Abstract No. 64. Investigations on the mechanism of activation, nucleation and zinc phosphate crystal growth on industrially produced steel sheet

Transcription

1 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, 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 (), 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. CETAS 20

2 , Düsseldorf, Investigations on the mechanism of activation, nucleation and zinc phosphate crystal growth on industrially produced steel sheet Junge, F.; Müller, G.; Weiher, N. TA Agenda S Phosphating process 2 Motivation 3 Activation-, nucleation- and zinc phosphate crystal growth CE 4 Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary

3 Agenda Phosphating process Motivation Activation-, nucleation- and zinc phosphate crystal growth Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary Phosphating process belt dryer phosphating hot air electrogalvanising rinse (multistage) activation CETAS 20

4 Phosphating process Previous assumption of the phosphating process Activator particle Phosphating process Previous assumption of the phosphating process Activator particle Zinc surface Phosphating solution Zinc surface CETAS 20

5 Phosphating process Previous assumption of the phosphating process Activator particle Phosphating process Previous assumption of the phosphating process Activator particle Phosphating solution Zinc surface Phosphating solution Zinc surface CETAS 20

6 Phosphating process Previous assumption of the phosphating process Activator particle Phosphating solution Phosphating process Schematic cross section of a phosphated steel sheet Zinc surface Phosphate coating electrolytically deposited zinc layer CETAS 20 cold-rolled steel

7 Phosphating process Further processing steps at the customers facility, zinc phosphate crystalls Agenda CETAS Phosphating process Motivation Activation-, nucleation- and zinc phosphate crystal growth Punching plates Pressing the sheet into components Cleaning (degreasing) Activation Phosphating Thermal treatment Cathodic dip painting Primer Top Coat Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary 20

8 Motivation Issues with paint adhesion on electrogalvanized and phosphated steel sheet in the past No approach to differentiate analytically between good and rejected material Mechanism of activation, nucleation and zinc phosphate crystal growth needs to be investigated fundamentally Issues of paint adhesion evaluation of paint adhesion with cross hatch testing in accordance with DIN EN ISO 2409 CETAS > 4

9 Agenda Phosphating process Motivation Activation-, nucleation- and zinc phosphate crystal growth Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary Clarification of the activation, nucleation and zinc phosphate crystal growth process Comparison of electrolytically galvanized surface before and after activation CETAS 20

10 Clarification of the activation, nucleation and zinc phosphate crystal growth process Comparison of electrolytically galvanized surface before and after activation Clarification of the activation, nucleation and zinc phosphate crystal growth process Comparison of electrolytically galvanized surface before and after activation CETAS 20

11 Clarification of the activation, nucleation and zinc phosphate crystal growth process Matrix STEM, activator particle, laterally radiate Clarification of the activation, nucleation and zinc phosphate crystal growth process Activator particle Activator seeds CETAS 20 STEM, activator particle, laterally radiate

12 Clarification of the activation, nucleation and zinc phosphate crystal growth process Dissolution of the activator particle matrix and self organisation of the activator seeds after release STEM Clarification of the activation, nucleation and zinc phosphate crystal growth process Activator particle after short phosphating, self organisation to spherical shape CETAS 20

13 Clarification of the activation, nucleation and zinc phosphate crystal growth process 20 Zinc phosphate crystal after short phosphating, lateral growth und timeresolved growth stages 3 2 S TA Clarification of the activation, nucleation and zinc phosphate crystal growth process CE Zinc phosphate crystal after short phosphating, crystallisation of the crystal shell

14 Clarification of the activation, nucleation and zinc phosphate crystal growth process Zinc phosphate crystal after short phosphating, crystallisation of the crystal shell, Nano Building Units inside the zinc phosphate crystal STEM pores / cavities Nano Building Units Clarification of the activation, nucleation and zinc phosphate crystal growth process crystal structure from transition clusters CETAS 20

15 20 Non-Classical Crystallization S CE TA Clarification of the activation, nucleation and zinc phosphate crystal growth process - Summary

16 Clarification of the activation, nucleation and zinc phosphate crystal growth process - Summary Clarification of the activation, nucleation and zinc phosphate crystal growth process - Summary CETAS 20

