For centuries, metals have been the go-to choice for multiples applications due to their durability, versatility and strength. However, among the challenges that people face when using metals, corrosion is arguably the most common and widely recognized.
Multiple solutions have been proposed to increase the longevity of metallic structures and enhance their corrosion resistance. Among them, metal coatings stand out as one of the most effective and convenient protection methods.
There are numerous methods for coating metallic surfaces, each with its own set of limitations and benefits. In the following sections we will take a detailed look at some of the most common types of metal coatings, and discuss their suitability for various applications.
How Metal Coatings Protect Surfaces and Structures
Metal corrosion is a deteriorative process that occurs under specific conditions. The most common type of corrosion occurs when metals react with moisture and oxygen to create various corrosion products. Iron, for example, reacts with water and oxygen in the atmosphere to form iron (III) oxide, or rust.
The logic behind metal coatings, therefore, is to create an inert (non-reactive) barrier around the metallic object being protected to prevent it from reacting with air and moisture.
Common Types of Metal Coatings and Their Benefits
Below, we have compiled a list of the most common types of metal coatings used across various industries, and the advantages and disadvantages of each.
Anodizing is a process used to promote the formation of a protective oxide layer on the surface of a metal. The resulting oxide layer forms more rapidly and is usually thicker than if it was produced naturally. While several non-ferrous metals can be anodized, aluminum responds most effectively to this process. (Background reading: Understanding Ferrous and Non-Ferrous Metals: Why You Should Understand These Key Differences.)
Anodizing is performed by immersing the aluminum component in a tank filled with an electrolytic solution along with a cathode (usually aluminum or lead). An electrical current is passed through the aluminum, causing it to oxidize and form a protective barrier.
Anodized finishes are perhaps the easiest to maintain of all the coatings mentioned in this article. Anodized surfaces can be easily periodically cleaned using mild detergents. Finished anodized surfaces are also chemically stable and do not decompose under normal conditions, allowing for a long-lasting coated surface. Furthermore, because anodizing is a natural process, it is non-toxic and does not produce any harmful or dangerous by-products.
The most significant drawback of this process is that it is only useful on a handful of metals. This process is unsuitable for ferrous metals, which means that common materials like steel and iron cannot be anodized. Additionally, due to the processes used, the colors that can be achieved by anodizing is limited.
Galvanizing involves immersing the metal (mostly steel or iron) in a molten zinc bath. Once removed, the coated metal reacts with oxygen and carbon dioxide in the atmosphere to form a protective zinc carbonate layer.
The galvanizing process has multiple advantages that make it a popular choice for numerous applications. For example, the zinc oxide coating is highly stable and adheres tightly to the metal substrate; it is very durable and does not flake off easily.
Galvanizing is also renowned for its galvanic protection. In other words, if the metal's surface becomes exposed due to scratches, cuts or dents, the zinc coating will sacrifice itself by corroding preferentially. This process helps protect the steel substrate between maintenance operations.
The biggest disadvantage of the galvanizing process is its cost. While hot-dip galvanizing (HDG) may be cheaper for coating large steel structures, it can be less cost-effective for smaller pieces such as nuts and fasteners. (To learn more, read Hot-dip vs Cold Galvanizing: What’s the Difference?) Additionally, galvanized surfaces have a dullish grey appearance that may not be aesthetically pleasing for some applications.
Electroplating, also known as electrodeposition, involves depositing a thin layer of one metal on the surface of another metal. During electroplating, both metals are placed in an electrolytic solution. The metal to be coated acts as the anode, while the coating metal acts as the cathode. An electric current is applied to the electrolytic cell, causing metal ions to move from the cathode to the anode, thus forming the coating. (Learn more about electroplating in Introduction to Electroplating Interview with Jane Debbrecht.)
Electroplating offers excellent corrosion resistance and can enhance some of the metal's mechanical properties. Electroplating also produces an aesthetically pleasing surface finish, making it ideal for coating jewelry and ornaments.
However, electroplating can produce non-uniform coating thicknesses, making it unsuitable for high-precision applications. Also, the process itself has numerous requirements and is too costly to be used on an industrial scale.
Potentially toxic and harmful compounds are used as electrolytes in the electroplating process. Therefore, care must be taken when discarding electrolyte chemicals to avoid environmental contamination.
Powder coating, as its name implies, involves coating an object with a powder-based substance. It is an electrostatic process, whereby the coating particles are electrically charged with a polarity that is opposite to the part to be coated. The difference in charge causes the powdered particles to adhere to the metal's surface. The coated object is then heat-treated in an oven to harden the coating.
Powder coatings are renowned for their durability and aesthetically pleasing appearance. Additionally, because powder coatings do not contain solvents, there are little to no volatile organic compound (VOC) emissions.
While powder coatings may be cost-efficient in the long-term, the initial start-up costs can be significant. The coating process requires special spray booths, ovens and spraying equipment. This can also limit the size of objects that can be coated.
It is also difficult, or even impossible, to achieve thin coating layers. Furthermore, the finished surface is not the smoothest when compared to other coating methods. Projects that require a coating thickness of less than six mils should rely on another coating process.
A painted coating is essentially the application of liquid paint. It is the most accessible and cost-effective type of coating. Different paint formulations can be used depending on the type of metal, the operating environment and the performance requirements.
For industrial applications, paint coatings are slowly being replaced by other coating methods. Some paints may contain toxic elements and other volatile compounds (VOCs), making them harmful to the environment. Their durability is also lower than other coating methods, as they are likely to fade, peel or flake off due to prolonged environmental exposure.
Industries in the United States alone bear a loss of about $7 billion every year due to corrosion. Metal coatings, if used correctly, can be effective in prolonging the service life of many metal assets. However, it's important to remember that all coatings are susceptible to failure. Therefore, it is essential to understand the advantages and disadvantages of each coating type to select the one that is best for your application.