Transcript MD-12-9 GalvInfo Center

Control and
Treatment of Hot-Dip
Galvanize Surfaces
Presented at the 97th Meeting of the
Galvanizers Association
October 16-19, 2005
Lexington, KY
GalvInfo Center
A zinc-coated steel sheet
technical information center
managed by ILZRO and
cosponsored by the steel, paint
and zinc industries.
Galvanize Surfaces
Hot-dip galvanize vary in appearance, and therefore
usability, because of:
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spangle size
zinc composition
mechanical treatment
chemical or oiling treatments.
Marketplace problems develop because customers find
the surface is:
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different in appearance than expected
affected by darkening, water staining, or field handling
marks
covered with unwanted substances that affect the ability
to pretreat, paint, or weld
Reasons for
Surface Treatments
 Improved surface uniformity
 Resistance to storage stain
 Improved adhesion/corrosion
resistance of prepainted steels
 Surface lubrication for forming
 Resistance to handling marks
 Preparing for field painting
Improving Surface Uniformity
 Temper passing gives a
more uniform and duller
surface topography
 Improves painted
appearance
 Typical TM is 4 Hi using
several hundred tons of
rolling force
 Percent extension can
be up to 2%
Improving Surface Uniformity
 Temper rolling hides
and smoothes spangle
 Improves painted
appearance by
controlling surface
roughness – i.e., average
roughness (peak height)
versus peaks-per-inch
 critical for exposed
automotive surfaces
needing a high DOI
after painting.
Spangle - Regular
 Dendrite growth dominates
during solidification
 Spangle or grain boundaries are
“depressed”
 Difficult to smooth by temper
passing
 Form due to impurities in the
zinc, historically Pb content in
the range of 0.05 to 0.1/0.15%
 Removal of Pb causes spangle
to disappear
Spangle-free
 Absence of Pb results in grains
growing by a cellular mode
 Grains are ~ 0.5 mm across barely visible to unaided eye
 Grain boundaries are very flat
 Very easy to smooth by temper
passing
 Satisfies the need for a
smooth surface and
environmental concerns
Galvanneal
 Produced by reheating to about
590°C for 10-15 seconds to
convert zinc to zinc-iron alloy
 Converts the appearance to
matte grey and has a surface
that results in very good paint
adhesion – needlelike crystals
into which the pretreatment
and/or paint can “lock”
 Temper passed to control the
topography
 Extensively used for high
quality auto exposed finishes
Improving Resistance to
Storage Stain
 Chemical Treatments – Cr Based
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Premature Spangle Darkening
Tests for passivation
 Chemical Treatments – Non Cr
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Removable and Permanent
RoHS issues
Oils
Chemical Treatments
 For decades these treatments have
been based on chromium solutions
 Primary purpose is to reduce the
susceptibility of metallic-coated sheet
to storage stain (white rust)
Storage Stain – “White Rust”
 Corrosion stain – typically
white zinc hydroxide – that
forms between sheets in close
contact that become wet
 Zinc hydroxide forms in the
absence of free air flow
 Can be grey, or black in color if
enough zinc is consumed to
allow iron to become involved
 Light white will abate aver time
if allowed to weather
Storage Stain on Galvanneal
 Stain that forms on water
damaged galvanneal is grey or
black
 Dark nature of the stain is the
result of the iron in the coating
 Can form easily on galvanneal
as much of it is produced as
unpassivated
Chromium Based
Chemical Treatments
 Use chromic acid, chromium salts and mineral
acids
 Dissolves some of the metal and forms a
protective film of complex chromium and metal
compounds
 Usually thin (<0.1 mm) and invisible but have yellow
or green tinge if applied heavier (0.1-0.6 mm)
 Total Cr 1-2 mg/ft2, with < 50% Cr+6 in complex
mixture of metal salts and oxides
Chromium Based
Chemical Treatments
 Galvanize has surface layer of Al2O3 (tens of
nanometers) that must be removed
 CT solution must dissolve Al2O3 layer with
fluoride to allow deposition of Cr compounds
 Zinc protected via barrier and passivation effects
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Cr oxide acts a barrier
Cr+6 re-passivates exposed metal
 Cr+6 is reason for the self-healing ability of
chromate passivation films
Effectiveness of Cr Based
Passivation
 Resistance to staining varies as
a function of the accelerated
test used
 Illustrates that oxygen may
play a role in stain formation
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Condensation test open to air
Water-film test isolated from
air
 Obvious that zinc has almost no
resistance to staining in the
absence of Cr on the surface
Premature Spangle Darkening
 Can occur after a few days of
exposure
 Reported characteristics
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Only in rural environments
Occurs within one week of
installation and only on outside
Some sheets remain bright
 Some spangles darken more
than others
 Appears related to spangle
forming elements (Pb & Sb)
 Not reported on spangle-free
coatings
Some Disadvantages of
Cr Passivation
 Paintability
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Decrease the adhesion of most paints to zinc
Severely interfere with the deposition of iron and zinc
phosphate treatments
For painting it is best to produce unpassivated sheet
 Weldability
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Interferes with spot weldability by “poisoning” copper
alloy welding electrodes – shortens electrode life
Use only unpassivated sheet
Is It Passivated?
