How to Apply Zinc Coating to Mixed Material Fasteners

Why Mixed Materials Complicate Zinc Coating

Zinc plating is a standard method for improving the corrosion resistance of steel fasteners. However, applying coatings to fasteners made from mixed materials, such as steel shanks with stainless steel heads or titanium washers, introduces complexity. The primary challenge is not just the zinc application itself, but how the different base metals react to the process and to each other.

Galvanic corrosion can occur when dissimilar metals are in electrical contact in the presence of an electrolyte. Zinc acts as a sacrificial anode. If you apply zinc to a steel fastener that retains a bare stainless steel head, the steel will corrode preferentially. The zinc will protect the steel, but the stainless steel head may suffer from pitting if the environment is aggressive.

Understanding these interactions is the first step. You cannot treat a mixed material fastener like a single alloy part. The process must account for the specific combination of metals involved.

Prerequisites for Successful Zinc Application

Before any coating is applied, you must establish a clear understanding of the part and the environment.

1. Material Identification
You need to know the exact composition of each part of the fastener. A spec sheet stating “steel” is insufficient. You need to know if it is low carbon steel, spring steel, or a stainless alloy. For the non-zinc parts, you must know the alloy grade. This determines the chemical compatibility with the plating bath and the expected galvanic series position.

2. Surface Condition Assessment
Inspect the fasteners for existing contaminants. Oil, grease, old coatings, or oxide scales will prevent adhesion. Mixed material parts often have different surface treatments on different sections. For example, a steel shank might be shot blasted while a brass insert remains polished. Each surface must be evaluated separately.

3. Environmental Requirements
Define the service environment. Is the fastener exposed to salt spray, high humidity, or chemical fumes? This dictates the required coating thickness and the type of zinc alloy. Standard zinc plating offers general protection. Thick-gauge plating or nickel over-plating offers specific corrosion protection for harsher conditions.

4. Logistics and Handling
Determine how the fasteners will be handled. Mixed material fasteners can be delicate. Some alloys are softer than others. Improper handling during transport to the coating plant can cause scratches that compromise the coating.

Step-by-Step Zinc Application Process

The application of zinc to mixed material fasteners follows a rigorous sequence. Each step serves a specific purpose in ensuring adhesion and uniformity.

1. Degreasing

   • Reason: Removes oil, grease, and handling residues. Zinc will not bond to a greasy surface.
   • Method: Use a hot alkaline or neutral degreaser. For mixed materials, ensure the degreaser is compatible with all base metals. Some acids can etch brass or aluminum components.

2. Rinsing

   • Reason: Removes degreasing chemicals to prevent them from entering the plating bath.
   • Method: Rinse with deionized water. Multiple rinses are often required to reach a specific conductivity level.

3. Pickling or Etching

   • Reason: Removes oxide scales and surface carbonates.
   • Method: Use a mild acid solution, such as hydrochloric or sulfuric acid. This step is critical for steel components. For stainless steel or titanium, pickling may be unnecessary or damaging. You must isolate these parts or use a different process.

4. Rinsing Again

   • Reason: Neutralizes the acid and removes pickling residue.
   • Method: Deionized water rinse. In some processes, a neutralizing bath of sodium bicarbonate is used to prevent acid from migrating into the zinc bath.

5. Pre-treatment or Alkaline Cleaning

   • Reason: Activates the surface for better zinc adhesion.
   • Method: An alkaline solution is applied. This cleans the surface and may slightly etch the metal. For mixed materials, this step requires careful pH control to avoid attacking softer alloys.

6. Zinc Application

   • Reason: Deposition of the zinc layer.
   • Method: The fasteners are immersed in a zinc plating bath. The bath composition determines the alloy. Bright zinc is smooth and uniform. Matte zinc is more porous and offers better corrosion protection. For mixed materials, the current density must be controlled to avoid burning the zinc on thin sections or under-depositing on complex geometries.

7. Rinsing

   • Reason: Removes plating solution residue.
   • Method: Water rinse to stop the plating reaction and remove bath components.

8. Passivation or Conversion Coating

   • Reason: Protects the zinc from immediate oxidation and reduces brittleness.
   • Method: A chemical bath, such as trichromate or chromate, is applied. This turns the zinc yellow or brown. For mixed materials, ensure the passivation chemistry does not stain or damage non-ferrous components.

9. Final Rinse and Drying

   • Reason: Removes residual chemicals and prevents water spots.
   • Method: Deionized water rinse followed by heated air drying.

Common Mistakes in Mixed Material Coating

Even with a correct process, mistakes happen. These are the most frequent errors in zinc application for mixed material fasteners.

• Ignoring Galvanic Pairs: Plating steel and leaving the stainless steel head bare creates a galvanic cell. The steel will corrode faster than if it had no zinc at all.
• Incompatible Degreasers: Using a strong acid degreaser on a brass or aluminum component can dissolve the softer metal.
• Inadequate Rinsing: Leaving acid residue in the zinc bath causes the bath to become acidic, leading to poor plating and corrosion of the tank.
• Over-Plating Thin Sections: High current density causes “burning,” where the zinc deposits too thickly and becomes brittle. This is common on thin steel shanks.
• Skipping the Passivation Step: Leaving zinc bare leads to rapid oxidation. The coating becomes dull and less effective.

Verifying Coating Quality

Applying the zinc is only half the job. You must verify that the coating is effective.

1. Visual Inspection
Check for coverage, especially in recesses and at material junctions. Look for bare spots, drips, or uneven color.

2. Coating Thickness Measurement
Use a magnetic induction gauge for ferrous parts. For non-ferrous parts, use a non-magnetic eddy current gauge. Measure multiple points per part. The thickness must meet the specification for the intended service life.

3. Salt Spray Testing
Submerge a sample in a salt solution. Look for rust or flaking after a set number of hours. This is a destructive test, so use sacrificial samples.

4. Adhesion Testing
Perform a tape test or a cross-cut test. Peel the tape or scratch the surface and see if the coating lifts. This confirms that the zinc is bonded to the base metal.

Inspection Method	Purpose	Typical Result for Good Coating
Visual Check	Detects major defects	No bare spots, uniform color
Thickness Gauge	Measures zinc layer	Within specified range (e.g., 5-10 microns)
Salt Spray Test	Measures corrosion resistance	No red rust after required hours
Tape Adhesion Test	Checks bond strength	No coating lift or flaking

Documentation and Traceability

For industrial fasteners, records are as important as the part.

• Material Certificates: Keep mill test reports for the base metals.
• Coating Report: The plating facility should provide a report detailing the bath chemistry, plating time, and thickness measurements.
• Inspection Records: Document the results of the visual, thickness, and adhesion tests.
• Service Environment Data: Record where the fasteners are installed. This helps in predicting failure modes.

If a fastener fails in service, the documentation allows you to determine if the failure was due to the coating, the base material, or the environment. Without records, you are guessing.

Maintenance and Re-Coating

Zinc coatings are sacrificial. They will eventually wear down.

• Regular Inspection: Check for coating loss during routine maintenance.
• Re-Coating: If the coating is thinning, re-plating is an option. The surface must be stripped of the old coating first. This is a labor-intensive process.
• Alternative Protection: For critical applications, consider a different fastener type that does not require plating. For example, a high-grade stainless steel fastener may offer better long-term corrosion protection without the need for re-coating.

Conclusion

Applying zinc coating to mixed material fasteners requires a disciplined approach. You must understand the materials, prepare the surface carefully, and verify the result. The goal is to create a uniform, adherent layer that provides the required level of corrosion protection. By following these steps and documenting your process, you ensure that your fasteners perform reliably in their intended service.