What Is Rust And How To Remove It

This article discusses the differences between floating rust and rust formation on steel pipes. If the pipe does get rusty, what are the common methods and techniques for removing the rust? What are the specific principles, how to apply and choose them, and what are the precautions?

The floating rust and rust on steel pipes represent two different degrees of corrosion states. The differences, treatment methods and selection criteria are as follows：

• Floating rust: This usually refers to an even, thin layer of oxide (Fe₂O₃ or Fe₃O₄) that forms on the surface of steel pipes during storage or transportation due to exposure to humid air. It adheres to the surface without penetrating deeply into the base material. It has a loose texture and can be easily scraped off using a hard object or sandpaper, revealing the intact metallic luster. Floating rust does not affect the structural strength of the material.

• Rusting: This refers to the situation where rust has penetrated deep into the metal, creating localized or extensive corrosion pits. The rust layer is thick and hard, presenting a reddish-brown color (Fe₂O₃·xH₂O), and is tightly bonded to the base material. After removal, the metal surface will leave uneven pits, significantly affecting the effective cross-section and load-bearing capacity of the material, and is a defect that must be addressed.

• Degree of Rusting: Light or floating rust can be handled using manual or power tools; moderate to severe rusting with oxide scales requires shot blasting or acid washing.

• Construction Conditions and Cost: On-site repairs often use mechanical methods; factory prefabrication and large-scale processing can use shot blasting or acid washing, which are highly efficient and ensure uniform quality.

• Subsequent Process Requirements: If painting is required, shot blasting can simultaneously achieve cleanliness levels (such as Sa2.5) and roughness, making it the best choice.

• Environmental Protection and Safety: Chemical acid washing requires handling of waste liquid, with high environmental requirements; on-site sandblasting requires dust protection measures.

• Safety first: During operation, be sure to wear protective glasses, masks (either dust-proof or anti-toxic), and gloves. For sandblasting and flame rust removal, strict fire isolation measures must be taken.

• Thorough cleaning: After rust removal, all dust, abrasives and residues on the surface must be removed using dry compressed air or a brush to achieve a "white" appearance.

• Timely protection: The freshly processed metal surface has high activity and is prone to rusting again within a few hours. It is necessary to apply primer or take other anti-rust measures within 4 to 8 hours.

• Inspection standard: Refer to the national standard "GB/T 8923.1-2011 Surface Treatment of Steel Before Coating". Select the appropriate treatment method and cleanliness level based on the degree of rust.

For surface rust, simple power tools can be used for grinding. For rusted steel pipes, to ensure long-term corrosion prevention, it is recommended to use shot (or blast) blasting for rust removal to the Sa2.5 standard and then apply coating as soon as possible.

1.Shot blasting for rust removal

• This is the most mainstream and efficient pre-treatment method in factories. Its principle is to use a high-speed rotating wheel to forcefully project a large number of small steel balls onto the surface of the steel under the effect of centrifugal force, like a "metal storm", instantly removing the rust layer and oxide scale. It has a fast processing speed, uniform quality, and can simultaneously form a uniform roughness on the surface of the steel, making it particularly suitable for large-scale steel sections and plates in the assembly line operation before coating.

2. Shot blasting rust removal

• The principle and effect are very similar to those of shot blasting. The main difference lies in the type of projectile medium used, which is steel wire cuttings (short cylindrical shapes). Steel wire cuttings have sharp edges and a stronger impact force, making them more effective in removing stubborn thick oxide layers and sand from the casting surface. Like shot blasting, the medium can be recycled and is usually carried out in a closed indoor environment.

3. Sandblasting for Rust Removal

• This process utilizes compressed air as the power source to create a high-speed jet, which then sprays abrasive materials (such as quartz sand, copper slag, etc.) onto the surface of the workpiece for cleaning. Its greatest advantage is its extreme flexibility, as it is not limited by the shape or size of the workpiece and can precisely handle any part. It is the preferred method for on-site construction, local repairs, and large-scale processing. However, its drawbacks include large amounts of dust and high noise levels, and strict environmental protection and labor protection measures are required.

4. Acid pickling for rust removal

• This is a chemical method where steel is immersed in an acid solution (such as hydrochloric acid or sulfuric acid), and through the chemical reaction between the acid and the rust, the rust layer and oxide scale are dissolved. Its advantage is that it can handle extremely complex-shaped workpieces (such as threads, inner pipe walls) without any blind spots, and the workpiece will not deform. However, this process generates waste acid solution, and there are high requirements for environmental protection treatment. Moreover, the surface after treatment is smooth, and additional phosphating or passivation treatment is required to increase the adhesion of the paint.

For batch processing in the factory, shot blasting is used; for processing castings or thick oxide layers, blast cleaning can be employed; on-site repairs and special components cannot do without sandblasting; for handling complex and precision parts, pickling is considered.