 Usually not possible to visually determine
 Producers use lab testing methods not available in
field
 Field tests:
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%5 HCL – drop will “fizz” on unpassivated
Diphenylcarbohydrazide – drops turns pink if Cr+6
present – ASTM D 6492
Use quick condensing humidity test – 140°F water in
beaker – test coupon as lid for 10-15 minutes
Condensing Humidity Test
Non-Chrome Treatments
 Alternatives being sought because of
environmental concerns, e.g., RoHS
 Removable non-chrome treatments
available now
 Permanent non-chrome treatments under
very active development
RoHS
 Article 4(1) of Directive 2002/95/c of the
European Parliament on the Restriction of certain
Hazardous Substances in electronic equipment
 From July 1, 2006n new equipment cannot contain:
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Lead
Mercury
Cadmium
Hexavalent Cr
PBB and PBDE flame retardants
 Request to exempt Cr+6 not yet ruled on
Oils
 Used sometimes as an alternative to passivation
 Specially formulated – contain polar products that
adsorb onto metal surfaces
 Effective in protecting against humidity rust prevent moisture condensing between contacting
sheet surfaces
 Not effective in preventing penetration of bulk
water – staining will occur quickly if this happens
 Used for prepaint products – can be cleaned off
 Provide lubrication during forming
Pretreatments
 Used to obtain good bonding between the metal
surface and paint
 Phosphate treatments
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Zinc phosphate
Iron Phosphate
 Chromate conversion treatments
Zinc Phosphate
 Widely used
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Final treatment on galvanize lines as base for field
painting
Pretreatment on coil prepainting lines
Post fabrication factory painting lines
Automotive – treating of entire body-in-white
 Applied via spray and dip method
 Zinc phosphate crystals provide an excellent
surface for paint bonding and resist disbondment
in corrosive atmospheres
Zinc Phosphate
 Several steps required including: cleaning, rinsing,
surface activation, ZnP application, rinse, and
often a sealing step (Cr or non Cr bearing)
 Key reaction involves an increase in the pH at the
surface, resulting in precipitation and deposition
of insoluble zinc phosphate
 Paint bonding is by:
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Mechanical keying – similar to galvanneal- micro porous
Oxygen in film promotes chemical hydrogen bonding
with the paint
Zinc Phosphate
 Experience has shown ZnP is effective in reducing
paint undercutting corrosion
 Particularly effective with coatings containing
high iron, i.e., galvanneal. May be a result of the
superior bond formed. Automotive body panels
made with ZnP treated 45A45A coatings have
excellent corrosion resistance.
“Bonderized” Steel
 Zinc phosphate treated
on galvanize line
 Intended to be field
painted with good paint
adhesion
 Being used in some
locales with the intent
of being left unpainted
– as shown here
“Bonderized” Steel
 Some producers offer
Bonderized sheet with
a clear or tinted
lacquer coating for
added durability
 Low lustre appearance
is an architectural look
preferred in some
areas
Chromate Conversion
Pretreatments
 Yellow to brown – contain complex oxides
 Thicker than passivation treatments – 0.5-3 mm
 Used on Zn and AlZn coatings to enhance the
corrosion resistance of prepainted sheet
 Applied using tank/spray or roll coaters (DIP)
 Galvanize must be unpassivated
 Contain both Cr+3 and Cr+6, thus RoHS is a concern
 Less resistance to paint undercutting than ZnP
Surface Lubrication
 Provides lubricity to forming and stamping
operations – prevents galling, scratching,
fracturing
 Typically applied with electrostatic oilers
 Types:
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Mineral “slushing” oils (most contain rust inhibitors)
Vanishing oils (high volatile content)
Dry lubricants
Dry film lubricants (typically water-borne, applied on
coating line)
Fingerprinting & Handling
Marks
 Salt in perspiration causes
permanent white stains on
galvanize – even if passivated
 AlZn coatings subject to roll
forming and handling marks
appearing as black smudges
 Clear acrylic coatings applied
to resist marking – may also
contain Cr
 Some are paintable and if not
painted will dissipate
 Others are not paintable and
can last for years
Field Painting
 Difficult to achieve adherence on passivated
galvanize
 Options:
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Weather for 12 – 18 months
Consider proprietary pretreatment solutions
Light sanding may be an option
Ensure surface is clean and dry (water break-free)
Use paint designed for bonding to zinc
Dulling the Surface
 Some users desire or are
mandated to have a dull
surface (max reflectivity index
of 0.35)
 If known beforehand, order
temper passed galvanize
 Commercial cleaning products
containing small amounts of
hydrochloric and/or phosphoric
acid will remove the sheen
Summary
 Many surface treatments in use
 Bath chemistry influences appearance and
performance
 Mechanical treatment aimed at appearance
 Many treatments involve application of carefully
formulated chemicals to:
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Protect from water damage
Improve corrosion resistance
Prepare for painting
Assist in metal forming
Alter the appearance