"Why is My Trailer Corroding?!"
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Why is My Trailer Corroding?! By: Bernardo Duran, Technical Sales Manager, AZZ Galvanizing Services
“Why is my trailer corroding?� Those are words no trailer company wants to hear from their customers. Fortunately, there are ways to slow the corrosion process and extend the life of metals used on trailers. These options apply even for trailers exposed to very corrosive environments, such as boat trailers used in seawater and on livestock trailers. This article will discuss common types of corrosion found on trailers. This will serve as a foundation for the next article in the series, which will discuss various corrosion protection options, and explain how they prevent or limit the types of corrosion discussed in this article. What is Corrosion? Corrosion is a natural process that causes steel to revert to the lower energy state it initially existed in as iron ore. Energy is put into iron during smelting, steelmaking, and fabrication processes, 60
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and corrosion acts to reduce this energy. This reduction in energy causes steel to lose its mechanical properties and often changes its appearance as it corrodes. Corrosion can occur on pure metals, alloys of steel such as carbon and stainless, and aluminum alloys, depending on the environment. Corrosion scientists and engineers have been describing different forms of corrosion since at least the early 1900s. The basic forms of corrosion in this article include general, crevice, pitting and galvanic corrosion. These forms of corrosion can all be identified with the naked eye. General Corrosion General corrosion is also known as uniform corrosion and is the most prevalent type of corrosion. This type of corrosion happens over the entire surface of a structure or large portions of it. The metal loss occurs at roughly the same rate over the corroded area, www.NATM.com
"Why is My Trailer Corroding?!"
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Mosaic of anodes and cathodes, electrically connected by the underlying steel.
Moisture in the air provides the electrical path between anodes and cathodes. Due to differences in potential, electric current begins to flow as the anodic areas are consumed. Iron ions produced at the anode combine with the environment to form the flaky iron oxide known as rust.
As anodic areas corrode, new material of different composition and structure is exposed. This results in a change of electrical potentials and changes the location of anodic and cathodic sites. Over time, previously uncorroded areas are attacked and uniform surface corrosion results. This continues until the steel is entirely consumed.
Changes in Anodic and Cathodic Areas on Steel (AGA)
which allows general corrosion rates to be predicted for many metals and alloys in different environments. This is in contrast to other types of corrosion rates, which are much less predictable. On a metal surface undergoing general corrosion, there are many very small anodes and cathodes on the metal which react in the presence of an electrolyte. The anode donates electrons to the cathode, and in so doing, the anode will corrode or dissolve. This is an electrochemical process.
As this process continues, areas on the metal surface constantly change to anodes or cathodes depending on the differing properties of the individual areas and environmental factors. For example, an electrolyte may dry in one area, which may stop the corrosion process at that location. Then, when an electrolyte deposits on the surface again, an area that was previously a cathode may be in electrical contact with a different area, and thus may switch to an anode as it reacts with the new area. As this process happens over a large surface, the outcome is general corrosion. Localized Corrosion Whereas general corrosion tends to happen over large areas of metal surfaces, localized corrosion happens on smaller, distinct areas. The effects of localized corrosion can be much more severe than general corrosion, since localized corrosion can happen much more quickly and severely. This makes it much harder to predict localized corrosion rates. The two most common types of localized corrosion include crevice and pitting corrosion, which are similar, but attack different areas on the metal surface. Crevice corrosion happens in closed off areas while pitting occurs on wide open surfaces.
Anodic and Cathodic Areas on Bare Steel www.NATM.com
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"Why is My Trailer Corroding?!"
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Crevice Corrosion under Painted Hardware
Crevice Corrosion As its name implies, crevice corrosion is corrosion that can occur in narrow and enclosed areas on steel and other metal fabrications. Moisture in the form of humidity, dew, rain and snow can enter these small gaps where it can stay until it forms an oxygen differential corrosion cell. In this type of corrosion cell, the moisture inside the crevice has a different level of oxygen than the moisture at the opening of the crevice. The metal inside the crevice becomes anodic and will corrode. The metal near the opening of the crevice becomes cathodic and is protected. Thus, corrosion takes place inside the crevice.
