Hydrophobic and superhydrophobic coatings for …860828/...Hydrophobic and superhydrophobic coatings for corrosion protection of steel LINA EJENSTAM Doctoral Thesis, 2015 KTH Royal

Hydrophobic and superhydrophobic coatings for corrosion protection of steel

LINA EJENSTAM

Doctoral Thesis, 2015

KTH Royal Institute of Technology

School of Chemical Science and Engineering

Department of Chemistry

Division of Surface and Corrosion Science

100 44, Stockholm, Sweden

TRITA-CHE Report 2015:54

ISSN 1654-1081

ISBN 978-91-7595-703-6

SP Chemistry, Materials and Surfaces Publication nr: A-3583

Copyright © 2015 Lina Ejenstam. All rights reserved. No part of this thesis may be

reproduced without permission from the author.

The following papers are printed with permission:

Paper I, III and IV: Copyright © Elsevier

Paper II: Copyright © Taylor & Francis

Printed at US-AB, Stockholm, Sweden 2015

Akademisk avhandling som med tillstånd av KTH i Stockholm framlägges till offentlig

granskning för avläggande av tekonologie doktorsexamen fredagen den 6 november kl.

10:00 i sal F3, KTH, Lindstedtsvägen 26, Stockholm. Avhandlingen försvaras på engelska.

iii

Abstract

Since metals in general, and steels in particular, are vital construction

materials in our modern society, the corrosion protection of said

materials is of great importance, both to ensure safety and to reduce costs

associated to corrosion. Previously, chromium (VI) and other harmful

substances were effectively used to provide corrosion protection to steel,

but since their use was heavily regulated around year 2000, no coating

has yet been developed that, in a fully satisfactory manner, replaces their

corrosion protective properties.

In this thesis, the use of hydrophobic and superhydrophobic surface

coatings as part of corrosion protective coating systems has been studied.

Since the corrosion mechanism relies on the presence of water to take

place, the use of a superhydrophobic coating to retard the penetration of

water to an underlying metal surface is intuitive. The evaluation of

corrosion protective properties of the hydrophobic and superhydrophobic

surfaces was performed using mainly contact angle measurements and

electrochemical measurements in severely corrosive 3 wt% NaCl water

solution.

First, the differences in corrosion protection achieved when employing

different hydrophobic wetting states were investigated using a model

alkyl ketene dimer wax system. It was found that superhydrophobicity in

the Lotus state is superior to the other states, when considering fairly

short immersion times of less than ten days. This is due to the continuous

air film that can form between such a superhydrophobic surface and the

electrolyte, which can retard the transport of electrolyte containing

corrosive ions to the metal surface to the point where the electrical circuit

is broken. Since corrosion cannot occur unless an electrical current is

flowing, this is a very efficient way of suppressing corrosion.

An air layer on an immersed superhydrophobic surface is, however,

not stable over long time, and to investigate long-term corrosion

protection using hydrophobic coatings a polydimethylsiloxane

formulation containing hydrophobic silica nanoparticles was developed.

This system showed enhancement in corrosion protective properties with

increasing particles loads, up until the point where the particle load

instead causes the coating to crack (at 40 wt%). The conclusion is that the

hydrophobicity of the matrix and filler, in combination with the elongated

iv

diffusion path supplied by the addition of particles, enhanced the

corrosion protection of the underlying substrate.

To further understand how hydrophobicity and particle addition affect

the corrosion protective properties of a coating a three layer composite

coating system was developed. Using this coating system, consisting of a

polyester acrylate base coating, covered by TiO2 particles (with diameter

< 100 nm) and finally coated with a thin hexamethyl disiloxane coating, it

was found that both hydrophobicity and particles are needed to reach a

great enhancement in corrosion protective properties also for this system.

