Regardless as to whether you prefer planes and trains, or boats and automobiles, we all rely upon modes of transportation in order to get from one place to another. Despite their differences, all four of these forms of transit share two common enemies: Corrosion and rust. Yet despite the commonality of these cancerous occurrences, there are some distinct difference between the two, none of which are familiar to the average person.
Here at AvalonKing, we have covered everything from undercarriage rust removal and protection tips, to holding discussions about the side-effects of road salt and de-icing chemicals. But for some reason, we’ve never tackled the topic of pitting and corrosion on aluminum alloy components.
So in the interest of the greater good, I began investigating the most common causes of aluminum corrosion, with high hopes of unearthing some removal and preventative maintenance tips along the way. What I discovered was startling…
Isn’t Aluminum Supposed to Never Rust?
According to a report that was put out recently by Monroe Engineering, aluminum accounts for about 8% of all elements in the Earth’s crust, making it the most readily available metal. It is also surprisingly lightweight, and from a manufacturing standpoint, incredibly easy to manipulate when heat is applied.
Today, you can find aluminum in virtually every aspect of daily human life. Automobiles, airplanes, farming equipment, roofing materials, weapons, elevators, electronics, hardware, appliances… the list of aluminum applications is endless. Commonly utilized for its ability to balance strength and lightweight rigidity with rust resistance and ductility, aluminum alloys are modern man’s space-age material.
Unlike iron, aluminum never rusts, a detrimental occurrence that contrary to common belief, can also eat into stainless steel, especially when put into contact with things like salt and water. In contrast, pure aluminum does not contain iron, therefore it will never rust. Instead, aluminum is susceptible to corrosion, which if allowed to fester, can also be quite detrimental.
As aluminum corrodes, it weakens, the same way a rust-riddled piece of iron is quick to break, so too will a heavily corroded piece of aluminum. Sure, both naturally occurring cancers consume metal at a snail’s pace, but once embedded, are a royal pain in the ass to remove and prevent from spreading.
So if aluminum won’t rust, why does it corrode?
For the answer to this puzzling question, we turn toward none other than bolt and hardware specialist, Fastenal. According to the Minnesota-based band: “Corrosion can be thought of as an electro-chemical action in which one metal is changed into a chemical or simply eaten away. When two metals are in contact with each other in the presence of some electrolyte [water], the less active metal will act as the cathode and attract electrons from the anode. The anode is the material which corrodes.”
Fastenal’s report goes on to offer an example of how an aluminum component can corrode rapidly. “If brass and aluminum plates are connected by a passivated [coated] 304 stainless steel bolt, both the brass and the aluminum will corrode severely where they touch the stainless steel because they are much more anodic than stainless steel. The aluminum plate will corrode more heavily due to it being more anodic to stainless steel than brass is. The aluminum will also corrode where its exposed surface contacts the brass plate because brass is more cathodic.”
In order to break this problem down to a molecular level, we turn toward a discovery made by Norwegian scientist, Kemal Nişancıoğlu. While examining the corrosive weaknesses commonly found within metal alloys, aluminum’s susceptibility to corrosion was exposed. “Pit initiation on multiphase commercial [aluminum] alloys occurs invariably at weak spots on the oxide around the intermetallic particles,” Nişancıoğlu explains. “The weakness results firstly from the presence of a flaw in the oxide at the particle-matrix interface.”
This nerd jargon will more than likely trigger a few nods of understanding from those of you who are familiar with the performance pros and cons of ceramic coating products. For the rest of you, here is a brief synopsis of what we are nerding-out over, and why it should even matter.
Surface flaws allow contaminants and moisture to creep into crevices where they do not belong, which over time, can cause a material like aluminum to become compromised. Like many other materials, aluminum is only as strong as its weakest link, so when pitting forms in a particular area, it will often spread outward if not removed in a timely fashion.
Quick Nerd Note: Aluminum alloys often form a smooth surface oxidation that can be anywhere from 0.001 to 0.0025 of an inch thick. This oxidized outer layer is not considered detrimental, as it forms a shell-like barrier, and is not prone to pitting.
Is there a way to prevent aluminum from pitting and corroding?
So there are several ways of protecting an aluminum surface from the damages associated with pitting and corrosion. The most common is the process of clear coating, which on many aluminum products, is pre-applied at the factory for superior corrosion protection. While the materials and techniques used may vary from the clear coat that is commonly found atop a vehicle’s paintwork, the purpose of both products is identical.
That’s not to say that either forms of these original factory coatings will be able to serve and protect until the end of time. There are a lot of factors at play that can affect an aluminum component, especially on something that sees a lot of abuse amid moisture-rich environments. Which is precisely why a vast majority of aluminum products come finished in an anodized coating.
While a brief paper on the matter by TECH-FAQ, certainly shines some light on anodized aluminum’s pros and cons, commercial-grade alloy coating specialist, Silcotek, delves a bit deeper into the reasoning behind the need to protect aluminum components.
