As a material that satisfies tough, industrial requirements and offers appealing aesthetics, metal is used for products everywhere. Metals boast resilience and strength in heavy-duty applications and hardscape supplies. In infrastructure, metal can be styled in versatile ways to blend in with both the traditional and contemporary.
When high-quality new metals are first milled or cast, the metal usually has one uniform surface color and no signs of etching or pitting. Noble metals like gold or platinum maintain this uniform, unblemished surface in most environments. However, non-noble metals go through a process of oxidation when exposed to moist air. This oxidization can be corrosive, and slowly eat into the metal, or it can create a surface-level patina, which changes the look of the metal, but protects the integrity of its structure.
Metals and oxidation
Oxidation occurs over time as raw metal is exposed to air and moisture in the atmosphere. In the formation of a permanent patina, the metal’s surface forms a scale, a combination of hard mineral coatings and corrosion deposits. Patinas can produce many different hues, shades, and colors because the chemical composition of each patina is unique to the alloy and its exposure to the elements. Rust refers to the oxides formed on iron and steel: the brown, black, and distinctive flaky red elements of rusted iron and steel are different types of ferrous oxide.
A patina can add character to a monument or architectural piece and give it historical presence. Take the Statue of Liberty, an iconic symbol of freedom and democracy—its rich, blue-green hue is brought about through oxidation. The outer layer of the statue is composed of copper. When it was originally assembled, it was uniformly brown. Over time, oxidation and weathering resulted in a thin layer of copper carbonate on the surface, sometimes called verdigris, producing the unique color achieved today.
Some alloys fair better than others when it comes to oxidation. In steel, rust is usually invasive and corrosive, eating and etching the metal. Steps to prevent corrosion should be performed throughout its service life. However, for alloys such as cast iron, copper, aluminum, chromium, and zinc, oxides can form a coating that protects the metal from further attack. This type of oxidization is not dangerous and its ability to change the metal’s color is often sought after for its rustic charm.
Not all rust is invasive
Finding traces of rust on the surface of metal does not have to mean that its structural integrity is being compromised. Both the alloy and the way the metal has been prepared will determine if oxidization is invasive or not. In alloys such as cast iron, aluminum, and copper, oxidization can create strong and protective compounds that prevent the metal underneath from pitting.
Stages of rust
During the first stage of rust, red, black, or white deposits become noticeable on the surface of the metal. This typically appears in small patches as the overall color of the metal’s surface becomes spottier and less uniform. In cast iron, this is the first step toward the formation of a permanent patina. Stainless steel depends on a chromium oxide as a protective film, not an iron oxide, so this rust is the failure of that protective film. Still, in stainless steel, Stage 1 represents a superficial problem that can be easily repaired if desired.
For steels and some types of iron, Stage 2 represents the first real degradation to the metal, as scale begins to form with the molecules that make up the first layers of the metal. In many ferrous metals, this scale is uneven, and begins the process of pitting or etching that starts to weaken the material. In cast iron, the scale formed is even and does not lead to etching or pitting.
Several oxides are being produced in this stage. The bright orange/red iron oxide (III) that we associate with rust can be brushed off. It will leave a powdery red pigment on hands or rainwater. The lack of adherence of this oxide is because the red iron oxide molecules are larger than the cast iron molecules that created them.
However, in well-prepared cast iron, oxides that do not flake or run off also form evenly. These are darker, usually brown or coppery colored. This layer of rust builds evenly so that the integrity of the base metal is unaffected.
At Stage 3, all surface molecules forming red iron oxides have been engaged and are in a powdered or granular state. Scaling continues. As the rusting powder of red iron becomes loose and fall off, the remaining darker iron oxides stay behind in a tough sealing layer. The metal’s color will darken to a bronze or dark brown as the bright orange and red brushes away. Over time, this patina will spread across the entire surface. Once the patina is achieved, it is permanent and protective, and requires no maintenance.
The amount of time it takes for patination to occur depends on the alloy type, wear and tear, and surrounding moisture conditions. For alloys such as cast iron, patination happens rather quickly. Metal products near the ocean will also experience fast patination as they are exposed to high moisture levels, and salt in the air, compared to those in drier environments.
Patinas on detectable warning plates
There are certain hardscape products where rusting patina is welcomed, as the effects on color are vital to their performance. This is the case for detectable warning plates, a form of tactile paving, where color contrast is essential.
Detectable warning plates deliver cues about urban navigation to visually impaired pedestrians by signaling changing levels or the transition from sidewalk to street. The truncated domes on the plates are felt underfoot, and the patina on the plates offer clear color contrast against the surrounding pavement.
Although color contrast could be achieved with paint or powder coating, there is no easier alternative than leaving the plates raw so that a natural patina can develop. This means the plates will not require maintenance or touchups for their entire service life, and the patina will also offer protection against harmful pitting.
Working with rust
The first step in appreciating rust is to realize that it is not always a threat, nor does it have to be unsightly. For many alloys, rust or patination is a practical solution to eliminate long-term maintenance. The alloy is protected by a thin, oxide layer, and experimenting with patinas allows for various shades and hues to come to life. Throughout history, people have played upon metal’s ability to patinate, rather than work against it, capitalizing on nature’s ability to protect and enhance.