What to do if workpieces rust easily after shot blasting?
In mechanical manufacturing, shot blasting is an essential step to prepare workpieces. It removes oxide scale and burrs, and introduces surface compressive stress that improves fatigue life. However, many factories find that workpieces quickly develop rust after shot blasting, sometimes faster than before the process.
This is more than an appearance issue; it is a quality problem. Rust damages the surface, causes paint to peel, and makes surfaces less accurate. Why does shot blasting increase rust risk? The answer involves electrochemical reactions. In this article, we will explain this problem in simple terms and provide practical solutions.
I. Unveiling the Mystery: Why are shot-blasted workpieces “most dangerously exposed”?
Many assume rusting after shot blasting results from inadequate cleaning. In reality, it is typically due to a combination of a highly active surface, micro-battery formation, and lack of protective film.
1. Extremely Hungry “Fresh Meat”
After rolling or forging, steel develops a dense oxide scale (Fe₃O₄/Fe₂O₃) on its surface. While unsightly, this acts as a passivation film, slowing down further oxidation of the internal iron. Shot blasting uses high-speed shot to remove this “old skin,” exposing the base metal (pure iron or solid solution).
From a metallurgical perspective, fresh metal surfaces have numerous lattice defects, high dislocation density, and extremely high chemical reactivity. This is akin to throwing a piece of fatty meat into air filled with bacteria; the oxidation rate increases exponentially.
2. Microscopic “Throat-Cutting” and Electrochemical Batteries
The shot blasting process is essentially countless micro-hammerings. If the shot is too hard, the impact angle is incorrect, or the material lacks sufficient toughness, microcracks or pits invisible to the naked eye will appear on the surface.
Even more deadly is embedded shot: fragments of broken steel shot become embedded in the workpiece surface. Steel shot typically has a high carbon content, while the workpiece matrix may have a low carbon content or contain impurities (such as SiO₂). In humid air, countless tiny “galvanic cells” form between the workpiece surface, embedded steel shot fragments, and impurities:
• Workpiece (anode): Loses electrons and corrodes (rusts).
• Steel shot fragments/impurities (cathode): Accelerate the reaction. This galvanic corrosion is much faster than simple chemical corrosion.
3. Residual Salts and Contaminants as “Accelerators”
If it is a casting or a workpiece that has previously been pickled, chloride ions (Cl⁻) or sulfate ions may remain in the pores. Shot blasting only cleans the surface; if not properly cleaned, these ions will be “pumped” into the surface microcracks. Chloride ions are notorious “stainless steel killers” and powerful catalysts for the rusting of carbon steel.
II. Diagnosis: Which type of “rust-prone” is your workpiece?
Before addressing the problem, it’s crucial to identify the specific pathology of the rust:
• Situation A: “Flash Rust”: Light yellow, hazy rust spots appear within 2-4 hours after shot blasting.
Cause: High ambient humidity (>60%), high surface activity, and lack of temporary protection. This is the most common cause, an inevitable result of “naked” blasting.
• Situation B: “Shot-Embedded Rust”: Isolated small red dots appear on the surface, sometimes surrounded by fine cracks.
Cause: Poor quality steel shot, high breakage rate, or excessive sandblasting pressure causing shot fragmentation and embedding into the surface.
• Situation C: “Mottled Rust”: Spotted or map-like rust distribution.
Cause: Uneven matrix material (e.g., looseness, segregation), or residual salts from previous processes (e.g., heat treatment) not properly cleaned.
• Situation D: “Rust Due to Over-Polishing”: Excessive surface roughness, even revealing a whitish metallic luster that quickly turns black.
Cause: An excessively thick work-hardened layer creates a network of micro-cracks on the surface, making it highly susceptible to trapping dirt and grime.
III. Solution: Five Lines of Defense from Source to Protection
To solve the problem of rust after shot blasting, simply applying oil is insufficient; a comprehensive defense system must be established.
