Branko N. Popov

1
Environmentally Friendly Coating Technologies
for Corrosion Protection Systems
Branko N. Popov
Center for Electrochemical Engineering, Department
of Chemical Engineering, University of South
Carolina, Columbia, SC 29208
AFOSR Corrosion Control Workshop December 4-5,
2006, Arlington, VA
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Deficiencies of Conventional Technologies
Conventional Cadmium Coatings

• Protects steel parts in Naval aircraft
  components, weapon systems and ground support
  equipment
• Fasteners, Landing gear, etc.

Advantages
Disadvantages

• Good barrier against corrosion
• Sacrificial coating
• Galvanically compatible with steel alloys

• Toxic
• Electrodeposition from cyanide baths
• Hydrogen embrittlement of steel due to hydrogen
  permeation during plating process

? ELV regulations restrict the use of cadmium
Alternatives to cadmium have become imminent.
3
Deficiencies of Conventional Technologies
Conventional Conversion Coatings
Hexavalent Chrome
Phosphating
Trivalent Chrome

• Advantages
• Higher heat tolerance
• Streamlined waste water management
• Disadvantages
• Less robust than hexavalent chrome
• No self-healing properties
• Unsatisfactory temporary solution

• Advantages
• Higher corrosion resistance
• Self-healing properties
• Disadvantages
• Highly toxic
• (e.g. Vehicles for the European market must
  contain less than 2 g of hexavalent chromium per
  vehicle)

• Advantages
• Environmentally benign
• Improved paint adhesion, lubrication and
  corrosion resistance
• Disadvantages
• Not as robust as chrome passivation processes

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Future Research Directions
Environmentally Friendly Technology for Cd
Replacement
Sandwich-type coatings
? Sandwich type coatings of electroless Ni-P,
pulse Zn-Ni and electroless Ni-Zn-P coatings. ?
Introduction of an underlying electroless
Ni-P/Ni-Zn-P or a thin layer of the same on the
surface of Zn-Ni alloy inhibits the corrosion
process.
? Compositionally modulate the coatings with
increasing nickel content (up to 21) in the
deposit by varying the potential sequence of
deposition. ? Presence of nickel-rich layer on
surface is expected to decrease the driving force
for corrosion of underlying metal.
? Surface modification meth-odology of Zn and
Zn-Ni has been developed. ? Introduction of thin
silicate layer (1 mm) decreases the corrosion
process.
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Diagnostic Model for Hydrogen Permeation through
High-Strength Alloys
Development of a user-friendly software for
predicting hydrogen permeation and embrittlement
failure under aerospace-use conditions
Unique features of new model developed at Univ.
of South Carolina
? Hydrogen permeation with discharge-chemical
recombination reactions under different
environmental corroding conditions ?
Quantitative determination of the rate constants
of (i) hydrogen discharge, (ii) recombination and
(iii) adsorption reactions, etc. ? Diagnostic
criteria for comparing the hydrogen entry
efficiency of different alloys ? Prediction of
hydrogen embrittlement failure ? Design of alloy
structures that are resistant to hydrogen
embrittlement cracking