How to Keep Your Trailers (and Reputation) from Corroding
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How to Keep Your Trailers (and Reputation) from Corroding By Bernardo Duran III, Coating Performance Engineer, AZZ Galvanizing Services simple options for protecting metal on your trailers from corrosion. Before we start that discussion, let’s first recap the common types of corrosion found on trailers. The first type of corrosion is called general or uniform corrosion, which happens over the entire metal surface, or large portions of it. General corrosion occurs at roughly the same rate over the corroded area, which makes the corrosion rate predictable.
Corrosion on unprotected carbon steel frames.
Corrosion protection on trailers is sometimes left as an afterthought, only considered after a trailer sits in a distributor’s lot or when a customer complaint hits your desk. By then, it may be too late if your trailer already shows signs of corrosion, and your hard-earned reputation might be damaged. Fortunately, you have options. Never before have there been so many effective and affordable corrosion protection options available. Spending a little time upfront considering these options can pay big dividends down the line, for your reputation and your customers’ investments in you and your products. Identifying and Fixing the Corrosion Problem This article is the second in a two-part series. The first article in the series, "Why is My Trailer Corroding?!" discussed common types of corrosion found on trailers and was featured in the March/April 2015 issue of Tracks magazine. In this second article in the series, we pick up where we left off, by discussing
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The second type of corrosion we want to protect against is called localized corrosion, which usually occurs on smaller and distinct areas. The rate of localized corrosion is much less predictable than generalized corrosion, making it potentially more destructive and dangerous. The most common types of localized corrosion found on trailers include crevice, pitting, and galvanic corrosion. Crevice corrosion occurs inside narrow and enclosed areas. Pitting corrosion occurs at defined areas and tends to create cavities on the metal. Galvanic or dissimilar metals corrosion occurs when two or more metals with different electrical potentials are electrically connected in the presence of an electrolyte. The more active and electronegative of the two metals corrodes in place of the less active metal. Corrosion scientists, engineers, and chemists have made great strides in protecting against the basic forms of corrosion described above. In fact, there are multiple affordable choices for corrosion protection today. We are going to discuss three of these choices, including barrier coatings, material selection, and good design practices. Barrier Coatings Barrier coatings are perhaps the most common form of corrosion protection in use today, and can protect metals, wood, and nearly
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How to Keep Your Trailers (and Reputation) from Corroding
Tracks any other construction material. Barrier coatings provide corrosion protection by preventing an electrolyte, such as water, from contacting the underlying steel or metal. If the electrolyte cannot contact the metal, a corrosion cell will not form and corrosion cannot occur. Remember from the first article, the four components necessary for corrosion to occur include the anode, cathode, metallic pathway, and electrolyte. If any of these four components are not present, corrosion will not occur. Paint and powder coating are two common types of barrier coatings in the trailer market. Other types of coatings, such as hot-dip galvanizing, can also act as barriers and prevent electrolytes from contacting the base steel.
The frame and superstructre of this trolley were hot dipped and then painted (called a duplex system) for corrosion protection and aesthetic appeal.
Preparation of a metal surface prior to coating with paint or powder coating is critical. If the metal surface is not prepared properly, which includes removing contaminants and applying a profile, the barrier coating will not adhere. If adhesion of paint or powder coating is bad, and the barrier coating detaches, the electrolyte will contact and corrode the metal. When this happens, corrosion can spread under adjacent barrier coating, swell, and cause the coating to flake from the steel, in a process called underfilm corrosion. This type of corrosion does not occur on hot-dip galvanizing.
outline various methods and techniques for cleaning and profiling steel and other materials which are to be overcoated. Choosing the Right Materials for Your Trailer Another popular form of corrosion protection for trailers is material selection. This simply means choosing materials that corrode more slowly than plain carbon steel. Options could include hot-dip galvanizing, stainless steel, or aluminum. Paint or powder coating can be applied over these materials for additional corrosion protection or aesthetics. When galvanized steel is painted or powder coated, it is called a duplex system.
Organizations such as NACE International and SSPC: The Society for Protective Coatings have produced specifications that
To give an idea of the difference materials selection can have on corrosion resistance, consider hot-dip galvanizing, which is a zinc coating metallurgically bonded to the underlying steel. According to the American Galvanizers Association, zinc corrodes at 1/10 to 1/40 the rate of steel. This means it is quite possible a galvanized trailer could last long enough to be passed down to future generations. The length of time a zinc coating will protect the underlying steel from corrosion depends on the environment the steel is used in and the zinc coating An electrolyte causes corrosion of the steel at breaks in the paint coating. Corrosion spreads under the thickness. paint, causing a bigger void and additional corrosion.
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How to Keep Your Trailers (and Reputation) from Corroding
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Time to First Maintenance for Hot-Dip Galvanized Steel
The reason zinc corrodes so slowly is because it forms a protective layer on its surface known as the zinc patina. The zinc patina is not aggressively attacked by rain, snow, and humidity, like uncoated carbon steel is. To understand how a zinc coating forms into the weather-resistant zinc patina, take for example a hot-dip galvanized trailer. Soon after being withdrawn from the galvanizing bath, the zinc begins reacting with oxygen in the air to form zinc oxide. As the galvanized trailer remains outdoors, and undergoes normal environmental wet/dry cycles, water reacts with zinc oxide to form zinc hydroxide on the galvanized surface. As the steel undergoes additional wet/dry cycles, zinc hydroxide reacts with carbon dioxide in the air to form basic zinc carbonate. The zinc carbonate layer is also known as the zinc patina and is responsible for the long-lasting corrosion protection hot-dip galvanizing is known for.
