Summary of Doctoral thesis in Materials science: Research on fabrication and characteristic properties of zirconium oxide film combination with silane on steel substrate as pretreatment for

Preparing of zirconia/silane combined films on steel subtrate for organic coatings to replace phosphate and chromate pretreatment. Proposing the mechanism of film formation process and assessing characteristics of morphology, composition, electrochemical properties and bonding of the zirconia/silane film.

MINISTRY OF

EDUCATION AND TRAINING

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

GRADATE UNIVERSIY OF SCIENCE AND TECHNOLOGY

-

NGUYEN VAN CHI

RESEARCH ON FABRICATION AND CHARACTERISTIC PROPERTIES OF ZIRCONIUM OXIDE FILM

COMBINATION WITH SILANE ON STEEL SUBSTRATE AS

PRETREATMENT FOR ORGANIC COATING

This thesis was done at: Graduate University of Science and Technology - Vietnam Academy of Science and Technology

Supervisors 1: Dr PHAM Trung San

Supervisors 2: Assoc.Prof Dr TO Thi Xuan Hang

This thesis could be found at National Library of Vietnam, Library of Graduate University of Science and Technology, Library of Institute of Materials and Science, Library of Vietnam Academy of Science and Technology

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INTRODUCTION The urgency of the thesis

Surface pretreatment of the steel substrate before painting had significant effect on coating adhesion and corrosion protection

performance Pretreatment not only increases the adhesion between the paint and the substrate, but also improves the long-term corrosion

protection Phosphate or chromate pretreatment has been widely used for this purpose However, these methods are often increasingly restricted by international conventions because of several drawbacks from environmental, energy and process points of view The trend of finding alternative methods has been interested in research and application recently These conversion coatings are usually transition metal oxides such as zirconium, titanium, vanadium, molybdenum Among them, A promising emerging pretreatment technique is one of potential replacements for phosphating and one of them is the application of zirconium oxide or organo-silane The outstanding advantage of zirconia is that the film is made of nano properties, environmentally friendly, cost-saving, simple technology, applicable on mutli-metal However, the disadvantage is that it is necessary to use deionized or ultrafiltration water because the formed film is quite sensitive to the ions in the washing water and easily forms rust during stages Besides, organo-silane is also considered as a promising surface pretreatment method because of its increased ability to bond between organic coating and metal substrate and effective corrosion protection However, the basic disadvantage of the silane is that it depends how the surface was treated, the cleanliness of the surface and the density of hydroxyl group Therefore, it is necessary to clean the surface well

before pretreatment to promote the effectiveness

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A combination of Zr based and silane based pretreatment on metal substrates in different ways has been performed recently such as: The hot-dip galvanized steel surface was treated, firstly, with zirconium nitrate salt then silane; the doped silane pretreatment on galvanised steel subtrate; Ce-silane-ZrO2 composite coating on 1060 aluminum The results confirmed that the combined film had higher corrosion resistance, less porosity, less microscopic cracking and better pre-treatment than that of each component

Several methods can be applied to combine between zirconia and silane Sol-gel is an atomic scale method which to form zirconia films with the advantage of high uniformity, the stages of the reaction can be controlled However, compared to the chemical dipping method, sol-gel method requires more stages, higher temperature, more by-products, it is limited in industrialization The zirconia/silane film could also be formed by two steps in two solutions, however, the one-solution method would allow a simpler process, at the same time, propose a mechanism for zirconia and silane to be together formed film on the substrate

Factors directly related to films forming by the method of immersion in hexaflorozirconic acid solution could be mentioned as: temperature, pH, concentration and dipping time Several studies have shown that when the temperature of the solution was increased, the corrosion performance and properties of the zirconia film were reduced Solution pH and concentration are two directly correlated parameters However, a low concentration hexaflorozirconic acid solution (with pH of about 3 to 4) usually leaded to a zirconia film with better pretreatment effect, and when the pH was changed, it would affect the forming film decisively

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Based on the above discusses, with the desire to manufacture a steel surface pretreatment film which is effective as phosphate and chromate, the topic of the thesis was chosen: "Research on fabrication and characteristic properties of zirconium oxide film combination with silane on steel substrate as pretreatment for organic coating”

Goals of the thesis

- Preparing of zirconia/silane combined films on steel subtrate for organic coatings to replace phosphate and chromate pretreatment;

- Proposing the mechanism of film formation process and assessing characteristics of morphology, composition, electro-chemical properties and bonding of the zirconia/silane film

Main contents of the thesis

- Research on preparation of zirconium oxide film on steel and selecting initial conditions on solution pH and dipping time as basis parameters for manufacturing zirconia/silane film;

- Research on manufacturing zirconia/silane films on steel substrates; explain the process of formation and their characterization

of morphology, composition, electrochemistry and bonding;

