The brown color that can appear on metal surfaces is often a result of oxidation or corrosion. When metal reacts with oxygen in the air, it forms a brown rust layer on the surface. This process is known as oxidation. For some metals like copper and bronze, the oxidation layer may impart an antique brown patina that is considered desirable. However, for structural metals like iron and steel, the formation of brown rust is detrimental and indicates corrosion that can weaken the metal. Understanding the factors that cause brown rust to form enables preventing and removing it when necessary.
Common Causes of Rust Formation
There are several common causes that allow rust to form on metal surfaces:
| Exposure to moisture | Water, humidity, rain, condensation |
| Exposure to air/oxygen | Unpainted metal reacts with oxygen |
| Salty environments | Seaside, ocean spray, road salt |
| Acids and chemicals | Industrial pollution, battery acid, cleaning agents |
Moisture is the primary factor that initiates rust formation. When bare metal is exposed to water or even just high humidity, the surface oxidizes to form hydrated iron oxide rust. Salts, acids, and chemicals accelerate the corrosion process. This is why metals corrode quickly near oceans or when exposed to road salt in winter.
How Rust Forms on Different Metals
While rust formation follows the same general process, the type of iron oxide created and the appearance of the rust layer can vary for different metals:
Iron and Steel:
Iron and steel are prone to rusting due to their combination of iron and carbon content. The rust formed is typically a reddish-brown color and consists of a mixture of iron oxides and hydroxides such as hematite, magnetite, goethite, and lepidocrocite. As the rust layer thickens, it takes on a flaky texture and dark brown color.
Stainless Steel:
Stainless steel contains chromium that makes it resistant to rusting. However, in salty or acidic environments, the chromium in the alloy can be depleted over time, allowing rust to form. The rust on stainless steel typically appears as red-brown patches rather than a uniform layer.
Cast Iron:
While cast iron has better corrosion resistance than carbon steel due to its graphite content, it can still develop surface rust. New cast iron may have a protective mill scale that flakes off as it rusts, revealing a pitting pattern. Old cast iron develops more uniform brown surface rust.
Copper:
Pure copper develops a green patina as it oxidizes, but copper alloys like bronze form brown rust layers. Bronze contains copper along with tin and sometimes other metals. As it oxidizes, the tin content separates out forming stannic oxide that appears brownish.
Aluminum:
Pure aluminum forms an impervious oxide layer that resists further corrosion. However, high-copper aluminum alloys develop a patchy brown and powdery surface rust as the copper component oxidizes.
Preventing Rust
To prevent the brown rust discoloration on metal surfaces, suitable protection is needed:
| Coatings and Plating | Paint, powders, zinc/nickel plating |
| Oil and Grease | Lubricants, waxes, corrosion inhibitors |
| Cathodic Protection | Sacrificial anodes, impressed current |
| Proper Design | Avoid galvanic corrosion, allow drainage |
The most common method is applying coatings like paint or powder to isolate the metal from exposure. For steel, zinc or nickel electroplating provides cathodic protection by oxidizing preferentially when damaged. Lubricating metals can displace moisture, while oil-based rust inhibitors chemically passivate the surface. Proper design considerations like avoiding contact between dissimilar metals can also prevent galvanic corrosion.
Removing Existing Rust
For metals with extensive rust damage, treatments are available to convert or remove the rust:
| Rust Converters | Chemically converts rust to stable complex |
| Rust Removers | Acids dissolve rust layer |
| Sandblasting | Physically abrasive removal |
| Grinding/Wire Brushing | Mechanical removal and smoothing |
Rust converters use chemicals like tannic acid to convert the rust into a stable metal complex that seals the surface. Rust removers contain acids like phosphoric or hydrochloric to dissolve the rust. Sandblasting uses abrasive particles to completely remove rust down to bare metal. Grinding or wire brushinggive more control for smoothing pits and defects.
Examples of Brown Rust on Various Metals
| Iron/Steel |  |
| Stainless Steel |  |
| Cast Iron |  |
| Copper Alloy |  |
| Aluminum Alloy |  |
The images illustrate the distinctive rust characteristics for different metals. Recognizing the specific form of corrosion enables identification of the metal alloy and causes.
