涂装基本知识之Zinc coatings

Accepted if not serious
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Hot dip galvanizing White rust:
Anode：Zn→ Zn2+ + 2e； Cathode：O2 + 4e- + 2H2O→ 4OH-； Reaction：2Zn+O2+2H2O→ 2Zn(OH)2 （white、loose:3-5 time larger）
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Hot dip galvanizing
White rust:
Storage
Storage
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Organic 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.
(Zinc ethyl silicate coatings)
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Hot dip galvanizing
Galvanizing is the process of cleaning steel free of all mill scale or other contamination and then dipping the steel into molten zinc. Technological process:
ZnCO3 cant be formed due to lack of CO2
镀锌层 水珠 镀锌层
低氧浓度区（阳极）
高氧浓度区（阴极）
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Hot 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 galvanized articles are not used in a short period
5. Cooling :by water can gain a very smooth and glossy surface
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Hot dip galvanizing
Microscopic cross section of hot dip coating
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Flux Inclusion Ash inclusion
Rejected
Ash burned on during dipping； Accepted unless gross Ash on top of bath lumps
White rust
Storage or packing under Damp condition
Hot dip galvanizing
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Hot dip galvanizing
The calculation of DFT
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Hot dip galvanizing
Condition Dull Gray appearance Cause Grounds for rejection Composition of original steel Normally, accepted (high silicon, phosphorous or Carbon) ； Server cold working Slow cooling； Composition of original steel；Normally, accepted Over pickling； Long immersion time High galvanizing temperature Grease or oil residue； Rejected if repairing is too Scale or rust residue; much Rolling defects； Excess aluminum，weld slag …
Rough surface
Bare spots
Ruststain
Blistering
Weeping of acid from seams
Defects or hydrogen out of steel
Accepted
Accepted
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Hot dip galvanizing
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Inorganic zinc coatings
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Inorganic zinc coatings
Comparation of physical properties
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Inorganic zinc coatings
Type 2 – Post cured inorganic zinc reaction Step 1: Dehydration-basic solvent water, leaving the zinc and the alkaline silicate(sodium 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 doesn’t react completely) ; at the same time , Sodium 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.
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Inorganic zinc coatings
Type 3 – Self-cured inorganic zinc coating Step 1: Dehydration-basic 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 detrimental!!!(binder leaching, powdery coating) Step 2: Silicic acid reacts with Fe and Zinc to form matrix to hold zinc(Silicic acid doesn’t 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 sillicate polymer.
1.Hot dip galvanizing
2.Organic zinc-rich coatings 3.Inorganic zinc coatings:
Hale Waihona Puke Baidu
(1)Post-cured inorganic zinc coatings
(Water based) (2)Self-cure inorganic zinc coatings (Water based) (3)Organic silicate zinc coatings
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Corrosion resistance zinc coatings
Jotun Korea Coatings Co., Ltd. Coating advisor : Danny (Dan Guo-qiang) 2015-07
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Contents
Cooling
Degreasing
Rinse
Acid pickling
Rinse
plating assistant dipping
Drying
Dipping
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Hot dip galvanizing
1.Degreasing(10-15min): 1)Alkali degreasing：～10％NaOH(or Na2CO3/Na3PO4 etc.)， above 60℃ 2)Acidic degreasing ：Dilute H3 PO4 2. Acidic pickling(30-40min): 1)H2SO4－Need heating，pollution、harmful 2)HCl (200g/L) －No need heating or little，widely used 3. plating assistant dipping(1-2min): NH4Cl:ZnCl2(mole ratio)＝1～4，200～400g/L, pH4~5, 60℃ Effect：Form Zn(NH3)2Cl2 to protect steel from rusting eliminate the oxide on the steel, purify the bath
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Organic zinc-rich coatings
• Good compatibility with both steel and organic • Cathodic protection (zinc powder particle to particle contact) • Easy for application ( RH limitation) • Touch up and repair of galvanized articles • Binder’s role(Different binder , different role) • Less subject to critical surface preparation
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Hot 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 . Generally , the working piece must attain the temperature of zinc bath , which may vary from 450 to 475 ℃.
Condition Dross protrusion Lump and run Dross High withdraw speed； Cold bath； Delay run off from seams； Contact between withdrawed Flux residue； Residue on steel surface； Flux picked on top of bath Cause Grounds for rejection Accepted unless dross Is heavy Depends on the prior agreement