1. Properties and Comparisons of Commonly Specified Coatings
Zinc/Zinc-Alloy, Copper/Alloys, and Some Precious Metals
By: Frank Altmayer
Technical Education Director, AESF Foundation/NASF
Data collected from a variety of sources, including ASTM and
“The Properties of Electroplated Metals & Alloys by Lowenheim”

2. Properties of Electroplated Zinc
Density: 7.14 g/cm3
Melting Point: °C/787.15°F
Brittle up to 100°C, Malleable 100°C-200°C, Brittle at >200°C
Para-magnetic
Electrical resistance: 5.9 mΩ cm
Not Solderable/brazeable
Weldable (fume problem)
Amphoteric
Metal ions are low in toxicity
Aquatic toxin (ion)
Powder is autophoretic
Incompatibles:
Chlorates, Chlorine, Acids, Nitrates, Sulfur, Calcium Chloride, Alkalis
Bare and zinc electroplated fastener

3. Properties of Electroplated Zinc
Sacrificial corrosion protection of ferrous substrates
Economical but yields unsightly corrosion products (photo)
Easily chromated to extend service life and achieve a desired appearance
Can be chromated and dyed/painted to produce a colored coating
Lower level of salt spray performance vs. Zn- alloys or Cd
Can grow zinc whiskers that may be a few microns in diameter and can grow at rates of up to 1 mm per year
Related to high compressive stress in the deposit and environment
Preventive measures include alloying and reduction of stress
Other metals that can grow whiskers include tin, silver, gold, cadmium
Zinc Whiskers, by Schtone, Wikipedia photo

4. Corrosion Resistance of Zinc
(From ASTM B633)
Atmosphere Corrosion Rate (μm/year)
Industrial 5.6
Urban nonindustrial 1.5
Suburban 1.3
Rural 0.8
Indoors <<0.5
Service Condition Min. Thickness, µm
SC4 (plumbing fixtures, pole line hardware) 25*
SC3 (furniture, builder’s hardware, bicycle parts) 12**
SC2 (tools, zippers, pull shelves, machine parts). 8
SC1 (buttons, wire goods, fasteners) 5
Note: The corrosion rate is greatly increased by frequent dew and fog, particularly if the evaporation rate is slow.

5. Corrosion of Zinc Bare Zinc: Zinc Plus Chromate
White gelatinous corrosion products
Zinc oxide plus zinc carbonate
Highly porous
Corroded fasteners may seize
Anti seizing compounds may be applied
Red rust indicates protection afforded by zinc has broken down
Zinc Plus Chromate
Chromate acts as barrier against corrosive
White corrosion products indicate failure of chromate to act as barrier (base metal is still protected
Red rust indicates protection afforded by zinc plus chromate has broken down
Corrosion of bare zinc over steel
Corrosion of zinc plus chromate over steel

6. Galvanic Series } } Metal E° [volts], 25°C E in Seawater @ 25°C
Zn → Zn e
Fe → Fe e
Cd → Cd e
Co → Co e
Ni → Ni e
Sn → Sn e }
0.4V }
0.1V
In a chloride environment, cadmium is less noble than iron
In a sulfate (acid rain) environment, cadmium is more noble than iron

7. Salt Spray Performance of Zinc Alloys
Hours to exposure for 12 microns over steel
Coating Hrs To WR Hrs To RR
Bare Cadmium NA >1000
Cadmium + Chromate >1000
Zinc-Nickel + Chromate (acidic solution)
Zinc-Nickel + Chromate (alkaline solution) >1000 >1000
Zinc-Tin + Chromate >1000
Zinc-Cobalt + Chromate (acidic solution)
Zinc-Cobalt + Chromate (alkaline solution)
Hours to red rust for 8 microns over steel

8. Zn-Ni vs. Zn-Co and Zn-Fe
Parameter
Acid
Zn-Ni
Alkaline Zn-Ni
Zn-Co
Alkaline Zn-Fe
Zn-Fe
Alkaline No-CN Zn
Alloy %
10-12
5-12
15-50
None
HV100
80-150
CCE
95-100
45-80
50-70
70-80
50-80
Throwing Power
Poor
Good
Formability
Fair
Solution Corrosive to Steel
Yes No
Appearance
Excellent

