Galvanic Corrosion In Vintage Campers

Do you have a vintage Airstream, Avion, Spartan, or another aluminum-bodied camper? Ever noticed screw heads covered in rust or pitting in aluminum around a steel screw? You, my friend, are the victim of galvanic corrosion. And if you don’t want to make it worse, you’ll want to know a thing or two about how this happens and how to avoid it.

What is Galvanic Corrosion?

When two dissimilar metals come into electrical contact, something has to give. Give electrons. This electrical contact doesn’t have to be anything near as sever as intentionally running a current between the metals. It is a pretty natural process, exacerbated by exposure to the elements. In such scenarios, one metal will be sacrificed to the other. Which metal that is depends on their ranking relative to each other and the severity varies by their degree of difference. We see that sacrifice as corrosion.

Cathodic. Anodic. It’s all Relative.

The relative difference between cathodic and anodic metals will also vary in the severity of the corrosion. For example, rather than using pure steal for components, which rusts easily, many steal components come with a zinc coating. You can see the difference: zinc coated steal has a blueish tint. As the difference between zinc and aluminum is far less than steal and aluminum, there is far less corrosion and, in fact, the zinc will sacrifice to the aluminum rather than aluminum sacrificing to the steal. But, here is the rub: that zinc coating will wear off over time. At that point, the already highly corrosive steal will come in direct contact with aluminum and all hell breaks loose. That is why it is important to look for early signs of corrosion. If zinc coated steal components are starting to corrode, they aren’t zinc coated anymore. That means it’s time to replace the the components with new, zinc-y ones. Early detection makes a huge difference!

Finding Metal In Odd Places

Even wood treated with copper, mercury, or fluoride based preservatives can increase the risk of corrosion for aluminum sheet. So it is very important to know your materials!

Galvanic Corrosion in Fasteners

This is the most common form of Galvanic Corrosion that you are likely to encounter in vintage campers. Idealy, the fastener and base metals are the same. In our Avion, most are. They are called rivets. But there are other scenarios where screws, nails, and other metal components will be needed that aren’t suited for aluminum or whatever the base metal may be. Each role demands different characteristics. What matters is that the difference between the metal types is managed to minimize corrosion. When using different base and fastener metals, the base should be more annodic and the fastener should be more cathodic. The table below demonstrates a range of relationships between cathodic and annodic materials.

Fastener Corrosion Chart

Base Metal	Fastener Metal
	Zinc & Galvanized Steel	Aluminum & Aluminum Alloys	Steel & Cast Iron	Brasses, Copper, Bronzes, Monel	Martensitic Stainless (Type 410)	Austenitic Stainless Steel (Type 302/304, 303, 305)
Key ○ Corrosion of the base metal is not increased by the fastener ◐ Corrosion of the base metal is marginally increased by the fastener ● Corrosion of the base metal is markedly increased by the fastener ⦾ Plating on the fastener is rapidly consumed, leaving the bare fastener metal ⦿ Corrosion of the fastener is increased by the base metal Based on chart from Preservation Science
Zinc & Galvanized Steel	○	◐	◐	●	●	●
Aluminum & Aluminum Alloys	○	○	◐	●	Not Recommended	◐
Steel & Cast Iron	○⦾	○	○	●	●	◐
Teme (Lead Tin) Plated Steel Sheets	○⦾⦿	○⦿	○⦿	●	●	◐
Brassess, Copper, Bronzes, Monel	○⦾⦿	○⦿	○⦿	○	○	◐
Ferritic Stainless Sheet (Type 430)	○⦾⦿	○⦿	○⦿	○	○	○
Austenitic Stainless Steel (Type 302/304)	○⦾⦿	○⦿	○⦿	○⦿	○	○

Making Up The Difference

Even when two different metals come in contact, that contact can be minimized. The severity of the galvanic corrosion is based on the relative difference between the cathodic and anodic materials, the exposure to a current (such as salt water), and surface area. Once the metals are selected, one can still minimize the later two variables—environment and surface area. Indoor metals or painted metals are protected from salt water and other environmental variables that could creative a current. Using plastic washers on screws or rubber mats between metals surfaces can insulate the metals from each other and cut down on the amount of surface area that is in contact.

What We Did

Aluminum screws are expensive. When 1½” screws costs 65¢ each, a project on the scale of ours is overwhelmed just by the price of fasteners. That isn’t to say we skimped on screws. Type 316 Marine Grade Stainless Steal screws are remarkably resistant to corrosion and a 1½” screw costs 16.5¢. What distinguishes type 316 stainless steal from other, already hearty 300 Series austenitic stainless steel is the presence of 2–3% molybdenum, giving it exceptional corrosion resistance. To further minimize risk of corrosion, we used plastic washers to limit the contact between stainless steal and aluminum.

Arguably more important, though, are the L brackets and jack points. After all, these represent a far larger surface area than screws. Some of the worst corrosion observed on the camper are at these points. For this reason, we are replacing all steel L brackets with aluminum. While aluminum jack points aren’t really an option, we can minimize the surface area exposure by introducing a rubber mat between the aluminum skin and the stainless steal jack point.

Have we solved the galvanic corrosion puzzle? Only time will tell.

More Reading On Galvanic Corrosion

• “Preventing Galvanic Corrosion,” Preservation Science