1.What are the requirements for coating thickness?
Measurement method: Magnetic thickness measurement (non-destructive, primary method).
Minimum requirements: Minimum local thickness and minimum average thickness are specified based on the substrate thickness (δ).
Importance:
→ The thickness of the coating directly determines the anti-corrosion life and is the core hard indicator of acceptance.
→ Important structural parts of high-rise buildings (such as beams and columns) usually require δ≥6mm, so local thickness ≥70μm and average thickness ≥85μm are common requirements.
2.What are the requirements for coating adhesion strength?
Test method:
Hammer method: Hit the coating with a hammer to see if it peels off (qualitative).
Grid/scratch method: Scratch a grid or straight line with a knife, and the coating does not fall off after the tape is torn off.
Bending test: After the specimen is bent 180°, the coating should not peel off (only applicable to bendable parts).
Requirements:
→ The coating is firmly bonded to the base metal without peeling or shedding.
→ High-rise building components are easily impacted during transportation and installation, and insufficient adhesion can lead to local corrosion failure.
3.What are the requirements for appearance quality?
Uniformity: The coating is continuous and complete, covering all surfaces (including internal corners and hole walls).
Surface condition: slight flow marks and slight color difference are allowed;
Not allowed:
Mis-plating (exposed steel)
Burrs, zinc nodules (affecting bolt connection or installation)
Blisters, peeling, cracks
Residual zinc slag (affecting the friction coefficient of the contact surface)
Special requirements for high-rise buildings:
Exposed components (such as curtain wall keels) must strictly control flow marks and color differences to meet aesthetic requirements.
The connection surface of friction-type high-strength bolts must be free of zinc nodules and burrs, otherwise additional treatment is required.
4.What are the uniformity requirements?
Applicable objects: Steel wire, small parts, etc.
Test method: The sample is immersed in copper sulfate solution (4 times, 1 minute each time), and the coating should not have red metallic copper deposition.
For building structural parts, the thickness and adhesion are more concerned, so this method is rarely used.
5.What are the requirements for the substrate?
Surface pretreatment:
Before galvanizing, oil stains, rust, scale, and welding slag must be thoroughly removed (by pickling and plating agent treatment).
The weld needs to be polished and smoothed, otherwise it is easy to cause leakage or uneven plating.
Plating ability:
High silicon steel may produce ultra-thick coatings but is brittle and requires process control.
Cold-rolled parts need to avoid the "Sanderin effect" (dull coating and poor adhesion).