Oxidation is by far the most common form of high temperature corrosion – almost all useful metals and alloys will oxidize above a certain temperature, leading to scaling, loss of material and changes in physical properties. Gaseous attack is not limited to oxygen however, with sulphur-bearing gases, carbon oxides, nitrous oxides, halogens and many more all attacking materials in a different manner.
Furthermore, high temperature corrosion is not restricted to the gaseous phase – solid ash and salt deposits contribute to the corrosive effect, with associated erosion and removal of scale. In the liquid phase, molten metals and molten salts pose their own unique variety of challenges, causing highly complex and environment-dependent corrosion.
Industrially, high temperature corrosion is a significant issue. Any component that is exposed to a high temperature in a non-inert environment is potentially at risk. This includes the aerospace, power, metal processing, automotive, waste incineration and chemical processing industries, with many more besides.
TWI has dealt with corrosion prevention/mitigation in a number of environments by means of coatings technology, materials selection and cleaning techniques, as well as a programme of work focused towards fundamental understanding of several high temperature corrosion processes.
We have carried out testing on metals, alloys, ceramics, graphite and graphite-based materials, composites and coatings. TWI has considerable expertise in designing and building equipment for high temperature testing in extreme environments. This includes commissioning and maintaining equipment capable of testing up to 40 samples simultaneously in a hydrogen chloride-based atmosphere at up to 620°C.
- Testing in HCl, CO and CO2 environments at up to 1150°C
- Molten salt immersion at up to 900°C in a controlled gaseous environment
- Determining erosion-corrosion rates in molten salts at up to 1100°C and simulated flow rates of up to 40m/s
- Assessing electrochemical behaviour in molten salts up to 900°C
In addition to the above, TWI maintains a full and adaptable heat treatment facility, capable of treating and testing samples at up to 1500°C.
An example of TWI's expertise is shown in a series of two recent projects. The first dealt with corrosion-resistant coatings for a biomass plant.
As an offshoot from this and other work, TWI has recently begun a group-sponsored project looking at developing new coating technologies for high temperature corrosion mitigation in process plants, each with its own unique highly corrosive environment.