Hot-dip Coating

8/10/2019 Hot-dip Coating

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Institut für Eisenhüttenkunde

der RWTH achen

Topic 13:Continuous hot-dip coating

Dipl.-Ing. Friedrich Luther

Outline

13.1 Corrosion basics• Definitions• Electrochemical terms

13.2 Galvanizing Processes• Corrosion protection with zinc• Electrolytic and hot-dip galvanizing process

13.3 Metallurgical Aspects during hot-dip galvanizing• Steel-zinc interface• Hot-dip galvanizing of AHSS

13.4 Coatings• Galvanized, Galvannealed, GALFAN, GALVALUME• Properties: Formability, weldability, paintability, corrosion resistance

Outline




13.4 Coatings

• Galvanized, Galvannealed, GALFAN, GALVALUME• Properties: Formability, weldability, paintability, corrosion resistance

The corrosion system

Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.

Corrosion is the reaction of a material with itsenvironment which occurs with a measurable changeat the material and/or the environment.Such reaction can lead to a corrosion damage at thematerial and /or the environment.

Definition of the term corrosionISO 8044


This definition is not restricted to metallic material.

It includes all materials:

• organic• inorganic (metallic, non-metallic)

Definition of the term corrosionISO 8044




What is a corrosion failure?

A corrosion failure has occured if the material (acomponent of a structure or the whole structure) and /or the environment is impaired in its further use.

RustedRailway Tracks

Corrosion Failure?

No !



Rusted Car

Corrosion Failure?

May be !




Rusted Car

Corrosion Failure?

YES !!



Material relatedcorrosion failure

Medium relatedcorrosion failure

Corrosion failures


Corrosion reactions

chemical

corrosion

electrochemical

corrosion


electrical influencing variables are effectless

Chemical corrosion

direct electron-exchange between the reactants(redox-reaction)

chemical

corrosion




Driving force of chemical reactions

Every atom tries to gain the 'electron costume‘ of thenearest noble gas atom in the periodic table of elements(Nobel Gas Rule):

• next higher position by up-take of electrons or • next lower position by give-away of electrons.


Example for achemical corrosion reaction

Surface metal atom A interchanges electronegativity its metallicproperty and is converted (oxidized) into a compound A-B.The atom A has now lost (electrons) with an atom or a molecule Bfrom the environment.This can produce a chemical bonding metal ion. The compound A-Bcan form a protective layer (scale) on the surface if A-B is not volatile.


It runs exclusively in presence of a ion-leadingphase ; dependent on electrical variables like e.g.potential

There are two places , one for the electron-absorptionand one for the electron-disposal

Electrochemical corrosion

...is the most common corrosionmechanismelectrochemical

corrosion


Example for aelectrochemical corrosion reaction

Rusting of iron:

Anodic reaction: 2Fe 2Fe2+ +4e-Kathodic Reaction: O2+ 4e- + 2H2O 4OH-


Schematic electrochemicalcorrosion mechanism


Schematic electrochemicalcorrosion mechanism




Outline





Electrochemical terms












Nernst equation


Galvanic series


Current density-potential curve

Net Current Density /Potential of the Single Electrode


Current density-potential curve


Mixed electrodes


Important cathode reactions




Corrosion protection with zinc

How does it work?

Zinc coatings add corrosion resistance to steel in two ways:

• Zinc as a barrier layer seperates the steel surface fromthe corrosive environment

• Zinc as galvanic protection acts as a sacrificial anode toprotect the underlying steel

Protection with zinc coatings

Source: Merkblatt 400, Korrosionsverhalten von feuerverzinktem Stahl, Stahl-Informations-Zentrum, Düsseldorf

