It’s not just a question of appearances. If untreated rust can completely decimate an entire structure. For example rust was a major factor in the Silver Bridge disaster of 1967 during which the steel suspension bridge collapsed in less than one minute.
steel products are widely used in many industries. In these circumstances, rust prevention should be a priority.
What is Rust?
Rust is a form of iron oxide. It occurs when iron combines with the oxygen in the air causing it to corrode. Rust can affect iron and its alloys, including steel. The main catalyst for rust to occur is water. Although iron and steel structures seem solid to the eye, water molecules are able to penetrate microscopic gaps in the metal. This starts the process of corrosion. If salt is present, for example in seawater, the corrosion will be more rapid. Exposure to sulfur dioxide and carbon dioxide will also hasten the corrosive process.
Rust causes the metal to expand, which can place great stress on the structure as a whole. At the same time, the metal will be weakened and become brittle and flaky. Rust is permeable to air and water, so the metal beneath the rust layer will continue to corrode.
How to Prevent Rust?
Exposure to outdoor conditions will increase the risk of rust, especially if the climate is rainy or humid. The best ways to prevent rust include:
- Using rust resistant alloys
- Design considerations
- Organic Coating
- Powder Coating
- Regular maintenance
Using Rust Resistant Alloys
The most popular rust resistant alloys are stainless steel and weathering steel.
Stainless steel contains a minimum of 11% chromium. This allows the formation of a protective film of chromium oxide which acts as a shield against rust. The protective film will re-form if damaged. Corrosion resistance can be further enhanced with the addition of nickel.
Weathering steel, also known as “COR-TEN” steel, contains up to 21% of alloying elements such as chromium, copper, nickel and phosphorous. The alloys form a protective rust patina which reduces the corrosion rate with time. COR TEN steel tends to be cheaper than stainless steel.
Proper planning during the design stage can minimize water penetration and reduce the risk of rust. Cavities and crevices should be avoided. Joints should be welded not bolted. Drainage holes for water should be considered if appropriate. The design should allow air to freely circulate. For large structures, adequate access should be enabled to allow for regular maintenance.
The process of galvanization involves coating the surface with an external layer of metallic zinc. This is accomplished through hot-dip galvanizing or electroplating. The zinc layer prevents corrosive substances from penetrating further into the metal. In addition the zinc acts as a sacrificial anode which means that the damaging oxidation process of rust will be transferred to the zinc layer.
Bluing is a useful technique which offers limited protection against rust for small steel items. The term “bluing” comes from the blue-black appearance of the finish when using this technique. Blueing is often used in firearms manufacture to provide a degree of corrosion resistance. It’s also used in fine clocks and other metal work.
Bluing is accomplished by immersing the steel parts into a solution of potassium nitrate, sodium hydroxide and water.
Organic coatings such as paint are a cost effective way to protect against rust. Organic coatings form a barrier against corrosive elements. Oil based coatings are ideal for preventing penetration of water and oxygen. Typical organic coatings are 15 to 25 micrometers thick.
A dry powder is evenly applied to a clean surface. Next, the object is heated, turning the powder into a thin film. There are a wide range of powders available including acrylic, polyester, nylon, vinyl, epoxy and urethane.
Powders are commonly applied using an electrostatic spray process. The electrically conductive object is sprayed with a charged, non-conducting powder. The charged particles are attracted the object and cling to its surface. The object is placed in a hot oven which fuses the particles into a continuous film. The film is typically between 25 and 125 micrometers.
Regular maintenance is advised to stop rust forming and halt the progress of any rust that occurred. It’s essential to remove any rust that has formed. A razor blade can be used for small areas. Next any surface grime should be removed using warm water and soap. Lastly a rust resistant coating should be applied to the surface.
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