Galvanize Surfaces Galv Assn 2005

Information about Galvanize Surfaces Galv Assn 2005

Published on January 4, 2008

Author: Ethan

Source: authorstream.com

Content

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 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:  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:  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 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:  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 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 of Cr 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 Condensing Humidity Test:  Condensing Humidity Test 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 RoHS:  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:  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 Phosphate:  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:  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” Steel:  “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

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