Abstract No. 64 2015 - stahl-online.de | No. 64 Investigations on the mechanism of activation,...

Abstract No. 64

Investigations on the mechanism of activation, nucleation and zinc phosphate crystal growth on industrially produced steel sheet

Nicole Weiher, Fabian Junge, Gregor Müller, ThyssenKrupp Steel Europe AG

Zinc phosphate conversion coatings are used for surface finishing of galvanized steel sheet to markedly improve relevant characteristics regarding to processing and product features. This includes particularly the increase of corrosion resistance as well as the improvement of formability and paint adhesion. Zinc phosphate coatings are anorganic crystalline conversion coatings which are precipitated from a liquid process bath. Characteristics of zinc phosphate coatings can be specifically influenced by a prior application of so-called activator particles. The applied activator particles lead to an increasing number of zinc phosphate crystals per unit area as well as to enhanced rate of crystal formations and surface coverage. That in turn leads to a positive effect on surface characteristics like coating weight, corrosion resistance and paint adhesion of the zinc phosphate coating.

In this work the whole process of activation, nucleation and zinc phosphate crystal growth was analyzed with Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM) and Dynamic Light Scattering (DLS) technique. Zinc phosphate crystals belong to the group of so called Mesocrystals which were understood as self-assembling super structures consisting of crystalline nano particles which are able to grow towards a single crystal. In this work it was possible to observe the relevant nucleation and zinc phosphate crystal growth process in characteristic stages and able to first assign these mechanisms to the non-classical crystallization. As a result of increasing activator bath-life, an activator particle agglomeration was observed. So the number of activator particles per unit area is highly increased if these agglomerated particles are applied to the metal surface. The activator particle agglomerate cannot be dissolved fast enough during the following phosphating step. Activation seeds located inside of the activator particles connecting to zinc phosphate crystals which are growing right where the seed particles are released, because of the phosphoric acid attacks on the activator particles. This turns to the formation of stacked zinc phosphate crystals or zinc phosphate crystal agglomerates.

These zinc phosphate crystal agglomerates are hollow, mechanically instable and there is only a very little contact between the zinc phosphate crystals and the metal surface which can have an adverse effect on the paint adhesion. The relation between aging of the activator bath, agglomeration of these particles and paint adhesion on the resulting zinc phosphate crystal conservation coating could be made. CE

TAS

2015

ThyssenKrupp Steel Europe

Investigations on the mechanism of activation, nucleation and zinc phosphate crystal growth on industrially produced steel sheetJunge, F.; Müller, G.; Weiher, N.

CETAS 9, Düsseldorf, 20.05.2015

CETAS 920.05.2015, DüsseldorfDr. Fabian Junge ThyssenKrupp Steel Europe

Phosphating process

Agenda

1

Motivation2

Activation-, nucleation- and zinc phosphate crystal growth3

Influence of agglomerated activator particles on the zinc phosphate crystal growth4

Measures for increasing the activator bath stability5

Summary5 CETA

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Phosphating process

Agenda

1

Motivation2




Summary6


belt dryer phosphating

hot air

activation

electrogalvanisingrinse

(multistage)

Phosphating process

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Activator particle

Zinc surface

Phosphating processPrevious assumption of the phosphating process


Activator particle

Zinc surface

Phosphating solution


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Activator particle

Zinc surface




Activator particle

Zinc surface



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Activator particle

Zinc surface




electrolytically deposited

zinc layer

Phosphate coating

cold-rolled steel

Phosphating processSchematic cross section of a phosphated steel sheet

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• Punching plates• Pressing the sheet into components• Cleaning (degreasing)• Activation• Phosphating • Thermal treatment• Cathodic dip painting • Primer• Top Coat

Phosphating process

SEM, zinc phosphate crystalls

Further processing steps at the customers facility


Motivation

Phosphating process

Agenda

1

2




Summary5 CETA

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• Issues with paint adhesion on electrogalvanized and phosphated steel sheet in the past

• No approach to differentiate analytically between good and rejected material

Motivation

Mechanism of activation, nucleation and zinc phosphate crystal growth needs to be investigated fundamentally


> 4

0 1 2 3 4 5

evaluation of paint adhesion with cross hatch testing in accordance with DIN EN ISO 2409

Issues of paint adhesion

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Activation-, nucleation- and zinc phosphate crystal growth

Motivation

Phosphating process

Agenda

1

2

3



Summary5


SEM

Clarification of the activation, nucleation and zinc phosphate crystal growth process Comparison of electrolytically galvanized surface before and after activation

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SEM




SEM CETA

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STEM, activator particle, laterally radiate

Clarification of the activation, nucleation and zinc phosphate crystal growth process


Matrix Activator seedsActivator particle

STEM, activator particle, laterally radiate


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Dissolution of the activator particle matrix and self organisation of theactivator seeds after release

STEM



Activator particle after short phosphating, self organisation to sphericalshape

SEM


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3

1

2

SEM

Zinc phosphate crystal after short phosphating, lateral growth und time-resolved growth stages



