In 1979, Canadian rock and roll artist Neil Young and American band Crazy Horse released their live album “Rust Never Sleeps.” By acknowledging the changing music scene in the late 1970s with punk rock and synth-pop in emergence, Young challenged rock and roll artists to stay current with the times.
The album has been described by one critic as a “sneak attack on entropy,” with rust as the metaphor for corrosion, and a warning to embrace change else one fades away. In chemistry, iron tends to fade away when exposed to the elements, becoming iron oxide Fe2O3. Rust.
Why does rust form?
Electrochemical oxidation is a natural process that will convert a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. This reaction is also the principal behind electrochemistry and energy storage. Corrosion degrades the useful properties of certain materials and structures, including strength, appearance, and permeability to liquids and gases.
According to the website corrosionpedia.com, rust encompasses a number of iron oxides occurring where there is unprotected steel or iron exposed to the water or air. “It can form in various colors like yellow, brown, orange and green, which is known as the green rust.”
“Green rust is produced in environments with low oxygen concentration like rebar in chlorine-rich environments, where there can be a release of iron hydroxides. In such a case, the steel’s depassivation leads to the formation of green rust when the ratio between hydroxide ions and chloride ions is higher than 1. Thus, rebar in materials such as concrete is protected from green rust if the concrete’s alkalinity is adequately high.”
Ways to protect iron and steel
Galvanizing, invented by Stanislas Sorel, a French engineer, is a method of protection that has been around for nearly 200 years. Steel galvanization creates a coating of zinc over metals that easily oxidize. In the process, clean, rust-free steel is bathed in molten zinc which hardens over the steel and created a moisture-resistant coating. Instead of iron particles being exposed to moisture and oxygen, the zinc molecules sacrifice themselves to the oxidation process. In other words, the zinc is corroded in preference to the underlying steel.
Types of oxides
While not all oxidations (i.e., the loss of electrons during a chemical reaction) involve the element oxygen, compounds that combine a positively charged element with one or more oxygen atoms are typically called oxides. Oxygen, a negatively charged ion (anion), naturally bonds with positively charged elements (cations). Most of the Earth’s crust consists of solid oxides, the result of elements oxidized by the oxygen in air or in water.
Not all oxides are corrosive but rather exist in stable and useful forms. The oxide of silicon with the chemical formula SiO₂ is commonly found in nature as quartz, in various living organisms, and on sandy beaches around the globe.
Commonly referred to as alumina, aluminum oxide is the source of aluminum materials. Alumina are also useful as adsorbents and have been traditionally used as desiccants. These oxides play a role in mitigating municipal wastes and in creating polymer composites and pharmaceuticals. A similar desiccant and dehydrating agent is phosphorus pentoxide (P4O10).
Titanium dioxide, due to its ability to efficiently scatter visible and absorb UV light, is widely used as a whitening material in everything from toothpastes to whiteboards.
Chromium(III) oxide (or chromia) is used in paints, inks, and cosmetics such as nail polish. Known as chrome green, it is one of the most commonly used and stable forms of green pigment available.
Physically and chemically stable at high temperatures, magnesium oxide is valued as a refractory material for crucibles and ovens, and as a fireproofing ingredient in construction materials. This oxide finds its way into cement (hydration), food products (anti-caking), and antacids (adsorbent). Spent MgO-carbon refractory bricks are used as a material for permeable reactive barriers to treat a nickel- and cobalt-contaminated groundwater.
Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It consists of several oxides, including silicon dioxide, aluminum oxide, iron oxide, calcium oxide, magnesium oxide, sulfur trioxide, sodium oxide, as well as smaller amounts of several other oxides (NIST SRM 1887b). Typically, manufactured glass contains around 15% sodium oxide, 70% silica (silicon dioxide) and 9% lime (calcium oxide).
How oxide materials are used
Mining and quarry companies, materials processors, and manufacturers of products using oxide materials need a way to reliably and accurately detect oxide compounds and determine their concentrates within a given material. Wavelength-dispersive X-ray fluorescence (WD-XRF) allows measurement of up to 83 elements of the periodic table. The technique features short analysis times, generally easy sample preparation, very good stability, precision and wide dynamic range (from ppm levels to 100%).
X-ray fluorescence of rust
The accuracy of an analysis of powders may be impaired by particle size effects and mineralogical effects in the sample. Although these effects can often be minimized by grinding the powder below 50 microns and pelletizing it at high pressure, often mineralogical effects cannot be completely removed, or harder particles cannot be broken down below the required size.
X-ray fluorescence sample preparation
Fusion is the most accurate method of sample preparation for X-ray fluorescence analysis of oxides. Fusing oxide materials removes both grain size and mineralogical effects in powders and inhomogeneous samples. This preparation method heats a mixture of the sample with a borate (or similar) flux in a gas or electric fusion machine at high temperature (1000 °C–1200 °C) so that the flux melts and dissolves the sample. The overall composition and cooling conditions must be such that the end product is a one-phase glass after cooling.
The Thermo Scientific™ ARL™ 9900 Series X-ray Fluorescence Spectrometer can be calibrated from the factory as a complete analytical package that provides the analysis of a large variety of minerals, using the general oxide calibration based on a sample preparation by fusion.
The application note Total oxide X-ray analysis with ARL 9900 IntelliPower Series Simultaneous/Sequential X-ray Fluorescence Spectrometers shows how WDXRF determines the concentration ranges of the various oxide types and shows typical limits of detection on ARL 9900 Intellipower Series obtained from various oxides fusions.
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