1.What are the main causes of unexpected surface blackening?
Residual rolling oil/emulsion carbonization: If the strip surface is not thoroughly cleaned before annealing, residual rolling oil or emulsion will decompose and carbonize in the high-temperature annealing furnace, forming firmly adhering black or grayish-black oil stains.
Water stains/rust:
Water stains: If the strip surface is not completely dried after pickling or cleaning, residual moisture evaporates, leaving mineral residue that forms watermarks, sometimes dark in color.
Rust: If moisture (such as rainwater or condensation) is trapped inside during coiling, uniform or strip-shaped dark red iron oxide (Fe₂O₃) will form during storage, appearing as if "blackened".
Oxidation:
Slight oxidation: In humid environments, a very thin oxide film forms on the surface, possibly exhibiting interference colors (blue-purple or dark yellow).
Severe oxidation: If exposed to corrosive media such as acids or salts, or in prolonged high-temperature and high-humidity environments, a thicker oxide scale (Fe₃O₄, etc.) will form, appearing black or dark brown.
2.What are the methods for handling and preventing this?
Treatment:
Physical Methods: For minor, loose contamination, dry (e.g., brushing, sandblasting) or wet (e.g., high-pressure water cleaning) cleaning lines can be used. However, for contaminants that have carbonized or formed a dense oxide layer, complete removal may be difficult and costly.
Chemical Methods: Acid pickling (e.g., hydrochloric acid, sulfuric acid) can remove the oxide layer, but it alters the surface condition and dimensions of the substrate, usually requiring rerolling or treatment as a defective product.
Prevention (Critical):
Enhance Cleaning: Ensure effective cleaning processes after rolling, controlling the concentration, temperature, and spray pressure of the cleaning solution.
Ensure Drying: After cleaning and before oiling, the strip surface must be thoroughly dried using drying equipment.
Control Oiling: Use appropriate rust-preventive oil and ensure even application and a complete oil film.
Improve Storage: Warehouses should be well-ventilated, dry, and kept at a constant temperature and humidity to prevent condensation. Use wooden blocks when stacking steel coils to avoid direct contact with the ground.
3.What are some intentional surface blackening treatment processes?
Phosphating Blackening (Black Phosphating):
Principle: Steel is immersed in an acidic phosphate solution containing manganese, iron, and zinc. A chemical reaction forms a black crystalline film on the surface, primarily composed of ferric manganese phosphate.
Characteristics:
The film is dark grayish-black to pure black, uniform and aesthetically pleasing.
The film has a microporous structure, good adsorption properties, and is often used in conjunction with oiling, providing excellent rust prevention.
It improves the adhesion of subsequent coatings.
The film itself also has some friction-reducing and wear-resistant properties.
Applications: Automotive parts, fasteners, firearms, optical instrument components, etc.
Alkaline Oxidation Blackening (Blue):
Principle: In a high-temperature (approximately 135-155℃) concentrated alkali (sodium hydroxide) and oxidant (sodium nitrite) solution, a dense layer of iron(III) oxide (Fe₃O₄) film forms on the steel surface.
Characteristics:
The color is usually bluish-black or dark black, with a glossy finish.
The film is very thin (about 0.5-1.5 micrometers) and does not affect the size of the part.
4.What is the process flow?
For cold-rolled coils, a continuous processing line is typically used: degreasing → water washing → pickling (rust removal and activation) → water washing → blackening treatment (phosphating or oxidation bath) → water washing → sealing (such as passivation or oiling) → drying.
5.How to determine the properties?
Observe whether the "blackening" is uniform and glossy, or mottled and stained. Can it be wiped off? If it's uneven stains, it's situation one (defect), and the cause needs to be found in the production and storage stages. If a uniform, functional black film is required, it's situation two (process).
Clarify the requirements: If a functional blackening is desired, clarify:
End use: Is the focus on rust prevention, wear resistance, or aesthetics?
Subsequent processes: Is coating, drawing, etc., required?