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The primary surface treatment methods for high-strength fasteners, along with their key characteristics and typical applications
When customers place orders, they often ask if our factory can do various surface treatments on their own. Our factory mainly does oxidation blackening treatment, and other surface treatments are done by outsourced units. Due to regulations from the Ningbo government, other surface treatments must be concentrated in one place for environmental protection reasons.
- Zinc Plating (Electroplating):
Process: The most common and economical method. Fasteners are immersed in an electrolyte solution containing zinc ions. An electric current deposits a thin layer of zinc onto the steel substrate.
Characteristics: Provides sacrificial (cathodic) corrosion protection. Appearance ranges from dull grey to shiny silver/blue. Corrosion resistance is moderate and directly proportional to coating thickness. Friction characteristics can be inconsistent without supplementary treatments. Risk of Hydrogen Embrittlement (HE) for high-strength grades (≥ 1000 MPa or Class 10.9/12.9), requiring strict post-plating baking.
Applications: General-purpose fasteners in automotive interiors, appliances, electronics, machinery, and construction where moderate corrosion resistance suffices (e.g., dry or mildly corrosive indoor environments). Often used with a chromate conversion coating (see below). Cost-sensitive applications.
Variants: Blue/White/Yellow/Black Chromate, Olive Drab Chromate (increasing corrosion resistance and color options).
- Zinc Flake Coatings (e.g., Dacromet®, Geomet®, Delta Protekt®):
Process: An inorganic coating applied by dipping or spraying. Typically consists of zinc and aluminum flakes suspended in an inorganic binder (chromium(VI)-based historically, now mostly chromium(III)-based). Cured at high temperatures.
Characteristics: Excellent corrosion resistance (significantly better than standard zinc plating), often exceeding 500-1000 hours salt spray. Provides good friction control (consistent clamp load). Excellent heat resistance (up to 300°C+). Sacrificial protection. Very low risk of HE as baking is integral to curing. Typically has a matte grey appearance. Environmentally friendlier chromium(III) versions are now standard.
Applications: Critical automotive underbody components (suspension, brakes, engines), agricultural machinery, construction equipment, offshore applications, high-temperature environments, and anywhere superior corrosion protection and reliable torque-tension are required for high-strength fasteners. Replacing cadmium in many aerospace applications.
- Mechanical Galvanizing:
Process: A "cold" process where fasteners are tumbled in a drum with zinc powder, impact media, and special chemicals. The mechanical energy causes the zinc to cold-weld onto the steel surface.
Characteristics: Produces a uniform, relatively thick zinc coating. Provides good sacrificial corrosion protection. Very low risk of Hydrogen Embrittlement as no acid cleaning or electrolysis is involved – ideal for very high-strength fasteners (≥ 1200 MPa / 12.9). Coating is somewhat porous but robust. Dull grey appearance.
Applications: High-strength structural bolts (ASTM A325, A490 historically, though often replaced by coatings like Zn-flake), earthmoving equipment fasteners, mining hardware, fasteners for pre-stressed concrete, and other applications where HE risk must be absolutely minimized.
- Hot-Dip Galvanizing (HDG):
Process: Fasteners are cleaned, fluxed, and immersed in molten zinc (typically around 450°C). A metallurgical reaction forms zinc-iron alloy layers topped with pure zinc.
Characteristics: Provides the thickest zinc coating, offering very long-term sacrificial corrosion protection in harsh environments. The coating is durable but rough. Dimensional tolerances are affected. Appearance is bright/spangled grey when new, weathers to dull grey. Risk of HE requires post-galvanizing baking for high-strength fasteners. Coating can be brittle.
Applications: Structural steelwork (bridges, power transmission towers, building frames), highway hardware, marine environments (less severe), utility poles, fencing, and heavy industrial settings exposed to weather. Commonly used for large bolts, nuts, and washers.
- Phosphating (Zinc or Manganese Phosphate):
Process: A chemical conversion coating. Fasteners are immersed in a dilute phosphoric acid solution containing zinc or manganese ions, forming a crystalline, insoluble phosphate layer on the steel surface.
Characteristics: Does not provide significant standalone corrosion protection. Its primary functions are to improve adhesion for subsequent paints/powder coatings, enhance oil retention (aiding lubrication and short-term corrosion resistance), and reduce friction/galling during assembly. Appearance is dark grey to black. Minimal HE risk.
Applications: Primarily used as a pre-treatment. Common for automotive engine and transmission fasteners that will be painted or require consistent friction characteristics with assembly oil.