Why Surface Cleanliness Matters
The surface of a cold-rolled coil carries residual rolling oil, fine iron fines and dust from the rolling and handling process. If these residues remain, paint and plating layers lose adhesion, welds and bonds weaken, and electroplating can develop pinholes. Surface cleanliness is therefore a specified quality parameter, not an incidental one, and it is expressed quantitatively in milligrams of residue per square metre (mg/m2). The required level depends on the downstream process: electroplating and high-performance coatings need the cleanest surfaces, while simple forming or no surface treatment tolerates more residue.
Cleanliness Grades and Standards
JFS A 3021, issued by the Japanese steel industry, classifies the surface cleanliness of cold-rolled steel sheets into grades that are widely cited in coil specifications. Grade A means extremely clean with very little residue, typically below 50 mg/m2, and is required for electroplating and high-requirement coatings. Grade B means clean with a small amount of residue, typically 50 to 100 mg/m2, suitable for general coatings. Grade C means standard cleanliness with a limited amount of residue, typically 100 to 200 mg/m2, acceptable for parts that need simple processing or no surface treatment.
| Grade | Typical residue (mg/m2) | Suitable processes |
|---|---|---|
| A | Below 50 | Electroplating, high-performance coatings |
| B | 50-100 | General painting and coating |
| C | 100-200 | Forming without surface treatment |
At the international level, the ISO 8502 series covers the assessment of surface cleanliness before painting, including qualitative and quantitative methods for detecting grease and salts, while ISO 11126-11128 series deal with surface preparation standards. These standards are used mainly for site assessment before coating application. In the automotive sector, European OEM material specifications often set even tighter limits, frequently below 100 mg/m2 and in some cases lower, with third-party test reports required; the limits apply to the coil surface as delivered and after degreasing.
Chinese Standards and Enterprise Specifications
GB/T 5213, the standard for cold-rolled low-carbon steel sheet and strip, gives descriptive surface quality requirements, such as freedom from defects that affect use, but does not set numeric cleanliness levels. In practice, major steel producers define numeric cleanliness targets in their enterprise specifications or technical agreements with customers, usually based on JFS A 3021 classes or on customer requirements. For a specific order, the cleanliness grade should be written into the technical agreement so that the acceptance test is unambiguous.
Test Method 1: Solvent Extraction and Gravimetric Measurement
The most objective method is solvent extraction. A sample of known area is wiped or soaked with a solvent such as acetone or petroleum ether to dissolve the surface oil and soluble residue. The solvent is then evaporated and the remaining residue is weighed. The result is expressed directly in mg/m2, which makes it the most comparable quantitative indicator between mills and between batches. The method requires careful laboratory practice to avoid solvent contamination and evaporation losses.
Test Method 2: Contact Angle Measurement
A drop of water is placed on the cleaned surface and the contact angle between the droplet and the surface is measured. A cleaner surface has higher surface energy, so the droplet spreads and the angle is small; oil or dirt raises the angle. The method is fast and non-destructive, but it is indirect and sensitive to surface roughness, so it is best used for rapid production-line monitoring rather than for acceptance testing.
Test Method 3: Dyne Pens and Dyne Solutions
Dyne test pens apply liquids with known surface tension to the surface. If a liquid with a given value spreads evenly without beading within about two seconds, the surface energy is at least that value, expressed in mN/m or dynes per centimetre. The method is common in printing and coating shops to verify that a surface is ready for treatment, for example a minimum of 38 mN/m before printing. Like the contact angle method, it is a quick indicator rather than a quantitative cleanliness measurement.
Choosing the Right Cleanliness Level
Set the cleanliness requirement according to the end process. For electro-galvanizing, electroplating or adhesive bonding, specify Grade A with a numeric limit below 50 mg/m2 and verify by solvent extraction. For powder coating or general wet painting, Grade B in the range 50 to 100 mg/m2 is typical. For parts that will be formed and used without surface treatment, Grade C is usually sufficient and cheaper to supply. Always record the test method in the specification, because different methods give different results on the same surface.
FAQ
How is surface cleanliness of cold-rolled coil measured?
The reference method is solvent extraction followed by gravimetric weighing, giving a result in mg/m2. Contact angle and dyne pen tests are used as fast production-line indicators.
What is the difference between cleanliness grades A, B and C?
Grade A is below about 50 mg/m2 and suits electroplating; Grade B is about 50-100 mg/m2 and suits general coatings; Grade C is about 100-200 mg/m2 and suits forming without surface treatment.
Does GB/T 5213 set a numeric cleanliness limit?
No. GB/T 5213 gives descriptive surface quality requirements. Numeric cleanliness limits are usually set by enterprise specifications or technical agreements based on JFS A 3021 classes or customer needs.
Why does the automotive industry require very clean surfaces?
Because body panels are painted, welded and bonded in automated lines; residues cause paint defects, weld porosity and bond failure. OEM specifications therefore set tight limits, often below 100 mg/m2, with third-party test reports.
Can the contact angle method replace solvent extraction?
Not for acceptance. Contact angle is fast and non-destructive but indirect and roughness-sensitive. It is used for line monitoring, while solvent extraction provides the objective mg/m2 value for acceptance.
What surface energy is needed before printing on cold-rolled steel?
A common shop-floor target is at least 38 mN/m measured with dyne pens, ensuring the surface is wettable and ready for ink or coating adhesion.