When you hear the term corrosion, you likely think of old, rusted metal. You might think of the orangey-brown tones of an old wagon wheel or the reddish hues of a deteriorating ship hull needing paint. Or, you might see automobile graveyards where once-classic cars rust back to their original iron state.
Corrosion is a natural occurrence that happens with all metal products over time. What you might consider "rusting" is just one form of corrosion where iron and steel products oxidize in the presence of oxygen and water. Many other metals suffer corrosion threats including aluminum, brass, bronze and even the highest stainless steel grades. Fortunately, metal corrosion is preventable. Corrosion protection can save the American economy vast sums lost annually by nature’s energy cycle built into metal.
The National Association of Corrosion Engineers (NACE) is considered a worldwide authority on corrosion, with members worldwide who collaborate on solutions to control corrosion. According to their 2016 International IMPACT Study, corrosion damage has a global cost of $2.5 trillion annually — a significant amount that could be saved with proper corrosion protection practices.
Despite corrosion being such a massive cost to the global economy, the fight to control and prevent corrosion gets little attention. Corrosion affects almost every part of daily infrastructure from transportation to utility providers. It can also result in catastrophic events like airplane crashes and bridge failures that cost money and human lives. Preventing metal corrosion and its far-reaching effects start with understanding what causes corrosion.
What Causes Corrosion?
To understand the causes of corrosion, it’s necessary to know what corrosion is. The National Academies Material Advisory Board (NMAB) defines corrosion through two National Research Council reports. Corrosion is the deterioration or degrading of a material’s physical properties through chemical reactions within its environment. Although non-metallic substances like glass, plastic and ceramics can technically corrode, by far the most common corrosion processes occur with manufactured metals.
The term corrosion comes from the Latin word corrodere, meaning “to gnaw to pieces,” which has the similar root word “rodent.” Corrosion is the slow destruction or eating away of things, which has a literal application such as with rusting or abstract implications like corroding emotions or relationships. In the material world, the highest risk for corrosion is metal.
Oxidation is the most prevalent metal corrosion form. Oxidation corrosion happens when metal objects react with oxygen and a fluid environment like air or water to form a more stable thermodynamic state. Synthetic metals are the highest risk for corrosive oxidation because they were changed from their original ore state by adding energy to create new compounds and alloys.
These manufactured products exist in a higher energy state than their ores once were. As part of a natural cycle, these materials release energy through corrosion in a long-term path of returning to their original state. When metal atoms such as iron experience oxidation, they release negatively charged ions that build up in the material and exacerbate the corrosive process. At the most basic form, corrosion is an electrochemical process. However, there are different causes.
Primary Causes of Metal Corrosion
In general, five main causes of metal corrosion can occur, along with some other less common reasons. Each contributing factor can act alone or in unison with another. All occur wherever metal has an active environment that’s suitable for creating corrosion.
1. Atmospheric Corrosion
By far, the most common form is atmospheric corrosion. It’s also called uniform or general corrosion. This is where oxidation takes place across a metal object’s entire surface that’s exposed to atmospheric conditions. These conditions include air or oxygen, moisture such as rain, snow, ice or dew, sunlight, airborne pollutants and temperature fluctuations. Although atmospheric corrosion typically happens in an open environment, the same processes occur underground and underwater.
2. Galvanic Corrosion
This corrosion cause occurs when two dissimilar metals electrically engage when they’re mated in a manufactured assembly. Galvanic corrosion, or bimetallic corrosion, happens when one material creates an electric charge that’s passed to the other causing an electrochemical event. The terms “noble” and “active” are associated with galvanic corrosion. Noble metals (cathode) are more inert than active (anode) metals. The further separated cathode and anode electrochemical reactions are, the faster they break down the affected metal.
3. Crevice Corrosion
This is a common corrosive cause in products manufactured with metal. Every assembled product has crevices like joints or seams susceptible to invasive conditions that bring on corrosion. Those crevices can also be cracks, splits or gaps occurring through wear and tear during a metal object’s life cycle. Crevices in shielded areas are at the highest risk for corrosion. These micro-environments create perfect conditions for trapped moisture, stagnant solutions and depleted oxygen. Often, crevices get contaminated with chloride or salt, which significantly speeds up corrosive electrochemical reactions.
