A schematic overview of coatings in the automotive industry and some of their properties

1. Overview of Coatings in the
Automotive Industry
Erik Galdames
Bach. Of Eng.
E-46800 Xativa (Spain)

2. Content
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Main groups of coating systems
Electroplating finishes
Zinc flake coatings
Conversion coatings
Mechanical plating
E-coating
Main applications for fasteners

3. Main groups of coating systems
Coating system
Coating
technique
Material
Used on
Post
treatments
Corrosion
protection
Colour
Electroplating
Electrolytic
Metallic
Metallic parts
Yes
Cathodic/Barrier
Various, mainly
silver
Zinc flake coatings
Dipspinning,
spray
Contains metals
Metallic parts
Yes
Cathodic
Silver
Organic coatings
Dipspinning,
spray
Organic
Metallic parts
Yes
Barrier
Various
Top-coats
Dipspinning
Organic/Inorgan
ic
Applied over
base coat
Possible
Barrier
Transparent/Black
E-coat
Electrophor
etic
Organic
Applied over
base coat
No
Barrier
Black
Conversion
coatings
Chemical
reaction
Metallic
parts/Applied
over metallic
coatings
Yes
Temporary
Silver
Yellow
Black
Lubricants
Dipspinning,
static
Organic
Metallic
parts/Applied
over metallic
coatings
No
Temporary
Transparent/Dark
Mechanical plating
Mechanical
Metallic
Metallic parts
Yes
Cathodic
Silver
Micro-encapsulated
Locking feature
Spraying
Organic
Coated parts
No
Barrier
Various

4. Main groups of coating systems
Coating system
Description
Electroplating
Electrolytic metallic coatings, pure or alloyed. Used in combination of passivation and top-coats and lubricant.
Good adhesion, uniform coating distribution, good temperature resistance. Non resistant to acids/alkaline products
except Ni, Cu, Cr, resistance to organic solvents. Susceptible to hydrogen embrittlement. ZnNi less susceptible to
hydrogen embrittlement. Good electrical conductivity. Reliable coating technology
Zinc flake coatings
Coatings contain Zn and Al flakes in an anorganic matrix. Dip-spin or spray. Used in combination with top-coats
and lubricants. Medium adhesion, fair ductitlity, sensitive to wear. Non resistant to acids/alkaline products, resistant
to organic solvents. Fair electrical conductivity. Modern coating technology
Organic coatings
Organic paints. Better to be used in combination of zinc flake coatings or phosphated surfaces. No cathodic
protection. Fair adhesion, good ductility, sensitive to wear. Good resistance to acids/alkaline products, good
resistant to organic solvents. Non-electrically conductive. Available in various colours. Modern coating technology
Top-coats
Organic and inorganic layers of nanoparticles. They offer sealing effect and increase corrosion protection of zinc
flake coatings and electroplated surfaces. With and without solid integrated lubricant. Non resistant to
acids/alkaline products. Fair electrical conductivity. Modern coating technology
E-coat
Organic coatings applied by electrophoretic technology. Used in combination of Zn or ZnNi electroplating. Excellent
black finish. Non-electrically conductive. Modern coating technology
Conversion
coatings
Applied by chemical reaction to a metallic substrate (phosphating) or metallic coating (Cr6-free passivation). No
increase in coating thickness. Temporary corrosion protection. Good friction properties. Reliable coating technlogy
Lubricants
They produce a regulation of coefficient of friction to reduce friction. Some provide temporary corrosion protection.
Reliable technology
Mechanical plating
Metallic coatings, pure or alloy applied by friction (mechanical interaction). Used in combination of passivation and
top-coats and lubricant. Good adhesion, uniform coating distribution, good temperature resistance. Non resistant to
acids/alkaline products, resistant to organic solvents. Non-susceptible to hydrogen embrittlement. Very good
electrical conductivity. Old fashioned coating technology. Reduced availability of suppliers.
Micro-encapsulated
Locking feature
Partial coatings with self-locking properties for threaded fasteners. Adhesive coatings and self-locking coatings.
Organic materials