Keywords

Superhydrophobic coating, hydrophobic coating, corrosion protection,

contact angles, electrochemical measurements, surface characterization

v

Sammanfattning

Eftersom metaller, och då särskilt stål, är viktigta

konstruktionsmaterial i vårt moderna samhälle är korrosionsskydd av

stor betydelse, både för att garantera säkerhet och för att minska

kostnader som uppkommer i samband med korrosion. Tidigare har

sexvärt krom och andra skadliga ämnen använts för att på ett effektivt

sätt skydda stål från korrosion, men efter att deras användning kraftigt

reglerades runt år 2000 har ännu ingen beläggning utvecklats som helt

kan ersätta krombeläggningarna med avseende på funktion.

I denna avhandling har hydrofoba och superhydrofoba ytbeläggningar

och deras möjliga applikation som en del av ett korrosionsskyddande

beläggningssystem studerats. Eftersom korrosionsmekanismen är

beroende av närvaron av vatten, är användandet av en superhydrofob

beläggning för att fördröja transporten av vatten till den underliggande

metallytan intuitiv. De korrosionsskyddande egenskaperna hos

superhydrofoba ytbeläggningar utvärderades här främst med hjälp av

kontaktvinkelmätningar och elektrokemisk utvärdering i korrosiv lösning

bestående av 3 vikts% NaCl i vatten.

Först undersöktes skillnaden i korrosionsskydd som uppnås vid

användandet av ytbeläggningar med olika hydrofoba vätningsregimer

med hjälp av ett modellsystem bestående av ett alkylketendimer vax. Det

konstaterades att superhydrofobicitet i Lotusregimen är överlägset bättre

än de andra hydrofoba vätningsregimerna, i alla fall när man ser till

relativt korta exponeringstider, typiskt mindre än tio dagar. Detta beror

på att den kontinuerliga luftfilm som kan bildas på en sådan typ av

superhydrofob yta kan minska transporten av elektrolyt (som innehåller

korrosiva joner) till metallytan till den grad att den elektriska kretsen

bryts. Eftersom korrosion inte kan ske utan en sluten elektrisk krets är

detta ett mycket effektivt sätt att förhindra korrosion från att ske.

Ett luftskikt på en superhydrofob yta nedsänkt i vatten är dock inte

stabilt under lång tid. För att undersöka möjligheten till korrosionsskydd

under längre tid med hjälp av hydrofoba beläggningar utvecklades en

hydrofob ytbeläggning bestående av polydimetylsiloxan och hydrofoba

nanopartiklar av kiseldioxid. Detta system visade en förbättring av

korrosionsskyddet vid ökat partikelinnehåll upp till den koncentration

(40 wt%) där i stället sprickbildning i ytbeläggningen observerades. Från

detta system kunde slutsatsen dras att matrisens och partiklarnas

vi

hydrofobicitet i kombination med den längre diffusionsvägen som

partiklarna orsakade förbättrade korrosionsskyddet av den underliggande

metallen.

För att ytterligare förstå hur hydrofobicitet och partikeltillsatser

påverkar en ytbeläggnings korrosionsskyddande egenskaper har

dessutom ett treskikts kompositbeläggningssystem utvecklats. Genom att

använda detta beläggningssystem, som består av en basbeläggning av

polyesterakrylat, ett lager TiO2-partiklar (med en diameter på <100 nm)

slutligen belagt med ett tunt ytskikt bestående av hexametyldisiloxan så

kunde slutsatsen dras att både en hydrofob matris och partiklar behövs

för att nå en markant förbättring av ytbeläggningens

korrosionsskyddande egenskaper.

