Chrome plating is a popular surface finishing process that improves corrosion and wear resistance, enhances aesthetics, and provides a durable protective layer on metal surfaces. However, despite its advantages, chrome plating can also have some disadvantages and defects that may arise during electroplating.
Chrome Plating Process
Chrome plating, also known as chromium plating, refers to the electroplating process where a thin layer of chromium is deposited onto a metal substrate through electrodeposition. The metal surface is cleaned and polished to prepare it for plating. It is then activated in an acid bath before being submerged into a chromium plating solution containing chromic acid and catalysts.
An electric current is applied, which causes the chromium ions in the solution to deposit onto the cathode (metal surface). As the chromium metal builds up, it creates a thin, microscopically rough chrome layer that adheres tightly to the base metal. The thickness of the coating typically ranges from 0.25 – 1.0 mil for decorative finishes.
Key Disadvantages and Defects
While chrome plating offers many benefits, the process also has some inherent disadvantages and defects that may arise, including:
Blistering
Blistering manifests as bubbles or dome-shaped protrusions in the chrome coating and is one of the most common plating defects. It occurs when hydrogen gas evolves at the interface between the chromium deposit and the base metal, leading to detachment of the coating.
Causes
- High current densities
- Improper bath chemistry
- Contaminated base metal surface
- Poor rinsing
Effects
- Reduced corrosion protection
- Poor adhesion
- Cosmetic defects
Prevention
- Optimize current density
- Maintain proper chemical balance
- Thorough cleaning and rinsing
- Use of degassers and agitation
Burned Deposits
Burned deposits appear as dark or black spots with a rough texture on the plated surface, resulting from the rapid buildup of chromium in high current density areas.
Causes
- Excessive current density
- Irregularities in base metal surface
- Improper racking of parts
Effects
- Poor aesthetics
- Reduced corrosion resistance
Prevention
- Careful control of current distribution
- Proper part racking
- Use of auxiliary anodes
Cleavage Cracks
These cracks form along the structural planes of the chromium deposit, reducing the plating’s strength and fracture toughness. They mainly occur in die castings.
Causes
- High internal stresses
- Cleavage planes in the substrate
- Embrittlement of the deposit
Effects
- Reduced structural integrity
- Increased susceptibility to cracking
Prevention
- Stress relieving operations
- Metal substrate preparation
- Use of additives to improve deposit ductility
Cold Shuts
Cold shuts appear as visible lines on the plated surface when different regions of the deposit cool and harden at different rates during electroplating.
Causes
- Non-uniform current distribution
- Complex part geometries
- Improper racking
Effects
- Reduced corrosion protection
- Cosmetic defects
- Sites for crack initiation
Prevention
- Optimize racking method
- Use auxiliary anodes
- Modify part geometry
Hydrogen Embrittlement
Absorption of hydrogen during plating leads to hydrogen embrittlement, reducing the substrate’s ductility and causing cracking.
Causes
- High cathode current densities
- Contaminated plating solutions
- Plating on high-strength steels
Effects
- Cracking and fractures
- Reduced ductility and toughness
Prevention
- Low current densities
- Monitoring solution contamination
- Baking to remove hydrogen
Density Variations
Variations in deposit density appear as matte or bright patches on the plated surface, resulting from solution chemistry fluctuations.
Causes
- Temperature variations
- Changes in current density
- Depletion of plating chemicals
Effects
- Inconsistent appearance
- Reduced corrosion protection
Prevention
- Precise temperature control
- Anode maintenance
- Timely chemical replenishment
Pitting
Pitting manifests as small holes or depressions in the chromium coating, resulting from evolving hydrogen and contamination.
Causes
- High cathode current density
- Contaminants in solution
- Poor rinsing
Effects
- Accelerated corrosion
- Reduced service life
Prevention
- Optimize current density
- Filtering and solution maintenance
- Improved rinsing practices
Poor Adhesion
Lack of proper adherence between the chromium layer and substrate arises from various factors.
