What is corrosion?

To some extent, everyone knows what corrosion is. It is a common phenomenon that can be observed in our daily lives — just think of the rust that can be found on a car. Knowing that corrosion is a chemical or electrochemical reaction involving electron transfer is far less intuitive.

In automotive applications, ferrous alloys can react with oxygen and humidity in the air to form hydrated iron oxide, commonly known as rust. Although seemingly simple, corrosion is actually more complex, as it affects a large variety of surfaces and can involve many chemical reactions. Corrosion affects mostly metallic parts. However, ceramics and polymers may also degrade in the presence of environmental factors such as acid rain or sulfates sometimes present in groundwater due to pollution. The combination of these factors can destroy materials considered very stable and thus difficult to degrade over time. Despite being part of our day-to-day life, from a scientific standpoint, corrosion is a world in itself.

You are probably familiar with the Statue of Liberty, which was gifted to the United States by France. The monument was originally built in Paris in the 1880s and designed by Frédéric Auguste Bartholdi, with the internal iron framework engineered by Gustave Eiffel. Made of copper, the statue was brown at first, but turned green over time. What happened? The answer is simple: corrosion or, more precisely, oxidation.

In this case, the oxidation of copper protected the rest of the statue by coating it in a green layer, also known as patina, composed primarily of copper carbonate and related compounds. This patina acts as a barrier, significantly reducing further corrosion of the underlying metal.

In other cases, corrosion can lead to severe consequences. The Statue of Liberty, for example, required significant restoration in the late 1900s due to severe corrosion of its iron framework. When the copper skin came into contact with the iron, it produced an electrical current similar to a battery, resulting in what is known as galvanic corrosion. Simply put, an improper combination of dissimilar metals can lead to an accelerated corrosion process.

The corrosion process is generally classified into two categories: wet corrosion (corrosion requiring an electrolyte) and dry corrosion (corrosion associated with elevated temperatures or sulfidation). Wet corrosion is the most common type and is the primary focus of this guide.

Dry corrosion, or oxidation, typically occurs at elevated temperatures when oxygen in the air reacts with the metal, forming an oxide layer. Since most engineering metals oxidize slowly at ambient temperature, dry corrosion is less common than wet corrosion in typical service environments.

Wet corrosion is a slow, measurable process that follows Faraday’s law. For this type of corrosion to occur, the following elements are required: an electrolyte, an anode, and a cathode.

The combination of these components forms a system called an electrochemical cell. Corrosion is essentially an electric circuit with a flow of current between two electrodes, commonly called an anode and a cathode. These electrodes may come from different materials or simply from dissimilar areas on the same surface.

In this system, these two elements are in metallic contact and are immersed in an electrolyte. This electrolyte is usually a liquid through which ionic conduction occurs, or that separates into ions when electricity passes through it. It is a phase through which the movement of ions carries a charge. This conductive substance is required in order for the corrosion circuit to be completed. A common example of an electrolyte would be salt water. This is a key element in corrosion protection. Indeed, if this conductive substance can be prevented from contacting the metal structure we want to protect, for example with a protective coating, then the electron exchange cannot proceed, and corrosion is significantly reduced.

The corrosion of a material involves a pair of reactions, which are called oxidoreduction (oxidation-reduction) reactions, that happen simultaneously. The first half-reaction is called the oxidation reaction. It is defined by the loss of electrons at the anodic electrode. At the same time, a reduction reaction, also called a cathodic reaction, occurs.

The reduction reaction refers to a gain of electrons, which will attach to the other material in the electrochemical cell called the cathode. In other words, there is a current in the electrochemical cell that induces a flow of electrons away from the anode and through the metallic path toward the cathode. Ions move through the electrolyte solution to maintain charge balance. These oxidation-reduction reactions induce the corrosion of the anode. The cathode is not consumed during corrosion.

Each reduction reaction of an element is rated on a scale by its reduction potential, which refers to its ability to accept electrons. It is measured relative to the standard reference electrode (hydrogen), which is assigned a potential of 0.00 V under standard conditions.

Negative values indicate the best electron donors (anodes) — also called the least noble materials — meaning they are more likely to oxidize. As a reference, Lithium (as in lithium-ion batteries) has a potential of −3.045V. Positive values indicate a good electron receiver (cathode) or noble materials for the reduction half-reaction. These indicators are also useful for evaluating the potential of an electrochemical cell, which will be defined by the added potentials of both its constituent electrodes.

There are many types of corrosion that can occur on materials. Sometimes, more than one corrosion type occurs at once, which can potentially compound the deleterious effects.

Let’s say you are home and want to prepare a good tomato sauce to go with your spaghetti. You take an aluminum pan and start following the recipe. After all your efforts, the sauce looks weird and tastes bitter. The aluminum pan had a chemical reaction due to the acidity of the tomatoes, forming an aluminum salt and some dihydrogen gas, which changed the taste and the look of your sauce. This type of corrosion is called uniform corrosion and occurs, in this case, in the presence of an acidic environment. This might surprise you as aluminum is well-known for being relatively resistant to corrosion, proving that understanding the phenomena of corrosion allows us to avoid the problems they generate as much as possible.
Contrary to the general belief, metallic aluminum is much more reactive than iron, therefore less resistant to corrosion. What happens is that aluminum oxidizes quickly to form an aluminum oxide layer, which prevents further corrosion under neutral conditions. If you expose the aluminum to an acidic solution, e.g. tomato sauce, the aluminum oxide layer dissolves, further exposing the bare reactive aluminum metal. To make a long story short, don’t use an aluminum pan to make your tomato sauce.

An issue that we came across more than once was an aluminum can into which a worker had poured methylene chloride to rinse paintbrushes. Methylene chloride, over the weekend, attacked the aluminum oxide layer, and the reactive aluminum reacted with it to produce phosgene and hydrochloric acid, leaving all metal surfaces in the immediate area with a corrosion film.

Large engineered systems employing many types of metal in their construction are susceptible to galvanic corrosion if care is not exercised during the design phase. Choosing metals that are as close together as practicable on the galvanic series helps reduce the risk of galvanic corrosion. The infrastructure of the Statue of Liberty, for example, had to be rebuilt using a duplex stainless steel structural frame, as copper alloys and stainless steels are quite close in their nobility. A small anode/cathode area ratio is also highly undesirable since the current will be concentrated onto a small anodic area, leading to rapid thickness loss. Preventing moisture/water from contacting the surface using an appropriate coating will also greatly reduce the corrosion risks.

Zinc protection is a very common type of sacrificial anode often used on ships in order to protect the hull, the ballast tanks, and the heat exchangers, as well as the storage chests close to the bottom of the vessel, from corrosion. Pieces of zinc are bolted to the shaft of the boats and are more reactive, thus protecting the hull, which is the body of the ship that is exposed to water. Even non-metal boats use zinc anodes, which are connected by copper wires throughout the boat to protect underwater metal parts.

Zinc coatings are also often applied to provide barrier protection. The coating starts to corrode sacrificially first, therefore protecting the material underneath. This type of protection is usually used on smaller parts, such as screws or light switch plates, that will be exposed to corrosive environments.