A selection of the SEM images of the corrosion deposit and a part of the crack with the analysis points indicated

Stress Corrosion Cracking in District Heating Pipes

Euroheat & Power / Fernwårme international, June 2001, pp 52-58.

Wally Huijbregts, Toon Beijers, René Leferink, André Zeijseijnk (KEMA)

Jan Peters, Peter Verbeek, Ruud Bilsen (PNEM Netwerk BV). (paper 52)

Paper available as pdf

Abstract

Failures due to stress corrosion cracking has occurred in the return pipes of a district heating system. Extensive in service failure investigations were carried out on the pipe sections. Stress analyses were made for various scenarios. The pipes are exposed to a slow strain rate of approximately 10-7 per second on heat consumption. It is concluded that the residual stresses along the welds, in combination with the critical strain rates, are sufficient to cause SCC given unfavourable water conditions.

The steel in the pipes complies with the normal specifications of St37.0 and St37.2

Iron carbonate is present in the corrosion products of the pits. As a result of the chemical equilibrium in the pits between iron hydroxide, iron carbonate and the high pH of the water, a high carbonate concentration arose in the pits. Cracking started from the carbonate filled pits. Intergranular SCC was simulated in laboratory experiments under various carbonate conditions. A first requirement to prevent carbonate SCC is maintenance of water quality with respect to pH value in accordance with the applicable guideline.

8 Conclusions

Due to the temperature changes in the return pipe in the event of heat consumption, intergranular SCC is initiated preferentially in the zone adjacent to the weld joints, where the axial residual stresses are also highest. As a result of the temperature changes in the return pipe, the critical strain rate of the steel is 10-7/s.

SCC only occurs in the return pipe as a result of temperature changes and as a consequence of the stability of carbonate at lower temperature. The carbonate concentration in the pits is governed by the iron carbonate-iron hydroxide equilibrium. At a higher pH value, the carbonate concentration rises sharply.

Intergranular SCC also probably occurs readily due to the large quantity of tertiary carbides present in the steel. In selecting materials for carbon steel components which are susceptible to SCC, greater attention will therefore have to be paid in future to the structure and chemical composition of the steel.

The leaks are the result of carbonate SCC. The high carbonate concentrations occur locally underneath the corrosion layer as a result of the high set pH value in the water.

A cracked zone near the weld.

The macro-etching technique shows the different areas in the heat-affected zone and the cracks.