Will metal buildings rust? They would…if the metal components were left uncoated. Today’s metal building components typically don’t leave manufacturing without at least one type of protective coating to make the surface of each piece rust resistant.
Don’t take your cues about steel buildings from those old corrugated metal buildings you see rotting in fields along the highway. Most of those have been there for decades and have little relationship to modern metal structures, whether they are simple garden sheds or a high-rise.
Here is what you need to know about rust and how metal buildings are made to resist it.
Rust is a four-letter word
You have seen rust; you know what it looks like. But do you know what causes rust and the impact it has on metal where it forms?
Bare steel is formed from iron, which is easily oxidized by moisture and air. All that means is that atoms of iron bond with atoms of oxygen to form a molecule that makes up that brown stain you see on metal. The iron gives it that signature color.
If you look closely you will see the rusted metal has a rather porous look. Get enough rust particles together and you will get metal with weak spots that can eventually crumble. You may have seen rusted out cars where the metal has a lacy edge; this is due to the breakdown of the metal bonds.
If you leave metal out in the elements or simply somewhere damp, eventually it will rust away to brown dust particles. Fortunately, we know how to keep that from happening.
The simplest way to prevent rust is to keep moisture away from the metal. Gardeners do it by cleaning all the dirt off their tools and coating the metal parts in oil. You can keep cast iron skillets from rusting the same way.
This doesn’t work so well for anything larger than a shovel, though. So another way has been found to protect steel from oxidation.
Galvanizing is a method of protection that has been around for nearly 200 years. (It was invented by Stanislas Sorel, a French engineer, if you are interested.) Steel galvanization is a process that creates a coating of zinc over metals that easily oxidize or corrode.
Clean, rust-free steel is bathed in molten zinc which hardens over the steel and created a moisture-resistant coating. Instead of iron particles being exposed to moisture and oxygen, the zinc molecules sacrifice themselves to the oxidation process. In other words, the zinc is corroded in preference to the underlying steel.
The only way the steel is exposed is if the zinc coating becomes scratched.
Galvanization is done by either a “hot-dip” process, where the metal is bathed in a molten zinc solution or by electro-galvanization, in which zinc particles are given a positive charge to make them tightly adhere to the surface of the steel member. You can recognize a galvanized surface by the crystallized appearance of the metal surface, called spangle. Electro-galvanizing can create a thinner, tighter bonding coat than the hot-dip process.
Paint is an excellent secondary coating barrier to moisture that has the added benefit of beautifying your metal building. Steel can be very attractive on its own, but you may need or want other options.
Architectural metal panels often come in a variety of colors directly from the manufacturer. A colored finish is sprayed and baked onto an already galvanized metal panel or another member. Generally, this is an organic paint such as polyester, acrylic, or fluorocarbon.
Acrylic and polyester paint are highly durable and common in residential and commercial use. For some uses, the polyester is supplemented with silicone to give even better corrosion resistance while maintaining a glossy appearance.
Fluorocarbon based paint provides resistance to heat, UV radiation, and chalking. The finish is very smooth and dense with a high resistance to staining. The color typically lasts longer than polyester or silicone-poly based paint.
More recently, two-ply coatings have become available; one is named PVC Plastisol. The primary coat is corrosion resistant primer and the top coat is a PVC resin dispersed in plasticizer. If your building will be used to store harsh chemicals (acids, bases, and inorganic compounds), this is the coating for you.
A reputable steel building manufacturer will provide a warranty that includes protection against corrosion or rust. Some of these warranties are for 40 to 50 years depending on the quality of the metal and finish. This tells you how confident the metal building manufacturers and vendors are that their products remain rust-free for decades given the proper, yet minimal, maintenance.
Some warranties come from the steel mill; Galvalume is a coating provided at the mill and comes with a warranty all its own. Simple galvanized steel is not included in the warranty, it is only for the trademarked Galvalume product.
Other types of warranties
Warranties can call out a variety of conditions that are covered. However, that doesn’t mean every condition is covered. You need to read your warranty.
For example, there is a warranty called a “red rust warranty” that protects against the accumulation of rust on a panel’s flat area. It does not cover cut edges, which are more likely to corrode because the coating has been interrupted.
Another term you may find is “film integrity.” This refers to the ability of the coating to remain adhered to the member surface. The warranty will cover areas where the coating has peeled away or otherwise separated from the steel but does not cover fading or chalking. These conditions are not causes of nor caused by film integrity failures.
Prevent rust with some TLC
Building with steel is the best investment you will ever make. Steel is durable, safe, and is environmentally friendly. But it will only remain so if its surface is properly protected with the appropriate coating. That coating, as well as the metal itself, requires some maintenance. Any scratches or gouges that penetrate the coating will leave the metal vulnerable to rust and other corrosion.
If cared for as required by the warranty, you will not only have a structure to last for many years, a warranty will protect you from financial harm in the rare instance of rust.