An introduction to rust reformation after steel shot blasting and its impact on the coating
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For cast iron and steel products whose rust and scale cannot be removed by pickling, mechanical methods are generally used for treatment. Sandblasting has been halted due to serious lung disease issues; metal shot blasting is currently used more frequently, and equipment such as rolling and shot-rolling is also employed for rust removal. The biggest problem after mechanical rust removal is rust reformation between processes. Although many people are aware of this, due to incorrect handling methods, quite a number of companies still suffer losses as a result. For example, oil stains, oxidation layers, and rust on the surface of steel radiators must be removed. Due to environmental regulations, many companies have switched from acid washing for rust removal to shot blasting. Heaters that have been descaled by shot blasting require a second pressure test, and the problem of rust reappearing after this testing poses a significant challenge for companies. There are many methods for rust prevention, yet the fact that the rust-proof coating should not cause damage to the paint layer is something that many people do not take into account. In the radiator industry, low-concentration sodium nitrite aqueous solutions are commonly used for passivation and rust prevention; other inorganic salts are also employed for rust prevention between different processing steps. All of these methods will have an adverse effect on the coating quality.
Rust recurrence on steel after shot blasting and its impact on coatings: The surface of steel that has undergone metal shot blasting is highly reactive, and it is prone to rapid oxidation when the air humidity is high or moisture is present, resulting in rust recurrence. Untreated rust not only significantly reduces the cleanliness of the workpiece surface but also leads to a substantial decrease in the adhesion and protective properties of subsequent coatings. Common rust prevention methods and their issues: 1. Passivation rust prevention using inorganic salts such as sodium nitrite. Advantages: Cost-effective, convenient, and widely used. Disadvantage: Residual inorganic salt ions may hinder the adhesion of the coating, leading to reduced coating quality or peeling of the film. 2. Common rust-proof oil or rust-proof film methods: Disadvantages include difficulty in complete removal, severe hindrance to direct bonding between the coating and the substrate, and poor adhesion of the coating. The main impact of rust reformation on coating quality: – It weakens the adhesion between the coating and the substrate, increasing the risk of cracking, bubbling, and peeling of the coating ； - Reduce the effective protective lifespan of the coating ； - Reduces the apparent quality and corrosion resistance of the product ； - It may lead to subsequent repair costs for the company as well as customer complaints. To address the above issues, it is recommended to: – Use rapid drying between processing steps as much as possible, and control the environmental humidity ； - Apply coating to the products after shot blasting as soon as possible ； - If an aqueous rust inhibitor is chosen, it is recommended to use a specialized passivator that has no effect on the coating ； - Optimize the processes appropriately to avoid secondary pollution and ensure stable and reliable coating quality. .