9. Zinc-nickel plus yellow chromate
Features:
High salt spray resistance
4-6 Times the performance of zinc alone
Good Kesternich SO2 performance
Zinc-cobalt is better
Favorable galvanic couple with aluminum
High abrasion resistance (HV100 = )
Good formability & weldability
Excellent corrosion resistance at elevated temperatures(up to 180°C)
Forms temperature stable chromates
Limitations:
Inferior to Zn-Co in SO2 (acid rain)
Zinc-nickel plus yellow chromate
 2005 American Electroplaters & Surface Finishers’ Society, Inc. R-3-05

10. Automotive parts, zinc-cobalt plated, illustrating a variety
Features:
Co may be unregulated
Less expensive v. zinc nickel
Low Co alloys are highly ductile
HV100 =
Parts can be deformed after plating
Deposit is easier to chromate
Limitations:
Chromates are dehydrated at elevated temperatures
Less corrosion resistant vs. Zn-Ni, Zn-Fe
Lower solderability vs Zn-Ni
Higher coefficient of friction vs Cd
Automotive parts, zinc-cobalt plated, illustrating a variety
of chromates

11. Zinc-Iron Features: Limitations:
Low cost (least expensive of the zinc alloys)
High ductility, suitable for toxing
Low hardness (HV25 = )
Limitations:
Corrosion resistance is heavily dependent on the chromate
Less corrosion resistance vs. Zn-Ni, Zn-Co
Corrosion resistance is obtained from interaction of chromate and iron (bare Zn-Fe @ % Fe is no better than bare Zn)
Prone to delayed blistering

12. Tin-zinc, as plated and chromated
Features:
High Corrosion resistance (especially in sulfur atmosphere)
Chromate performance better that other alloys of zinc
Excellent solderability
Low coefficient of friction
Excellent lubricity
Excellent ductility
Soft (HV25 =13-28)
Excellent couple with Al
Near neutral plating solutions are available (pH 5.5-7)
Limitations:
Tin is expensive
Unpleasing appearance
Tin-zinc, as plated and chromated
 2005 American Electroplaters & Surface Finishers’ Society, Inc. R-3-05

13. Impact of Zinc Alloys on Hydrogen Embrittlement Relief Bake
Coating Pass/Fail Hrs. to Failure
Bare Cadmium PASS (3) >200
Cadmium + Chromate PASS (1) 183, (5) >200
Acid Zinc-Nickel FAIL (3) 0.5, 0.4, 0.5
Acid Zinc-Nickel* PASS (3) >200
Alkaline Zinc-Nickel PASS (1) 141, (5) >200
Zinc-Tin FAIL (3) 0, 0, 0.2
Acid Zinc-Cobalt FAIL (3) 0.8, 5.7, 4.4
Alkaline Zinc-Cobalt FAIL (3) 4.5, 4.5, 8.3
Notes:
All alloy deposits were chromated
* Designates an alternate supplier’s process was used
 2005 American Electroplaters & Surface Finishers’ Society, Inc. R-3-05

14. Electroplated Copper Melting Point: 1083°C
Stable in water, tarnishes quickly in air
Eventually forms a patina and stops corroding
Ductile, malleable
Elongation is %
Easily buffed/machined
Excellent conductor of heat and electricity
Hardness is typically HV25 =
Low internal stress
Chemically attacked by nitric, hot sulfuric, sulfides
Diffusion barrier over brass
Heat Treat Stop-off
Under-plate for nickel
Most commonly plated from sulfuric acid, cyanide or alkaline non-cyanide based solutions