Formation of the Zinc Patina
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This evolution from a zinc coating, right after galvanizing, to complete formation of the zinc patina can take six months to two years to form, depending on the specific environmental variables. Throughout that time, the underlying steel is always protected by the zinc that covers its surface, which acts as a barrier to electrolytes.
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How to Keep Your Trailers (and Reputation) from Corroding Protecting Trailers from Scratches In addition to corroding much slower than uncoated steel and providing barrier protection, hot-dip galvanizing also provides an additional benefit known as galvanic cathodic protection. Galvanic cathodic protection protects the underlying steel by sacrificial corrosion of the zinc coating in place of the steel. To give an example how cathodic protection on a galvanized trailer works, suppose a piece of heavy machinery were loaded onto a trailer. If the galvanizing were scratched through the coating all the way down to the bare steel, the zinc immediately adjacent to the scratch would donate its electrons to prevent the steel from corroding. By donating its electrons in this manner, the
Tracks exposing the steel to electrolytes and eliminating the corrosion protection the coatings offer. Some materials, such as hot-dip galvanizing, provide increased abrasion resistance. The galvanized coating is made of four layers, three of which are zinc-iron intermetallic layers that are harder than the base steel. The outer layer, nearly pure zinc, is softer and more flexible. The combination of a flexible outer layer and hard inner layers greatly reduces scratches through the galvanized coating while loading and unloading trailers. So, in addition to providing galvanic cathodic protection at scratches down to the base steel, the galvanized coating greatly reduces the likelihood of scratches to begin with. Good Design Practices
Scratch on galvanizing down to the bare steel. The surrounding zinc coating provides galvanic cathodic protection to protect the steel from corrosion.
zinc sacrificially corrodes in place of the steel and will continue doing so until the zinc coating has been depleted. The amount of cathodic protection provided by galvanizing is dependent on the conductivity of the electrolyte and the thickness of the zinc coating, but scratches up to 2-5 mm wide on galvanized steel will usually be protected.
In addition to the corrosion protection options discussed above, using simple recommended design practices can also greatly reduce corrosion on trailers. For example, including drain holes in key locations can prevent water from pooling on trailers. If water does not pool, the corrosive effects it might have on the trailer will thus be decreased or eliminated. This underscores the importance of paying close attention to and reducing design details that could collect water. In particular, areas around wheel wells and close to the rear wheels of the tow vehicle should be inspected for crevices and overlapping areas where water might collect. Careful selection of weld design can also help reduce corrosion. Including seal welds can prevent water from collecting in
Scratches are an important topic to consider since trailer owners often load and unload items, thereby increasing the potential for scratches. Barrier coatings such as paint or powder coating can be scratched off the steel, thus Micrograph showing dpn hardness values for the layers of the galvanized coating relative to the base steel.
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How to Keep Your Trailers (and Reputation) from Corroding
Tracks crevices, and can thus prevent crevice corrosion. When seal welding is not possible, careful consideration for weld quality and placement can also decrease pooling of water. If welding is necessary after a trailer has been painted, powder coated, or galvanized, touch-up will be necessary to restore corrosion protection on welded areas. Manufacturers can unintentionally cause galvanic corrosion (dissimilar metals corrosion) on their trailers by connecting different types of metals or alloys. If the metals are in physical contact, they are electrically connected and galvanic corrosion can occur. When a design calls for dissimilar metals to be in contact, installing dielectrics between the metals stops electrical current from flowing between the metals, and will thus eliminate the concern for galvanic corrosion. Corrosion protection will last as long as the dielectric material is intact and the metals are not in electrical contact. Some examples of dielectrics include rubber or plastic sleeves or grommets. Alternatively, there are dielectric paints on the market specifically designed to prevent galvanic corrosion. Dielectric paints would be expected to provide corrosion protection until the paint wears away or flakes from the metal(s). Conclusion and Next Steps
The author of this article is a NACE certified coatings inspector, participates in NACE and ASTM technical committees aimed at reducing corrosion, and serves as chairman on an SSPC technical committee. He can be contacted for additional information and literature for corrosion protection options for your trailers. About AZZ Galvanizing AZZ Galvanizing Services, a division of AZZ Inc., headquartered in Fort Worth, Texas owns and operates 42 hot-dip galvanizing plants strategically located across the U.S. and Canada. AZZ operates kettles ranging from 16’ to 62’ in length. With the company’s network of plants, they are able to accommodate the largest projects with customized turnaround times at a competitive price. GalvXtra, their unique nickel-zinc alloy process, helps meet the highest quality standards necessary in the highly competitive transportation market. Photos courtesy of the American Galvanizers Association www.galvanizeit.org
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Protecting trailers from corrosion from the time they leave your shop, until long after your customer has taken delivery, has never been easier or more affordable. The corrosion industry offers many good options to keep your customers smiling and maintain your reputation for providing high quality trailers. In this article, we discussed commonly used methods for reducing corrosion on trailers, including barrier coatings and materials that corrode more slowly than carbon steel. We learned some materials, such as hot-dip galvanizing, use their zinc coating to offer the benefits of barrier and cathodic protection, as well as the weather-resistant zinc patina. Lastly, we discussed several good design practices to limit corrosion. This article is only an introduction to common corrosion protection options available for trailer manufacturers. You can obtain additional information specific to your application by contacting your coatings supplier or hot-dip galvanizer. Additionally, trade associations such as the American Galvanizers Association (AGA), NACE International, and SSPC: The Society for Protective Coatings, can provide additional information.
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