In the search for non-toxic anti-corrosion treatments for carbon steel, A*STAR researchers have developed a technique to study the effectiveness of a safer and more environmentally friendly corrosion inhibitor .
Carbon steel, an alloy made from iron and carbon, is the largest class of alloys in use today. It is used to make a range of products from fences and springs to steel wires and pipelines, and for structural support in buildings, bridges, as well as nuclear and fossil fuel power plants.
The corrosion of carbon steel, however, represents an enormous cost to industry and is of enormous practical importance. A common corrosion inhibitor used in the construction industry, calcium nitrite, is very toxic to humans, impairing the ability of red blood cells to carry oxygen.
In search of safer corrosion inhibitors, Yong Teck Tan and his colleagues at the National University of Singapore and the Singapore Institute of Manufacturing Technology investigated molybdate as a potential alternative and developed a technique to determine its adequacy.
Molybdate is non-toxic and protects carbon steel from corrosion by competitive adsorption against chloride on the passive film surface and, in the presence of calcium cations, can also deposit a layer of calcium molybdate.
“Our goal was first to determine the suitability of molybdate as a corrosion inhibitor for carbon steel in alkaline environments, and then to study its effect on the passivation of carbon steel,” says Tan.
“Previous studies using electrochemical techniques have focused on the effectiveness of corrosion inhibition at a given time, which provides a snapshot of the level of corrosion at that time,” says Tan. “Depending on whether it was assessed on short or long timescales, different conclusions were drawn.”
So the research team took a closer look. They used an electrochemical method to estimate the extent of the corrosion over the entire duration of the investigation and were able to assess the overall effectiveness of the molybdate.
“Even though the molybdate resulted in slightly higher passive current in the later stages, faster passivation in the early stages resulted in a lower overall level of corrosion,” says Tan.
Researchers found that incomplete coverage of carbon steel with calcium molybdate resulted in slightly higher corrosion rates compared to untreated surfaces. By controlling the composition of the molybdate solution, however, the calcium molybdate film covered the entire surface, resulting in improved corrosion resistance.
“Overall, molybdate has proven to be an effective corrosion inhibitor,” says Tan. “We will now explore its effectiveness in solutions containing other ions.”
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Yong Teck Tan et al. Effect of molybdate on the passivation of carbon steel in alkaline solutions under open circuit conditions, Journal of the Electrochemical Society (2016). DOI: 10.1149/2.0651610jes
Quote: New technique to study the action of molybdate on carbon steel (April 19, 2017) retrieved on January 14, 2022 from https://phys.org/news/2017-04-technique-action-molybdate-carbon-steel .html
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