Nyckelord

Superhydrofoba ytbeläggningar, hydrofoba ytbeläggningar,

korrosionsskydd, kontaktvinklar, elektrokemiska mätningar,

ytkaraktärisering

vii

Preface

The work presented in this thesis was performed as a joint project

between the Division of Surface and Corrosion Science at KTH, and SP

Chemistry, Materials and Surfaces. My PhD project was designed to

further promote the collaboration between KTH and SP, with me

performing my work at both sites, and was started as a part of the SSF

(Stiftelsen för strategisk forskning) funded program: Microstructure,

Corrosion and Friction Control. Additional funding was provided by

RISE Research Institutes of Sweden. Valuable supervision has been given

by professor Per Claesson and professor Jinshan Pan at KTH, and by

adjunct professor Agne Swerin at SP, throughout the project. This joint

project has been highly educational to me and I am grateful I got the

chance to work with people, and procedures, both in academia and at a

research institute.

Since the topic of corrosion protection is interesting to a broad

audience, both in academia and in industry, the technical level of this

thesis has been adapted to be, hopefully, attractive to anybody with

interest in the field, without requiring specific background in either

wetting or electrochemistry.

Stockholm, October 2015

Lina Ejenstam

viii

ix

List of papers

This doctoral thesis is based on the following papers, which are referred

to in the text by their Roman numerals.

I. The effect of superhydrophobic wetting state on

corrosion protection – The AKD example

Lina Ejenstam, Louise Ovaskainen, Irene Rodriguez-Meizoso,

Lars Wågberg, Jinshan Pan, Agne Swerin and Per M. Claesson.

Journal of Colloid and Interface Science, 2013, 412, 56-64

doi: 10.1016/j.jcis.2012.09.006

II. Towards superhydrophobic polydimethylsiloxane-silica

particle coatings

Lina Ejenstam, Agne Swerin and Per M. Claesson.

Preprint, accepted for publication in Journal of Dispersion

Science and Technology, 2015

doi: 10.1080/01932691.2015.1101610

III. Corrosion protection by hydrophobic silica particle-

polydimethylsiloxane composite coatings

Lina Ejenstam, Jinshan Pan, Agne Swerin and Per M. Claesson.

Corrosion Science, 2015, 99, 89-97

doi: 10.1016/j.corsci.2015.06.018

IV. Long-term corrosion protection by a thin nano-

composite coating

Lina Ejenstam, Mikko Tuominen, Janne Haapanen, Jyrki M.

Mäkelä, Jinshan Pan, Agne Swerin and Per M. Claesson.

Applied Surface Science, In press, available online, 2015

doi: 10.1016/j.apsusc.2015.09.238

V. Comparison between AFM-based methods for assessing

local surface mechanical properties of PDMS-silica

composite layers

Hui Huang, Lina Ejenstam, Jinshan Pan, Matthew Fielden,

David Haviland and Per M. Claesson

Manuscript

x

The author’s contribution to the included papers:

Paper I: Major part of experimental work, major part of

manuscript preparation. The surface coatings were

prepared by Dr. Louise Ovaskainen, at the division of

Fibre and Polymer technology, KTH.

Paper II: All experimental work, major part of manuscript

preparation.

Paper III: All experimental work, major part of manuscript

preparation.

Paper IV: Major part of experimental work, major part of

manuscript preparation. The polyester base coat was

prepared at Akzo Nobel in Malmö in cooperation with

Dr. Majid Sababi, and the TiO2 coating was prepared by

Dr. Mikko Tuominen at Tampere University of

Technology, Finland.

Paper V: Part of initiating measurements, part of manuscript

preparation. All measurements shown in the manuscript

were performed by Hui Huang at the division of Surface

and Corrosion Science, KTH, in cooperation with the

section of Nanostructure Physics, KTH. Hui Huang also

prepared the major part of the manuscript.

xi

Summary of papers

Paper I:

The effect on corrosion protective properties of coatings representing

different wetting states was evaluated using a model system consisting of

alkyl ketene dimer (AKD) wax coatings exhibiting different hierarchical

structures. All coatings had the same surface chemistry and the effect

seen could therefore be attributed only to the difference in surface

structure. A remarkable increase in the measured electrochemical

impedance was found for the coating wetting in the superhydrophobic

Lotus state, originating from the air layer present at that surface, which

slows down transport of corrosive ions from solution to substrate, to the

point where the electrical circuit is broken and no corrosion can proceed.

