Hot Dip Galvanizing [PDF]

Zitiervorschau

Material & Metalurgi

Hot dip Galvanizing

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Applications

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Zinc galvanizing ≠ Zinc electroplating

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Definition of Hot-dip galvanizing Dipping steel pieces into a molten zinc bath

Creating a metallurgical reaction between iron and zinc

• Zinc will not react with unclean steel •

integrity of the coating is immediately apparent as the steel is removed from the galvanizing bath

• The galvanized coating is metallurgically bonded to the steel •

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• Classification

• Galvalume: 55% Al, 43.4% Zn and 1.6% Si • Galvan: 95% Zn and 5% Al

The zinc coating is more than just a barrier coating, actually become part of the steel and protect the substrate steel cathodically

• Improved deformation behavior • Higher corrosion resistance at thermal stress and atmospheric

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Hot-dip galvanizing process source: American Galvanizers Association (AGA)

Surface preparation

Bath method 3/9/2020

(Discontinuous (batch) methods (for profiles cut to length, construction parts, and small parts).

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Hot-dip galvanizing for steel strips and wire

Hot-dip sheet (Continuous methods for steel strips and wire) 3/9/2020

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Surface preparation: Solution cleaning • The surface preparation step → quality control because zinc will not react with unclean steel. • Surface preparation for galvanizing consists of three steps:

DEGREASING • • •

Degreasing - A hot alkali solution (pH range 11 and 12), mild acidic bath, or • biological cleaning bath removes organic contaminants such as dirt, paint markings, grease, and oil • from the metal surface. Epoxies, vinyls, asphalt, or welding slag, must be removed before galvanizing by sand-blasting, or other mechanical.

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FLUXING

PICKLING

Pickling – A dilute solution of heated sulfuric acid or ambient hydrochloric acid removes mill scale and iron oxides (rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using abrasive cleaning or air blasting sand, metallic shot, or grit onto the steel.

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Fluxing – The final surface preparation step in the galvanizing process, a zinc ammonium chloride ((NH4)2ZnCl4) solution, serves two purposes. It removes any remaining oxides and deposits a protective layer on the steel to prevent any further oxides from forming on the surface prior to immersion in the molten zinc. Slide - 8

Solution cleaning • High pH (pH > 13) → will damage the zinc coating • The best solution temperature range → 60 – 85oC • For newly galvanized steel → a water – based emulsifier can be used to remove contaminant. After that rinse with hot water and followed by drying

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Galvanizing Process Immersed in molten Zinc The material is completely immersed in a bath of molten zinc. The bath chemistry is specified by ASTM B6, and requires at least 98% pure zinc maintained at approximately 840 F (449 C).

Reacts with iron (Fe) While immersed → the zinc reacts with the iron in the steel to form a series of zinc/iron intermetallic alloy layers.

Withdraw slowly Once the fabricated items coating growth is complete, they are withdrawn slowly from the galvanizing bath, and the excess zinc is removed by draining, vibrating, and/or centrifuging.

The metallurgical reaction will continue after the articles are withdrawn from the bath, as long as the article remains near bath temperature. Articles are cooled either by immersion in a passivation solution or water or by being left in open air. 3/9/2020

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Schematic representation of galvanizing reactions with as a function of dipping time

Dissolution of Fe

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Nucleation and growth of inhibition layer

Diffusion of Fe through inhibition layer

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Microstructure • Most of the Zinc alloy is nearly pure zinc. • Structure:

• Near to the steelcoating interface are prismatic particles of zeta phase, • Alpha zinc-iron intermetallic compound containing about 6% Fe.

Layer Formation in Hot - dip Batch Galvanizing Between 435 ° C and 620 ° C • The basis for layer - formation processes in batch galvanizing are the reactions between zinc and iron. Through reciprocal diffusion, intermetallic Fe - Zn phases are formed 3/9/2020

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Microstructure: galvanized vs electroplated

http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10709/ 3/9/2020

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Microstructure of galvanized coating (h)

(z)

(d)

(G)

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Hot – dip galvanizing product

https://www.askthebuilder.com/galvanized-nails-rust/

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Material & Metalurgi

Silicon content in steel killed

Parameters: The steel type → content • Apart from the immersion time and, above all, from the melting temperature, layer growth is also dependent on the steel type (content of Si and P). • 0.015 – 0.020% P, its influence is negligible • < 0.035% Si, which has to be taken into consideration in explanations of the galvanizing behavior.

