Corrosion is the enemy of all metal. Even though metals are the strongest materials known to man, corrosion acts as metal’s kryptonite. Like weeds in a lawn, corrosion can spread like wildfire across the surface of most metals and render the items essentially useless.
Sometimes this happens on replaceable items such as fasteners and latches, but in other cases, it can result in the loss of high-cost items, such as vehicles and machinery. Therefore, it’s crucial to know how to identify the most common types of metal corrosion and how to protect metal surfaces with corrosion-resistant powder coatings.
Galvanic Corrosion
What is Galvanic Corrosion?
Galvanic corrosion, also known as bimetallic corrosion, occurs when the ions of two metals with contradictory properties cross paths along an electronically conductive path. The corrosion can form along any machine or structure that consists of parts made from more than one metal type. The conflict comes down to different electrochemical charges, which can often stem from contradicting metals.
Specifically, the conflict between ions of anodized and cathodic metals are bound to lead to corrosion if the two make contact from a conductive path. When this happens, corrosion will take root along the merging pathways and gradually weaken the surfaces of each metal. As time passes, the problem can spread across larger portions of both metals.
What Causes Galvanic Corrosion?
The formation of galvanic corrosion is also possible without the presence of two dissimilar metals. When one metal is made of alloys with contradictory charges, an internal conflict can ignite that could lead to corrosion along the surface. In cases where there’s no electrical current to ignite the conflict, the corrosion tends to spread in a more generalized manner across the metal as a whole.
A common example of internalized galvanic corrosion will occur with household batteries, which are prone to conflict due to the presence of carbon-zinc cells:
- The trouble manifests as the zinc corrodes within the cells along the electron pathway.
- The problem can also occur with metal structures placed underwater, where sacrificial anodes end up corroding while guarding the cathode metal within a galvanic couple.
In a system with components that consist of mixed metals, sodium will sometimes be applied to reduce the possibility of galvanic corrosion. For example, if a metal piece consists of copper and cast iron, the production of said metal might also involve the injection of sodium nitrite or molybdate, which can serve as galvanic inhibitors.
Still, the effects of these sodiums are not guaranteed. Therefore, metal pieces consisting of mixed metals and galvanic inhibitors must be inspected regularly for signs of corrosion, especially if the components are submerged for any length of time.
In fact, galvanic inhibitors can sometimes have the reverse of their intended effects. For instance, if a sodium manages to boost the conductivity of the lake or tank water that surrounds the metal component, there could actually be a greater risk of galvanic corrosion than if no sodium had been applied to the metal.
Famous Examples of Galvanic Corrosion: The Statue of Liberty
One of the foremost examples of galvanic corrosion is what formed on the Statue of Liberty during the iconic structure’s first 100 years of existence. As restoration work on the statue commenced during the 1980s, it workers discovered galvanic formations had occurred between the copper exterior and wrought iron structural elements.
Granted, the potential for galvanic corrosion had been perceived a century beforehand by Alexandre Gustave Eiffel, who built the statue. Even though a layer of shellac was appended to Frédéric Bartholdi’s original design for the Statue of Liberty — in order to serve as a protective barrier between the copper and iron — the galvanic process still occurred as the decades passed. As the shellac layer failed, rust formations appeared along the statue’s supportive iron components.
Due to these findings, workers encaged the Statue of Liberty and gave it a full reworking to address the issues of conflict between the skin and supporting parts of the structure. Spanning a period of two years between 1984 and 1986, the restoration included a full reworking of the statue’s interior structure, which from there on out rendered the inside much more hospitable for tourists.
It must be noted galvanic corrosion had only affected a portion of the statue’s connecting points between the copper and iron parts. Therefore, the statue had indeed remained sturdy and safe for visitors in the years leading up to the restoration. Nonetheless, the large-scale effort that went into Lady Liberty’s restoration was seen as an essential investment due to the statue’s iconic place in the hearts and minds of most Americans.
Galvanic Corrosion on the HMS Alarm and USS Independence
During the 18th century, the discovery of galvanic corrosion occurred the hard way during inspections of ships with mysteriously corroded nails. In 1763, inspectors of the HMS Alarm found that iron nails — which had been fitted to the copper-plated hulls of the ship only two years earlier — had turned to paste within the hull.
However, the problem only affected certain nails. On the nails that weren’t affected, brown paper had prevented contact between the iron and copper. The presence of paper was due to the wrapping in which the copper sheathing had been delivered to the ship’s work crew.
On certain sheaths, the paper wasn’t removed before the fastening work was completed, and thus nails were applied straight through the paper. As such, the discovery of galvanic copper was accompanied with the realization that iron and copper should never be allowed to make contact under seawater.
