HT Holden Pattern&LM Holden Bearing-AU-850-A522 | China Trailer Brake Drum Factory

Characteristic	Black Painting Process	Galvanizing Process	Dacromet Coating Process
Core Principle	Forms an organic coating through spraying to isolate air and moisture.	Forms a zinc metal protective layer on the substrate surface through electroplating or hot-dip galvanizing.	A water-based coating containing zinc/aluminum flakes and chromate (or environmentally friendly chromium-free formulas) is applied by dipping or spraying and then baked to form an inorganic coating.
Key Process Steps	1. Pretreatment (alkali cleaning, rinsing, drying) 2. Surface Oxidation Treatment (enhances adhesion) 3. Spraying and Curing (often using high-temperature paint).	1. Pretreatment (cleaning, pickling) 2. Electrogalvanizing or Hot-dip Galvanizing 3. Possible Chromate Passivation (e.g., yellow chromate, black chromate) for increased corrosion resistance.	1. Pretreatment (degreasing, shot blasting) 2. Dip or Spray application of Dacromet coating 3. Baking and Curing (typically around 300°C).
Appearance	Black matte or semi-gloss, uniform and aesthetically pleasing.	Silvery white (zinc layer), iridescent (color passivation), or black (black passivation).	Characteristic silvery-gray metallic luster. Environmentally friendly chromium-free types also appear silvery white.
Corrosion Resistance	Fair. Depends on coating integrity and paint film quality; prone to rusting from damaged areas.	Good. The zinc layer provides sacrificial anode protection, offering continuous protection even with minor scratches.	Excellent. Salt spray resistance is typically 5-8 times or more better than traditional electrogalvanizing.
Heat Resistance	Requires specialized high-temperature resistant paint (>600°C), otherwise prone to scorching.	Fair. The zinc layer oxidizes and diffuses at high temperatures, with limited long-term high-temperature oxidation resistance.	Excellent. The cured inorganic coating can withstand higher brake operating temperatures.
Impact on Braking Performance	Strictly prohibited on the friction surface, as it would severely reduce the friction coefficient and cause brake failure. Only used on non-friction surfaces (e.g., cooling fins, inner cavity) for rust prevention.	Same as left. The friction surface must remain clean, bare metal.	Primarily applied to non-friction surfaces. If applied to the friction surface, coating uniformity and friction coefficient must be strictly controlled to avoid affecting braking stability.
Environmental Impact	Traditional solvent-based paints have VOC emission issues; water-based paints are more environmentally friendly.	Traditional galvanizing (especially with hexavalent chromium passivation) requires strict wastewater treatment and faces significant environmental pressure.	Traditional Dacromet contains toxic and harmful hexavalent chromium. The mainstream now is “Chromium-Free Dacromet”, complying with environmental standards like RoHS.
Other Advantages	Lower cost, multiple color options, relatively simple application.	Mature technology, moderate cost, good electrical conductivity.	No hydrogen embrittlement risk (important for high-strength parts), good penetration, strong adhesion (can reach grade 0-1).
Main Disadvantages	Poor wear resistance, easily scratched by stones, etc.; may age and chalk under prolonged high temperatures.	Challenging to control coating uniformity on complex workpieces; hot-dip galvanizing may affect dimensional accuracy.	Relatively low coating hardness and wear resistance; higher equipment investment and energy consumption.

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