Paper II:

The development of a one-pot superhydrophobic coating formulation

was addressed through systematic addition of hydrophobic silica

nanoparticles (16, 60-70, 250 and 500 nm) to a polydimethylsiloxane

(PDMS) matrix. Superhydrophobicity in the Lotus state was only reached

when particles were added at such high amount (40 wt%) it caused the

coating to crack, while the superhydrophobic rose state was achieved for

non-cracked coatings, which lead to further understanding of how the

shapes of the protrusions affect the resulting wetting state.

Paper III:

The corrosion protective properties of the most promising coating

system developed in Paper II was evaluated by coating carbon steel

substrates with formulations consisting of PDMS containing 0, 10, 20 or

40 wt% hydrophobic 16 nm particles. Due to crack formation allowing

easy passage of electrolyte through the coating, the 40 wt% coating did

not provide any corrosion protection to the underlying substrate. The 20

wt% coating performed very well, although it did not reach

superhydrophobicity but displayed hydrophobic wetting behaviour. The

protective properties were assigned to the synergistic effect of the

hydrophobicity of the matrix and particles, as well as the elongated

diffusion path arising from particle addition. Beyond this, another very

interesting effect was seen. The coatings with 0 and 10 wt% particles

exhibited a surprising increase in measured impedance during the first 24

xii

hours. This effect is so large that the most likely reason is passivation of

the underlying metal, promoted by the coating properties.

Paper IV:

To further understand the corrosion protective properties seen in

paper III, a coating system consisting of a polyester acrylate (PEA) base

coat, covered by a layer of TiO2 nanoparticles (< 100 nm) which in turn

was covered by a thin hexamethyl disiloxane (HMDSO) coating was

developed. The systematic evaluation of the protective properties of the

layers one by one, two by two, and in the three layer combination showed

that to reach a long-term stable, reproducible, protective coating a

combination of properties from all three layers is needed.

Paper V:

A comparison of coating properties on the nanoscale level using three

different operating modes of atomic force microscopy (AFM) was

conducted using the polydimethylsiloxane coating (Papers II and III)

without and with the addition of 20 wt% hydrophobic silica nanoparticles

(16 nm). The surfaces were found to be quite heterogeneous on the

nanoscale level, and different regions of softer and stiffer polymer was

encountered and attributed to variations in crosslinking degree. The 16

nm hydrophobic silica particles were found to aggregate and form

structures in the size 20 - 110 nm. In addition to the particles, spherical

shapes were also seen on the surface which were recognized as air

bubbles trapped in the matrix during curing.

xiii

Nomenclature

Scientific abbreviations

ACA Advancing contact angle

AFM Atomic force microscopy

AKD Alkyl ketene dimer

CA Contact angle

CAH Contact angle hysteresis

CE Counter electrode

CPE Constant phase element

DMT Derjaguin-Muller-Toporov

EIS Electrochemical impedance spectroscopy

HMDSO Hexamethyl disiloxane

LFS Liquid flame spray

OCP Open circuit potential

PDMS Polydimethylsiloxane

PEA Polyester acrylate

RCA Receding contact angle

RE Reference electrode

RESS Rapid expansion of supercritical solutions

WE Working electrode

Symbols

C capacitance (F)

θ contact angle (°)

γ surface energy (J/m2)

r roughness ratio

λ wavelength (nm)

xiv

xv

Table of contents

Abstract ……………………………………….……….………..…….iii

Sammanfattning………………………………………….…………...v

Preface…………………………………………………….…………..vii

List of papers…………………………………………………………ix

Summary of papers………………………………………………….xi

Nomenclature……………………………………….……………….xiii

Table of contents……………………………………………………xv

1. Introduction ............................................................................. 1 1.1. Aim of this work .................................................................................... 2

