Will an Aluminum Deck Rust?

It’s a question that we get asked often here at Wahoo Decks and it’s a relevant question. Many fear that because aluminum is metal, like most metal products, it will eventually rust. Even so, the fact of the matter is that aluminum does not rust; however, aluminum can corrode. First, let’s address how rust can actually affect aluminum and what you can do to prevent it.

“When iron-containing metals combine with oxygen, oxidation, and eventually rusting, occurs. Because aluminum, like copper, is not an iron-containing metal, it doesn’t actually rust itself. If you have a rust stain on an aluminum surface, it’s not because the aluminum itself has rusted. More likely, it has been stained from a nearby metal. If an iron-containing metal, such as steel, rest on, is attached to, or drips water on another surface, it can create a reddish-brown stain and make it appear that the surface has rusted” (Rust on Aluminum).

A common decking example of this happening would be if you used stainless steel fasteners on an aluminum deck. The aluminum will not rust, but the steel fasteners might, which could cause rust to stain the aluminum. The best way of preventing this from happening is by using Zinc coated steel fasteners. All of Wahoo Decks’ stainless steel fasteners are Zinc coated to prevent rusting. In the Galvanic series of metals and alloys, aluminum and Zinc are side by side. Therefore, corrosion of the base metal is not increased by Zinc and is safe to use with aluminum. However, if you use a stainless steel fastener that is not Zinc coated, rust and corrosion will occur. When it comes to building with an aluminum product always use stainless steel that is Zinc coated to avoid the risk of rust staining.

Now that we’ve answered the question of how rust can affect aluminum, it’s time to take a look at how aluminum can corrode, which can happen in certain scenarios. To understand this, you need a better comprehension of the physics of aluminum and the process known as Galvanic Corrosion.

galvanic corrosion occurring between aluminum and copper“When two dissimilar metals are immersed in an electrolyte solution, a battery is created. The electrolyte solution serves as a bridge between the two metals and effectively closes half of an electrical loop. When the two dissimilar metals come into contact, the electrical loop is closed, and the natural voltage differential between them causes electron flow. One metal will become the anode (negative) and one will become the cathode (positive)” (Corrosion Resistance of Aluminum).

Simply put, often Galvanic Corrosion takes place when aluminum (the anode) comes in contact with a metal like copper (the cathode), in or near saltwater (the electrolyte). Saltwater is a perfect example of an electrolyte solution. A very common board that is used in decking is ACQ lumber, but due to its high copper content, ACQ is corrosive to many metals, such as, aluminum.  The ACQs’ highly conductive copper, along with oxygen and rain water cause Galvanic Corrosion of the bare metals. Therefore, untreated, mill finish steel or aluminum should not have intimate contact with ACQ treated lumber for extended periods of time, especially in the presence of water. Keep in mind, the severity of corrosion will depend on the two metals that are in contact with one another.

“The relative area of the anode and cathode has a pronounced effect upon the amount of corrosion that occurs due to Bimetallic Corrosion. A small anode (the less noble metal, such as aluminium) joined to a large cathode (the more noble metal, such as stainless steel) will result in a high current density on the aluminium, and hence a high rate of corrosion.

The corrosion is concentrated by the area difference. Conversely if the area of the anode is large compared to that of the cathode this dilutes the corrosive effect, in most cases to the extent that no problem occurs. It is common practice to use stainless steel fasteners to fix aluminum sheeting or signs, but if aluminium screws were used to fix stainless steel the screws may rapidly corrode” (Galvanic Corrosion Bimetallic Corrosion).

If aluminum is the primary metal, the Galvanic Corrosion process will take many years to occur or may not happen at all. If concrete is the primary metal that is contact with aluminum, Galvanic Corrosion will occur at a faster rate and will be more severe.

galvanic corrosion on aluminum

In this particular case, Galvanic Corrosion took place on an aluminum rail post. Inside the aluminum post was a steel bolt that began to corrode once the post was in contact with saltwater

Fortunately there are preventative measures you can take to avoid Galvanic Corrosion from occurring:

  • Using a rubber membrane along joists is an added protective step to take to prevent Galvanic Corrosion.

    Using a rubber membrane along joists is an added protective step to take to prevent Galvanic Corrosion.

    Use a Butyl rubber membrane deck protector on joists if the decking comes in contact with a product like pressure-treated wood, especially if you are located near a saltwater environment like the coast. A Butyl rubber membrane is highly weather resistant and flexible. In addition, use a rubber membrane when installing a deck on or near concrete to prevent Galvanic Corrosion occurring.

  • Use a marine grade alloy to begin with. “There are hundreds of different aluminum variations. Like steel, each exhibits different qualities and is formulated for different and specific end uses. It is imperative that the proper alloy is selected to realize aluminum’s reputation as highly corrosion-resistant metal in marine applications” (Corrosion Resistance of Aluminum).

How Wahoo Decks’ products resist aluminum corrosion:

  1. Wahoo Decks’ aluminum boards are watertight. This prevents water (electrolytes) from becoming the catalyst for Galvanic Corrosion.
  2. Wahoo Decks uses a 6005-T5 series marine grade aluminum with a polyester powder coated finish; the finish provides a protective barrier between dissimilar metals.
  3. All of Wahoo Decks’ stainless steel fasteners are 410 Zinc coated. The Zinc coating prevents the corrosion of the fastener and also protects against a Galvanic Corrosion between the aluminum and the steel in the fastener.

About Wahoo Decks:

Wahoo Decks is a leading manufacturer of award winning outdoor decking products. Wahoo products have been engineered to be uniquely low maintenance and more resistant to natural elements than traditional decking products. Wahoo Decks offers AridDek, a waterproof aluminum decking product, DryJoist and DryJoistEZ a revolutionary waterproof and structural exterior aluminum joist, and Wahoo Rail a complete line of aluminum railing kits. Additional information about Wahoo Decks can be found on www.WahooDecks.com or by calling (877) 270-2317.

Sources:

Rust on Aluminum

Corrosion Resistance of Aluminum

Galvanic Corrosion Bimetallic Corrosion

Photo Credit:

Aluminum’s Corrosion Resistance

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