5. Electroplating finishes
Coating
system
Layer
thickness
Cathodic
protection
WR/RR
Conversion
coating
capability
Top-coat
Weldabil
ity
Conductivity
Service temp.
Zn
8 µm
YES
48/96 h
YES
YES
Good
YES
90ºC
ZnFe
8 µm
YES
120/380 h
YES
YES
Medium
YES
120ºC
ZnNi
8 µm
YES
120/720 h
YES
YES
Medium
YES
150ºC, 300ºC**
ZnSn
8 µm
YES
120/720 h
YES
NO
Very
good
YES
120ºC
Cu
3 µm
NO
N.A.
NO
NO
Good
YES
250ºC
Ni
10 to 20 µm
NO
N.A.
NO
NO
--
YES
250ºC
Sn
5 to 15 µ
NO
N.A.
Mainly not
NO
Very
good
YES
90ºC
CuNiCr
10/25/0,3
µm
NO
480
NO
NO
--
YES
150ºC
5 µm
NO
N.A.
NO
NO
--
YES
150ºC
Ag
* Service temperature is the maximum temperature at which coatings maintain their main properties.
Though their melting temperature is higher, some characteristics like corrosion resistance will be
reduced
** Temporary local temperatures. E.g. normal service temperature 150ºC, short exposition to peaks of
300ºC can be reached

6. Zinc flake coatings
Coating
system
Layer
thickness
Cathodic
protection
WR/RR
Curing
temp.
Top-coat
Weldabil
ity
Conductivity
Service temp.
Geomet
6 µm
YES
200/600
200/1000
300ºC
YES
Poor
Poor
300ºC
Delta
Protekt
9 µm
YES
240/840
240ºC
YES
Poor
Poor
240ºC
Magni
10 µm
YES
240/840
240ºC
YES
Poor
Poor
240ºC
Zintek
10 µm
YES
200/720
240ºC
YES
Poor
Poor
240ºC

7. Conversion coatings
Coating
system
Layer
thickness
Cathodic
protection
WR
Substrate
Top-coat
Colour
Cr
passivation
0,1 to 0,3
µm
NO
Zn 48 h
ZnFe 96 h
ZnNi 120
h
Zn
ZnFe
ZnNi
Yes
Transp.
Yellow
Black
3 µm
NO
6h
(oiled) 24
h
Fe
Yes
Oil
Dark grey
ZnPh
MnPh
Application
Service
temp.
Improves corrosion
protection (WR)
90ºC
Temporary corrosion
protection
Base for paints
Lubrication properties
90ºC

8. Mechanical plating
Coating
system
Layer
thickness
Cathodic
protection
WR/RR
Conversion
coating
Top-coat
Weldabil
ity
Conductivity
Service temp.
Zn
5 µm
YES
48/96 h
YES
NO
Very
good
Very good
90ºC
Sn
3 µm
YES
48/96 h
YES
NO
Very
good
Very good
90ºC

9. E-coating
Coating
system
Layer
thickness
Cathodic
protection
WR/RR
Colour
Top-coat
Substrat
e
Conductivity
Service temp.
KTL
5 µm
NO
-/120 h
Black
YES
Fe
Nonconductive
200ºC
Zn+KTL
8 µm (only
KTL)
NO (only
the base
coat
provides it)
CC Test
Black
YES
Zn
Nonconductive
90ºC
ZnNi+KTL
8 µm (only
KTL)
NO (only
the base
coat
provides it)
CC Test
Black
YES
ZnNi
Nonconductive
90ºC

10. Main applications for fasteners
Coating
Typical applications
Adequate for
Less adequate for
Zinc flake coatings
Chassis
Motor compartment (areas
exposed to high corrosivity)
Aluminium joints
Bolts ≥ 10.9
Microencapsulated coating
allowed (without lubricated topcoat)
Ground connections
External thread <M6
Internal thread <M10
Internal drive features T30
ZnNi
Chassis
Motor compartment (areas
exposed to high corrosivity)
Aluminium joints
External thread < M6
Internal thread < M10
Bolts ≥ 10.9, ≥ 1000 N/mm 2
Though less hydrogen
embrittlement risk than pure eplated Zn.
Service temperature > 150ºC
ZnNi black
Chassis
Motor compartment (areas
exposed to high corrosivity)
Aluminium joints
External thread < M6 with
internal drive feature
Internal thread <M10
Self-tapping screws
Bolts ≥ 10.9, ≥ 1000 N/mm 2
Though less hydrogen
embrittlement risk than pure eplated Zn.
Service temperature > 150ºC
Phosphating + oil
Multicomponent, oil circuits or
transportation
Parts in motion without
corrosion exposure
Transport and limited storage
in warehouses
Parts not multi-component
Service temperature > 180ºC
Zn+passivation+top-coat
(silicate)
Mg alloys
Parts in contact with
magnesium alloys
Bolts ≥ 10.9
Zn+passivation
Weld parts
Weld parts
Bolts ≥ 10.9