1.What are the dangers of insufficient roughness (surface that is too smooth)?
Mechanical anchor failure: The smooth surface cannot provide enough microscopic "anchor points", and the coating and the galvanized layer mainly rely on intermolecular forces, and the bonding force is weak.
Under temperature stress or mechanical impact, the coating is easy to peel off from the interface (manifested as large-area delamination).
Poor interface wettability: The low roughness surface energy is low, and the liquid coating is difficult to spread and penetrate fully, which is easy to produce microbubbles or shrinkage holes, forming bonding defects.
Salt spray test performance: The cross-hatch adhesion test may be qualified, but the edge blistering rate increases by more than 50% after 500h of salt spray.
2.What is the core value of appropriate roughness?
Mechanical locking effect: The ideal roughness forms a uniform and dense micro-peak-micro-valley structure. After the liquid coating penetrates into the valley bottom and solidifies, it forms a "hook-type" mechanical interlocking, and the adhesion is increased by 3 to 5 times.
Increase the effective contact area: Roughening increases the actual surface area by 30% to 70% compared with the apparent area, providing more reaction sites for chemical bonding.
Improve the anti-permeability of the coating: The dense anchoring structure blocks the lateral diffusion of the corrosive medium along the interface, and the scratch expansion width in the salt spray test is reduced by more than 40%.
3.What are the effects of excessive roughness (excessive surface roughness)?
Tip effect causes corrosion: When the radius of curvature of the rough peak is too small, it becomes the first point for electrochemical corrosion to occur, and the galvanized layer itself will be consumed faster.
Incomplete coating coverage: Deep valleys are prone to insufficient paint film thickness, forming local anti-corrosion weak points. Stress concentration leads to cracking: The coating at the sharp micro-peaks is subjected to concentrated stress, inducing micro-cracks under temperature cycle or vibration load.
4.What are the key control points of engineering applications?
Roughness test standard: Ra and Rz dual parameter control specified in ISO 4287 is preferred.
Qualitative evaluation using only visual inspection or tape is prohibited.
Surface treatment before painting
Sandblasting grade: Sa 2.5 is recommended for hot-dip galvanized parts, and the roughness target is Ra 2.5~3.5μm.
Chemical treatment: Phosphating or passivation generates a porous conversion film, the roughness increases by about 0.5μm and the activity is enhanced.
Coating matching selection
For high roughness surfaces (Ra>4μm), high solid epoxy primer (strong permeability) is selected;
For low roughness surfaces (Ra<1.5μm), modified primer containing silane coupling agent (enhanced chemical bonding) is selected.
5.What is the "golden range" of roughness?
There is an optimal range for the surface roughness of galvanized steel - Ra 1.5~4.0μm (or Rz 10~25μm). In this range: Mechanical anchoring and chemical bonding are balanced, and adhesion is maximized;
The coating thickness uniformity is controllable and has strong resistance to medium penetration;
Avoid local corrosion caused by excessive roughness.
In engineering, it is necessary to actively control the roughness through sandblasting/chemical treatment and match the adaptive coating system to achieve long-term protection of the galvanized steel-coating system for more than 30 years.