Sorry Mister Stark, but iron is a lousy choice for a suit. It’s heavy, which will limit your ability to fly, and worse than that, it rusts. Spend too long in damp environments and you’ll find reddish brown flakes lifting off the surface. Aluminum would be better because its much lighter and doesn’t rust. Aluminum does corrode though, especially if exposed to seawater. Here’s an explanation, along with some tips on what to do about it.
Rust vs Corrosion
Most metals want to corrode back to some form of ore. (Gold is one notable exception.) Corrosion starts with oxidation, where atoms of metal link up with oxygen, followed by a gradual, or not so gradual, breakdown. Rusting is a specialized form of corrosion that only iron and steel go through.
Rust is when the iron oxidizes and flakes off. It’s accelerated by moisture. Flaking exposes fresh metal beneath, which in turn oxidizes and flakes.
Aluminum oxidation happens faster than that of steel, because aluminum has a really strong affinity for oxygen. Rather than flaking though, aluminum oxide just forms a hard, whitish-colored surface skin. When all the aluminum atoms have bonded with oxygen the oxidation process stops.
Scratching this oxide skin exposes bare metal, and the process begins again. It won’t eat the metal away though, except under two conditions. First, if chlorides or sulfides are around they’ll attack the aluminum oxide layer.
Chlorides are compounds of chlorine. Sodium chloride would be an example, which is the chemical name for salt. And where do you find lots of salt? In the ocean. Likewise, sulfides are sulfur compounds. They’re prevalent in areas of polluted air.
Second, if conditions are right you could experience galvanic corrosion. this is an electrical effect experienced when dissimilar metals are brought close together in a conducting liquid. For example, immerse brass and aluminum in seawater and electrons move from the aluminum to the brass. This can be a problem in boats where brass fittings are close to or even in contact with aluminum. (Fuel tanks are a prime example.)
Preventing Aluminum Corrosion
You can’t do much about aluminum oxidation, and unless appearance matters, it’s not a big problem. Aluminum corrosion could however be a serious issue. If there’s any possibility of it happening you have two options:
- Apply a protective coating.
- Minimize or mitigate the effect of galvanic corrosion.
You should also consider the grade or series of aluminum you’re using. Some of theses, notably 5052 and 3003, have better corrosion-resistance properties than others. More generally, 1xxx, 3xxx, 5xxx and 6xxx series alloys offer good corrosion resistance.
There are three options:
- Powder coat
If you’re concerned about galvanic corrosion look for paint or powder with high electrical resistance. Anodizing is a kind of surface oxidation that can produce some very attractive finishes. However, it’s not usually practical for larger fabrications.
If taking the coating approach, don’t forget that any damage needs immediate attention. Leave some aluminum exposed and you risk corrosion getting a toehold.
Preventing Galvanic Corrosion
Ideally, keep the aluminum dry. The galvanic effect can’t work without an electrically-conductive liquid between the two metals. If that’s not possible, try to use electrically insulating coatings.
Many boaters also use a sacrificial anode made of zinc. This corrodes faster than the aluminum, in effect, sacrificing itself. Sacrificial anodes do need replacing periodically, but then so will most coatings.
Building a Better Iron Man Suit
In the movie Tony Stark says he moved on from the original iron suit to one made of a gold-titanium alloy. As the American Chemical Society point out in this YouTube video, that doesn’t make a lot of sense because gold is really dense. They suggest a better option would be Nitinol, an alloy of nickel and titanium.
You can request Nitinol for your next fabrication project if you want, but it may work out to be a little expensive. We’d suggest aluminum might be a better choice, and if you’re worried about corrosion, well you know how to handle that.