Note: The steel is fully deoxidised (killed) before casting by the addition usually of silicon, manganese and aluminium, but also sometimes vanadium, titanium and zirconium. 3/9/2020

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The effect of silicon in the steel on galvanizing The effect of silicon in the steel on galvanizing

• In hot - dip galvanizing release the hydrogen absorbed during pickling.

Classification of Si content • • • •

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Low – silicon range ( 0.28% Si )

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The effect of silicon in the steel on galvanizing Hydrogen Concatenated pore chains in the zinc coating typical for absorbed molecular hydrogen

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Influence of Alloying Elements of the Melt on Layer Formation • Conventional Zinc Melts • Alloyed Zinc Melts

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Conventional Zinc Melts • Pure zinc → lead was added (during the initial years of the development of hot - dip galvanizing). • Contains small quantities of other metal contaminants like copper • Small quantities of tin and aluminum were added for optical reasons (brilliance, spangle).

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Alloyed Zinc Melts

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Common alloy elements • Pb → precipitates at different spots in the coating, however, in particular near the surface in the form of small droplets (1 to 5 μ m). • Sn → accumulates at the external boundary of the palisade layer ( ζ phase) between the dross crystals. • If Bi is alloyed, the Sn/Bi/Pb mixture between the palisade shaped crystals can penetrate the zinc coating up to the compact δ 1 layer and loosen the bond in this layer.

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Common alloy elements • Ni, Ti and V, → small amount (Ni amounts to approximately 0.05%) • Al → The formation of a thin intermetallic phase of FeAl3 or Fe2Al5 on the steel surface that is stable at least in the initial galvanizing phase and inhibits the galvanizing process for a certain time.

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Common alloy elements • Pb, up to approximately 1%→ because its high density causes it to sag to the bottom of the zinc tank facilitating the removal of the dross. • It also protects the tank bottom from the direct attack of the zinc melt. • On the other hand, it reduces the surface tension of the zinc melt and results in smooth coatings. 3/9/2020

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Reduction of coating thickness through the addition of 0.054% Ni to the zinc me(Steel with 0.08% Si, 445 ° C, 5 min) (a) Conventional melt (b) alloyed melt with nickel addition 3/9/2020

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Material & Metalurgi

Inspections

Inspection

Simple and quick. 3/9/2020

• The two properties: coating thickness and coating appearance. • A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence, and appearance. • Products are galvanized according to long established, accepted, and approved standards (i.e. ASTM, ISO, CSA etc) and these standards cover everything from the minimum coating thicknesses required for various categories of galvanized items to the composition of the zinc metal used in the process. Slide - 28

Post - treatment • The purpose of after - treatment of galvanized surfaces is their protection against white rust. • The usual treatment, however, rarely applied, is • chemical passivation, → Chemical passivation by means of chromate containing and chromate - free solutions or phosphating principally requires methods otherwise only used in electroplating • lubrication with oil, or • coating with organic or inorganic materials.

• It has to be stated that the best after - treatment is the respective proper storage of the galvanized products 3/9/2020

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Surface profiling • The galvanized surface must be flat (no protrusions and slightly roughened to provide anchor for paint • Tool: hand tools or power grinder (be careful with specific thickness) • Sweep blasting

• Particle size → 200 – 500 microns • Materials → Aluminum/magnesium silicate or organic media (corn cobs, walnut shell), corundum, limestone and sands (Mohs 21oC

Sweep basting ≠ near white cleanness standard 3/9/2020

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Zinc galvanizing coating protection on steel

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Zinc coating corrosion

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Zinc coating corrosion

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References • Peter Maaß and Peter Peißker, Handbook of Hot-dip Galvanization, Wiley-Vch Verlag GmbH & Co. KGaA, 2011 • Indra Laksmana, Coating over hot-dip galvanized steel, 10 years of service coating summit 2016, ASCOATINDO Conference and Exhibition, Bali, 2016 • J.F.H. van Eijnsbergen, Duplex Systems hot-dip Galvanizing plus Painting, The Hague, The Netherlands, 1994

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Material & Metalurgi

Duplex system

Combination zinc galvanizing and painting

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Duplex coating system

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Paint selection

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Sample: Painting over galvanized steel

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Duplex system performance

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