Rusting, also known as corrosion, under a metal washer is primarily caused by a combination of moisture and oxygen interacting with the metal surface. This type of corrosion is known as atmospheric corrosion or oxidation, and it specifically affects metals that contain iron, such as steel.
Here's a breakdown of the process:
Moisture: Water is the main catalyst for rusting. When a metal washer comes into contact with moisture, either from the environment or during use, it creates an electrolytic environment that facilitates the rusting process.
Oxygen: Oxygen from the air also plays a crucial role. It reacts with the metal in the presence of moisture to form iron oxide, which is the reddish-brown scale we commonly recognize as rust.
Electrochemical reaction: When moisture is trapped between the washer and the surface it's in contact with (such as metal or wood), it forms a small electrolytic cell. This cell allows the flow of electrons between different regions of the metal surface, leading to the oxidation of iron atoms into iron ions (Fe2+ or Fe3+). These iron ions then combine with oxygen and water to form rust (iron oxide).
Dissolved impurities: Sometimes, dissolved impurities in water can accelerate the corrosion process by facilitating the flow of electrons in the electrolytic cell.
Preventing rusting under a metal washer can be achieved by using materials that are less susceptible to corrosion, such as stainless steel washers or galvanized washers. Stainless steel contains chromium, which forms a protective layer on the surface, preventing direct contact between the iron in the steel and the surrounding environment. Galvanized washers are coated with a layer of zinc, which acts as a sacrificial anode, corroding before the underlying metal does.
Moreover, keeping metal surfaces clean, dry, and protected from prolonged exposure to moisture can help delay or prevent the rusting process. Regular maintenance and applying protective coatings, such as paint or sealants, can also be effective in minimizing rust formation.