First Line of Defense: Process Optimization (Root Cause Solution)
• Controlling Shot Quality: This is crucial! Steel shot must be screened regularly to remove dust and broken shot. Bainitic steel shot or diced shot is recommended; avoid using brittle cast iron shot.
• Adjusting Shot Blasting Parameters: Avoid over-blasting. Observe the Almen Intensity value; while achieving a cleanliness level (Sa2.5), minimize impact time. Surface roughness (Ra) should ideally be controlled between 12.5-25μm; excessive roughness prevents rust inhibitor adhesion and allows water to accumulate.
• Shot blasting removal: For high-precision workpieces, an additional “tumbling” or “brushing” process should be added after shot blasting to remove embedded shot fragments.
Second line of defense: Post-treatment cleaning (detoxification)
The shot blasting machine outlet must be connected to a cleaning machine (spray or ultrasonic).
• Key: Use a water-based rust-preventive cleaner, not pure water. The cleaner displaces chloride ions in the micropores and forms a very thin organic protective film on the surface.
• Fast drying: Drying must be done within 30 minutes after cleaning (hot air drying). Never allow it to air dry, otherwise watermarks will become a source of rust.
Third line of defense: Chemical conversion film (armor)
For workpieces that need to be stored for more than a week, simple rust-preventive oil is insufficient; a chemical reaction film is required:
• Phosphating treatment: The most classic and cost-effective method. It forms a dense phosphate crystal film on the surface, which both prevents rust and increases the adhesion of subsequent paint.
• Blackening/Blueing: For structural components, alkaline oxidation provides some rust prevention and a beautiful appearance.
• Passivation Treatment: For stainless steel or alloy steel, chromate or chromium-free passivation solutions are used to seal surface micropores.
Fourth Line of Defense: Rust-Preventive Oil/Grease (Temporary Preservation)
If processing or assembly is required in the short term, rust-preventive oil is the last line of defense:
• Choosing the Right Oil:
• Short-Term Rust Prevention (1-3 Months): Use dehydrated rust-preventive oil, which has low viscosity, is easily volatile, and leaves the workpiece surface non-sticky.
• Long-Term Rust Prevention (Over 3 Months): Use soft-film rust-preventive oil or grease, or even wax sealing, for good air isolation.
• Spraying the Right Locations: Spray not only the surface but also the bottom of holes, grooves, and threads. Vapor phase rust inhibitors (VCI) are recommended; their volatile gases fill the packaging bag and even cover micro-cracks invisible to the naked eye.
Fifth Line of Defense: Environmental Control (External Shielding)
• Workshop Dehumidification: The relative humidity in the shot blasting area and storage area must be controlled below 50%-60%.
• Isolated Storage: Freshly shot blasted workpieces must not be placed directly on cement floors or steel frames (cement floors absorb and release moisture, steel frames conduct electricity and heat, easily causing condensation). Wooden pallets or VCI packaging bags must be used for sealing.
IV. Emergency Response in Special Circumstances: What to do if rust has already formed?
If the workpiece has already shown slight rust, do not assemble or paint it directly.
- Light Surface Rust: Use a wire brush or gauze to polish, then immediately spray with anti-rust primer.
- Severe Rust: Rework is necessary. Remove rust by entering an acid pickling tank (pay attention to neutralization and cleaning after acid pickling to prevent residual acid), then re-shot blast.
- Localized Rust Spots: Wipe with a rust remover (such as a phosphoric acid + organic acid formula), clean and dry, then apply anti-rust oil.
V. Summary: Rust Prevention is a Mindset
Rusting after shot blasting is a natural result of surface reactions. While these cannot be completely prevented, surface energy can be reduced through process control and protective coatings to mitigate the effect.
Remember this formula:
No rust = Clean shot + Suitable roughness + Thorough cleaning and drying + Timely chemical sealing + Dry storage environment.
Do not rely only on rust-preventive oil at the end; this is like taking action after damage has occurred. The real key to keeping workpieces rust-free over time starts with selecting the right steel shot. Treat shot blasting as the beginning of protecting the surface, not the final step, to set up long-term success against rust.