Causes
- Oil or dirt on base metal
- Inadequate surface activation
- Oxide formation on substrate
Effects
- Plating peeling or flaking off
- Reduced corrosion protection
Prevention
- Thorough cleaning and rinsing
- Shorter activation times
- Use of oxide removers
Rough Surfaces
The as-plated surface replicates the substrate’s topography, so rough base metals produce rough chrome surfaces.
Causes
- Poor metal preparation
- Burrs and sharp edges on substrate
- Heavy grinding or machining marks
Effects
- Poor aesthetics
- Reduced corrosion resistance
Prevention
- Fine grinding and polishing of base metal
- Deburring sharp edges
- Mechanical finishing
Cracking
Cracks can form in the chromium layer due to hydrogen embrittlement, internal stresses, or mechanical damage.
Causes
- Embrittled deposit
- Residual stresses
- External impacts
Effects
- Corrosion sites
- Plating detachment
- Propagation of cracks
Prevention
- Baking to remove hydrogen
- Stress-relief annealing
- Careful handling to avoid impacts
Overcoming Plating Defects
While chrome plating has some inherent drawbacks, there are ways to minimize defects through proper control of the process:
- Surface preparation – Thorough cleaning and polishing of the substrate is crucial prior to plating to ensure good adhesion.
- Solution maintenance – Precise control of bath chemistry and temperature is needed to obtain high-quality deposits. Filtration and replenishment of chemicals is critical.
- Current density optimization – Careful current regulation minimizes defects like burning, pitting, and hydrogen embrittlement. Auxiliary anodes can improve current distribution.
- Post-treatment – Baking and annealing after plating can relieve stresses and remove hydrogen from the deposit to prevent cracking and embrittlement.
- Quality control – Rigorous process monitoring, inspection testing, and adherence to specifications is key to reducing defects.
Effect on Performance and Applications
The defects described above can negatively impact the performance of chrome-plated parts in various applications:
- In decorative chrome plating, defects like pitting, roughness, and burning result in unsightly blemishes on finished surfaces.
- For engineering applications, cracks, poor adhesion, and embrittlement can lead to premature failures of plated components through corrosion, wear, or fracture.
- In automotive applications, chrome defects can reduce the durability of trim pieces and lead to chipping, tarnishing, and corrosion.
- For industrial equipment, hydrogen embrittlement and cleavage cracks reduce the strength and fatigue resistance of critical plated parts like hydraulic cylinders and shafts.
- In marine settings, blistering, pitting, and density variations degrade the corrosion protection provided by chrome plating on components exposed to seawater.
Overall, plating defects can undermine the corrosion protection, wear resistance, and aesthetic qualities that make chrome plating desirable across many industries. Careful process control and testing is required to ensure satisfactory performance.
Conclusion
Chrome electroplating provides a versatile and durable coating for metal substrates across a range of applications. However, the plating process is prone to defects like blistering, cracking, and hydrogen embrittlement that can occur from various process parameters. Careful control of surface preparation, solution chemistry, current regulation, and post-treatment is crucial to minimize these disadvantages. While not always preventable, being aware of the common plating defects allows informed steps to be taken to reduce their risks and produce high-quality chrome coatings. Understanding the limitations and intricacies of the process is key to leveraging the benefits of this useful metal finishing technique.
References
[1] Sur, G. S., & Mandich, N. V. (2006). Electroplating and electroforming. In ASM Handbook (Vol. 5, pp. 351-362). ASM International.
[2] Safranek, W. H. (1986). The properties of electrodeposited metals and alloys. Amer Electroplaters & Surface Finishers Society.
[3] Lowenheim, F. A. (1978). Electroplating fundamentals of surface finishing. McGraw-Hill.
[4] Jain, R. (2017). Advances in material electroplating. In Electroplating (pp. 51-79). IntechOpen.
[5] Chrome plating. (n.d.). Sharretts Plating Company. https://www.sharrettsplating.com/chrome-plating/