17 Aufklärung des Aktivierungs-, Keimbildungs- und Zinkphosphatwachstumsprozess - Zusammenfassung Agenda CETAS Phosphating process Motivation Ideal growth path Activation-, nucleation- and zinc phosphate crystal growth Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary 20

18 Influence of activator bath stability on the paint adhesion belt dryer phosphating electrogalvanizing hot air Influence of activator bath stability on paint adhesion belt dryer phosphating electrogalvanising hot air rinse (multistage) activation rinse (multistage) activation CETAS 20

19 Influence of activator bath stability on paint adhesion Agglomeration of activator particles, measured with Dynamic Light Scattering Particle size [nm] Particle size [nm] D0 D0 D D0 D0 D Time of measurement [h after start] Influence of activator bath stability on paint adhesion Agglomeration of activator particles, measured with Dynamic Light Scattering cross cut ratings CETAS Time of measurement [h after start]

20 Influence of activator bath stability on paint adhesion Agglomeration of activator particles inside the activator bath leads to agglomerated particles on the metal surface, agglomerated activator particle Influence of activator bath stability on paint adhesion Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area CETAS 20

21 Influence of activator bath stability on paint adhesion Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area Influence of activator bath stability on paint adhesion Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area CETAS 20

22 Influence of activator bath stability on paint adhesion Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area Influence of activator bath stability on paint adhesion Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area CETAS 20

23 Influence of activator bath stability on paint adhesion Agglomerated activator particle after short time of phosphating, dissolving of the agglomerate, zinc phosphate crystal growth preferred at the flank area Influence of activator bath stability on paint adhesion Agglomerated activator particle after longer time of phosphating, zinc phosphate crystal agglomerat CETAS 20

24 Influence of activator bath stability on paint adhesion Cavities inside a zinc phosphate crystal agglomerate (FIB-cut) Influence of activator bath stability on paint adhesion Cavities inside a zinc phosphate crystal agglomerate (FIB-cut) CETAS 20

25 Influence of activator bath stability on paint adhesion Particle size [nm] D0 D0 D cross cut ratings good cross cut ratings Time of measurement [h after start D 90 : 64 ± 27 nm D 90 : 233 ± 74 nm Influence of activator bath stability on paint adhesion FIB-cuts at good and rejected material () Platinum layer Zinc phosphate crystals Zinc layer cross cut ratings poor Cavities CETAS 20

26 Influence of activator bath stability on paint adhesion brittle fracture on sample with poor cross cut ratings Zinc layer Zinc phosphate crystals Influence of activator bath stability on paint adhesion brittle fracture on sample with poor cross cut ratings Zinc layer Zinc phosphate crystals Cavities CETAS 20

27 Influence of activator bath stability on paint adhesion FIB-cut on a painted sample with poor cross cut ratings Summary Ideal Zinc Phosphate Building Process CETAS 20

28 Summary Agenda CETAS Phosphating process Motivation Activation-, nucleation- and zinc phosphate crystal growth Ideal Zinc Phosphate Building Process Zinc Phosphate Building Process With Agglomerated Activator Particles Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary 20

29 Measures for increasing the activator bath stability Particle size [nm] Particle size [nm] , D 90 - Without countermeasure D90 ohne Gegenmaßnahmen , 2 2, Time of measurement [h after start] Measures for increasing the activator bath stability D90 ohne Gegenmaßnahmen 90 - Without countermeasure DD90 mit Gegenmaßnahmen 90 - With countermeasure CETAS , , Time of measurement [h after start]

30 Measures for increasing the activator bath stability Particle size [nm] , Agenda CETAS DD90 ohne - Without Gegenmaßnahmen countermeasure D 90 - With countermeasure D90 mit Gegenmaßnahmen , 2 2, 3 Phosphating process Motivation , , Time of measurement [h after start] Activation-, nucleation- and zinc phosphate crystal growth Influence of agglomerated activator particles on the zinc phosphate crystal growth Measures for increasing the activator bath stability Summary 20

31 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 CETAS 20