This month’s case study describes a very interesting failure of a steam condensate reboiler line.
R-Tech Materials received a section of pipework from a 6” steam condensate reboiler with the request to determine the cause of failure (see Figure 1). The leak was detected on the underside of a bend. The operating pressure was 3 kg/cm2 and the operating temperature was 130°C.
Figure 1 Failed pipework
Visual examination of the pipework revealed a large area of perforation in the vicinity of the circumferential weld which had initiated from the internal surface. Clusters of pits were also present close to the area of perforation in addition to some deep grooving (see Figure 2).
Figure 2 Clusters of pits and grooving on internal surface
Examination of a section taken through the pitting/grooving revealed undercutting and significant corrosion product build-up (see Figure 3). The corrosion product was found to consist of magnetite, hematite and wustite with trace amounts of sulphur. The chemistry, microstructure and mechanical properties of the parent and weld materials were found to be in accordance with the given specification.
Figure 3 Section through pitting/grooving
The presence of localised grooving, clusters of pits and the locality of the failure to the weld suggests that the pipework had failed due to CO2 corrosion. CO2 corrosion generally occurs in areas of turbulence and sometimes at the root of piping welds. In addition, this form of corrosion can be influenced by metallurgy, such as welds, which causes transverse grooving. This corrosion is primarily related to the carbide microstructure. Pearlite in normalised steels generally promotes resistance to CO2 corrosion, where the cementite lamellae reinforce the protective carbonate layers. Spheroidised carbides in normalised steels affected by heat treatment, particularly welding, do not have this favourable effect which leads to preferential attack.
The major source of CO2 in boilers is the thermal breakdown within the boiler of bicarbonate and carbonate present in the feed water. Other sources of CO2 include the breakdown of small amounts of organic material. Carbon dioxide by itself does not cause corrosion, though when it dissolves in water; carbonic acid is formed, which subsequently lowers the pH level to 4.5-5.5 which then causes localised attack of the steel. Corrosion inhibitors can be used in the system to reduce corrosion rates. Furthermore, 300 series, 400 series and duplex stainless steels are more resistance to carbon dioxide corrosion than carbon steels.
A failure analysis of this type is a valuable and informative tool to reduce the likelihood of a repeat incident which improves safety and productivity and can enable evolution towards a better product.