The people that tend to be most interested in aluminum are manufacturers and engineers. Although aluminum has amazing mechanical properties that make it a prized material for all kinds of products and modern designs, the real wonders of this indispensible metal must be understood at a molecular level. The chemistry of aluminum tells us a lot about how it interacts with its environment.
Pure aluminum is an element on the periodic table, classified with the atomic number 13. In its natural state, it is a soft, silvery-white, non-magnetic metal in the boron group. Because aluminum is so reactive, it is rarely found in its purest form. Instead, it can be combined with as many as 270 different minerals, and bauxite is the most common aluminum ore.
Although aluminum is the third most abundant element in the Earth’s crust, there is no known form of life that uses aluminum salts metabolically. Fortunately, aluminum is well tolerated by plants and animals, making it safe for a wide variety of applications, from food and beverage to the medical industry.
The history of aluminum chemistry
Because of its affinity to react with other elements, early scientists did not know of aluminum’s existence. In the 1700’s, chemists such as Friedrich Hoffman hypothesized that alum, a common chemical compound used by the ancient Greeks and common in the textile industry, was made up primarily of a mysterious chemical element that had yet to be discovered which they referred to as aluminum.
The hunt was on beginning in 1760 to see who would be the first to isolate pure aluminum. It would not be until the Danish chemist Hans Christian Ørsted was able to react anhydrous aluminum chloride with potassium amalgam that anyone successfully produced aluminum. His experiment could not be replicated, however, and so it was the German chemist Friedrich Wohler who was credited with its discovery a few years later.
Still, the process these early chemists used was arduous and difficult to repeat. As excited as people were by the prospect of using aluminum for commercial purposes, without a cost effective method of production, it would be a distant goal. In fact, for many decades after its discovery, aluminum was valued as a precious metal worth more than gold or silver.
Near the end of the nineteenth century, the first large-scale production method for extracting aluminum from alumina (another aluminum ore) was developed. It is known as the Hall–Héroult process, after Paul Héroult and Charles Martin Hall. A few years later, Austrian chemist Carl Joseph Bayer came up with a way of converting bauxite into alumina, which opened the doors for the commercialization of aluminum.
A look at the chemistry of aluminum
Aluminum has characteristics of both pre- and post-transition metals. On the periodic table, it is listed among the boron group. Besides boron, the heavier elements in this group all have few available electrons for metallic bonding, meaning it has the characteristic physical properties of a post-transition metal. Aluminum therefore has longer-than-expected interatomic distances, is a small and highly charged cation, is strongly polarizing and tends towards covalency.
However, it is unique among the post-transition metals, in that the core underneath aluminum’s valence shell is identical to the preceding noble gas. Thus, aluminum exhibits electropositive behavior, a high affinity for oxygen and highly negative standard electrode potential, like pre-transition metals. This helps to explain aluminum’s unique properties.
As previously mentioned, aluminum is highly reactive. When heated, aluminum reacts with most nonmetals. Typical compounds include aluminum nitride (AlN), aluminum sulfide (Al2S3) and aluminum halides (AlX3). Aluminum especially has a close chemical relationship with oxygen.
It also forms a wide range of intermetallic compounds involving metals from every group on the periodic table. In fact, an interesting experiment is to take a fine aluminum powder, which will react explosively when it comes into contact with liquid oxygen and observe it. Under natural conditions, aluminum will form a thin oxide layer on its surface. This passivation layer is what makes aluminum resistant to corrosion.
Its chemical makeup and corrosion resistance mean that aluminum can be used to store compounds such as nitric acid, concentrated sulfuric acid and organic acids. Also, because aluminum retains its silvery appearance even in powdered form, it is a main ingredient of silver-colored paints. Other chemical reactions of note include in hot concentrated hydrochloric acid and in aqueous sodium hydroxide or potassium hydroxide. Aluminum can be corroded by dissolved chlorides, such as common sodium chloride, meaning regular aluminum alloys are not great protection against salt water, though certain alloys have been developed that offer better protection in marine environments.
The chemistry of aluminum alloys
The key to unlocking aluminum’s true potential was the development of aluminum alloys. Typical alloying elements include copper, magnesium, manganese, silicon, tin and zinc. Metallurgists use different combinations of alloying agents to develop grades suited for a variety of applications. This enhances aluminum’s natural abilities, like its strength-to-weight ratio, while bolstering other properties like corrosion resistance or thermal conductivity.
It’s important to note the particular characteristics of any grade you chose to work with. For example, if you know that your application will be exposed to salt water, then you’ll want to select an alloy that is extra corrosion resistant. A good choice for such an environment would be 5086. Whatever your needs are, do your research first.
Your Technical Services Professional
Most people who work closely with aluminum do not have a good understanding of aluminum’s chemistry, and it’s not really necessary for manufacturers and designers to have a metallurgical background. But you want your metal supplier to have a thorough knowledge of aluminum’s many properties and the advantages it can offer, including how different alloys respond to different application environments. Working with a good supplier means you have a knowledgeable and professional resource that can assist you in selecting just the right material.
At Clinton Aluminum, our mission is to always live up to the high standards our customers expect from us. We are committed to working with our customers at every step of the procurement process. Contact one of our friendly customer service representatives today to learn more about what aluminum alloy might be right for you.