Aluminum is the most abundant metal in the Earth’s crust, accounting for over 8% of all metals. However, unlike gold, silver, copper, and platinum, it’s rarely found uncombined in its pure form because it is inherently unstable and highly chemically reactive.
Resistance to corrosion is one of aluminum’s many positive attributes, and as long as specific protective measures are taken, it will hold up in salt water. Under certain conditions, though, aluminum will corrode and eventually break down unless these measures are taken.
Aluminum is a popular choice with anglers as a material for their boats, so clearly, its ability to resist the corrosive action of saltwater is recognized. The metal is also used in the manufacture of aircraft, in the construction industry, in packaging, and in many other fields where it is exposed to salt water in its various forms. Aluminum certainly holds up in salt water, but it needs the proper treatment.
Aluminum Is A Manufactured Metal With Unique Properties
A metal ornament found in the tomb of a third-century Chinese military leader was found to be 85% pure aluminum, but how it was created is an unanswered mystery.
Aluminum has an affinity to oxygen, and while scientists were aware of the existence of aluminum oxide as early as the late 18th century, it was only in 1827 that a German, Friedrich Wohler, was able for the first time to extract the pure metal from the oxide.
Today aluminum is obtained mainly by the Hall–Héroult method, which is exceptionally energy-intensive – nearly 5% of electricity generated in the US is utilized in the production of aluminum. However, the country is not among the world’s leading producers.
China (where it all began) is responsible for almost two-thirds of global aluminum manufacture, followed by the Gulf States and Asia – America is ranked only 9th in the world.
Reasons For Choosing Aluminum Over Other Metals
Aluminum has many properties which make it highly sought-after in many industries. Because it performs effectively and efficiently in many applications, it is preferred over many other metals.
- It can be recycled while still retaining all its properties, so it is a sustainable product. From a cost-effectiveness point of view, this gives aluminum the edge.
- Aluminum has the lowest density of any commercially used metal except magnesium, making it lightweight but still durable and strong. In the construction industry, as an example, this will result in considerable cost savings as fewer support structures, and load-bearing components need to be provided.
- Aluminum reacts with air to form a coating of aluminum oxide, which protects the metal from corrosion. This resistance to corrosion will lead to less maintenance and repair costs.
- Aluminum is flexible, so it can be cut, molded, extruded, melted, and cast into any shape required while retaining its corrosion-resistant properties. As a result, it can be used in a multitude of different applications.
Uses Of Aluminum
Let’s look at the use of aluminum specifically where we need to consider the effects of salt water on the metal. In the most extreme case, aluminum may be immersed in salt water, as with aluminum boats, or it may be affected by the salt in the humid air at the coast. Whichever way it is used, whether in the sea or on land, it’s crucial to examine how aluminum holds up to the effects of exposure to salt and moisture.
- Construction: If you’re building a beach house or a multi-story office block in a coastal city, aluminum is a preferred material for a number of products that will be incorporated into the building. Some of these are doors, window frames, balcony and stairway railings, shelving, staircases, and even roofs and exterior facades.
- In the home: aluminum is widely used in manufacturing outdoor furniture, cutlery, and kitchenware (pots, pans, and cooking foil), and in mobile phones, tablets, and laptops (Apple use it extensively in their product range).
- Packaging: Extensive use of aluminum is made in the canning industry and in the packaging and preserving of foodstuffs.
- Electrical goods: aluminum is not as good a conductor as copper but is more ductile, lighter, and resistant to corrosion. It is used in appliances, televisions, satellite dishes, and even in power lines.
- Transport: Almost all modes of transport utilize aluminum extensively – aircraft in particular but also cars, trains, boats, and even space shuttles. The weight advantage compared to other metals, as well as the durability, flexibility, and strength of aluminum, make it the ideal material. Even many sea-going fishing boats are constructed from aluminum and can last for years with proper maintenance.
The Effect Of Salt Water On Aluminum
For our purposes in this discussion, the term “saltwater” will cover not only sea water but also salt-laden, humid air as experienced in warm, coastal regions.
We’ve noted that when exposed to air, aluminum forms a thin coating of aluminum oxide on its surface, inhibiting further corrosion. When salt is added to the mix, however, it changes the reaction.
Salt doesn’t have a direct effect on aluminum, but it causes an electrochemical reaction that results in the formation of the chalky white substance, aluminum oxide, but also causes pitting of the aluminum.
- Atmospheric corrosion: This will occur if aluminum is exposed to salty, moist air, in the same way as if placed in seawater. The chlorides in the seawater and sulfides in polluted air will result in the aluminum oxide being attacked and thus exposing the aluminum to further corrosion until it breaks down completely.
- Galvanic corrosion: When other metals are close to aluminum, for example, with brass fittings on a boat, galvanic corrosion also occurs to speed up the breakdown of the aluminum.
- Pitting corrosion: As mentioned, pitting is the formation of tiny holes on the surface of aluminum. It is unsightly but doesn’t affect the aluminum’s strength unless left untreated for an extended period.