Protection, years

T h

i c k n e s s o

f z

i n c c o a

t i n g ,

µ m

0 10 20 30 40 50 60 70 80

200

150

100

50

i n d u

s t r i a

l c l i m

a t e

m a r

i n e c l i m

a t e

u r b a n c l

i m a t e

c o u n t r y

c l i m a t e

i n d o o r s

Comparison of ionizationtendency among elements

Source: Nippon Steel News, No. 307 September2003

Mechanism of sacrificial protection

Source: Nippon Steel News, No. 307 September2003

anode: Zn ⇒ Zn 2+ + 2 e -

cathode: 4e - + 2H 2O + O 2 ⇒ 4(OH) -

Zinc layer + painting

30-40µm

30-40µm

20-25µm

5-20µm

Corrosion warrantiesof car producers

0

5

10

15

20

25

30

35

A u d i

B M W

C i t r o e n F i a t F o r

d N i s s

a n

O p e l ( G

M ) P e u

g e o t P o r

s c h e R e n

a u l t S a a b

S k o d a

T o y o t a V o l v

o V W W a r r a n t y a g a i n s t c o r r o s i o n

d a m a g e s ,

Y e a r s

D a i m l e

r C h r y s l

e r

„ „mobilomobilo -life-life “ “

Average warranty: 10-12 Years

TÜV-Report 2000:TÜV-Report 2000: Year Share of 10 year old cars with

corrosion damages

1990 25%

2000 5%

Source: Androsch et al, stahl und eisen 121 (2001)Nr.6



Restriction!

The sacrificial protection of a zinc layer is limited to a distance of about 1 mm!Relevant for voids, scratches and cut edges.

Usage of galvanized steels

Building industry:fences, rails, roofing sheets, garages, ...

Steel construction:steel frame construction, bridges, ...

Electrical Engineering:transformer, pylons, ...

Mechanical engineering:cases, facilities, …

Sheet fabricating industry :washer, fridges, household equipment, ...

Automotive industry:car body, accesories , …

Examples

Sources:Feuerverzinken interaktiv, Institut Feuerverzinken GmbH, Düsseldorf http://www.volkswagen.de

Outline




13.4 Coatings


Galvanizing of steel strip

ContinuousHot-Dip Galvanizing

ources: http://www.dassnagar.com/ Feuerverzinken interaktiv, Institut Feuerverzinken GmbH, Düsseldorf

Electro-zinc coating


Hot-Rolling Pickling Cold Rolling

Continuous Annealing

Batch Annealing

Coil-Coating

Temper Rolling


Shipment

Hot-DipGalvanizing

C.C.



Hot-dip galvanizing

Hot-Rolling Pickling Cold Rolling

Continuous Annealing

Batch Annealing

Coil-Coating

Temper Rolling


Shipment

Hot-DipGalvanizing

C.C.

Steel utilization in EU 15and GER in 2005

EU 15Long Products 65.799.000 tFlat Products 90.796.000 t

Cold rolled sheet steel 43.349.000 tMetallic coated 26.739.000 tOrganic coated 4.685.000 t

GermanyLong Products 12.724.000 tFlat Products 25.047.000 t

Cold rolled sheet steel 12.004.000 tHot-dip coated 5.858.000 tElectr. coated 1.801.000 tOrganic coated 986.000 t

Source: Statistisches Jahrbus der Stahlindustrie 2006/2007, Verlag Stahleisen GmbH, Düsseldorf 2006.

Development of steelutilization in Germany

ource: Statistisches Jahrbus der Stahlindustrie 2006/2007, Verlag Stahleisen GmbH, Düsseldorf 2006.

Electro-zinc coating line

07.) Electrolytic cleaning08.) Rinsing tank09.) Picklingtank10.) Electro-zinc coating11.) Phosphate treatment12.) Chromate passivation

01.) Pay-offreel02.) Cropping shear 03.) Welding machine04.) Entry loopaccumulator 05.) Strip pretreatment06.) Tension leveller

13.) Strip dryer 14.) Exit loopaccumulator 15.) Inspection station16.) Trimming shear 17.) Electrostatic oiler 18.) Tension reel