More recently, the U.S. Navy has had to deal with galvanic corrosion on the hull of the USS Independence. Built during the late 50s, the littoral combat ship was discovered to have serious galvanic formations along the aluminum hull. The problem stemmed from the jet propulsion system connected to the hull, which resulted in anode conflict between the aluminum of the hull and the stainless steel jets. Inspectors would ultimately realize that without an electrical isolation barrier in place, corrosion would be inevitable between the two metal components.
Beware Corrosive Refrigerators, Too
One of the silliest occurrence of galvanic corrosion has occurred inside refrigerators. More specifically, the notorious “lasagna cell” — a case of spot–specific galvanic activity — has been found to occur when the pasta is wrapped in aluminum foil. Contact between aluminum foil and steel pans causes the cell, in which lasagna acts as a go–between.
When the problem occurs:
- The aluminum acts as the anode
- The steel acts as the cathode
- The salt within the lasagna acts as an electrolyte
While the contact between the aluminum and actual salt might be limited, the galvanic corrosion can spread very quickly in places where it does occur and ultimately cause holes in the foil.
Stress Corrosion and How It Forms
In order for corrosion to form, a metal doesn’t need to have its ions conflicted with those of an opposing metal via electrolytes, nor is it necessary for internalized alloy friction to take place. In fact, some of the worst corrosion can form like an infection in an injured part of a metal object. When a crack forms in a metal panel or bar, the ruptured area will often be vulnerable to stress corrosion.
In many cases, corrosion will form first, then the crack will follow. This is known as stress–corrosion cracking, where a piece of metal will weaken at a certain spot due to the presence of rust and form a hole or rupture. The damaged area, in turn, is then rendered even more vulnerable to further corrosion, which is likely to spread even faster across the intact areas of the metal surface.
A range of stress factors, such as temperatures or work environments, can cause stress-corrosion cracking. Examples of environments where stress cracks are common include welding facilities and thermal treatment operations. In many cases, the damage caused by stress cracks will render a machine inoperable or a fixture unusable.
Cracks from corrosion stress are liable to spread from places along a metal panel that contain a fastening hole. For example:
- If the inner lining of a rivet hole corrodes due to conflict between the ions of the panel and fastener, the stress around that hole could ultimately lead to a cracked edge at some point along the circumference.
- Once this crack has formed, further corrosion is liable to form within the ruptured opening and in turn spread along the panel.
Stress cracks can also form on a metal surface where the flat texture has been compromised by rust formation. If the rust has led to the formation of holes, stress cracks are liable to form between those holes. In cases such as these, the panel in question will likely require replacement, whether it’s the metal enclosure to a piece of machinery or a supportive piece to an outdoor fixture.
General Corrosion and How it Forms
General corrosion is any type of corrosion that stems from rust, regardless of whether two ions come into conflict, or whether a crack has formed to render a piece of metal more vulnerable. For example, when steel comes into contact with water, rusting is liable to take place due to oxidization of the metal surface. The main similarity with galvanic formations is that general corrosion also results when electrochemical activity takes place.
Examples of general corrosion can be found on metal items that have been exposed to the elements and have ultimately formed rust, such as on vehicles, sheds and outdoor fixtures. If a car loses paint across one of its exterior panels, rust can form along the metal of that exposed area as the vehicle comes into contact with rain. It doesn’t matter how large the exposed area might be, as rust can form along small paint cracks as well as fully stripped doors and hoods.
General corrosion is also common on the metal parts of abandoned docks, where rain has taken its toll on the fasteners and post brackets that join the beams and decks together. On ships that have long ran aground and been abandoned at shore, anything that remains of the hull will likely be fully covered in rust, especially after several decades as a withering artifact of a once-mighty ship. Many public outdoor fixtures still in use will also show hints of rust, such as mailboxes and newsstands.
In order to stop the process of oxidation that allows for general corrosion, outdoor steel surfaces must be given preventative coatings that can withstand the elements:
- On vehicles, paint jobs serve as the ultimate corrosion protection against rust.
- On newsstands and other sidewalk fixtures, paint has the same protective effect under rain and hail for as long as coatings are maintained.
Localized Corrosion and How It Forms
When corrosion occurs at a single spot on a metal surface with no surrounding signs of rust, the problem is known as localized corrosion. The corroded spot could be due to an exposure on that area that doesn’t apply to the surrounding surface, such as when a crack on the paintjob of an automobile allows rust to form at the opening. Localized corrosion can also be caused when conflicting ions make electrically charged contact — but only at an isolated spot — with an anodized metal.
The effects of localized corrosion can ultimately be far more damaging than more generalized occurrences of rust formation, because corrosion can seriously weaken a specific spot of metal when the problem is confined to a small area. In many cases, the problem will have gotten way out of hand by the time it’s discovered, and the surface or component will either need to be replaced or scrapped.