Anodization drastically changes the texture of an aluminum alloy, giving it a far more porous surface, which in turn allows subsequent protective and/or pigmented coatings to adhere to the metal. Even without an additional coating atop its surface, an anodized aluminum material increases aluminum’s resistance to corrosion behavior and wear-and-tear. This semi-rough surface not only negates cases where cracking or peeling may occur, but its thick oxide layer is also completely undetectable by the naked eye.
But anodizing also has its weaknesses. Unlike titanium, iron, and stainless steel, untreated aluminum is weakened once temps surpass the 212° Fahrenheit (100° Celsius) mark. And while anodized aluminum’s porous surface may make it easier to coat, and therefore more resistant to flaking and corrosion, it suffers when the heat is on, with a surprisingly low 176° Fahrenheit (80° Celsius) cracking point.
Quick Nerd Note: In its simplest form, anodization is little more than the exposure of raw aluminum alloy materials to a torturous blend of acid solutions and violent electrical currents. This killer combo forces the aluminum to form hydrogen at its negative electrode (cathode), with oxygen along its physical aluminum surface serving as the positive electrode (anode). While the hydrogen is little more than an aluminum by-product, the oxygen produced is transformed into an aluminum oxide, which in essence, is the basis for anodization.
Don’t Stress, and Keep Clean
Another common risk to aluminum is stress corrosion cracking (SCC), a name that should be fairly self explanatory. Being that aluminum is prone to being compromised under pressure, the right mixture of corrosion-forming foreign elements and stress can force the lightweight material to corrode prematurely.
This means that things like aluminum boat hulls, stamped truck beds, aircraft exteriors, and construction and agriculture equipment are all prone to stress corrosion cracking. If it’s aluminum and it gets bludgeoned by another object or a pressing force on a regular basis, chances are it is susceptible to SCC. This is why routine surface cleaning and inspections are so crucial, for if left unattended for too long, the structural integrity of said section could become unfit for use.
So while it may be impervious to iron oxide degradation, and far more lightweight than, say, stainless steel, aluminum can weaken rapidly once regular corrosion or stress corrosion cracking sets in.
In order to remove said “metallic cancer” from an aluminum surface, most people will take one of two approaches. The first, and perhaps most appealing, is a highly acidic, easy-to-make, super affordable, all-natural aluminum oxidation remover of unrestrained awesomeness.
By combining distilled water with either pure lemon juice or white vinegar, and then gently agitating the corroded area with a mild scrubbing pad, most mild cases of aluminum corrosion can be removed. To create this magical elixir, simply combine a quart of distilled water with two tablespoons of an acidic liquid of your choosing, and stir. With a little bit of elbow grease, ample amounts of exposure time, and a dash of luck, you should be able to remove most garden variety forms of aluminum corrosion.
However, if you feel like something stronger is required, there is a seemingly endless array of aluminum cleaner chemical products specifically engineered for oxidation removal on untreated and anodized surfaces. And while things like non-woven abrasive pads work wonders on corroded anodized aluminum surfaces, one must be careful to not damage the metal’s protective film. However, if the anodization becomes compromised in some way, a splash of chromic acid or another inhibitive treatment can be used to restore the oxide film.
Quick Tip: NEVER use steel wool or steel wire brushes on aluminum surfaces, or any other highly abrasive cleaning media for that matter. It can easily damage soft aluminum surfaces, thus causing further corrosion.
How to Protect Aluminum From Corrosion
One way to protect an aluminum surface from corrosion is to store it in a climate-controlled environment. This may be ideal for something small that is only used occasionally, but preventing an automobile from coming into contact with rain, humidity, or other moisture-rich natural elements is virtually impossible. Which leads us back around to the most widely practiced form of aluminum protection: Clear coating.
Clear coating protection can be as simple as the application of a layer of ceramic coating to an aluminum surface area. An invisible layer of clear coat not only protects the raw alloy from the elements, but it also has the ability to add depth to any previously painted or powder coated surface. While a basic, “rattle can approach” to clear coating can be applied to smaller aluminum objects, larger surface areas typically require a more potent solution.
A report by Boeing on the subject of aluminum corrosion on aircraft illustrates some of the countermeasures being made to fight this destructive occurrence within the aviation arena. While much of it is inapplicable to those of us in the private sector, it is intriguing to see the measures that are being taken to keep aluminum corrosion at bay on airplanes, and what that means for automotive applications.
The aerospace powerhouse’s report goes on to explain that if you were to affix something made from stainless steel to an aluminum surface, the steel should be cadmium plated in order to prevent it from galvanically corroding the aluminum. The same goes for titanium, which Boeing says should receive a corrosion-inhibiting primer like a Skydrol-resistant epoxy in order to withstand the damages commonly caused by jet fuel. Polyurethane topcoats can also be applied over aluminum primer for functional reasons, with these decorative polyurethane topcoats offering greater resistance to filiform corrosion.
Fortunately, most of us won’t ever need Skydrol-resistant epoxies. A simple coating of Armor Shield IX should be more than enough to protect the average person’s aluminum possessions. From automotive components and body panels, to boats, bicycles, motorcycles, and more, protecting an aluminum surface has never been easier nor more resilient thanks to the advent of ceramic coating.
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