15. Electroplated Copper Property General Range High Strength Deposit
UTS (kpsi) 64 – – 93
Yield (kpsi) 41 – 57
Elongation (%) 0 – 40 3 – 18
Hardness (HV) 48 – – 159
Resistivity (micro-ohms-cm) – 2.1
Internal Stress (psi) -6, ,000 +5,000 – +7,000
Fatigue Limit (106 cycles, kpsi) 10 – 20 No data
Notes:
Thinner deposits from copper sulfate solutions are usually stronger than thicker
Thicker deposits from cyanide solutions are usually stronger than thinner
Fatigue limit for copper deposited from a cyanide copper solution (using periodic reverse rectification) was twice that for sulfuric acid

16. Bronze (Copper-Tin) Applications: Most commonly plated alloy: 8-15% Sn
Powertrain, hydraulic and bearing components
Hardware exposed to marine environments (corrosion rate = 0.002ipy)
Nitride stop-off,
Most commonly plated alloy: 8-15% Sn
High hardness (HV = )
Low coefficient of friction (0.06 on steel vs for hard chrome)
Color ranges:
Red (8-10%Sn)
Golden yellow (12-15%Sn)
White (>22%Sn)
Bronze plated hydraulic pump cavities
Photo by F. Altmayer
Wikipedia photo
Bronze plating on a German ship hull (1914)

17. Bronze (Copper-Tin) 120 600 100 500 80 400 Resistivity Micro-ohms-cmHV 60
300 40
200 20
100 20 40 60 80
Weight % Tin

18. White Bronze (55-60%Cu, 25-28%Sn, 14-18%Zn)
Features:
55-60%Cu, 25-28%Sn, 14-18%Zn
Alternative to electroplated nickel
Bright
Solderable
High Hardness (HV )
Non magnetic
Effective diffusion barrier between gold and copper
Limitations:
35% the conductivity of copper
Cyanide based process
White bronze plated spring loaded washers for electronic terminals

19. Properties of Plated Silver
Features:
Best conductor of heat and electricity of all metals
Electrical resistance = µ-ohms-cm, unless brightened with antimony (10 times more resistive)
Ductile, Malleable
UTS: 34-48ksi
Elongation: 12-19%
HV25 = )
Melting point 960.5°C
Boiling point 1950 ° C
Density 10.5 g/cc
Excellent reflection of visible light
Internal stress: psi (tensile)
Anti-galling
Anti-bacterial
Automotive fuse with silver plated contacts
Photo by F. Altmayer
Limitations:
Silver readily reacts with sulfur producing silver sulfide (Ag2S)
Silver sulfide whiskers growing out of surface-mount resistors
Photo by J. Reinhart

20. Comparing Plated Precious Metals
Property Ag Au Rh Pd Pt
Color White Yellow White White Grey
Density, g/cm
Melting Point, °C
Hardness, HK
Ductility (% elongation)
Catalytic Activity No No Yes Yes Yes
Crystal Structure FCC Nano FCC FCC FCC
Visible Light Reflection Excel. Poor Excel. Good Fair
Internal Stress Kpsi
Corrosion Resistance Poor Excel. Excel. Good Excel.
Wear Cycles* < ,
Contact Res. Ohms < * .6 Arc Resistance Poor Poor Excel. Poor Poor
Solderability Excel Excel. Good V Good V Good Thermal Exp. Coeff
Thermal Conductivity**
*Cross wire, 200 g load
**Cal/cm/sec/°C

21. Properties of Electroplated Gold
Features:
Typically alloyed with nickel, cobalt and other heavy metals
Additive-free hard gold is available
Hardness, HK25 =40-90 Pure, Alloy
Extremely malleable
0.5mm diameter pellet can be hammered to >0.5 square meter
Highly corrosion resistant to most environments
Low internal stress: -8,000ksi to +40,000ksi
Liabilities:
Can grow “whiskers”
Chemically attacked by: Cyanide, Aqua Regia
Gold Plated Contacts
Gold Whiskers
Gold Plate
Nickel Plate
Cross Section of Gold over Nickel, 1000X

