Pure chromium metal does not rust, but chrome plating often develops the reddish-brown corrosion known as rust. This occurs because the rust is not a failure of the chromium layer, but a sign that the underlying base metal, usually iron or steel, has been exposed to moisture and oxygen. Chrome plating is a surface finishing technique that applies a thin layer of chromium to a metal substrate, primarily for decorative appeal and corrosion resistance.
The Difference Between Chromium and Rust
Rust is the common term for iron oxide, which forms when iron reacts with oxygen and water. This oxidation process results in a flaky, porous substance that does not adhere strongly to the underlying metal. It continuously flakes away, exposing fresh metal to further oxidation.
Chromium, however, is highly resistant to corrosion due to passivation. When chromium metal is exposed to air, it instantly reacts with oxygen to form a microscopically thin layer of chromium oxide. This oxide layer is stable, non-porous, and adheres tightly to the surface, acting as an impermeable shield that prevents further oxygen from reaching the chromium atoms underneath.
How Rust Forms on Chrome Plated Objects
The rust that appears on chrome-plated items is the base metal corroding beneath the coating. Commercial decorative chrome plating is a multi-layer system applied over a substrate, typically steel. This system consists of a thick base layer of nickel, followed by an extremely thin top layer of chromium (often less than 0.5 micrometers thick). The nickel layer provides corrosion resistance, while the chromium layer offers the bright finish.
Compromising the Protective System
The integrity of this protective system is compromised in three primary ways. Mechanical damage, such as scratches or chips, can physically break through both the chromium and nickel layers, exposing the bare steel beneath. The chromium layer is also naturally porous or micro-cracked. Manufacturing defects can create microscopic pathways that allow moisture, road salt, and oxygen to seep through to the substrate metal below.
Galvanic Corrosion
Once the iron base metal is exposed, the corrosive process is accelerated by galvanic corrosion. The base metal (iron) and the plating metals (nickel and chromium) form an electrochemical cell when an electrolyte, such as salt water, is present. Since iron is more chemically active than chromium, it corrodes preferentially. This causes rust to form rapidly beneath the plating, and the internal pressure from the growing iron oxide causes the surrounding chrome layer to blister and flake away.
Stopping and Treating Rust Damage
Preventing rust involves maintaining the integrity of the chrome plating and reducing exposure to corrosive agents. Regular cleaning with mild soap and water removes contaminants like road salt and grime that accelerate corrosion. Applying quality car wax or a specialized chrome sealant helps fill micro-pores and repel moisture, effectively sealing the protective layers. This maintenance prevents the initial penetration of moisture.
For minor spots of surface rust, non-abrasive methods can remove the iron oxide without further damaging the plating. A common technique involves rubbing the rust spot with crumpled aluminum foil dipped in water. The softer aluminum acts as a gentle abrasive, and a mild chemical reaction helps lift the iron oxide without scratching the chromium finish. Alternatively, mild household acids like vinegar or lemon juice can dissolve the rust before gently scrubbing and thoroughly rinsing the area.
If the rust has penetrated deeply, causing the plating to bubble or flake extensively, the damage is too severe for simple surface treatment. Once the base metal is compromised, the only permanent solution is to have the component professionally stripped and re-plated.