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Control and Treatment of Hot-Dip Galvanize Surfaces. Presented at the 97 th Meeting of the Galvanizers Association October 16-19, 2005 Lexington, KY. GalvInfo Center.

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control and treatment of hot dip galvanize surfaces
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

GalvInfo Center

A zinc-coated steel sheet technical information center managed by ILZRO and cosponsored by the steel, paint and zinc industries.

galvanize surfaces
Galvanize Surfaces

Hot-dip galvanize vary in appearance, and therefore usability, because of:

  • spangle size
  • zinc composition
  • mechanical treatment
  • chemical or oiling treatments.

Marketplace problems develop because customers find the surface is:

  • 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
Reasons forSurface 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
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 uniformity6
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
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
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
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
Improving Resistance to Storage Stain
  • Chemical Treatments – Cr Based
    • Premature Spangle Darkening
    • Tests for passivation
  • Chemical Treatments – Non Cr
    • Removable and Permanent
    • RoHS issues
    • Oils
chemical treatments
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
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
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
Chromium BasedChemical 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 treatments15
Chromium BasedChemical 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
    • 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
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
    • 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
Premature Spangle Darkening
  • Can occur after a few days of exposure
  • Reported characteristics
    • 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
Some Disadvantages ofCr Passivation
  • Paintability
    • 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
    • Interferes with spot weldability by “poisoning” copper alloy welding electrodes – shortens electrode life
    • Use only unpassivated sheet
is it passivated
Is It Passivated?
  • Usually not possible to visually determine
  • Producers use lab testing methods not available in field
  • Field tests:
    • %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
non chrome treatments
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
slide22
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:
    • Lead
    • Mercury
    • Cadmium
    • Hexavalent Cr
    • PBB and PBDE flame retardants
  • Request to exempt Cr+6 not yet ruled on
slide23
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
Pretreatments
  • Used to obtain good bonding between the metal surface and paint
  • Phosphate treatments
    • Zinc phosphate
    • Iron Phosphate
  • Chromate conversion treatments
zinc phosphate
Zinc Phosphate
  • Widely used
    • 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 phosphate26
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:
    • Mechanical keying – similar to galvanneal- micro porous
    • Oxygen in film promotes chemical hydrogen bonding with the paint
zinc phosphate27
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
“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 steel29
“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
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
Surface Lubrication
  • Provides lubricity to forming and stamping operations – prevents galling, scratching, fracturing
  • Typically applied with electrostatic oilers
  • Types:
    • 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
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
Field Painting
  • Difficult to achieve adherence on passivated galvanize
  • Options:
    • 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
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
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:
    • Protect from water damage
    • Improve corrosion resistance
    • Prepare for painting
    • Assist in metal forming
    • Alter the appearance