4. Pitting Corrosion
Pitting usually occurs on a metal object’s exterior where it is uniformly exposed to atmospheric conditions, but the surface has been protected by a film like plating, painting or powder coating. Over time, tiny holes perforate or pit the protective coating and allow subsurface penetration of water, chemicals and oxygen. This also creates a mini-environment under the film which is virtually invisible to the naked eye. These pits grow under the film surface until they blister and present themselves. By then, the corrosion has caused significant structural damage.
5. Microbial Corrosion
This form of corrosion happens when unprotected metal stays in contact with sludge or soil. Both air-bearing (aerobic) and air-void (anaerobic) conditions lead to corrosive action. Excessive water presence accelerates microbial growth, which literally “eats away” at the metal. Sulfate-reducing bacteria are the most aggressive microbes. They can destroy an unprotected metal product in a short time unless electrochemical control measures are in place.
6. Other Corrosion Causes
Lesser known and rarer corrosion causes exist, too. One is high-temperature corrosion that happens where metal objects experience great heat continually. Jet engine exhaust ports are a prime example. Meta dusting is another corrosion cause. This occurs in high carbon and sulfuric gas situations where metal quickly corrodes from bulk to a fine powder. Weld decay and knifeline attacks also cause corrosion on metal fabricated equipment. Here, openings in seams allow ingress of corrosion-causing substances.
Corrosion is a natural process. Metal deterioration is inevitable and part of nature’s energy cycle. That’s unless metal manufacturers and maintainers take preventive steps to preserve their products.
How to Prevent Metal Corrosion
The key to preventing metal corrosion is to stop or slow the electrochemical forces that cause corrosion. Some metals are much more at-risk for corrosion than others. One of the main corrosion prevention factors is to choose a metal base that’s a low corrosion risk given its intended application and the environment it’s used in.
It’s also crucial to assess which corrosion causes a product may endure in order to prevent electrochemical breakdown. Above-ground applications have the highest atmospheric conditions that threaten a product. Generally, the best corrosion prevention is a top coating like paint or a baked-on powder. Below-ground applications also benefit from usable, but they usually need an anti-galvanizing treatment to stop electrical activity.
Metal corrosion may be inevitable given the right time and conditions. However, corrosion and engineering research has discovered short- and medium-term solutions to slow the kinetic process that causes corrosion and put the economic burden onto society. Here are the main types of corrosion prevention methods scientists and engineers work with:
Product Design: Scientists and engineers constantly thrive to improve existing technologies and design new corrosion-resistant metals. This includes developing advanced computer models that simulate actual conditions without the time and expense necessary to test products in real environments. Design work takes in accelerated testing in controlled conditions. Here, newly designed metal alloys provide the least corrosive metals. Plus, testing advanced coatings and finishes provides accurate performance predictions without the need for field tests.
Risk Mitigation: The same product design tools and databases allow metallurgists to mitigate the risk of product failure in real-time situations. Over the years, corrosion risk mitigation came from long-term study and experience of what metal alloys and protective coatings performed with the least corrosive action. Today, risk mitigation starts with applying the right corrosion resistant metals to their performance environment and then matching the correct protection in the way of coatings and electrochemical grounding.
Corrosion Detection: Metal scientists and structural engineers monitor existing products, buildings and infrastructure components to detect corrosion at different stages. Highly-technical sensors and remote monitors provide information on corrosive reactions that simply can’t be found by human sight and touch. Detecting metal corrosion plays a large part in prevention programs. Assessing current corrosion damage detected in existing materials provides a prognosis for predicting degradation and preventing serious failures.
Corrosion Prediction: Research and development, along with detection and mitigation techniques, allow scientific models to predict which metal products will withstand environmental forces. Scientific data also lets designers predict which materials are doomed for failure. Predictions based on metal properties extend into providing an accurate prescription of corrosion-resistant finishes and the successful way they can be applied to protect products. From information-based predictions, better materials and better protective coatings continue to evolve and make the world a safer place.