2. Background ............................................................................. 3 2.1. The corrosion mechanism ................................................................... 3 2.2. Passive corrosion protection .............................................................. 4 2.3. Hydrophobic and superhydrophobic coatings .................................. 6 2.3.1. Wetting ...................................................................................................... 6 2.3.2. Fabrication of superhydrophobic surfaces .............................................. 10

3. Experimental .......................................................................... 13 3.1. Coatings .............................................................................................. 13 3.1.1. Alkyl ketene dimer wax ........................................................................... 13 3.1.2. Polydimethylsiloxane with hydrophobic silica particles............................ 14 3.1.3. Commercial polydimethylsiloxane ........................................................... 15 3.1.4. Layered composite coating ..................................................................... 15 3.1.5. Summary of coating properties ............................................................... 16

3.2. Methods ............................................................................................... 18 3.2.1. Environmental scanning electron microscopy ......................................... 18 3.2.2. Atomic force microscopy ......................................................................... 18 3.2.3. Confocal Raman microspectroscopy....................................................... 20 3.2.4. Contact angle measurements ................................................................. 21 3.2.5. Electrochemical measurements .............................................................. 22

4. Results and discussion......................................................... 25 4.1. The AKD system ................................................................................. 25 4.2. The PDMS-hydrophobic silica nanoparticle system ....................... 27

xvi

4.3. The three layered composite system ................................................ 29 4.4. Unifying concept 1: Superhydrophobicity........................................ 30 4.4.1. Shape, size and distribution of protrusions .............................................. 30 4.4.2. Air film stability under water .................................................................... 34

4.5. Unifying concept 2: Corrosion protection ........................................ 36 4.5.1. Effect of wetting state on corrosion protection ......................................... 36 4.5.2. Barrier effect from particles ..................................................................... 39 4.5.3. Possible passivation of underlying substrate ........................................... 41

4.6. Commercial PDMS .............................................................................. 43 4.7. Mechanical properties of PDMS coating........................................... 45 4.7.1. Nanomechanical properties of PDMS coating ......................................... 45

5. Conclusions .......................................................................... 47

6. Future work ........................................................................... 49

7. Acknowledgements .............................................................. 51

8. References ............................................................................ 53

INTRODUCTION | 1

1. Introduction

Metals are important construction materials in the world today, and,

for example, steel is used in almost every bridge, building and

transportation vehicle built. Due to this, corrosion protection of metals

has become an issue of great importance, foremost to ensure the safety of

constructions and vehicles, but also to great extent out of economic

concerns. In the United States costs associated with corrosion were

concluded to be circa 3 % of the gross domestic product (GDP) in 2002

[1], and today all industrialized countries, in general, experiences costs

related to corrosion corresponding to 1 – 4 % of their country’s GDP [2].

Until recently corrosion protection could be achieved efficiently

through the use of chromium containing coatings. However, chromium

(VI) ions have been proven harmful, both to humans and to the

environment [3, 4] and around year 2000 the use of chromium (VI) was

heavily restricted. Today its use is not acceptable in any concentrations

[4]. Due to these regulations there is a need to develop efficient and

environmentally friendly, non-toxic, corrosion protection systems. One

approach is to use polymeric coatings, which gains extra interest due to

their flexibility, the possibility to make them water based or solvent free,

and their comparatively low cost [5].

From this point of view it is not surprising that the interest in the

development of environmentally friendly corrosion protective coatings

has escalated recently [6]. This is clearly visualized in Figure 1, where the

number of patents and research publications returned by a search for

“corrosi* AND protect* AND coating” are plotted versus publication year

[7]. Since the regulation of chromium (VI) use came into place, there has

been a continuous increase in reported research regarding alternative

ways to protect metal from corrosion. An increase in filed patents can also

be seen just around year 2000 when these regulations came into place.

