What paint is applied on steel and NOT enclosed in concrete?
🔬 Understanding the Challenge: Protecting Steel
Steel, an alloy of iron and carbon, is incredibly strong but has a major weakness: it rusts. Rust (iron oxide) is a chemical reaction between iron, oxygen, and water. When steel is exposed to the atmosphere ("NOT enclosed in concrete"), it needs a protective coating to prevent this corrosive process.
Anticorrosive Paint: A specialized paint designed to protect metal surfaces from corrosion. Its primary job is not decorative but protective, acting as a barrier and often containing chemical inhibitors to stop the rusting process.
⚖️ Detailed Analysis of the Options
The question is asking for the most common and effective traditional primer for exposed steelwork.
(d) Red Lead (Pb₃O₄)
Function: Red lead is the classic and most well-known anticorrosive primer for iron and steel. It works by creating a passive (non-reactive) layer on the steel surface, which chemically inhibits the formation of rust. It is mixed with a vehicle like linseed oil to create a tough, durable, and water-resistant primer coat.
Application: It is almost exclusively used as a first coat (primer) on structural steel, bridges, and other ironwork that will be exposed to the elements.
(a) Yellow lead, (b) Green lead, (c) Black lead
These terms are generally distractors in this context.
- Yellow Lead (Lead Chromate): A pigment known for its bright yellow color (chrome yellow), but it is not primarily used as an anticorrosive primer in the same way as red lead.
- Green Lead (Chrome Green): This is typically a mixture of chrome yellow and Prussian blue pigments. It is used for its color, not for its rust-inhibiting properties.
- Black Lead (Graphite): Graphite is a form of carbon. It can be used in some protective paints due to its water resistance and lubricity, but it is not the standard rust-inhibiting primer for steel. Red lead is far more common and effective for this purpose.
🎨 Other Types of Protective Paints
While red lead is a classic choice, other paints are used for specific protective purposes:
- Bituminous Paint: Made from asphalt or tar, this paint is black and provides excellent water resistance. It is ideal for painting ironwork that is underwater, underground, or in contact with moisture.
- Asbestos Paint: A fire-resistant paint used for surfaces exposed to high heat, acidic gases, or steam. (Note: Due to health risks, asbestos use is now heavily restricted).
- Plastic Paint: Modern emulsion paints that offer a durable, washable, and decorative finish. They are widely used for interior and exterior walls but are not the primary choice for priming structural steel.
💡 Study Tips for Protective Paints
- Red for Rust: The easiest way to remember is to associate the color "Red" with "Rust." Red lead is the primary defense against rust on steel.
- Primer vs. Topcoat: Recognize that red lead is a primer. Its job is protection, not decoration. It will always be covered by subsequent undercoats and finishing coats.
- Know the Application: Associate the paint type with its specific use. Red Lead = Exposed Steel. Bituminous = Underwater/Underground Steel.
- Distinguish from Pigments: While red lead is a pigment, its primary *function* in this context is anticorrosion. Don't confuse it with pigments used purely for color, like chrome yellow or green.