A common misconception involves “anodized steel” and its resistance to rust. Steel, an iron alloy, is generally not anodized in the same way other metals are. This article clarifies what anodizing entails for the metals it is typically applied to, explains why steel is not anodized, and details effective methods used to protect steel from rust.
What Anodizing Involves
Anodizing is an electrochemical process that increases the thickness of a metal’s natural oxide layer, enhancing its surface properties. This process is primarily used on non-ferrous metals like aluminum, titanium, and magnesium. During anodizing, the metal part acts as the anode in an electrolytic cell, submerged in an acid electrolyte bath. An electric current is then applied, causing oxygen ions to react with the metal surface and form a controlled, thicker oxide layer.
For aluminum, this process creates a hard, durable, and corrosion-resistant aluminum oxide layer that is integrated with the base metal, preventing chipping or peeling. This anodized layer can be porous, allowing it to absorb dyes for various colors, or sealed for enhanced corrosion resistance. Titanium anodizing also forms a protective oxide film, improving wear properties and allowing for vibrant color changes based on the oxide layer’s thickness. Similarly, magnesium can be anodized to form a stable, thick oxide layer, significantly improving its corrosion resistance, physical strength, and paint adhesion.
Why Steel Is Not Anodized
Steel, an alloy primarily composed of iron, cannot be anodized effectively using the traditional process applied to metals like aluminum. The fundamental difference lies in their electrochemical properties and how they react to oxidation. When steel is subjected to an anodizing-like process in acidic solutions, it tends to form iron oxides, commonly known as rust. This rust layer is porous, brittle, and flaky; it does not adhere well to the surface and promotes further corrosion. Therefore, traditional anodizing would accelerate steel degradation instead of providing a stable, protective coating. While specialized laboratory methods can induce a magnetite layer on steel using specific alkaline mediums, these are not economically viable or practical for widespread industrial corrosion protection.
How Steel Is Protected From Rust
Since anodizing is not a suitable method for protecting steel, various other techniques are employed to prevent rust and enhance its durability:
- Galvanizing: This involves applying a protective zinc coating. The zinc layer acts as both a physical barrier and a sacrificial anode, corroding preferentially to the steel even if scratched.
- Painting or Powder Coating: This creates a physical barrier between the steel and corrosive elements. Powder coating applies a dry powder material that is melted and cured to form a smooth, durable film resisting chipping, scratching, and UV damage.
- Bluing: A chemical process that forms a thin, controlled black oxide layer (magnetite) on the steel surface, offering limited corrosion resistance and a distinctive appearance.
- Electroplating: Methods like chrome plating deposit a thin layer of another metal onto the steel for improved corrosion resistance and aesthetics.
- Stainless Steel: This alloy is inherently resistant to rust due to its chromium content (typically 10.5% or more), which forms a thin, self-healing passive film of chromium oxide protecting the underlying iron.
Identifying Rust On Steel
If a metal object begins to show signs of deterioration, it is important to accurately identify the cause. Rust on steel typically appears as a reddish-brown, flaky, or powdery substance. This distinct coloration and texture are indicative of iron oxide formation. If an object you believed to be “anodized steel” is exhibiting these characteristics, it confirms that it is either unprotected steel or a steel object with a coating that has failed.
Genuine anodized metals like aluminum, titanium, or magnesium do not rust; instead, they may show signs of corrosion specific to their oxide layers if damaged, which often appears as a white or gray powdery substance in the case of aluminum. When rust is observed on a steel item, it means the metal’s protective measures, if any, have been compromised or were never present. Steps can then be taken to address the rust, such as cleaning the affected area and applying new protective coatings to prevent further damage.