Concentrated solar power (CSP) is a growing renewable energy technology, with some plants already in operation. It has the potential to provide energy on the scale of coal-fired power stations. An array of mirrors concentrates the sun’s energy onto a receiver where it heats a heat transfer fluid (HTF). This can be either stored until thermal energy is required or used to generate steam to drive a turbine, producing electricity in the traditional way.
The effective use of CSP requires cost-effective and corrosion-resistant materials for the tubes that receive the concentrated sun rays and carry the HTF. Such components must operate for extended periods at high temperatures and withstand thermal cycling between around 900ºC in the day and ambient temperature at night. Nickel-based alloys can withstand these extremes but are prohibitively expensive. The most practical compromise materials are high-temperature austenitic stainless steels (ASSs), but these can only operate at considerably lower temperatures than required for the next-generation plants. Development of cost-effective, heat-resistant steels will enable solar thermal plants to reach their maximum potential, bringing greater efficiencies and cheaper electricity.
Failure mechanisms of current steels in practical service in CSIRO’s pilot CSP plant in Newcastle are the focus of Mr Alex La Fontaine, a PhD student with A/Prof. Julie Cairney at USyd. Pipes cycled between 970ºC and ambient temperature for four days and nights were examined using complementary microscopy techniques, including the flagship atom probe.
Large intergranular corrosion cracks extended deep into the tube wall, indicating a dramatic and rapid structural failure. Microscopy showed that the extreme heat causes chromium within the steel to move away from the grain boundaries, allowing corrosion-causing oxygen to penetrate from the surface and oxidise the surrounding steel. When the temperature drops, the crystal structure of the oxidised steel changes, opening up cracks along the grain boundaries allowing more oxygen in and creating a vicious circle of corrosion.
The atom probe was used to look closely at these corroded surfaces revealing that iron-rich oxides are present, rather than the protective chromium oxides (see below image), weakening the overall structure. A silica layer, intended to improve corrosion resistance, also cracked during thermal cycling, weakening the metal still further.
The design of more corrosion-resistant steel should result from the identification of these high-temperature failure mechanisms. Compositional changes to include aluminium as a more effective corrosion-proof layer will be created, tested in service, then it too will be evaluated.
Researchers in the field recognise that this kind of analysis is particularly challenging without the high-resolution and 3D capabilities offered by the atom probe.
CSIRO’s pilot CSP plant in Newcastle
October 20, 2014
