Corrosion is a naturally occurring process that converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion not only causes economic losses but can also have severe health and safety consequences like bridge collapses, boiler explosions, toxic leaks, etc. To protect assets and ensure safety, it is important to prevent or minimize corrosion using various methods. One such effective and widely used method is applying anti corrosion coatings.
Types
There are different types of Anti-Corrosion Coatings available that provide corrosion protection in various environments. Some of the major types are:
Paints and Epoxy Coatings: Paints are the most common and cost-effective anti corrosion coatings. They form a protective film over metal that acts as a barrier between the substrate and the environment. Epoxy paints offer superior protection due to their excellent adhesion and ability to form a thicker film. They are suitable for surfaces constantly or frequently exposed to humidity, water, chemical exposure, etc.
Galvanizing: This involves coating iron or steel with a layer of zinc by hot-dipping or electroplating. Zinc has an anodic index higher than iron. So it sacrificially corrodes to protect iron/steel from corrosion if the zinc coating is damaged. Galvanized coatings can last for decades in normal outdoor conditions.
Metallic/Ceramic Coatings: Metallic coatings like aluminum and rare earth provide outstanding barrier protection. They are applied using thermal or cold spray technology. Ceramic coatings like aluminide and chromide form an impervious oxide layer offering excellent high-temperature corrosion resistance up to 1600°C.
Polyurea and Polyurethane Coatings: These elastic coatings have high mechanical strength and can flex with metal movements. They protect pipelines, valves, offshore structures from corrosion under insulation. The coatings expand and contract with temperature fluctuations without cracking.
Required Properties
Other than the type, certain key properties determine the effectiveness of anti corrosion coatings:
Adhesion: Coatings must adhere strongly to the metal surface to avoid peel off and remain effective for a long time. Adhesion depends on surface preparation and coating formulation.
Barrier Protection: Coatings should form a uniform, dense, non-permeable, and defect-free film to act as an effective barrier against aggressive ions and molecules.
Mechanical Strength: Coatings must have high tensile strength, hardness, and flexibility to withstand stresses from metal movements, vibrations, thermal cycling without cracking or peeling off.
Chemical Resistance: Resistance to acids, alkalis, solvents and other corrosive chemicals present in the exposure environment. Certain specialized coatings offer resistance to multiple chemicals.
Dielectric Strength: Coatings used for buried pipes and offshore structures must be electrically non-conductive and have high dielectric strength to mitigate corrosion due to external electrical interference.
Aesthetics: Coatings applied externally also require aesthetically pleasing colors and gloss retention properties.
Surface Preparation for Effective Corrosion Protection
Proper surface preparation of the metal substrate is a critical step for achieving maximum coating performance and service life. It ensures strong adhesion of the coating to the substrate. Some key steps in surface preparation are:
Cleaning: Removing grease, oil, dust, and other contaminants using solvent/alkaline cleaning.
Blast Cleaning: Achieving the required surface profile by blast cleaning methods like abrasive blasting, water jetting, etc.
Pretreatment: Applying pretreatments like phosphating, chromating for improved adhesion.
Priming: Priming the prepared surface for better coating adhesion. Compatible epoxy/zinc primers are commonly used.
Many coatings are also specially formulated for application over dirty/rusted surfaces without extensive surface preparation using advanced resins. However, cleaning and roughening always improve coating performance.
Application and Testing of Anti Corrosion Coatings
Coatings are applied using various techniques depending on the type - brush/roller, air/airless spray, plural component spray, etc. Curing/drying accelerators may be required for fast curing.
Coating thickness and properties should be checked using instruments like DFT gauges, holiday detectors, pencil hardness testers, adhesion test kits, impact testers, etc. as per specification.
Coatings must undergo accelerated lab tests as per ISO, ASTM standards to verify corrosion protection ability under various conditions like - neutral salt spray, Prohesion, Cyclic corrosion, Cathodic disbondment, etc. before use in specific aggressive environments.
Periodic field inspections and coating lifetime monitoring also help ascertain actual corrosion protection performance over time in real exposure conditions. Overall, proper application and testing ensure coatings deliver as per requirements.
a variety of specially formulated and tested anti corrosion coatings provide effective long-term corrosion protection to metals when applied with suitable surface preparation. Their barrier protection, mechanical strength and resistance to specific environments contribute to enhanced asset integrity, safety, and reduced maintenance costs.
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Money Singh is a seasoned content writer with over four years of experience in the market research sector. Her expertise spans various industries, including food and beverages, biotechnology, chemical and materials, defense and aerospace, consumer goods, etc. (https://www.linkedin.com/in/money-singh-590844163)