涂装基本知识之Zinccoatings

1、Jotun ProtectiveCorrosion resistance zinc coatings Jotun Korea Coatings Co., Ltd.Coating advisor : Danny (Dan Guo-qiang)2015-07 Jotun ProtectiveContents 1.Hot dip galvanizing2.Organic zinc-rich coatings3.Inorganic zinc coatings: (1)Post-cured inorganic zinc coatings(1)Post-cured inorganic zinc coati

2、ngs (Water based)(Water based) (2)Self-cure inorganic zinc coatings (2)Self-cure inorganic zinc coatings (Water based) (Water based) (3)Organic silicate zinc coatings (3)Organic silicate zinc coatings (Zinc ethyl silicate coatings) (Zinc ethyl silicate coatings) Jotun ProtectiveHot dip galvanizing G

3、alvanizing is the process of cleaning steel free of all mill scale or other contamination and then dipping the steel into molten zinc. Technological process:DegreasingRinseRinseAcid picklingDryingCoolingDippingplating assistant dippingJotun ProtectiveHot dip galvanizing 1.Degreasing(10-15min): 1)Alk

4、ali degreasing：10NaOH(or Na2CO3/Na3PO4 etc.)，above 60 2)Acidic degreasing ：Dilute H3 PO4 2. Acidic pickling(30-40min): 1)H2SO4Need heating，pollution、harmful 2)HCl (200g/L) No need heating or little，widely used 3. plating assistant dipping(1-2min): NH4Cl:ZnCl2(mole ratio)14，200400g/L, pH45, 60 Effect

5、：Form Zn(NH3)2Cl2 to protect steel from rusting eliminate the oxide on the steel, purify the bath Jotun ProtectiveHot dip galvanizing 4. Galvanizing: The dipping process may take from 30 seconds to 8 hours depending on the composition of steel and the configuration and mass of the coated articles .

6、Generally , the working piece must attain the temperature of zinc bath , which may vary from 450 to 475 . 5. Cooling :by water can gain a very smooth and glossy surfaceJotun ProtectiveHot dip galvanizing Microscopic cross section of hot dip coating Jotun ProtectiveHot dip galvanizingJotun Protective

7、Hot dip galvanizing The calculation of DFT Jotun ProtectiveHot dip galvanizingConditionConditionCauseCauseGrounds for rejectionGrounds for rejectionDullDullGrayappearanceComposition of original steelComposition of original steel(high silicon, phosphorous or (high silicon, phosphorous or Carbon) Carb

8、on) ；Server cold workingServer cold workingSlow coolingSlow cooling；Normally, acceptedNormally, acceptedRough Rough surfacesurfaceComposition of original steelComposition of original steel；Over picklingOver pickling；Long immersion timeLong immersion timeHigh galvanizing temperatureHigh galvanizing t

9、emperatureNormally, acceptedNormally, acceptedBareBarespotsspotsGrease or oil residueGrease or oil residue；Scale or rust residue;Scale or rust residue;Rolling defectsRolling defects；Excess aluminumExcess aluminum，weld slag weld slag Rejected if repairing is tooRejected if repairing is toomuchmuchRus

10、tstainRuststainWeeping of acid from seamsWeeping of acid from seamsAccepted Accepted BlisteringBlisteringDefects or hydrogen out of steelAccepted Accepted Jotun ProtectiveHot dip galvanizingConditionConditionCauseCauseGrounds for rejectionGrounds for rejectionDross Dross protrusionprotrusionDrossDro

11、ssAccepted unless dross Accepted unless dross Is heavyIs heavyLump and runLump and runHigh withdraw speedHigh withdraw speed；Cold bathCold bath；Delay run off from seamsDelay run off from seams；Contact between withdrawed Contact between withdrawed Depends on the prior agreementFlux Flux Inclusion Inc

12、lusion Flux residueFlux residue；Residue on steel surfaceResidue on steel surface；Flux picked on top of bathFlux picked on top of bathRejected Rejected AshAshinclusioninclusionAsh burned on during dippingAsh burned on during dipping；Ash on top of bathAsh on top of bathAccepted unless grossAccepted un

13、less grosslumpslumpsWhite rustWhite rustStorage or packing under Storage or packing under Damp conditionDamp conditionAccepted if not Accepted if not seriousseriousJotun ProtectiveHot dip galvanizing低氧浓度区（阳极）镀锌层水珠镀锌层高氧浓度区（阴极）White rust:Anode：Zn Zn2+ + 2e；Cathode：O2 + 4e- + 2H2O 4OH-；Reaction：2Zn+O2+

14、2H2O 2Zn(OH)2 （white、loose:3-5 time larger） ZnCO3 cant be formed due to lack of CO2Jotun ProtectiveHot dip galvanizing White rust: Medium or heavy white should be removed by hard brush(no wire brush !- damage) White rust will slow the formation of protective ZnCO3 Storage is key important if the gal

15、vanized articles are not used in a short periodJotun ProtectiveHot dip galvanizing White rust: StorageStorageJotun ProtectiveOrganic zinc-rich coatings Organic zinc coatings are a mixture of zinc dust or metallic zinc pigment (typically 50 to 95% by weight in the dried film) in an organic vehicle. J

16、otun ProtectiveOrganic zinc-rich coatingsGood compatibility with both steel and organicCathodic protection (zinc powder particle to particle contact)Easy for application ( RH limitation)Touch up and repair of galvanized articlesBinders role(Different binder , different role)Less subject to critical

17、surface preparationJotun ProtectiveInorganic zinc coatingsJotun ProtectiveInorganic zinc coatings Comparation of physical properties Jotun ProtectiveInorganic zinc coatingsType 2 Post cured inorganic zinc reaction Step 1: Dehydration-basic solvent water, leaving the zinc and the alkaline silicate(so

18、dium silicate) on the surface as a coating (hard and metalic but sensitive to water) Step 2: Amine+H3PO4 are applied on the surface by brush or spraying Step 3: Silicic acid is formed, and react with Fe and Zinc to form matrix to hold zinc(Silicic acid doesnt react completely) ; at the same time , S

19、odium phosphates are also formed. Step 4: Zinc continues react with H2O and CO2 into zinc ionizes which react with silicate polymer until it becomes essentially a saturated zinc sillicate polymer.Jotun ProtectiveInorganic zinc coatingsType 3 Self-cured inorganic zinc coating Step 1: Dehydration-basi

20、c solvent water, leaving the zinc and the alkaline silicate(sodium silicate , lithium silicate, etc.) on the surface (the alkaline silicate is much less alkaline than type 2 due to more form of poly-silicate acid, this allows faster reaction of zinc into coating) Rewetting by water or rain is detrim

21、ental!(binder leaching, powdery coating) Step 2: Silicic acid reacts with Fe and Zinc to form matrix to hold zinc(Silicic acid doesnt react completely) Step 3: Zinc continues react with H2O and CO2 into zinc ionizes which react with silicate polymer until it becomes essentially a saturated zinc sill

22、icate polymer. Jotun ProtectiveInorganic zinc coatingsType 4 Organic silicate zinc coating(Ethyl silicate zinc) Step 1: Evaporation of the solvent in the organic silicate vehicle, leaving the organic silicate zinc mixture on the metal surface. Step 2: moisture from the air continues to hydrolyze the ethyl silicate to silicic acid; Silicic acid reacts with Fe and Zinc to form matrix to hold zinc(Silicic acid doesnt react completely) Step 3: Zinc continues react with