- Study the role of zirconia/silane pretreatment film for powder coating

Scientific and application of the thesis

On the scientific side, the thesis has contributed new points in the research of steel surface treatment films for coatings to replace phosphates and chromate

In practice, the results of the thesis are the basis for the development of technology for manufacturing steel surface treatment films for environmentally friendly coatings in Vietnam

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Detail goals of the thesis

- Preparing of zirconium oxide and zirconium oxide/silane films

on steel subtrate by chemical immersion method

- Interpreting the mechanism of forming ZrO2/silane film and describing their characterization of morphology, composition, electrochemistry and bonding

- Identifying some basic factors affecting the film forming process; selecting suitable conditions to form film

- Manufacturing of zirconium oxide/silane film with high corrosion resistance, improved adhesion and long-term protection

performance of powder coating compared to zinc phosphate

CHAPTER 1 OVERVIEW

1.1 The traditional method of steel surface treatment Overview

of mechanical treatment methods

Concept, development history, formation mechanism, properties and technological diagrams of chemical treatment methods: phosphate, chromate

1.2 Zirconia-based treatment method: Mechanism of formation, pretreatment efficiency, characterization and influencing factors; 1.3 Silane-based treatment method: Mechanism of formation, pretreatment efficiency, characterization and influencing factors; 1.4 Zirconia and silane combined treatment method: Presentation

of several methods were applied to combine between zirconia and silane; The advantages of these combining methods compared to an individual method;

CHAPTER 2 EXPERIMENTAL AND METHODS 2.1 Research scheme

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2.2 Main materials and chemicals

- Carbon steel samples (Quoc Viet Company) were abraded with SiC polishing paper, degreased, rusted and rinsed with distilled water and stored in a dehumidifier (bare samples)

- ZrF4 crystal, 99,99% purity, white (Sigma), Silane A-1100: γ- APS (China, 99% purity)

2.3 Preparation of surface treatment solution

Preparation of H2ZrF6 solution: ZrF4 was completely dissolved in

HF solution and then distilled water was added into H2ZrF6 acid solution obtained Zr4+ = 50 ppm Preparation of H2ZrF6/silane solution: Silane A-1100 with different concentration added to H2ZrF6solution to form H2ZrF6/silane solution

Fabrication of zirconia/silane films Fabrication of zirconium oxide films

Characterization of

morphology, composition,

electrochemistry and bonding

Preparation of chemicals, samples and H 2 ZrF 6 solution

Adhesion and corrosion

resistance under paint,

long-term protection

performance of coating Discussion,

conclusion

Determining the appropriate

conditions of pH and dipping time

according to corrosion resistance

and adhesion

Mechanism of film formation process The influence of dipping time and silane concentration on characteristic properties

Preparation of chemicals, samples and solution of

H 2 ZrF 6 /silane

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2.4 Methods for substrate treatment of samples

2.4.1 Surface treatment in H 2 ZrF 6 solution

To form zirconia film, the bare samples were immersed in H2ZrF6solution in a combination of pH varying from 1 to 6, the division was

1 and the time varying from 1 to 6 minutes, the division was 0,5

2.4.2 Surface treatment in H 2 ZrF 6 solution combined with silane

To form zirconia/silane film, the bare samples were immersed in

H2ZrF6/silane solution in a combination of silane concentration varying from 0 %  0,05 % (v/v), the division was 0,0125 %, the time varying from 1 to 6 minutes, the division was 0,5

2.4.3 Surface treatment with a two steps by immersion

The bare samples were treated in H2ZrF6 solution to form zirconia film then in silane solution to form silane film (two steps)

After surface treatments, the samples were dried with a dry air stream (70 ± 3 °C) for about 15 minutes in the laboratory

2.5 Methods, equipments and technique

EIS and DC were conducted using PGSTAT204N with electrode cell in 3,5% NaCl Frequency: 100kHz10mHz; ± 100 mV Scan rate of 1mV/s, step of 1 mV The OCP were performed during film formation for 6 minutes

3-Surface morphology was investigated by FE-SEM on Jeol 7401F (Japan) Components and bonds in the film were studied by FT-IR on Bruker Alpha (Germany) in wave number of 3000500 cm-1, EDS was investigated by Jeol 7401F and XRD patterns in the following

mode: 2: 20  80o; speed: 0,05o/giây; Cu (K α) = 1,5406 Å

To quickly assess the decrease in adhesion and the degree of corrosion under the incision, the samples were immersed in 3,5 % NaCl with different exposure time, according to ASTM D 1654-5

35 ÷ 37 oC; saturation temperature: 47 ÷ 49 oC; speed: 2 mL/h

Natural testing was conducted in accordance with ISO 4628: 2016 (Part 8) at the Marine research and testing station, Vietnam - Russia Tropical Center, Hon Tre Islands, Nha Trang city, Khanh Hoa province