This can sharply increase the rate of attack in crevices. Deicing salts can similarly affect electrolytes when they deposit on the underbodies and outer areas of trailers. Pitting Corrosion Pitting occurs when corrosion occurs at very small, defined areas. The start of the corrosion process might be due to soluble salts depositing on the metal surface. The salts could be chlorides from coastal air, or they might be sulfates from industrial pollutants.
Crevices on trailers can be found in many areas. Some examples include in moving parts on couplers, axles, under bolt heads, between washers and bolted surfaces, on clamps, areas that have been stitch welded or are very close together, and even under protective coatings. Any place on a trailer that has a tight area which can trap water could be a candidate for crevice corrosion. The special case of crevice corrosion under barrier coatings, such as paint, is called filiform corrosion. On livestock trailers, animal waste can add to the crevice corrosion problem. The waste can affect the conductivity and pH of electrolytes. 64
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Severe Pitting Corrosion on the Steel above the Weld (AGA) www.NATM.com
"Why is My Trailer Corroding?!" The result of pitting corrosion is a cavity, which is sometimes covered by its own corrosion products. The corrosion tends to be deep, narrow, and can be directed into the substrate thickness. Because of this, pitting corrosion can be extremely dangerous. Although very little area might corrode in terms of the overall surface area, perforation completely through a member can occur, which may result in catastrophic failures.
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Galvanic Corrosion This type of corrosion is also known as dissimilar metals corrosion. It occurs when two (or more) dissimilar metals or alloys are electrically connected in the presence of an electrolyte that covers both metals. An example of this could be a galvanized boat trailer that has been bolted with stainless steel bolts, and will be used to frequently launch a boat in seawater. The high conductivity of the seawater can serve as a potent electrolyte and cause galvanic corrosion. There are four components necessary for galvanic corrosion to occur, including an anode, cathode, metallic pathway (or return current path) and an electrolyte. When all four components are present and electrically connected, a bimetallic couple is formed. Corrosion will occur at the anode, which provides corrosion protection to the cathode.
Galvanic Corrosion between Bare Steel and Zinc-Coated Hardware
Right: Bimetallic Couple (AGA) www.NATM.com
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Galvanic Series (AGA) Galvanic corrosion occurs due to the electrical potential difference between the anode and cathode. The anode is the more electronegative (or active) metal and the cathode is the more electropositive (or noble) metal. The chart known as the Galvanic Series arranges metals in order of their electrical potentials in seawater. In the Galvanic Series, the seawater serves as the electrolyte (electrical potentials will vary in different electrolytes). Aspects that affect the corrosion rate of the anode in a bimetallic couple include the electrical potentials of the anode and cathode in a given environment, surface area of the anode relative to the cathode, conductivity of the electrolyte, and temperature.
About AZZ Galvanizing Services AZZ Galvanizing Services, a division of AZZ Inc., headquartered in Fort Worth, Texas owns and operates 36 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
Conclusion
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Although corrosion is a natural and inevitable process, this does not mean trailer manufacturers can do nothing to control it. The field of corrosion science and engineering has made incredible discoveries and inventions that give designers many ways to limit corrosion. In addition to maintaining design integrity, corrosion protection keeps trailer owners happy since it prolongs the life of their investment. This article discussed the types of corrosion commonly found on trailers, including general, crevice, pitting and galvanic corrosion. The next article in this series will discuss ways to prevent or at
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least decrease the rate of attack from these forms of corrosion. To learn more about the various types of corrosion, the book Forms of Corrosion – Recognition and Prevention, and the week-long NACE International course Basic Corrosion are good starting points. The author has used these sources while conducting research for this article. Additionally, the American Galvanizers Association’s website www.galvanizeit.org offers a tremendous amount of free information and downloads relating to corrosion protection offered by hot-dip galvanized steel.
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