22. Properties of Electroplated Gold
Stress: to +40,000
Contact Electrical Resistance: Milliohm/in2
Electrical Resistivity: micro-ohms-cm
Wear Properties
Transitional metal Alloys Best
Organic Brighteners Good
Semi-metallic Brighteners Poor
Porosity
Semi-metallic Brighteners Best
Transitional metal Alloys Poor
Micro-circuit manufactured using gold plating
Notes:
Porosity of a gold deposit is also related to thickness, temperature, solution purity, and current density
Gold plating offers excellent corrosion resistance, excellent solderability, weldability and infra-red reflectivity

23. Electronic Connections
Issues:
Corrosive environments
Atmospheric
Body corrosives (wearables)
Abrasive Wear
Connection cycles may be very frequent
Thermal effects
Heat dissipation requirements
Formation of inter-metallics
Diffusion
Contact Between Mating Surfaces
Occurs only at the high spots, creating constriction resistance
Surface roughness has an impact on contact resistance
Resistance due to current constriction
High Spot
Illustration by F. Altmayer

24. Electronic Connections
More Issues:
Connectors are smaller, more dense and often carry a higher current density
Soldering nearer to the mating surfaces can heat the connectors to ~240°C
Connectors may need to operate at higher temperatures (up to 300°C) vs. older designs
Co hardened gold is not suitable at > 125°C
Ni hardened gold is limited to about 200°C
Soft gold over hard gold may allow service at higher temperatures
Selectively plated connector
USB connector
Apple type connector
Illustration courtesy of Molex Inc.
Photo by F. Altmayer

25. Electronic Connections
Contact Electrical Resistance (CER):
Films and abrasive residues from sliding friction increase CER
Total CER is a combination of resistance due to constriction plus film resistance plus the force between mating surfaces
Mating force may be <10 grams
Resistance due to current constriction
High Spot
1.0
0.0016” Ni under-plate
CER (mΩ)
0.5
0.4
0.0008” Ni under-plate
0.3
0.0002” Ni under-plate
Gram Load
Deposit % Au Resistivity (µΩ-cm) CER (mΩ)
Pure Gold
CoHG
NiHG
Data from “Gold Plating Technology” by Reid and Goldie

26. Electronic Connectors
Soldering Issues:
Gold dissolves into the solder, bond is created by the formation of an intermetallic with the under-plated metal/alloy
Solder containing > 4% Au can be brittle and may form a weak joint due to formation of intermetallics with (preferentially) Sn and Pb
Surface Roughness:
Smoother surfaces yield low levels of porosity and lower CER values:
Roughness (RMS) Pores/cm (CoHG)
Data from “Gold Plating Technology” by Reid and Goldie

27. Gold Plating on Connectors
Features:
Reliability
Especially at current as low as 1 nano-amp
Corrosion resistance
Wear properties
Suitable for low force connections
Low/consistent contact electrical resistance
Not subject to degradation by fretting
Usable at elevated temperatures
Liabilities:
Cannot be mated to tin plated surfaces
Requires nickel (or other suitable) under-plate
Low arc resistance
Whiskers can produce short circuits
High cost of gold
Photo by F. Altmayer

28. Properties of Palladium
Features:
Melting Point 1555°C, Boiling Point 2200°C
Density: g/cm3
Internal Stress: +10,000 to +100,000 psi
Good Tarnish Resistance
Hardness: HK25
Contact Resistance, ohms
Resistivity, 10.7 ohm-cm
Solderability: excellent
Thermal Expansion coefficient: 11.8
Thermal conductivity: 0.17 cal/cm/°C
Liabilities:
Attacked by HCl and HNO3
Arc resistance: low
Low ductility (Elongation: 2-4%)
Relatively Poor Wear Properties
Polymer Adsorption on Surface
Palladium plated jewelry

29. Palladium-Nickel Alloys
40 to 99+% palladium
Bright
Hard electrodeposits (450 to 600 KHN100)
Much lower tendency toward micro-cracking and polymer adsorption
Good chemical stability
Good wear resistance
Deposit crystal orientation is (100) or (110)
More economical than pure palladium
Outperformed by pure palladium deposits in applications serving at high temperatures

30. The End, Thank You!