CHAPTER 3 RESULTS AND DISCUSSION

3.1 Research on manufacturing zirconia film

3.1.1 Effect of pH of hexaflorozirconic acid solution

3.1.1.1 Effect of pH on corrosion resistance of samples

EIS spectrum, polarization curve were at different pHs (Figure 3.1, 3.3) had similar shape but different radians

Figure 3.1, 3.3 The EIS spectrum and PD curve at different pHs

Nova 2.0 software, equivalent diagram, capacitance formula:

and Tafel extrapolation were used to identify the typical parameters (Table 3.1, 3.2) The zirconia film formation has increased the corrosion resistance Rp value was higher (Jcorr was lower) when the pH was between 3 and 5 and reached the largest value at pH = 4 When pH <3, the acidity was high, Fe was dissolved quickly and the Zr film if formed was easy also dissolved When

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pH> 5 alkalinity increased, the anode reaction decreased, the cathode

reaction slowed, so the pH at the surface-solution interface increased

not enough to form Zr oxide

Table 3.1 Electrochemical parameters of the film at different pHs

Parameter Bare pH of H2ZrF6 solution

Table 3.2 Tafel extrapolation results of samples at different pHs

Parameter Bare pH of H2ZrF6 solution

E (- mV/SCE) 560,8 633,2 676,4 690,2 683,9 637,6

J corr (µA/cm 2 ) 137 38,8 7,7 7,2 8,0 74

3.1.1.2 Effect of pH on the adhesion of powder coating

Figure 3.4 Adhesion of powder coating at different pHs

Base sample pH=2 pH=3

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Flaking degrees (Figure 3.4) showed that the samples were treated at solution pH 3 or 4, achieving the best results, the incisions were almost unchanged (level 5) The rests appeared certain flaking marks, showing lower levels of adhesion testing

3.1.2 Effect of immersion time in H 2 ZrF 6 solution

3.1.2.1 Effect of immersion time on corrosion resistance of samples

EIS spectrum, polarization curve of the samples with different time (figures 3.5, 3.6), datas archieved from EIS, PC (tables 3.3, 3.4)

Figure 3.5, 3.6 EIS spectrum and PD curve with different time Table 3.3 Electrochemical parameters of films with different time

2 mins 3 mins 4 mins 5 mins 6 mins

5 mins

6 mins

E (- mV/SCE) 560,8 630,5 649,3 690,2 650,7 634,7

J corr (µA/cm 2 ) 137 16,1 12,5 7,2 16,1 18,1 The formation of zirconia films increased corrosion resistance of the substrate from 2 to 5 times Jcorr of treated steel samples were greatly reduced from 7,5 to 19 times, compared to the bare sample

As the immersion time increased, the increased Rp value (Jcorrdecreased) because of completing of film formation But , immersion time was too much leading to decrease Rp value (Jcorr increased) because of film was too thick, heterogeneous, cracking due to heat drying, dehydration

3.1.2.2 Effect of immersion time on the adhesion of powder coating

Assessment of flaking (Figure 3.7) showed that samples treated with immersion time from 3 to 5 minutes achieved good results, the incision almost unchanged (level 5), better than the samples treated with time of 6 or 2 minutes

Figure 3.7 Adhesion of powder coating with different immersion

time

3.2 Fabrication and characterization of zirconia/silane film

4 mins 5 mins 6 mins

t

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3.2.1 Process dynamics and film composition

The trend of OCP (Figure 3.8) showed that the steel electrode was gradually moving toward the positive side during film formation The film was quickly formed within the first 2 minutes, slowed down until 4 minutes and stabilized to 6 minutes

Figure 3.8 Trend of OCP value of bare sample in H2ZrF6/silane Initially, when the bare sample was immersed in H2ZrF6/silane,

Fe was oxidized into the solution by anode reaction (Fe-2e→Fe2+) The Fe2+ ion would combine with ZrF6-2 to release Zr4+ into the solution (Fe2+ + ZrF6-2 → Zr4+ + FeF6-4) H+ ions were reduced by local cathode reaction on the surface, releasing H2 (2H+ + 2e → H2↑) The local pH result on the sample surface increased, resulting in precipitation of hydrated zirconium oxide The crystal is germinated and then spread to the entire surface to form a zirconia film according to the equation (Zr4++ 3H2O→ ZrO2·H2O +4H+)

Siloxane network formation reactions could also occur:

In silane solution, ethoxy groups switch to silanol group (–

Si(OC2H5)3+3H2O→–Si(OH)3+3C2H5OH) The silanol group was adsorbed by (Fe-OH) through hydrogen bonding to metal-siloxane (Si-O-Fe) bonds according to (–Si(OH)3+Fe-OH→H2N(CH2)3 Si(OH)2-O-Fe) Si-OH groups also formed stable siloxane (Si-O-Si) network by the equation (SiOH+SiOH→Si-O-Si+H2O)

Immersion time (mins)