Rust, the reddish-brown coating on metals, is formed by exposure to oxygen and moisture. Responsible for this phenomenon are several types of microscopic bacteria. The bacteria induce the decay of metal into rust.
Rust is corrosion of iron and its alloys. Understanding the microbiological aspects of rust formation and the role of bacteria in rusting gives a closer look into the invisible natural world. Many unseen factors are involved in familiar processes.
How Does Rust Form?
Rust primarily occurs through chemical reaction of oxidation. When iron metal is exposed to water (H₂O) and oxygen (O₂) over time, it undergoes an electrochemical process. The reaction can be summarized in two main phases: oxidation and reduction.
Oxidation Phase: Iron loses electrons and is converted into iron ions (Fe²⁺).
Reduction Phase: Oxygen from the surrounding environment gains electrons in the presence of moisture, forming hydroxide ions (OH⁻).
When these iron ions react with hydroxide ions, they ultimately form rust, specifically hydrated iron(III) oxide. This compound gives rust its characteristic reddish-brown color.
Industrious bacteria accelerate this process.
Bacteria Behind Rust Formation
Several types of bacteria have been identified as crucial players in the rusting process. Here are four major types:
Sulfate-Reducing Bacteria (SRB):
Examples: Desulphovibrio, Desulphotomaculum, Desulphomonas
Role: SRBs thrive in anaerobic environments, where they reduce sulfate ions to hydrogen sulfide (H₂S). Production of hydrogen sulfide can react with iron, forming iron sulfide, which further accelerates corrosion. Iron(II,III) sulfide is a blue-black chemical compound of iron and sulfur.
Iron-Reducing Bacteria (IRB):
Examples: Gallionella ferrugine and Ferrobacillus sp. esp. Ferrobacillus ferrooxidans
Role: IRBs obtain energy by reducing ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), ultimately leading to the dissolution of iron in water, promoting further rust formation.
Acid-Producing Bacteria:
Examples: Pseudomonas, Aerobacter, Bacillus
Role: These bacteria produce organic acids as metabolic byproducts, lowering the pH of their environment. The acidic conditions not only accelerate the corrosion of metal but can also promote the prevalence of more corrosive species.
Sulfur Oxidizing Bacteria:
Examples: Thiobacillus sp.
Role: These bacteria oxidize sulfur compounds, leading to further acidity in their environment and creating conditions that are ripe for rusting processes.
Other Microorganisms: Various other bacteria and microorganisms can contribute to rust formation, though their roles may not be fully understood. Biofilms, for instance, can harbor diverse microbial communities. These can collectively influence corrosion rates.
The Biological Process of Rust Formation
The interaction between bacteria and metal surfaces can occur in several stages, each contributing to the overall rusting process:
Biofilm Formation: Bacteria attach to metal surfaces and establish biofilms. This protective layer can retain moisture and create localized microenvironments favoring corrosion.
Metabolite Production: Bacterial metabolism leads to the production of various metabolites, including organic acids, hydrogen sulfide, and other corrosive compounds.
Electrochemical Reactions: As bacteria metabolize and interact with the iron, they alter the local pH and redox conditions, facilitating electrochemical reactions promoting rust development.
Nutrient Cycling: Bacterial processes contribute to a cycling of nutrients that may further exacerbate rusting. This includes the reduction of ferric to ferrous iron, providing a constant supply of soluble iron ions that can easily react to form rust.
Through a combination of chemical reactions, microbial metabolism, and environmental factors, these bacteria orchestrate the transformation of pristine metal into a weathered, corroded patina of rust. The colors of iron oxides are found in pigments such as ocher.
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