Source: http://www.salzgitter-flachstahl.de/

Plating cells


One Side Galvanizing Both Sides Galvanizing

Pump

Conductor Roll

Anode (+)

Electrolyte

Technical data

Strip width range 900 - 1.850 mmStrip thickness range 0,5 - 2,0 mmSpeed max. 180 m/minCoil weight max. 32 tCurrent intensity 50 kA/cellRectifier capacity 850 kANumber of cells 17Coating range 2,5 - 15 µ mCapacity 33.000 t/mo




Hot-dip galvanizing line

coiler weldingmachine

shear jet-cooling

looptower preheater

cleaning section

RTFRTF

coolingtower

zinc potcoating gauge

measurement(hot)

coating gaugemeasurement (cold)

skinpass mill

phosphate coater / passivation

looptower

inspection

electrostaticoiler

shear

coiler

ource: voestalpine AG, Linz

Strip width range: 650 – 1.600 mmStrip thickness range:(cold rolled): 0,5 – 3 mm(hot rolled): 1,4 – 3,5 mmSpeed:Coil weight max. 32 tCoating range: 70 – 600 g/m 2Capacity: 33.000 t/mo

Functions of a hot-dip galvanizing line

• Pre-cleaning (removing of rolling oils, iron fines, loose soils)• Heat treatment (recrystallization, mech. properties)• Hot-dipping (e.g. Z, ZA, AZ)• Heat treatment of coated strip (ZF)• Skin pass (mechanical properties, roughness)• Post treatments (corrosion protection, …)

Annealing furnace

ource: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.

Pot region

Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.

Post treatments

passivated Ccorrosion protection

oiled Ocorrosion protection

phosphated Pimprovement of adhesion/protection of coatings,

improvement of forming properties

passivated and oiled PO

phosphated and oiled CO

sealed Scorrosion protection, improvement of forming properties

ource: Charakteristische Merkmale 095: „ Schmelztauchveredeltes Band u nd Blech“, Stahl-Informations-Zentrum

Outline







Phase diagram Fe-Zn

0 10 20 30 40 50 60 70 80 90 100

Zinc in weight%

Zinc in atom%

T e m p e r a t u r e

i n ° C

300

500

700

1500

1300

1100

900

Fe0 10 20 30 40 50 60 70 80 90

Zn

1394 °C

1538 °C

912 °C

770 °C42

782 °C

665 °C

530°C425

550°C

419.58 °C

(γ − Fe)

(α −Fe, δ−Fe)

magnetic transformation

623 °C Γ

Γ1

δ

ζ

100

L

BC Ch. III. 3 Transparency 21

Zinc-rich corner of theFe-Zn phase diagram


Phase transformations

3 peritectic transformations:S + α ΓS + Γ δS + δ ζ

1 peritectoid transformationΓ + δ Γ1

Zinc is referred to as η -Phase. The intermetallic phases ζ , δ,Γ1 and Γ form depending on the annealing and coating

process during and after hot-dip galvanizing.

Phases in the phase diagram Fe-Zn

mass% atom%

Eta η hexagonal Zn 0 0 52 very ductile

Zeta ζ monoclinic FeZn 13 5-6.2 6.7-7.2 208 ductile

Delta δ hexagonal FeZn 7 7-10 8.5-13.5 358 brittle

Gamma 1 Γ1 fcc Fe 5 Zn21 15.8-23.5 18.5-23.5 505 hard + brittle

Gamma Γ bcc Fe 3Zn10 23.2-31 23.2-31 326 brittle

microhardness

characteristicFe

phasespacelattice

formula

FeZn intermetallic phases

Microstructure of Zn coating formed after 300 s immersion in a

450°C, 0.00 wt% Al bath on a ULC steel substrate.


(1) gamma ( Γ )(2) delta ( δ )(3) zeta (ζ )

Fe-Zn phase layer formation in a0.00 wt-% Al-Zn bath

t 0 corresponds to zero time

t 1 < t 2 < t 3 <t 4

Source: CE Jordan et al., J . Mater. Sci. 32 (1997) 5593.



