue gas contains high concentrations of NOx. Nitric acid from the flue gas can react with carbon steel and insulation material forming ammonium nitrate and calcium nitrate. Both materials have hygroscopic properties and are very corrosive, even above the water dewpoint of the gases. For this reason, the resistance of carbon steels and low-alloyed steels in ammonium nitrate solutions was studied. A correlation was found between steel composition and concentration of ammonium nitrate in which intergranular corrosion could commence.

Introduction

Corrosion arising from condensing gases has been a problem in industry for many years. The basics of corrosion due to condensing gases are well understood, and it might be expected, therefore, that this type of attack should not occur. However, the many different combinations of operating temperature and chemical stoichiometry of combustion reactions, combined with constructional features of a combustor design, can lead to many possible corrosion reactions. In recent years, many corrosion problems have arisen from condensing gases. When there is a risk of condensation, the designer should have the answers on the following questions:

• Which condensed liquid can be formed? (The dewpoints of the various gases should be calculated).
• What amount of condensed liquid can be expected?
• What concentration of corrosive liquid can be expected?
• What is the corrosion resistance of the material in the environment to be expected?

These aspects should be considered at an early stage of a new or modified design as such consideration will reduce drastically the likelihood of premature failure or the necessity for later modifications of the design. A brief overview of the basic chemical and physical processes taking place during condensation is presented below, then some examples and their analysis are discussed to illustrate the types of situation in which combustion gas condensation corrosion may occur, and what measures can be taken to avoid or mitigate attack. Specific examples associated with condensation in blast stoves for the steel production, heat recovery steam generators (HRSG's) and waste incineration boilers are considered.

Conclusions

• Even though knowledge on condensation of corrosive gases is well available in literature, design failures due to condensation phenomena still occur in technical installations. The operation of equipment which can generate condensing gases makes it necessary to think carefully about the operational regimes (both on and off line), and of the possible risk of acid condensation attack.
• Condensation of nitric acid can cause stress corrosion cracking of carbon steel. However, reaction products of nitric acid with the steel or insulation also can result in the formation of corrosive ammonium nitrate or calcium nitrate.
• Calcium and ammonium nitrate both are hygroscopic materials that can take up water even above the dewpoint of a gas. Up to a temperature of 120°C this can create a very corrosive environment, and may result in a severe risk of nitrate SCC to susceptible materials.
• As nitrate stress corrosion cracking is often a precursor to intergranular corrosion, SCC is particularly influenced by chemical corrosion. Annealing a material, or the selection of a higher strength material, will be of minimal help in preventing SCC on the longer term. The selection of a low-alloyed 2% chromium steel as a replacement for carbon steel in critical locations is a better choice for preventing the intergranular corrosion in dissolved nitrate environments.

Figure 1: The water vapour pressures from the water vapour table. A gas with 6.5 v% H2O has a vapour pressure of 49.7 mm Hg (100 v% water has a vapour pressure of 758 mm Hg) and a dewpoint of 38°C

Figure 2: Dewpoint behaviour of SO3 at various water contents of the gas, calculated from the formula of Verhoff

Figure 3: Dewpoint behaviour of SO2 at various water contents of the gas, calculated from the formula of Kiang. The SO2 dewpoints for all gases are lower than is the water dewpoint of the gases.

Figure 4: Dewpoint behaviour of HCl at various water contents of the gas, calculated from the formula of Kiang and the water vapour table

Figure 5: Dewpoint behaviour of NO2 at various water contents of the gas, calculated from the formula of Perry and the water vapour table.