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
13.1 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
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
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
This definition is not restricted to metallic material.
It includes all materials:
• organic• inorganic (metallic, non-metallic)
Definition of the term corrosionISO 8044
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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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 !
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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.
Rusted Car
Corrosion Failure?
May be !
What is a corrosion failure?
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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.
Rusted Car
Corrosion Failure?
YES !!
What is a corrosion failure?
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Material relatedcorrosion failure
Medium relatedcorrosion failure
Corrosion failures
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Corrosion reactions
chemical
corrosion
electrochemical
corrosion
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
electrical influencing variables are effectless
Chemical corrosion
direct electron-exchange between the reactants(redox-reaction)
chemical
corrosion
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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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.
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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.
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Example for aelectrochemical corrosion reaction
Rusting of iron:
Anodic reaction: 2Fe 2Fe2+ +4e-Kathodic Reaction: O2+ 4e- + 2H2O 4OH-
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Schematic electrochemicalcorrosion mechanism
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Schematic electrochemicalcorrosion mechanism
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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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
Electrochemical terms
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Electrochemical terms
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Electrochemical terms
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Electrochemical terms
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Electrochemical terms
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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Nernst equation
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Galvanic series
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Current density-potential curve
Net Current Density /Potential of the Single Electrode
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Current density-potential curve
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Mixed electrodes
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
Important cathode reactions
Source: Schmitt, G.: Corrosion & Corrosion Protection, Lecture IEHK, 2006.
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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
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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
13.1 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
Galvanizing of steel strip
ContinuousHot-Dip Galvanizing
ources: http://www.dassnagar.com/ Feuerverzinken interaktiv, Institut Feuerverzinken GmbH, Düsseldorf
Electro-zinc coating
Electro-zinc coating
Hot-Rolling Pickling Cold Rolling
Continuous Annealing
Batch Annealing
Coil-Coating
Temper Rolling
Electro-zinc coating
Shipment
Hot-DipGalvanizing
C.C.
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Hot-dip galvanizing
Hot-Rolling Pickling Cold Rolling
Continuous Annealing
Batch Annealing
Coil-Coating
Temper Rolling
Electro-zinc coating
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
Source: http://www.salzgitter-flachstahl.de/
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
Source: http://www.salzgitter-flachstahl.de/
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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
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
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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
Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
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.
ource: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
(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.
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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.
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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
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
Steel Usage in theinternational project ULSAB-AVC
0
ULSAB-AVC
(UltraLight Steel Auto Body - Advandced Vehicle Concepts)
PNGV-ClassAuto Body
74% dual phase steels
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• 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
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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
Inhibition layer (Fe 2 Al5)
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
Inhibition layer (Fe 2 Al5)
Zinc coating
Segregation of Mn, Si, Cr duringannealing
Precoating, e.g. Fe
PrecoatingSurface Layer
Removal
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
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.
Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
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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.
ource: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
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).
Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
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)
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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
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
Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
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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.
Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
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
Source: A.R. Marder, Progress in Materials Science 45 (2000) 191-271.
Properties: general
alkali resistancetemperature resistance
• electrostatic
• electrophoretic
acid resistance
• coil coating
• conventional
coatabilitybest surface
AS AZZAZFZ
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-
Standard
+
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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
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-
Standard
+
++
Source: Charakteristische Merkmale 095: „ Schmelztauchveredeltes Band u nd Blech“, Stahl-Informations-Zentrum
• adhere
• mechanical joining
joining• spot welding
• solder
• high deformation degrees
• abrasion
• (micro) cracking
forming
AS AZZAZFZ
--
-
Standard
+
++
Source: Charakteristische Merkmale 095: „ Schmelztauchveredeltes Band u nd Blech“, Stahl-Informations-Zentrum