2 | INTRODUCTION

Figure 1. Number of patents and research publications returned when searching for

“corrosi* AND protect* AND coating” versus year of publication. A continuous increase of

research articles can be seen as a reaction to the restriction of the chromium (VI) use.

1.1. Aim of this work

The overall aim of this project was initially to develop a thin top coat

layer, to be used as part of a corrosion protective coating system. The top

coat layer should retard transport of water to the underlying metallic

substrate. Subsequently an investigation of other applications for such

coating was intended, with specific focus on possible friction control and

wear. Originating from these aspirations the decision to focus the work on

superhydrophobic surface coatings was simple, especially since their

positive effect on corrosion and friction control already had been

reported. Following promising results attained at the beginning of the

corrosion protection evaluation, a need to understand the underlying

mechanism arose, and focus was put on that during the second part of

this work.

Ultimately, the two most important goals of this work became:

Develop an easy to use, one-pot, superhydrophobic surface coating,

preferably without the use of environmentally harmful compounds.

Evaluate and understand the corrosion protective properties of such

coating systems.

BACKGROUND | 3

2. Background

Corrosion protection by hydrophobic and superhydrophobic coatings

falls into an interdisciplinary field, between classical corrosion science

and wetting, and to help the reader to more easily follow the discussions

when looking at the results, the most important features of these two

fields are first briefly presented in this chapter.

2.1. The corrosion mechanism

By corrosion one commonly refers to the oxidation, and following

dissolution, of a metallic material. The oxidation is balanced by a

reduction of a non-metal, for example oxygen, to create a system of redox

reactions. The oxidation of the metal typically occurs at the

metal/environment interface and constitutes the anodic reaction, while

the reduction of oxygen, which typically occurs in solution, often

constitutes the cathodic reaction [6]. The anodic and cathodic reactions

together form an electrical circuit, which is completed by conduction of

electrons in the metal substrate and by ionic conduction through the

electrolyte. In conductive materials the anodic and cathodic reactions can

take place at widely separated locations of the surface, or just next to each

other. If there is a separation of the sites, the anodic one tends to become

acidic and the cathodic one becomes alkaline, due to the species released

by the corrosion reactions [6].

Iron is one of the most frequently used construction metals and the

corrosion mechanism of iron is therefore also one of the most well

studied, and it constitutes an important example of metallic corrosion. In

an environment including water and oxygen, corrosion products of iron

will entail a complicated mixture of hydrated iron oxides and other

related species. If these corrosion products can form a dense, insoluble,

film with good adherence to the underlying metallic surface, the corrosion

reaction may be self-limiting due to passivation [6, 8]. However, many of

the iron oxides are porous, and the more stable ones are very sensitive to

the presence of chloride ions, which will cause the oxide layer to degrade

4 | BACKGROUND

rapidly [9]. The simplest form of reactions associated with corrosion of

steel, in the presence of water and oxygen, is given below.

Fe Fe2+ + 2 e- (anodic reaction)

O2 + 4 e- + 2 H2O 4OH- (cathodic reaction)

Fe2+ + 2 OH- Fe(OH)2

4 Fe(OH)2 + O2 2 Fe2O3 + 4 H2O (formation of iron oxide)

2.2. Passive corrosion protection

Applying an organic coating to a metal surface is often an efficient way

of protecting the metal from corrosion, while still maintaining the

desirable mechanical properties [5, 6, 10], and the use of

superhydrophobic, or hydrophobic, coatings to slow down the transport

of water containing corrosive ions to the underlying substrate has been

studied before [11-18].

The corrosion process on a bare metal is complicated in itself, and the

corrosion rate and the morphology of the corrosion products depend on

several factors [19]. However, when applying an organic coating on top of

the surface, the number of parameters to take into account in an

evaluation dramatically increases. Coating thickness, size and

distribution of pores, barrier properties to water and oxygen, adhesion to

the substrate and ionic conductivity are only a few of the parameters that

greatly affect the corrosion protective properties of a coating [19].

