Synchrotron and Neutron Users' Group  

The Science of Corrosion

– Economic losses from corrosion of metal equals about the 3% of the world gross domestic product. It is an everyday phenomenon that occurs with cars, pipelines, and even in your home air conditioning unit. Maintenance procedures to delay or prevent corrosion are often times labor intensive. While large industrial operations can plan and effectively mitigate corrosion, every day consumer products such as air conditioning condensers or pumps end up being replaced. Thus the research of today will help the quality of life for everybody, from less expensive repairs on home HVAC systems, heat pumps for swimming pools and businesses, and longer lasting car parts. It has been known for a hundred years that steels with 10% chromium are highly resistant to corrosion. Over time, various other additives and processes were developed to improve quality of steel types-broadly referred to as stainless steel.

- In a recent article by the Las Vegas Sun newspaper, one HVAC contractor reported that upto 20 percent of their work was replacing heat exchangers. Including grills and mounts and ducting. Commercial heat exchangers in corrosive environments do not last more than a few years. This creates the need for frequent maintenance and in Las Vegas the recycling program for waste metals is not efficient. Las Vegas is known for liberal filling of its cheap landfill space. Silver Fox Air Conditioning has been tracking the life-cycle of metals, costs, and carbon-foot print of repair and installation activities for all of its Air Conditioning and HVAC work. They hope to recycle more than 99% of all metal wastes but also to introduce the newest and most inert HVAC materials to extend the life cycle of equipment and reduce maintenance. The general manager of Silver Fox Air Conditioning said that he became interested in helping a group of researchers who approached him. The researchers from the University of Nevada, Las Vegas, are trying to quantify the exact cost-benefit and total life cycle cost in terms of carbon emissions and other environmental impacts for novel materials, coatings, and treatments for HVAC equipment using state of the art methods for characterizing the molecular structure of metal surfaces. In addition, Silver Fox has been field testing the newest surface treated filters for improving air quality in buildings for demanding applications (removing smoke from Las Vegas Casino indoor air).

- There are many types of stainless steel. Some are good for steam generators and some are good for kitchen sinks. But in critical applications where chemical resistance is needed such as with heat exchangers in pool heat pumps or outdoor air conditioning units, use of titanium has proven to provide higher life spans. But research continues into making steels and other metals more corrosion resistant. This research continues in understanding the metal on a microscopic level of atoms looking at crystal structure and defects caused by mechanical loads, operation, or fabrication processes. High strains can cause metal materials to be more susceptible to corrosion.

- Researchers from Max Planck Institute, the University of Ulm (Germany), and the ESRF used the European synchrotron light source to study the corrosive process in a gold-copper alloy. This led to a fundamental understanding of the corrosion process used in gold-copper alloy plating processes used for hundreds of years by the Inca Indians. Inca metal-smiths used salts to cover a gold and copper alloy covering a objected to be plated with gold. The salts formed an acid that dissolved the copper leaving a gold-rich surface suitable for polishing. Thus, the Incas could use less gold.

- By creating these detailed structural pictures, researcher have been able to under how corrosion begins and how to prevent corrosion by modification of the first surface layer. A group of researchers (Laboratoire de Restauration des Monuments Historiques, ESRF (The European Synchrotron Radiation Facility) and SLS, Swiss Light Source) have been using synchrotron based micro X-ray Absorption Spectroscopy to study micro scale chemical information such as coordination and oxidation state of phases constituting corrosion products within archaeological iron artifacts buried in soil. These developed methods helped answer questions about iron corrodes in the presence of chlorine. The group had one focus of how to restore and conserve of metallic artifacts of historical significance.

-A new grain mapping technique developed by researchers at the ESRF has been used to study stress corrosion cracking in stainless steels. Cracks can appear in stainless steel components when stress or strain is combined with a corrosive environment that attacks sensitive grain boundaries. These cracks represent a critical failure mechanism. In power generation plants, certain grain boundaries can become sensitive during heat treatments or during fast neutron irradiation in nuclear power stations.

-A team of researchers from the University of Manchester, the National Institute of Applied Sciences (INSA) in Lyon (France) and the ESRF has revealed how a growing crack interacts with the 3D crystal structure of stainless steel. By using a new grain mapping technique it was possible to determine the internal 3D structure of the material without destroying the sample. Afterwards, a crack was initiated in the stainless steel, and the scientists were able to study how the crack grew between the grains. This is the experiment has using a 3D grain mapping technique was published in the Journal of Science Different visions of the cracks forming in a stainless steel wire. In the first image, the sample is visible, as well as some cracks. In the middle one, the cracks inside are colored red, and the last picture shows the different grains that make the wire, as well as the crack, superimposed, in red, that starts to form following grain boundaries



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