Fe-Zn phase layer groth

Fe–Zn gamma ( Γ ) phase, delta ( δ ) phase, and zeta ( ζ )phase layer growth for a ULC steel substrate hot dippedat 450°C in a 0.00 wt% Al–Zn bath

ource: CE Jordan et al., J . Mater. Sci. 32 (1997) 5593.

Influence of the Zn-phaseson the forming behavior

3.0

2.5

2.0

1.5

1.0

0.5

04 5 6 7 8 9 10 11

Fe-content in %

A b r a s i o n

i n g

/ m 2

Fe-Zn-Al phase diagram

Isothermal section of the Fe–Al–Zn phase diagram at

450°C, (left) overall section, (right) zinc rich corner ource: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.

Zinc-rich corner of the 450°C isothermalsection of the Fe–Zn–Al phase diagram

Source: N.Y. Tang, On determining effective Al in continuous galvanizing baths, Proceedings of GALVATECH 1995, Chicago, 777.

Mechanism of formationof the inhibiting layers

Source: F. E. Goodwin (Herausgeber): Zinc-BasedSteel Coating Systems: Productionand Performance, TMS, Warrendale, 1998.

dissolution of ferrite

precipitation of Fe2Al5

Fe–Zn phase layer formationin a 0.20 wt% Al–Zn galvanizing bath

t 0 corresponds to zero time,

t 1 < t 2 < t 3 < t 4

Source: CE Jordan et al., J . Mater. Sci. 32 (1997) 5603.



Summary of coating microstructurein continous galvanizing

Source: N.Y. Tang, Zinc-BasedSteel Coating Systems: Productionand Performance, TMS, Warrendale, 1998.

Al-rich Interfacial Layer

0,00E+00

5,00E+02

1,00E+03

1,50E+03

2,00E+03

2,50E+03

3,00E+03

3,50E+03

0 50 100 150 200 250 300 350 400 450 500

Position, nm

I n t e n s

i t ä t , C o u n

t s

FeZn

Mn

Al

Si

Zinc

Fe2Al5Znx

Steel

HAADF-STEM-Picture of theSteel-Zinc Interface

EDX-Linescan of the Steel-Zinc Interface

Role of aluminum

Adhesion for the zinc layer

Diffusion barrier for Iron

Setting of soluted iron as top dross

1 2 3 4

Fe2Al5Znx-Layer zinc layer

steel

Industrial zinc coating

Quelle: B. Schumacher et al.: Element distributionof aluminium and leadin hot-dip galvanizedcoatingd and their influences on thecoating properties, in 4th International Conference on Zinc and Zinc Alloy CoatedSteel Sheet, September 1998, Chiba, Japan, 819-823.

Outline





Steel Usage in theinternational project ULSAB-AVC

0

ULSAB-AVC

(UltraLight Steel Auto Body - Advandced Vehicle Concepts)

PNGV-ClassAuto Body

74% dual phase steels



• bad wettability by zinc• inhomogeneous formation of inhibition layer and zinc coating

• high content of alloying elements (Si, Mn, Cr, Al, P)

• segregation of alloying elements, formation of external oxides

Mahieu, J.; UniversiteitGent, 2004

Mn SiO2 4

SiO 2

MnAl O2 4

MnO

CMnSi CMnAl CMnP

Technical issues duringhot-dip galvanizing of AHSS

Selective oxidation of alloying elements:REM pictures and GDOES analysis

0

2

4

6

8

10

12

0 0,1 0,2 0,3 0,4 0,5

Tiefe, m

G e w

i c h t s p r o z e n

t , %

O

Mn

Cr

dual phase steel

TRIP steel

Elemental mapping of a CMnCr DP-steel surface after Rx annealing

BF Mn

Cr Al O

Si

Elemental mapping of a CMnAlSiTRIP-steel surface after Rx annealing

BF Si

Cr Al O

Mn

Formation of theAl-rich interfacial layer

DP600

TRIP700

Coating quality

DP600 TRIP700



Base sheet alloying approaches

Steel substrate

Oxide layer

Inhibition layer (Fe 2 Al5)