Localized corrosion can also be due to peculiarities with select spots on a metal component, which could cause such problems to form more rapidly than in other areas. In cases such as these, the localized spot could be the tipping point for slower problems, such as stress or fatigue, across the broader surface.
There are several types of localized corrosion that can take place on a surface:
- When the problem forms within a crevice or along a shielded area, it’s often referred to as crevice corrosion.
- Pits of a certain diameter, both wide and narrow, mark another manifestation of localized corrosion. When a cavity or cavities form on a metal panel with roughly equal depth and diameter, the problem is known as pitting corrosion.
- A much rarer type of localized corrosion can occur when reactions within grains occur. While the boundaries of grain have little impact on most applications of metal, those that do can have harsh reactions and lead to corrosion on metal surfaces.
Caustic-Agent Corrosion and How it Forms
Water, salt and conflicting ions are not the only causes of corrosion. In rare cases, corrosion can form when metal comes into contact with particulates of caustic agents. Impurities in gas, for example, can have a corrosive effect on metals if distributed along a surface in droplet form. That same gas wouldn’t, however, have an effect on the metal while in a gaseous state.
Some of the most pronounced examples of caustic–agent corrosion result from contact with dry particulates of hydrogen sulfide on moisturized metal surfaces. When this occurs along a vast stretch of metal, the effects can lead to extreme discoloration and peeling across the surface. Other types of caustic–agent corrosion occur when metal comes into contact with impure liquids and solid materials.
Corrosion-Protectant Metal Coatings
The effects of corrosion-resistant metal coatings can all depend on two crucial factors — the type of metal in question and the type of corrosion that needs to be prevented. When it comes to iron and steel alloys that could be vulnerable to galvanic corrosion, zinc and aluminum-based anti-corrosion coatings are most effective at keeping the metals safe.
Corrosion-protectant metal coatings made of aluminum and zinc are often applied to treat the metal surfaces of large outdoor fixtures exposed to the elements 24/7, year after year, such as bridges. Meanwhile, cadmium coating is usually applied to fasteners and bolts on public fixtures in order to block the absorption of hydrogen.
Coatings made of nickel and cobalt chromium are also applied on metal surfaces to prevent the formation and spread of corrosion. Chromium coatings are often valued for their low porosity levels. As moisture-resistant coatings, chromiums are highly effective at preventing rust and keeping metals preserved and intact for many years. Oxide ceramic coatings are also effective at keeping metals free of damaging rusts for decades on end.
Fusion-Bonded Epoxy Coating
When it comes to transformer components, the best coating types of corrosion-resistant powder coatings consist of epoxy powder. More than three decades ago, when some of the first powder coating systems were implemented, products for switchgear and transformer arsenals were among the first items treated by the powder. Fusion-bonded epoxy coating offers the strongest shield against corrosion on pilings, sheeting and steel reinforcement layers.
Nonetheless, the use of a powder coating to prevent corrosion is just one of the steps to ensure the long life of a steel component. An environment should also be created that allows metals to breathe and drain properly so the agents that cause corrosion don’t fester on the surfaces of metal parts.
After all, the purpose of corrosion-protectant powder coatings is not to leave metals safe for endless abuse, but to seal out the chance of rust formations if and when metal parts undergo prolonged exposure to the elements
TGIC Polyester Powder Coating
For corrosion resistance with a touch of style, TGIC polyester powder coating application is the most powerful treatment currently in use for metal components made to endure heavy weather on an ongoing basis. With its decorative finish that comes in a range of colors, TGIC is suited to all kinds of metal machines, fixtures and other outdoor structures. As the most effective of the corrosion-protectant powder coatings, TGIC has been applied to everything from gates, fences and guide rails to traffic signs, railings and bollards.
Get Corrosion-Resistant Powder Coatings From Lane
At Lane Coatings, our goal is to offer the most powerful corrosion-resistant metal coatings for a range of structural components that need the utmost protection from the elements. To that end, we employ the strictest standards for each coating application by using the highest-quality products with state-of-the-art coating technology.
Over the years, various industries have learned the hard way that corrosion can ruin some of the largest and most expensive objects imaginable, which happen to contain metal pieces. For operations large and small in which metal machines and parts are among the key components, it’s crucial to have all metals coated to help ensure the durability of everything in a given arsenal. The anti–corrosion service provided at Lane Coatings has helped numerous operations get longer life and superior performance from their equipment.
At Lane Coatings, we do powder coatings for everything from rebar, railings, bollards and bridges to solar carports, traffic poles and various other structural components. We have performed a range of work for towns and municipalities in upscale areas that value high-quality outdoor fixtures. With more than 30 years in the powder-coating business, we know there’s virtually no project that can’t be handled at our 65,000-square-foot facility. To learn more about the service we offer, explore our powder-coating solutions and give us a call.