Chemical Composition of Rust
While rust is generally considered just iron oxide, there are actually various chemical forms it can take:
| Name | Chemical Formula |
| Hematite | Fe2O3 |
| Magnetite | Fe3O4 |
| Goethite | FeO(OH) |
| Lepidocrocite | FeO(OH) |
| Ferric Oxyhydroxide | FeO(OH) |
| Ferrous Hydroxide | Fe(OH)2 |
Hematite, magnetite, goethite, lepidocrocite and ferric oxyhydroxide are all iron oxides compounds consisting of iron and oxygen. Ferrous hydroxide is an iron hydroxide. The exact species formed depends on environmental factors like moisture and oxygen levels during corrosion.
Removing Rust Through Reduction Reactions
Since rust formation involves the oxidation of metal, the process can be reversed through reduction reactions. Some common reducing agents used:
| Acid Solutions | Phosphoric, hydrochloric, sulfuric |
| Molten Salts | Alkali chlorides, nitrates, carbonates |
| Hydrogen Gas | H2 atmosphere at elevated temperature |
| Electrochemical | Cathodic process on rusted metal |
Acid solutions like phosphoric acid directly reduce iron oxide rust back to iron metal. Molten salts can dissolve and reduce rust through chemical or electrolytic means. Hydrogen gas heat treatment reduces surface rust through a high temperature reaction with hydrogen. Electrochemical reduction uses a negative voltage applied to the rusted metal to convert rust back to metallic iron.
Effects of Rust on Strength of Metal
The corrosion and material loss caused by rust can significantly weaken the strength of the affected metal:
| Metal | Loss of Strength |
| Carbon Steel | Up to 50% when heavily rusted |
| Cast Iron | Up to 75% in worst rusting |
| Stainless Steel | Up to 25% strength reduction |
| Copper Alloys | Negligible effect on strength |
Carbon steels and cast irons have the greatest loss of cross sectional area from surface rusting and pitting. Stainless steel is more resistant but still has substantial strength reduction if the chromium content is depleted. Copper alloys maintain most of their strength as the rust layer is very thin.
Threshold Rust Levels for Structural Failure
For load bearing iron and steel structures, the following rust levels canproduce structural failure:
| Application | Failure Rust Level |
| Pipe and Tubing | 20-40% rust |
| Beams | 25-50% rust |
| Reinforced Concrete | 15-25% rebar rust |
| Wire Ropes | 30-50% loss of area |
| Fasteners | 10-20% rust |
Pipes, beams and reinforced structures can endure some rust level but undergo brittle fracture when corrosion advances. Wire ropes and fasteners are more sensitive to the material losses from rusting. These thresholds ensure intervention occurs before failure.
Economic Impact of Rust
The economic costs associated with metallic rust are estimated at billions of dollars annually:
| Industry | Annual Cost (Billions) |
| Infrastructure | $20-$25 |
| Transportation | $15-$20 |
| Processing Plants | $10-$15 |
| Utilities | $5-$10 |
| Production Loss | $15-$20 |
| Total | $75-$100 |
Corrosion of bridges, buildings, vehicles, equipment, and services disrupts operations and requires constant maintenance and replacement. The impacts of rust are pervasive across industrial sectors, contributing to production losses and downtime. Overall it represents a major economic problem worldwide.
Environmental Factors in Rust Formation
Several environmental conditions accelerate the oxidation and rusting of metals:
– Moisture – Higher humidity, condensation, and direct water exposure encourage rust formation.
– Salt – Saline environments from seawater or deicing salts damage protective coatings and promote corrosion.
– Pollutants – Industrial fumes and acidic particulates attack surface oxide films and boost rusting.
– Temperature – Warm conditions increase corrosion rates from moisture and accelerated chemical reactions.
– Abrasion – Mechanical wear removes protective coatings, exposing the bare metal.
– UV Light – Solar radiation can degrade organic coatings and protective oils and greases.
Control of these parameters reduces the environmental damage driving corrosion. Proper design is also key, such as allowing drainage, providing shelters and using sacrificial anodes.
Conclusion
The formation of brown rust on metals is a complex oxidative process that degrades structural integrity. Controlling humidity, salts, pollutants and temperature mitigates corrosion, along with protective coatings, cathodic protection and design improvements. Swift action must occur once rusting is identified to prevent costly damage and failures from advancing rust growth. With diligent prevention and maintenance, the costly menace of metallic rust can be contained.