Zinc coating

Segregation of Mn, Si, Cr during

annealing

Mo for Cr(DP)

Al for Si(TRIP)

Atmospheric approaches

Steel substrate

Oxide layer


Zinc coating

Segregation of Mn, Si, Cr during

annealingPure reduced Fe

Diffusion zone of C or N

OxidisingReducing

Reactive Atmosphere

Pretreatment of incoming strip

Steel substrate

Oxide layer


Zinc coating

Segregation of Mn, Si, Cr duringannealing

Precoating, e.g. Fe

PrecoatingSurface Layer

Removal

Outline




13.4 Coatings


Coating types

50-150(8,5-25 m/Seite)

3,010% Si + 90% AlASFAL

75-185(10-25 m/Seite)

3,855% Al + 43,4% Zn+ 1,6% Si

AZGALVALUME

65-300(5-23 m/Seite)

6,65% Al + 95% Zn+ 0,05% Cer/lanthan

ZAGALFAN

100-140(7-10 m/Seite)

7,110% Fe + 90% ZnZFGalvannealed

•Regular spangle•Minimum spangle• Extra-smoothtemper roll finish

70-600(5-42 m/Seite)

7,1100% ZnZGalvanized

surface finishescoating massg/m² (bothsides)

densitykg/dm3

compositionproduct

Galvanized

Galvanized hot-dip coatings often have a structure consisting

of very large grains called “spangles”. Spangle size isinfluenced by the cooling conditions during solidification. Thethree surface finishes commonly produced are:

• Regular spangle , where the coating solidifies from thedipping temperature by air cooling, producing the well-knownspangle finish.

• Minimum spangle , where the coating is quenched usingwater, steam, chemical solutions, or by zinc powderspraying.

• Extra-smooth temper roll finish carried out as anadditional operation with regular and minimum spanglematerial.




Microstructure of GALFAN coating

The microstructure of GALFAN is characterized by a two-phasestructure, a zinc-rich eta ( η) proeutectoid phase surrounded bya eutectic type phase consisting of beta ( β) aluminum and eta(η) zinc lamellae.


GALVALUME

GALVALUME, is a 55%–Al alloy coating containing about1.5% Si added for the purpose of preventing an exothermicreaction at the coating overlay/substrate steel interface. Duringthe coating process an interfacial Fe–Al–Zn intermetallic alloylayer forms at the interface between the steel substrate andthe overlay coating.

The surface of the GALVALUME coating containscharacteristic spangles that consist of aluminum dendrites witha clearly measurable dendrite arm spacing (DAS).


Microstructure of GALVALUME coating

(a) spangle finish;

(b) dendrite arm spacingource: Marder AR. Microstructuralcharacterization of zinc coatings. In: Krauss G, Matlock DK, editors. Zinc-basedsteel coating systems: metallurgy anderformance. Warrendale, PA: TMS, 1990. p. 55.

The coating contains beta ( β)aluminum dendrites, Zn-richinterdendritic regions and a finedispersion of Si particles. The Aldendrites were reported tocontain approximately 18 wt%Zn and up to 1.8 wt% Si whichis in good agreement with theAl–Zn phase diagram

Schematic of thesolidification of GALVALUME

t 1 < t 2 < t 3 < t 4

Source: Marder AR. Microstructuralcharacterization of zinc coatings. In: Krauss G, Matlock DK, editors. Zinc-basedsteel coating systems: metallurgy andperformance. Warrendale, PA: TMS, 1990. p. 55.

Applications I

Hot-dip galvanized (Z)• automotive industry (outer and inner

parts/components)• building industry e.g. insulation

and trapezoidal sheets (roof and wall)• plant engineering e.g. construction

elements, castings for machines• substrate for coil coating

Galvannealed (ZF)• automotive industry

Applications II

GALFAN (ZA)• automotive area (components)• plant engineering and

mechanical engineering, e.g.components which are formedto a high degree.