What Are the Most and Least Corrosive Metals?
True metals are rarely found in the earth’s mineral supply. Most true metals like gold, silver and platinum are non-corrosive by nature. They inherently resist corrosion and are in high demand, which is why they can be so expensive.
Other metals like copper, aluminum and brass also have excellent corrosion resistance properties. These materials are more abundant than precious metals and less costly by volume. The downside to brass, copper and aluminum is that they require considerable amounts of energy to process into usable products. That energy stores in their molecular makeup and makes them vulnerable to nature’s energy recycling program of electrochemical corrosion.
Copper is an interesting metal. It’s in relatively plentiful supply and is easy to work with. However, copper doesn’t need paint or powder coatings to preserve it from corrosion. When exposed to air and water, copper builds its own protection called passivation. Think of America’s famous landmark, the Statue of Liberty. Its copper sheathing has a rich greenish patina that naturally resists corrosion without other help.
Aluminum also forms a passivation protection layer. Without its greyish and mottled patina, shiny raw aluminum is somewhat corrosive. Boat builders often use aluminum for hulls and superstructures, which is partially because aluminum is lightweight and partially because it works well with products called sacrificial anodes. These small zinc or magnesium blocks or anodes absorb corrosive electrochemical reactions from aluminum and self-sacrifice by corroding first.
Because of corrosion threats, even resistant metals like aluminum often receive a surface protection coat. Many aluminum products destined for atmospheric exposure receive treatments during their manufacturing stage. Aluminum building products like siding and gutters have powder coats applied that last through years of harsh weather exposure.
Other metal alloys stand up well against corrosive conditions. Stainless steel is a blend of iron and chromium. As corrosion-resistant as stainless steel is, products manufactured from stainless steel often require protective coatings or regular anti-corrosion maintenance. Here is a list of common metals ranging from the most to least corrosive metals:
Magnesium and Alloys: Either cast or wrought
Zinc and Alloys: Wrought, die-cast or plated
Iron: Wrought, cast or carbon alloys
Steel: Refined iron and alloys like stainless steel
Aluminum: Smelted or cadmium plated
Lead: Solid or plated
Tin: Raw and lead-soldered
Chromium: Used to alloy stainless steel
Brass: Including bronze and alloys
Copper: Solid or plated
Nickel: Including titanium alloys
Silver: Solid or plated
Gold: Solid or plated
Platinum: Including gold-platinum alloys
Best Metals to Use
For most applications, you can use four reliable and economical metals. Each has distinct properties, and your metal choice depends on your specific application. That might be custom sheet metal fabricating, custom enclosure fabrication, steel fabrication or aluminum fabrication. No matter what your purpose, matching the best metal to use always includes providing it with the proper finish, such as powder coating.
Powder coating is an exceptionally dependable corrosion resistance process. This involves energizing a clean metal product and spraying a dry powder over it. The electrostatic reaction allows the powder to stick or adhere to the product. Following this, the metal product enters an over where it’s baked at 400 degrees Fahrenheit. Powder coated metal is one of the most cost-effective and long-lasting metal treatments available today.
APX York Sheet Metal provides first-rate metal fabrication and powder coating. Our services include product design, metal bending, metal rolling, laser cutting, machining, metal shearing and welding. Resisting corrosion is at the top of priorities at APX York Sheet Metal, which is why we always use these four best metals for building corrosion-resistant products:
Contact APX York Sheet Metal
We’re proud to be a leading custom metal fabricator in central Pennsylvania and northern Maryland. For more than 70 years, APX York Sheet Metal has built a reputation for excellence and dependability in both fabricating sheet metal and serving customers. As a valued customer, you’re faced with short lead times and rising costs. At APX York Sheet Metal, we understand that and strive to deliver low-cost value along with quick turnaround times.
Contact APX York Sheet Metal today for all your corrosion-resistant metal work. We’re just a call away at 717-767-2704, or you can always reach us online.