As manufacturers of cast iron and steel, we think a lot about preventing rust. Our bollards and bike racks are outdoors year-round, and we often weather-proof them through powder-coating or plastic covering. However, iron and steel are used in many different applications. The protection appropriate to our outdoor furniture might not be appropriate for utensils, engines, or marine applications. Happily, there are many strategies available.
First, a short primer on rust.
Many metals corrode, but when people talk about “rust,” they’re usually referring to the corrosion of “ferrous,” or iron-containing, metals. Rust is the flaky red oxide that appears on ferrous metals in the presence of air and water. Known as iron oxide III, rust can turn a strong iron object into a lace of holes, as it flakes and falls away, exposing the next layer of metal. Many other metals corrode into oxides or other compounds, but unlike rust, many of these products of corrosion stay tightly bound to the metal underneath, sealing and protecting it. Rust sloughs off, exposing the layer beneath to the same corrosive forces. (More info on rust here.)
In iron and steel, we cannot depend on a “passive film” created by top-level oxidation. Instead, we must seal the material.
Ways to seal ferrous metals
Cutlery is now often made of stainless steel, a special alloy that is rust resistant due to added chromium, which forms a passive film before iron oxide III has a chance. In the before stainless steel was invented, cutlery and bladed weapons were kept up through regular use, keeping them dry, and polishing. It was common to wax or oil knives or swords being put into storage, to create a seal around the metal. Raw iron or steel is still in use: engines and mufflers are often not sealed, because the heat of a running car and the oil in the car both work to keep them dry and rust free. (This is why not driving a car long enough for it to warm up will sometimes show up as rust blooms on the muffler!)
Other ways to protect against rust include:
- Paint — Watch out for pinholes and inspect regularly if outside.
- Powder-Coating — Done with a colorful resin, this gets complete coverage using electrostatic charge and then baking.
- Hot-dip galvanization — Applying zinc in a hot-bath to iron and steel protects in two ways: it uses galvanic corrosion for protection of the iron and also creates a seal with a layer of zinc.
- Electroplating — Electrostatic charges help apply a protective metal to iron or steel.
- Oil-treating — as with a cast iron pan, applying heat to a light coating of oil can lead to the polymerization of that oil. For your cast iron, flaxseed oil wiped over a clean pan, then baked at 500 degrees upside down in the oven for an hour, will create an oil-treatment that is food safe, but it will break down in the presence of acids.
- Forced passivation — blacking and blueing are both ways that iron can be forced to create an iron oxide II layer, rather than an iron oxide III layer, and this black iron oxide is far less “flaky” and ready to fall away than the red oxides are.
Iron and steel are our most common metals, and protecting against rust is one of humanity’s ongoing challenges. Fortunately, even rusty metal can be recycled, so once a steel part has lost its functionality due to rust, it can be melted, adjusted, and used again.