Here’s what you need to know about aluminum.

Many fear that because aluminum is metal, like most metal products, it will rust. So, would an aluminum dock rust?
Even so, the fact of the matter is that aluminum does not rust; however, aluminum can corrode.

How Rust Affects Aluminum

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, the stain is 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 have a Zinc coating to prevent rusting. In the Galvanic series of metals and alloys, aluminum and Zinc are side by side. Therefore, Zinc does not increase the corrosion of the base metal. It is safe to use with aluminum. However, rust and corrosion will occur if the stainless steel fastener does not have a Zinc coating. When it comes to building with an aluminum product always use stainless steel with a Zinc coating to avoid the risk of rust staining.

Physics of Aluminum

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 closes, 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).

Galvanic Corrosion

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. Decking commonly uses ACQ. However, due to its high copper content, ACQ is corrosive to many metals, such as aluminum. The ACQs’ highly conductive copper, along with oxygen and rainwater 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 area difference concentrates the corrosion. Conversely if the area of the anode is large compared to that of the cathode, it 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).

 
Sources:
Rust on Aluminum
Corrosion Resistance of Aluminum
Galvanic Corrosion Bimetallic Corrosion
Photo Credit:
Aluminum’s Corrosion Resistance