Rust: An age old problem

There is not a lot about rust that has not already been studied extensively, yet the problem of corrosion remains. Due to the diverse use of iron and steel in many industries for many different applications, iron and steel corrosion is a common, sometimes inevitable problem worldwide. While iron is a crucial building block of steel, iron corrosion is often nature’s effort to return it to its most stable form. Although many processes and preventative measures have been developed, such as galvanization, cathodic protection, and painting, in practicality, not all environmental conditions can be predicted. As such, there is a probability that some steel corrosion will occur. Corrosion currently accounts for billions of dollars each year to fix or replace corroded items. Just in the United States alone, corrosion damage cost over $1.1 trillion a year, affecting all industries. This cost estimate includes replacing or repairing everything affected by the failed part due to corrosion, such as damage to an entire floor of a building due to leaky pipes, or loss of running water in a neighborhood due to an underground water main burst, as well as lost production output due to machinery failing in a factory line.

At Eurofins EAG Laboratories, Los Angeles (formerly SEAL Laboratories), many clients seek our expertise with corrosion issues. As a metallurgical laboratory with failure analysis capabilities, we document and analyze the corrosion using many different testing capabilities available in our family of laboratories across the country. Working with the client, we can determine the corrosion mechanism, the probable cause, and if needed, recommend further actions to mitigate or prevent further corrosion damage.

This issue’s cover is an electron micrograph showing a crystallographic structure of rust tubercles formed on the inside surface of a hot water tank over an extended period of time. Such formation occurred over a long period of time with high humidity and temperature via corrosion of the plain carbon steel hot water tank after the protective thin glass coating had been locally compromised. Optically, the rust tubercles consisted of shiny black particles attached to a red-brown substrate layer of corrosion product. Additionally, when a magnet was applied, the rust tubercles were found to have a magnetic behavior.

After initial examination, the sample was placed in a Tescan Vega II XMU scanning electron microscope under standard high vacuum conditions and photographed with a 328 micron field of view. The tubercles are seen to have a cubic structure and cubic platelets have formed over each platelet as they grew with time. Some amorphous iron hydroxide formation was also seen with a rough structure. Energy dispersive X-ray spectroscopy (EDX/EDS) of the rust tubercle revealed the presence of iron and oxygen, indicating some form of iron oxide/hydroxide. The EDX spectra showed a small amount of manganese, which was from the corroded steel and carbon, which was from organic contamination in the corrosion product. Further analysis of the rust tubercle was done by X-ray diffraction (XRD), which is a widely accepted testing method for determining the form of iron oxide present, as magnetite was one of the first minerals whose crystallographic structure was analyzed using XRD in 1915. The results revealed the presence of magnetite (Fe3O4) with a trace amount of SiO2 from a residual glass coating of the steel tank. Additional analysis of the rust using X-ray photoelectron spectroscopy (XPS) indicated a mixture of iron oxide and hydroxide, which correlated with the XRD results. It was determined that the rust tubercle was a hydrated form of magnetite with a chemical formula of Fe3O4·H2O.

The tank was made from plain carbon steel, which, despite carbon steel being prone to corrosion, is commonly used for numerous industrial or construction applications where the carbon steel will be subjected to aggressive conditions. A glass coating on the interior surface of hot water tanks is a common way to protect carbon steel from a highly corrosive environment, such as steam. A glass coating is fused to a clean steel substrate and inspected to ensure a pore and defect free surface, as well as good adhesion to the substrate. Once in service, the presence of moisture in the surrounding environment played a significant role to corrosion progression. The high humidity and temperature of the hot water tank, along with a defect, pore or similar breach in the glass coating caused the steel substrate to be exposed and hydrated iron oxides started to form the fascinating structures shown in the micrograph. The rounded structures and cubic crystals were formed by the hydrated nature of the magnetite, influenced by the changing iron to hydroxide ratios in the surrounding environment. When iron is in excess of hydroxide, spheres are formed, and when hydroxide is in excess of iron, cubic crystals form.

This case demonstrates that even with proper materials selection, corrosion can still occur. Obviously, it is beneficial to prolong the life of machinery and equipment; this is why corrosion engineering and materials science engineering are such important, yet overlooked areas of study. A combined effort of these two disciplines, along with design engineering can mitigate corrosion problems worldwide and timely measures can be taken to prevent or minimize costly damage.

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