1.How to improve corrosion resistance by optimizing zinc coating properties?
Increase the thickness of the zinc layer
Principle: The zinc layer is a physical barrier that isolates the steel plate from the corrosive medium. The thicker the zinc layer is, the longer it can resist corrosion penetration.
Operation: Increasing the zinc layer thickness from the conventional 5-15μm (single-sided) to 10-20μm (such as a total double-sided zinc coating of 20-40g/m²) can significantly extend the corrosion resistance period.
Improving the composition of the zinc coating
Principle: Adding alloy elements (such as nickel, cobalt, iron, etc.) to the zinc coating to form alloy coatings such as zinc-nickel (Zn-Ni) and zinc-cobalt (Zn-Co), improve the density and electrode potential of the coating, and enhance corrosion resistance.
Effect: The corrosion resistance of the zinc-nickel alloy coating can be increased by 2-3 times compared with that of the pure zinc coating, and the time for white rust to appear in the salt spray test is significantly extended (for example, the pure zinc coating will have white rust in the 72-hour salt spray test, while the zinc-nickel alloy may need more than 1000 hours).
2.Can passivation improve corrosion resistance?
Chromate passivation (traditional method)
Principle: The chromate solution reacts with the zinc layer to form a chromium-containing passivation film (such as trivalent chromium and hexavalent chromium compounds) to isolate the corrosive medium.
Effect: The hexavalent chromium passivation film has excellent corrosion resistance, but it is gradually restricted due to environmental issues; the trivalent chromium passivation film is environmentally friendly and has corrosion resistance close to that of hexavalent chromium, making it a mainstream alternative (such as no white rust in 72 hours in the salt spray test).
Chromium-free passivation (environmental trend)
Method: Use silane, titanate, molybdate, etc. to replace chromate, and form a protective film through chemical film formation or molecular adsorption.
Advantages: Environmentally friendly and non-toxic, the corrosion resistance can reach more than 80% of trivalent chromium passivation (such as no white rust in the salt spray test for 48 hours after silane treatment), suitable for home appliances and food contact fields.
3.Will the corrosion resistance be better after phosphating?
Principle: The zinc phosphate solution reacts with the zinc layer to form a dense phosphate film (zinc phosphate crystals), which is firmly bonded to the zinc layer and can enhance the adhesion of subsequent coatings. At the same time, it has a certain corrosion resistance.
Application: Commonly used in automotive parts. After the phosphate film is combined with the paint coating, the salt spray resistance can reach more than 1,000 hours (such as phosphating + electrophoretic coating of body steel plates).
4.How to control temperature and humidity to improve corrosion resistance?
Humidity and temperature: Avoid using in high humidity (>80% RH) or environments with drastic temperature differences to reduce electrochemical corrosion of the zinc layer by condensed water (e.g., keep it ventilated and dry when stored indoors).
Corrosive media: Keep away from acidic and alkaline gases, salt spray (e.g., seaside environments), industrial pollutants (e.g., SO₂, Cl⁻), and add isolation protection (e.g., sealed housing) if necessary.
5.How to improve corrosion resistance through process treatment?
Principle: Optimize the composition of the electroplating solution (such as zinc ion concentration, pH value), current density and temperature, improve the uniformity and density of the zinc coating, and reduce the porosity (the lower the porosity, the more difficult it is for the corrosive medium to penetrate).
Case: By replacing DC electroplating with pulse electroplating technology, the porosity of the coating can be reduced by 50% and the corrosion resistance can be improved by about 40%.