Zinc phosphate crystal after short phosphating, crystallisation of the crystalshell

SEM


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Zinc phosphate crystal after short phosphating, crystallisation of the crystal shell, Nano Building Units inside the zinc phosphate crystal

pores / cavities

Nano Building Units

STEM



crystal structure from transition clusters


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Non-ClassicalCrystallization


Clarification of the activation, nucleation and zinc phosphate crystal growth process - Summary

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Aufklärung des Aktivierungs-, Keimbildungs- und Zinkphosphatwachstumsprozess - Zusammenfassung

Ideal growth path



Motivation

Phosphating process

Agenda

1

2

3



Summary5 CETA

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hot air

activation

electrogalvanizingrinse

(multistage)

Influence of activator bath stability on the paint adhesion



hot air

activation

electrogalvanisingrinse

(multistage)

Influence of activator bath stability on paint adhesion

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1

10

100

1000

10000

0 25 45 51 140

Particlesize

[nm]

Time of measurement [h after start]

D10 D50 D90

Influence of activator bath stability on paint adhesionAgglomeration of activator particles, measured with Dynamic Light Scattering


1

10

100

1000

10000

0 25 45 51 140

Part

icle

size

[nm

]


D10 D50 D90

1-21-21-2

1-21-21-2

231

555

555

cross cut ratings

Influence of activator bath stability on paint adhesionAgglomeration of activator particles, measured with Dynamic Light Scattering

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SEM, agglomerated activator particle

Agglomeration of activator particles inside the activator bath leads to agglomerated particles on the metal surface



Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area

SEM


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Agglomerated activator particle after short time of phosphating, dissolving ofthe agglomerate, zinc phosphate crystal growth preferred at the flank area

SEM




SEM


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SEM




SEM



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SEM




SEM

Agglomerated activator particle after longer time of phosphating, zincphosphate crystal agglomerat


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SEM

Cavities inside a zinc phosphate crystal agglomerate (FIB-cut)Influence of activator bath stability on paint adhesion


SEM

Cavities inside a zinc phosphate crystal agglomerate (FIB-cut)Influence of activator bath stability on paint adhesion

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1

10

100

1000

10000

0 25 45 51 140

Particlesize

[nm]

Time of measurement [h after start

D10 D50 D90

1-21-21-2

1-21-21-2

231

555

555

cross cut ratings



1-21-21-2

555

D90: 2533 ± 754 nm

D90: 164 ± 27 nm

cross cut ratingsgood

cross cut ratingspoor

FIB-cuts at good and rejected material (SEM)

Cavities

Zinc phosphate crystals

Zinc layer

Platinumlayer


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Zinc layer


SEM

brittle fracture on sample with poor cross cut ratingsInfluence of activator bath stability on paint adhesion


Cavities

SEM

Zinc layer


brittle fracture on sample with poor cross cut ratingsInfluence of activator bath stability on paint adhesion

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SEM

FIB-cut on a painted sample with poor cross cut ratingsInfluence of activator bath stability on paint adhesion


Ideal Zinc Phosphate Building ProcessSummary

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Zinc Phosphate Building Process WithAgglomerated Activator Particles

Ideal Zinc Phosphate Building ProcessSummary



Motivation

Phosphating process

Agenda

1

2

3



Summary5 CETA

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10

510

1010

1510

2010

2510

3010

3510

4010

4510

Particlesize

[nm]


D90‐ohne Gegenmaßnahmen

1-21-21-2

1-21-21-2

231

555

555

Measures for increasing the activator bath stability

D90 - Without countermeasure


10

510

1010

1510

2010

2510

3010

3510

4010

4510

0

0,5 1

1,5 2

2,5 3

20

21

22

23

24

25

26

27

27,5 44

45

46

47

48

49

50

50,5 51

68

69

70

71

72

73

74

140

141

142

Particlesize

[nm]



D90‐mit Gegenmaßnahmen

1-21-21-2

1-21-21-2

231

555

555


D90 - Without countermeasureD90 - With countermeasure

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10

510

1010

1510

2010

2510

3010

3510

4010

4510

0

0,5 1

1,5 2

2,5 3

20

21

22

23

24

25

26

27

27,5 44

45

46

47

48

49

50

50,5 51

68

69

70

71

72

73

74

140

141

142

Particlesize

[nm]



D90‐mit Gegenmaßnahmen

1-21-21-2

1-21-21-2

231

555

555

111

111

111

111

111


D90 - Without countermeasureD90 - With countermeasure



Motivation

Phosphating process

Agenda

1

2

3



Summary5 CETA

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Summary

• Technology to monitor the activator bath condition (Particle Size Distribution) was found in form of Dynamic Light Scattering (DLS)

• Correlation between agglomeration of the activator particles and decreased paint adhesion was shown

• Process of activation, nucleation and zinc phosphate crystal growth process was clarified

• Countermeasures against agglomeration of activator particles can be taken in order to improve paint adhesion


Thank you for your attention

CETA

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Abstract No. 64 2015 - stahl-online.de | No. 64 Investigations on the mechanism of activation,...

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