Paint Anti-Corrosion

Paint anti-corrosion refers to the ability of paint to resist the electrochemical reaction with its surroundings. It protects the substrate from rust, UV damage, moisture, and free radicals. Corrosion is often linked with metal and steel substrates. It also damages the non-metallic materials like polymers, ceramics, and plaster. Anti-corrosion paint contains rust-resistant pigments, binders, and additives. Homeowners rely on oil-based paints to protect the metal and steel substrates from oxidation. All oil-based (alkyd, enamel) paints are not rust-resistant, as it depends on the type of pigment used in paint formulation. Anti-corrosion paints act as a protective barrier to resist moisture and air from trapping in the paint surface. They are breathable paints that allow the air to escape. Anti-rust resistant paints reduce the future expense of frequent repainting. It is used to paint decorative items, building exteriors, and industrial equipment.

Table of Contents

How does Corrosion Degradation Occur in Paints?

Here is how rust degradation occurs in paints: 

  1. Moisture and Oxygen Penetration: Moisture and air enter the paint film through cracks or loopholes. It does not escape the surface and starts building up under the paint surface. 
  2. Oxidation Reaction: The trapped components create paint bubbles over time and undergo freeze-thaw cycles. It deteriorates the chemical bonds and weakens the structural components of paint. Paint starts to peel off, and the external components react with metal to produce iron oxide (rust).  
  3. Accelerating Factors: Accelerating factors are present in paint formulation and the external environment. There are six types of corrosion that occur in metals, such as galvanic, SCC, crevice, intergranular, and pitting corrosion. Galvanic corrosion takes place when two different metals, like aluminum with an anode and copper with a cathode ion interact. They resist each other in the electrolyte conductive medium, and one gets corroded. The most common type of corrosion is stress corrosion cracking due to environmental factors (acid rain, air). Crevice corrosion occurs at tight spots like nuts and bolts. Intergranular and pitting corrosion damage the metal surface and form grains and deep holes. 

How Does Anti-Corrosion Paint Work?

Here is the step-by-step guide on how anti-corrosion paints work:

  1. Barrier Protection: Anti-corrosion paints contain epoxy resins, chlorinated rubber, alkyd resins, and silicate-based binders, which create a strong film after solvent vapourization. They hardened the binder bonding and do not allow air and dirt particles to penetrate. 
  2. Chemical Inhibition: Anti-corrosion paints contain anodic (zinc, aluminum salts), cathodic, organic amines, phosphates), and natural (tannins) inhibitors to disrupt the chemical reactions of rusting. These inhibitors stop the electrochemical reactions, free radicals, and moisture from causing damage to the metal surface.  
  3. Sacrificial Coating: Zinc coatings are used to protect the substrate from corrosion. These coatings with anode rust after oxidation and protect the cathode ion-containing surfaces (galvanic corrosion). Zinc acts as a barrier layer and sacrificial material against corrosion.
  4. Moisture Control: Anti-corrosion paints contain pigments zinc chromate, zinc oxide, red lead, lead chromate, zinc dust, and inhibitive pigments (calcium, strontium) to resist rust. They are composed of corrosion-resistant resins (epoxy, vinyl), which protect against highly humid environmental conditions.
  5. Surface Adhesion: Apply zinc-rich primers to improve the paint adhesion and rust protection. It corrodes itself to save the substrate. Poor paint adhesion leads to blisters, bubbles, and orange peels. 

What are the Types of Anti-Corrosion Paints

Here are the six types of anti-corrosion paints:

  1. Zinc-Rich Primers: Zinc-rich primers contain epoxy or inorganic silicate resins to bind with zinc dust particles. These primers are applied under a topcoat, which oxidizes to protect the metal and steel surfaces. Zinc-rich primers are cost-effective, even with high zinc content. They are used for their barrier effect, weather-resistant abilities, and durability. 
  2. Epoxy-Based Paints: Epoxy-based paints are made up of thermosetting polymers and epoxy resins, which cross-link to create a 3D polymer structure. This crosslinking structure acts as a barrier against environmental factors. Different types of amine hardeners are used to create a weather and chemical-resistant strong epoxy paint film. Epoxy polymers disrupt all the electrochemical reactions to prevent the substrate from corrosion. 
  3. Polyurethane Coatings: Polyurethane coatings are composed of polyisocyanates and polyols. PU coatings are rust-resistant due to their non-porous surface. They create a protective barrier and do not let the air and water penetrate through it. PU coatings are classified into 3 types on the basis of their nature and components. It includes solvent-based, water-based, two-component, single-component, aliphatic, and aromatic polyurethane coatings. 
  4. Acrylic Anti-Rust Paints: Acrylic anti-rust paints protect the metal surfaces from corrosion with quick evaporation. Acrylic-urethane coatings and Acrylic anti-corrosion coatings prevent the chemical reaction that produces iron oxide. They are unfit due to water as their primary solvent and slow curing time in highly humid regions.
  5. Bituminous Paints: Bituminous paint, also known as asphalt paint, is made up of sticky black material. This is derived from petroleum jelly and later mixed with naphtha or acetone to achieve a paint emulsion. Its formulations make it waterproof, fade-resistant, and anti-corrosive. Bituminous paint is used to paint concrete and metal substrates.
  6. Ceramic Coatings: Ceramic coatings are made up of silicon dioxide (SiO2), which is water-repellent. It also contains aluminum oxide (AL₂O₃), titanium dioxide (TiO₂), titanium nitride, tungsten carbide, and chemically bonded phosphate ceramics (CBPC). Ceramic coatings slide the moisture on the paint surface like a droplet moves over oil-based paints. They are highly anti-corrosive, water-resistant, and durable coatings. Plasma spray, chemical vapor deposition, sol-gel, ALD, and thermal spray methods are used to apply ceramic coatings.