• white goods industry,• home electronics industry

GALVALUME (AZ)• building industry for outdoor roof

and wall elements• accessories and construction

components in the buildingsector, plant engineering andmechanical engineering (specialcorrosion forces)



Outline





Coating properties

In general, the coating microstructure consists of:

• substrate• interfacial alloy layer • overlay cast structure

Microstructure and composition of these constituents willcontrol the desired properties!

Coating properties II

Properties that concern the use of zinc coatings are primarily:

• Corrosion resistance• Formability• Weldability• Paintability

Corrosion loss of coating layer after atmospheric exposure test

Source: UchimaY, Hasaka M, Koga H. Effect of structureand mischmetaladdition on the corrosionbehavior of Zn-5 mass%Al alloy.GALVATECH '89. Tokyo: TheIron and Steel Institute of Japan. 1989. p. 545.

Corrosion losses of hot-dip coatingsin an industrial environment

Source: Marder AR. Effects of surface treatmentson materials performance. Materials selectionand design. ASM Handbook, vol. 20. 1997. p. 470.

Formability – Coating failures

The deformation and fracture behavior of zinc based coatings on sheetsteels can alter the performance in stamping operations. Zinc coatings failas a result of particle removal during forming.

Coating failures are classified as:

• Powderingparticles smaller than coating thickness

• Flakingflat particles similar to coating thickness

• Gallingdamage resulting from particles that bond to the tool surface – resulting inadditional coating damage




Schematic model for thecoating exfoliation process

Source: ArimuraM, UraiM, IwayaJ, Iwai M. Effects of press formingfactors and flash plating on coating exfoliationof galvannealedsteel sheets.GALVATECH ’95. Chicago, IL: Iron and Steel Society, 1995. p. 733

Weldability

Weldability of zinc coatings is an important property of the coating, since mostgalvanized product is joined in this manner.

• Arc weldingof galvanized steel sheet produces defects such as gas cavities(blowholes) and spatters.

• Spot weldingzinc coatings reduce the life of welding electrodes due to alloying of thecopper electrode with zinc. In the case of galvanized steel, the electrodelife may be as little as 1500–2000 welds, compared to a tip life for baresteel of 10,000 welds

Spot weldability of galvanneal coatings is improved over galvanizedcoating since it is more difficult for these Fe–Zn phases to alloy with thecopper electrode, thus improving electrode life.


Paintability

Although zinc coatings are often used in the as-coated state, someapplications call for a painted surface and therefore paintability is animportant design property of the coating. It has been shown that largespangle material is difficult to paint, therefore most painted products areeither minimum spangle or temper rolled.

In most painted products, a complex coating composite is used for corrosion protection. In addition to the Zn coated sheet steel, the compositeincludes a zinc phosphate pretreatment or complex oxide thin coating, theprimer and various top coats

Galvanneal coating paintability is better than galvanized coatings becauseof the microscopically rough surface formed as a result of the Fe–Zn alloyphases throughout the coating


Properties: general

alkali resistancetemperature resistance

• electrostatic

• electrophoretic

acid resistance

• coil coating

• conventional

coatabilitybest surface

AS AZZAZFZ

--

-

Standard

+

++

Source: Charakteristische Merkmale 095: „ Schmelztauchveredeltes Band u nd Blech“, Stahl-Informations-Zentrum

• cutting edge

painted (automotive)

painted (coil coating)• unformed surface

• formed surface

• cutting edge

• formed surface

• unformed surface

unpainted

AS AZZAZFZ

--

-

Standard

+

++


• adhere

• mechanical joining

joining• spot welding

• solder

• high deformation degrees

• abrasion

• (micro) cracking

forming

AS AZZAZFZ

--

-

Standard

+

++


Hot-dip Coating

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