- Intergranular corrosion occurs in aluminum alloys when there is a movement of electrons between the aluminum and the alloy, similar to the process that occurs in galvanic corrosion. This is why alloys corrode more quickly than pure aluminum, and the process is accelerated in the presence of salt water.
Corrosion rates vary: some factors that also affect the rate of corrosion are heat (warmer waters and warmer days will result in quicker corrosion), humidity levels in the case of atmospheric corrosion, and the salinity of the salt water, which can vary from region to region.
Preventing Corrosion So Aluminum Can Hold Up In Salt Water
Corrosion is a reality when aluminum is in seawater for extended periods, and there may be detectable signs of corrosion within weeks. But several simple steps will slow the process down so that an aluminum boat, for example, will last for fifteen to thirty years if adequately cared for.
Retain Natural Protection
Remember that the aluminum oxide that forms on the surface is a corrosion retardant, don’t remove it unless you plan to take other protective measures. However, if the aluminum has been contaminated with iron, the surface will show rust marks in the aluminum oxide. If that is the case, you might be better off removing the oxide coating and replacing it with another form of protection.
Separate Aluminum From Other Metals
Try to avoid galvanic corrosion by not using other metals in the seawater close to the aluminum. A stainless steel propellor, for example, would create just such a chemical reaction. Still, the most galvanic corrosion will occur when the two metals are in contact with one another, such as brass brackets on an aluminum railing.
Cathodic protection is particularly effective when used on aluminum boats spending considerable time in salt water. This process entails using a “sacrificial “ metal such as zinc as the cathode in an electrochemical reaction with aluminum as the anode. The cathode gets corroded in this galvanic corrosion process and saves the aluminum.
Cathodic protection is used not only on boats but also on a much larger scale on ships’ hulls, underground storage tanks, and oil rigs.
Passivation Of Aluminum
Passivation is a process in which a protective coating is applied through a chemical process that binds it to the underlying layer. This coating then inhibits further corrosion by reducing the reactivity of the surface.
Anodizing is a chemical process where a hard layer of aluminum oxide is integrated with the aluminum underneath. It is similar to the natural process where aluminum oxide forms a protective layer, but anodizing is a controlled electrolytic process that creates a much more durable protective barrier that forms part of the metal and will not crack or break off.
It is possible to add color to the anodized surface, which makes the process suitable not only for the prevention of corrosion but also for aesthetic appeal.
You will commonly see anodized aluminum being used in manufacturing aluminum doors, window frames, and railings, but it has a broad spectrum of applications in the construction industry.
Powder Coating Is A Common Form Of Corrosion Prevention
Electrostatically charged powder particles are applied to the aluminum, forming a tough coating between 2mm and 4mm thick.
A complex cleaning process is needed before the powder coating can be applied, as any impurities, dirt, or grease on the aluminum surface will cause the coating not to adhere properly. The cleaning also requires removing any aluminum oxide already on the surface.
Finally, there is the curing process, where heat is applied to the powder to create the finished product, a hard coating that is very much stronger than conventional paint.
Powder coating is flexible, extremely durable, and can be applied in any color, and a variety of finishes, making it a popular method of protecting aluminum.
Painting Is An Inexpensive Option
There are protective paint products that are made specifically for use with aluminum. Marine paints are developed particularly for painting aluminum boats, which will require regular applications to the hull to protect it from seawater and its corrosive properties.
The main disadvantage of painting an aluminum boat is that it is effective in preventing corrosion only as long as it remains intact. Unfortunately, boatowners will confirm that paint tends to bubble and lift because the salt in the water oxidizes and becomes brittle, and it’s easily damaged.
Painting aluminum is probably the least effective method of preserving aluminum, but a good option for items that are not subject to harsh treatment, such as windows and doors and outdoor furniture, and that are easy to repaint when necessary.
Washing Down With Fresh Water
This precaution applies to boats but, in fact, is a practical method of preserving all kinds of aluminum products that bear the attack by salt water in its various forms.
Regular pressure cleaning with fresh water will remove the salt residue and reduces the atmospheric and galvanic corrosion potential. Similarly, wiping down and drying your aluminum items will also prevent corrosion.
Using The Best Grade Of Aluminum To Minimize Corrosion
Not all aluminum alloys are created to withstand corrosion from salt water, and some are very much better than others. Without going into detail, every aluminum alloy is numbered according to the metal used and the amount of that metal in the alloy.
The 3000-series and the 5000-series are considered to be the most resistant to galvanic corrosion, and within those ranges, certain alloys are better suited, say, for boatbuilding than others. It would be best to get the advice of experts before deciding which alloy will hold up best to salt water in your specific case.
There’s no doubt that aluminum has enormous advantages for boat owners, owners of beach houses, developers of coastal properties, and anyone who has to contend with the corrosive action of salt water in all its forms.
In many cases, aluminum will hold up in saltwater better than other metals. But, although resistant to corrosion, it will last many years longer if care is taken to protect and preserve the metal by all the means we’ve mentioned. There is a cost involved, but it will save a whole lot more in the long run.