How to Know If Paint Is Anti-Corrosive

Here are the things to check if a paint is anti-corrosive or not:

  1. Check the Label or Technical Datasheet: Check the Material Safety Data Sheet (MSDS) and technical datasheet (TDS) to read all the product details. TDS contains the product ingredients, composition, physical properties, characteristics, and application instructions. MSDS tells about the safety concerns and health hazards (high VOC paints) related to paint. Look at the label and these datasheets to check if the paint is anti-corrosive, zinc-rich, or rust protective. 
  2. Manufacturer’s Specifications: Read the manufacturer’s specifications to check its compatibility with the type of substrate. Ferrous metal contains high iron, which produces iron oxide (corrosion) after chemical reactions. 
  3. Ingredients & Additives: Paints with zinc, epoxy, alkyd resin, PU, or bitumen are rust-resistant. Anti-corrosive paints also contain inhibitors, driers, rheology modifiers, calcium, and ferrite pigments. 
  4. Application Zones: Look at the application instructions to check where it can be used. Paints with marine, industrial, or external building usages have anti-corrosive and fade-resistant properties. They contain the UV stabilizers, absorbers, and heat-resistant components, which protect them from frequent contractions. 
  5. Coating Systems: Read the application guide, as corrosion-resistant paints follow the sandwich method. It includes the primer, mid coat (paint), and top coats (varnish). Paint without primer falls off quickly and leaves the metal surfaces prone to oxidation.

What are the Installation and Application Considerations

Here are the installation and application considerations of corrosion-resistant paints:

  1. Surface Preparation: Remove the dirt and previous paint residues from the metallic or non-metallic surface before painting. Use wire brushes or sandpaper to activate the substrate and improve its adhesion. 
  2. Primer Use: Apply water-proof, rust-resistant, and silicon-based primers to seal the paint surface. Primer increases the flexibility, durability, and lifespan of the paint. Apply shellac-based,  water-based (Acrylic/Latex), multi-purpose, and mildew-resistant primers to achieve the desired results. 
  3. Application Method: Opt for the right technique and method to apply the paint. Use brushes, roller, or sprays depending on the type of substrate. Avoid hard-bristle brushes as they cause scratches on metal. 
  4. Cure Time and Environment: Every paint type and coating has different formulations, solvents, and cure times. Standard paints require 24 hours to dry touch and seven days to fully cure. Curing time depends on the substrate, humidity, and weather conditions. 
  5. Layering System: Single coat paints do not resist water, air, and oxidation processes due to their low opacity and refractive index. They are heat and UV-sensitive, which makes them prone to freeze-thaw cycles. Multi-coat of paint creates a high opacity surface which blocks the water penetration, hides stains, and resists corrosion. 

Where Is Anti-Corrosion Paint Commonly Used?

Anti-corrosion paint is used to paint the following surfaces:

  1. Bridges, Towers & Railings: Anti-corrosive paints are used to paint electric transmission towers, cell phone towers, stair railings, and guardrails on roads. Anti-corrosive paint is also applied on Observation platforms and walkways, which are prone to damage due to sunlight, free radicals, and acid rain.
  2. Ships, Docks & Marine Equipment:  Ship hulls, cargo containers, fishing boats, port structures, oil rigs, and marine ladders are painted with rust-resistant paint. It is also used on dock equipment, buoys, and cranes.
  3. Industrial Machinery & Tanks: Anti-corrosive paint is applied on chemical storage tanks, boilers, frames of machinery, conveyor systems, equipment, and power plant machinery at industrial units. 
  4. Pipelines & Underground Structures: Corrosion-resistant paint is used on gas pipelines, underground storage tanks, manholes, concrete foundations, cable trays, and support brackets. It protects the hard-to-reach areas from rust in the long run.
  5. Vehicles, Trailers & Chassis: Car bodies, undercarriages, truck chassis, trailers, agricultural vehicles, motorcycle and bicycle frames are painted with zinc-rich paints. It increases the value of vehicles, protects them from pits and stress corrosion. 
  6. Household Applications: Homeowners use anti-corrosive paint on iron gates, outdoor furniture, metal window frames, garden tools, and water tanks. 

What are the Methods to Test Paint Corrosion Resistance?

Here are the widely used methods to test the corrosion resistance of paints:

  1. ASTM D2803: In this standard film-form corrosion test, the level of rust is measured under the paint film. The metal surface is exposed to highly humid conditions (70-95%) to check if the corrosion increases. 
  2. ASTM D7893: This coil-coated metal test measures the ability of metal panels to resist rust after being scratched. 
  3. ASTM D1654: In this general coating test method, a salt spray chamber is used to sprinkle salt on a painted metal substrate. It is used to measure the ability of paint to resist moisture and rust after galvanic or crevice corrosion.

About The Author