The corrosion protective mechanism of an organic coating system can

roughly be divided into three groups: the electrochemical effect, the

physicochemical effect and the adhesion to the substrate [8]. The

adhesion is very important since the protective effect of the coating will

completely fail at areas where delamination occur, creating easy access to

areas where corrosion can proceed without restriction. The focus of this

thesis has, however, not been on coating adhesion, and therefore this

issue will not be further discussed.

By the electrochemical effect the, often high, electrical resistance of

organic coatings is referred to. This will severely slow down the corrosion

process since corrosion is dependent on electrical flow, and the possibility

to create a closed electrical circuit. No organic coating is impermeable to

BACKGROUND | 5

water [8], and will therefore not avert the reduction of water to form

hydroxide ions. However, organic coatings exhibit high electrical

resistance and low ion solubility, which will block ionic pathways between

anodic areas and cathodic areas from being formed, and effectively

suppress the formation of corrosion products and thereby also the

progress of corrosion [5, 6].

Physicochemical effects include coating properties such as wetting and

effects due to addition of fillers, pigments or inhibitors. Inhibitors are

compounds, which actively react when corrosion starts, in a way designed

to counteract the corrosion. Wetting properties and inactive fillers, such

as pigments, play a great role in the work presented here. The pigments

and fillers enhance the barrier effect of the coating by prolonging the

diffusion pathway through the coating to the substrate. Increasing the

coating thickness will also result in an increase of its barrier properties [3,

8, 9, 20, 21]. Pigments may also prevent direct ionic conduction through

the coating by blocking the formation of pathways [6]. In Figure 2, the

possible reactions associated with corrosion are visualized on a steel

substrate carrying a polymeric coating. Dotted arrows indicate how the

diffusion of iron oxides and corrosive ions from solution are slowed down

due to elongated diffusion path from fillers and selective permeability of

the polymer matrix.

Figure 2. Schematic overview of the corrosion mechanisms of a polymer coated metallic

substrate. Dotted arrows indicate partly restricted diffusion paths.

In industry, polymer coatings designed to prevent corrosion are

combined in several layers, including pigments and/or inhibitors,

reaching a final combined thickness ranging from a few micrometers up

to one millimeter [22, 23]. For coatings used for research, the thicknesses

6 | BACKGROUND

often range from a few micrometres up to several hundreds of

micrometres, and the dominating polymeric matrices used to achieve

corrosion protection are epoxy, polyester, acrylate and polyurethane [24,

25]. Since the barrier effect is directly dependent on the coating thickness

it can be difficult to compare the performance of different coatings,

especially since the coating thickness it not always reported.

2.3. Hydrophobic and superhydrophobic coatings

Since the group lead by professor Barthlott discovered the extreme

water repellency and unusual self-cleaning properties of the Lotus leaf

(Nelumbo nucifera), and coined the notion of the lotus effect,

superhydrophobic surfaces have become widely studied in the field of

surface science [26, 27]. The most robust superhydrophobic structures in

nature exhibit a hierarchical structure [28, 29], but superhydrophobic

surfaces have been created using only one level structure as well [29]. In

addition to surface roughness, low surface energy is needed to render a

surface superhydrophobic.

2.3.1. Wetting

The contact angle of a liquid droplet is a convenient way of describing

the wetting properties of a surface. The contact angle is defined as the

angle where the droplet base line and the droplet tangent meet. The

contact angle can be related to the surface energies of the three interfaces

where air, liquid and solid meet as described by Young’s equation:

cos 𝜃 =𝛾𝑆𝐺−𝛾𝑆𝐿

𝛾𝐿𝐺 (1)

where θ is the contact angle, γSG , γSL, and γLG are the surface energies of

the solid-gas, solid-liquid and liquid-gas interfaces, respectively. See

Figure 3. Surfaces exhibiting a water contact angle higher than 150° are

termed superhydrophobic. A distinction should be made between the

local microscopic contact angle and the macroscopic contact angle, which

can be different, and recently it has been suggested to term the

macroscopic one “apparent contact angle” to diminish confusion [30-32].

