Corrosion resistant coatings for metals and alloys

1
Corrosion resistant coatings for metals and
alloys Goals  Robust coatings technologies for
demanding service applications, performance
equivalence to current technologies, low toxic
hazards, low added costs, do not impair
recycling.  Provide a scientific basis for
performance expectations in chromate coating
systems. Replacement Objectives Cr-free coating
systems SrCrO4-epoxy primers Chromate
conversion coatings Degreasing and
deoxidizing Chromic acid anodizing and
dichromate seals Cadmium coatings Cr and Ni
plate Additional Objectives Corrosion sensing
Sensing coating failure
2

• Technical challenges- conversion coatings.
• The objective is a performance equivalent,
  drop-in replacement for chromate conversion
  coating.
• Emerging technologies require deeper assessment
  of function.
• Surface pretreatment issues require systematic
  study.
• 3) The relative contributions of adhesion
  promotion versus corrosion protection by a
  pretreatment remain unquantified.
• 4) Coating chemistries robust enough for use on
  different classes of alloy or on different types
  of metals.
• 5) What is good enough in terms of performance?
  Corrosion performance threshold of 336 or 1000
  hours salt spray resistance?
• 6) Systematic approaches for incorporating
  corrosion inhibitors are not well used.
• 7) Relative merits of approaches? Inducing
  coating formation by sol-gel or by precipitation?
  
• 8) Performance on Al, Zn, Mg and steel (if
  possible).

3
A range of Cr-free conversion coatings are under
development.
A.E. Hughes in preparation for ASM Handbook on
Corrosion (2002).
4
Chromated Primers - Technical challenges Objectiv
e a long-term highly protective coating system
for condensing atmospheric exposure. 1) No
inorganic pigment possesses needed solubility and
corrosion inhibition characteristics. 2) Role of
adhesion versus inhibition not understood. 3)
Need for barrier properties not agreed upon. 4)
Few inhibitor storage and delivery schemes being
pursued. 5) Coating aging effects on corrosion
protection not well known. 6) Mechanical failure
of organic coatings not well integrated. 7)
Suitability of ion exchange compound pigments
need further development. 8) The concept of
inhibitor synergy should be better to
understand its general applicability and whether
there are gereral rules.
5
A remaining challenge--highly protective Cr-free
coating system for thin-coat applications.
Coating systems on Al alloy substrates do not
meet salt spray resistance requirements (2000h
)unless chromate is present in either the
conversion coating or the primer coating.
J. Osborne, et al., ACRAC Final Report 2000.
6
Is the technology stuck? If so, it wont be for
long.
EU End-of-life directive 2000/53/EC places a
ban on the use of Cr6 (among other heavy metals)
in automobiles put on the market after July 1,
2003.  Currently, Cr6 usage is 1 -
2g/vehicle.  Government authorities control the
destiny of end-of life vehicles.  Manufacturers
must cover most or all of the take-back
costs. The California EPAs Air Resources
Board has banned use of Cr6 and Cd in automotive
coatings starting January 1, 2003.  CA has
been a bellwether of trends in environmental
policy across the U.S.
P.C. Wynn, C.V Bishop, Automotive Finishing
Online, Gardiner Publications(2002). California
EPA Air Resources Board News Release, Sept. 20,
2001.
7
(No Transcript)
8
Technical Challenges - Chromic acid anodizing and
dichromate seals.
Objective is a replacement for chromic acid
anodizing and dichromate sealing of porous
anodized coatings.
9
Technical challenges - Cadmium coatings. Objectiv
e an environmentally friendly replacement for
metallic Cd coatings for steel componentry.
10
Technical challenges - Cr and Ni plate
Objectives Hard coat replacements for corrosion
protection, abrasion and wear resistance.