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

Control and Treatment of Hot-

Dip Galvanize Surfaces

Presented at the 97th Meeting of theGalvanizers Association

October 16-19, 2005Lexington, KY

GalvInfo Center

A zinc-coated steel sheet technical information center

managed by ILZRO and cosponsored by the steel, paint

and zinc industries.

Galvanize SurfacesHot-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 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

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 Premature Spangle Darkening Tests for passivation

Chemical Treatments – Non Cr 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 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 m) and invisible but have yellow or green tinge if applied heavier (0.1-0.6 m)

Total Cr 1-2 mg/ft2, with < 50% Cr+6 in complex mixture of metal salts and oxides

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

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

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 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?

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

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:

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 Zinc phosphate Iron Phosphate

Chromate conversion treatments

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 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 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 m 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:

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: 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: Protect from water damage Improve corrosion resistance Prepare for painting Assist in metal forming Alter the appearance