This document discusses various types of coatings used to protect metals from corrosion. It covers metallic coatings such as those that are more or less noble than the base metal. It also discusses organic coatings and how they provide protection through barrier properties and active inhibition. Various coating application methods are outlined. Finally, common corrosion issues that can occur with coatings like blistering, rust, and delamination are described along with examples of how coatings can solve corrosion problems in applications.

1. Coatings
Hans van der Weijde
Why coat metals ?
protection colour function

2. Subjects (problem, cause, solution)
• Metallic coatings (norm site)
• Anodic / cathodic (+ noble on noble)
• Organic coatings
• Combination
• Solving corrosion by coating (case)
Metallic coatings
• Various ways to categorize:
• Protection mechanism
• Application method
• Layer more noble than base metal
• Layer less noble than base metal

3. Layer more noble than base metal
• decorative, functional (wear), corrosion protecion…
• Uses :
Layer more noble than base metal
• Both metals “noble”:generally OK, be aware for exceptions
• (One) metal prone to corrosion: defect leads to pitting
• Multi layers normal (also for other reasons)y ( )

4. Layer less noble than base metal
• Sacrificial actions coating
• May work over a (short) distance : cut edge
• Also barier function: Zinc as example

5. Applications methods
• Plating: thin layers possible, controlled finish/structure (Ni, Cr, Sn,
Zn )Zn,…)
• Hot Dip : thicker layers, surface quality less (Zn, Al, …), dense
S thi k l it• Spray : thicker layers, porosity
• Gas phase (PVD, CVD): thin layers, controlled finish/ structure but
tendency for porositytendency for porosity
Organic Coatings & Protection
• Protection through:
• Barrier
ņ Real barrier
Adhesion/interface integrityņ Adhesion/interface integrity
• Active inhibition
ņ Passivating : chromates, phosphates
ņ Ion exchanging : clays/hydrotalcite
ņ Electrochemical : Zn powder

6. Barrier
• Polymer properties
• Pigments (micacious iron oxide, aluminium flakes)
?• Real Barrier ?
• Water/oxygen
• Ions : yes, but…..y ,
• Resistance
inhibition Temperature
[°C ]
Diffusion rate
[mol/m2s][ C ] [mol/m2s]
20 1.27E-08
60 1.37E-07
90 5.82E-03
Interface integrity
• Prevent formation of an electrolyte in
• Pores
• Delaminated areas
• “areas of lower density”areas of lower density
• If electrolyte is formed corrosion starts but type of interface is
determining
• Type of bond
• Type of surface (=type of oxide)
Example: dampafzuiging

7. Corrosion & Coatings
• Wet “adhesion”
• Blistering (blaarvorming)
• Early rust (vroeg roesten)y ( g )
• Flash rust (vliegroesten)
• Anodic undermining• Anodic undermining
• Filiform corrosion
• Cathodic delamination• Cathodic delamination
Wet adhesion
• Better: wet delamination
• Delamination from what surface ?
• When: if bonds to “metal surface” are hydrolised by incoming watery y g
• Purely matter of bond energy, no electrochemistry involved
• No corrosion (yet)• No corrosion (yet)
Example : hydrogen bond

8. Wet adhesion (2)
Fe-OH-OH-C
Turned into : Fe-OH-H2O-OH-CTurned into : Fe OH H2O OH C
And further : Fe-OH-H2O-H2O-OH-C
Etc.
Strongly depending on the bondStrongly depending on the bond
strenght in comparison to the bond
strenght to waterg
Solution : coating & pre-treatment
selection
Side step: surface conditioning
• Metal surface not a metal: (conditioned) odixe
• Natural
• “passivated”
C (VI) C (III)ņ Cr(VI) or Cr(III)
ņ Phospates
ņ Reare earth oxides (Ce)
ņ More and more also mixed with organics

9. Blistering
Areas where adhesion is lost and water (and ions) can accumulate, as a
lt fresult of
-extreme swelling
-solvent retention & porosity
-Osmosis (surface debris)
-Phase separation during filmforming
Early rust
• If moisture “dissolves” in solvent before final cure
• Low temperature
• High humidity
( i hibi d) b d• (uninhibited) water based systems
• May show after second coating layer
C d f b i ti• Concern: decrease of barrier properties

10. Flash rust
• (waterbased) coating on grit blasted surfaces.
• Some grit remains and acts as corrosion
accelerator
Differential aeration cell
O2 + 2H2O + 4e- o 4OH-
Anodic metal
dissolution
Cathodic Oxygen
Reduction
Cathodic Oxygen
Reduction
M o Mn+ + ne-
O2 + 2H2O + 4e- o 4OH-
Water droplet
Mn+ Mn+
Mene-
OH-O2 O2+H2O
M(OH)n
OH- +H2O
M(OH)n
Me
tal
ne ne-
.

11. Anodic undermining
• Caused by corrosion from a defect or cut-edge
• When defect is not blocked by corrosion products
Anodic undermining
Electrolyteand O2Electrolyteand O2
oxide
PolymerPolymerPolymerPolymer
coatingcoating
Al3+/OH-
coatingcoating
oxideoxideoxideoxide
SteelSteelSteelAl

12. Anodic underminding
intact
O2
electrolyte
intact
O2
electrolyte
intact
zone
intact
zone
intact
zone
intact
zonezonezone
oxideoxide Al3+/H+
Al3+/H+OH-
SteelSteelSteelAl
Filiform corrosion
• Thread like filaments
• High RH (60-95%)
• Permeable coating
C i i ( hl id ) d f• Contamination (chlorides) + defects

13. Cathodic delamination
Another differential aeration cell
When defect is blocked by corrosion products
Cathodic delamination
OO
Electrolyteand O2
O2
id
Electrolyteand O2
O2
idFerrousoxideFerrousoxide
Polymer
coating
Polymer
coating
Polymer
coating
Polymer
coating
FeOH+
coatingcoating
FeOH+
coatingcoating
oxideoxideoxideoxide
SteelSteelSteelSteel

14. Cathodic delamination
O2
intact
O2
electrolytecathode
cathodeO2
intact
O2
electrolytecathode
cathode
zone
intact
zone
O2
PET
zone
intact
zone
O2
PET
O2 + e- Æ O2
- .
O2
- . + H2O Æ O2H . + OH-
FexOyFexOyFexOyFexOy
O2H . + e -- Æ O2H -
O2H - + H2O Æ H2O2 + OH
oxide
FeOH+
oxide
e-e-e-e-e-e-e-
FeOH+-
H2O2 + e- Æ OH . + OH -
SteelSteelSteelSteelSteelSteelOH . + e - Æ OH -
Cathodic delamination
More reaction steps and radicals exist during the processes and can cause more
coating damage than expected from pH alone
O2 + e- Æ O2
- .
O - + H O Æ O H + OH-O2
- . + H2O Æ O2H . + OH-
O2H . + e -- Æ O2H -
O2H - + H2O Æ H2O2 + OH -
H2O2 + e- Æ OH . + OH -
OH . + e - Æ OH -

15. Cathodic delamination
• cations partly determine rate !
Cathodic delamination
½ O2
Katn+
Fe2+
Ann-Ann-
O2-
2e-2e-

16. Solving corrosion by coating (1)g y g ( )
Solving corrosion by coatings(2)
• Sn coating solves Corrosion (Sn + Sn Alloy)
• But :FeSn2 not an acceptable look (cosmetic) when locally dissolved
• Sn coatings adds functionality
• Partial coat problem area and leave rest of surface “as is”