In this work, however, the macroscopic contact angle is the only contact

angle reported and the simpler notation “contact angle” will be kept when

BACKGROUND | 7

referring to macroscopic contact angles, in accordance with the notation

used in Papers I - IV.

Figure 3. Showing the interfacial energies experienced by a droplet resting on a surface.

On structurally rough surfaces the contact angle can be described

using one of two models. Wenzel first described the situation where the

liquid fully penetrates the surface structure [33] by the following

equation:

cos 𝜃 = 𝑟 cos 𝜃0 (2)

where θ is the contact angle, r is the roughness ratio (ratio of the true

solid surface area over the apparent area), and θ0 is the contact angle of a

corresponding flat surface of the same material. Cassie and Baxter then

described the situation when air is trapped in the surface structure

creating a composite layer for the droplet to rest on [34]. The composite

situation is described by the Cassie-Baxter equation:

cos 𝜃 = 𝑟𝑓𝑆𝐿 cos 𝜃0 − 1 + 𝑓𝑆𝐿 (3)

where fSL is the fraction of the composite consisting of solid-liquid

interface [26, 28, 35, 36]. However, the model equations proposed by

Wenzel and Cassie-Baxter are not without shortcomings, and recently

Gao and McCarthy [37-39] received significant attention when they

published several papers stating that Wenzel and Cassie were “wrong”.

However, their main concern was to encourage the equations to be used

with care. They also put emphasis on how only the situation beneath the

three phase line determines the contact angle of the sample. This was

shown by adding patches of different structures and surface energies to a

sample, which did not affect the measured contact angle as long as the

different patches were fully covered by the water droplet. Also Extrand

[40] came to similar conclusions.

8 | BACKGROUND

External stimuli such as pressure, vibration and evaporation can cause

a transition from partial wetting in the Cassie-Baxter state, to full wetting

in the Wenzel state [26, 28]. The transition is permanent, i.e. no

spontaneous restoring has been reported, however, if the wetting was

induced by wetting agents or surfactants restoring may be possible [41,

42]. Regardless, understanding of how to decrease the risk for such

transition is of importance when designing superhydrophobic surfaces. It

has been found that reducing the micro-structural scales increase the

threshold pressure of the wetting transition [26].

However, when dual roughness, i.e. a hierarchical structure, is

present, the Cassie-Baxter model is no longer sufficient to describe the

wetting properties since several intermediate states arise [35]. Here only

the two most important for this work will be discussed. Assuming that the

surface energy of the sample is low, air can be stuck in the larger

structures, in the smaller structures, or in both, giving rise to two types of

superhydrophobicity. To distinguish between these states the advancing

and receding contact angles of a surface become important. The

advancing contact angle (ACA) is defined as the contact angle measured

when the droplet front advances over the surface, while the receding

contact angle (RCA) is the contact angle measured when the droplet front

recedes. The contact angle hysteresis (CAH) is defined as the difference

between advancing and receding contact angles. The advancing and

receding contact angles are commonly measured using either the needle

in droplet method (further discussed in section 3.2.4) or by tilting the

sample and evaluating the ACA and RCA at the droplet front and rear at

the inclination where the droplet starts to slide. Figure 4 shows how ACA

and RCA are defined in these two cases.

Figure 4. In (a) definition of ACA (θAdv) and RCA (θRec) using needle in method are shown,

and in (b) the corresponding definition using the tilting method.

BACKGROUND | 9

When air is present in both structures, see Figure 5, the sample will

exhibit a very low contact angle hysteresis, and a droplet placed onto the

surface will easily roll off. The most famous example of this in nature is

the leaf of the Lotus flower, and therefore the low adhesion

superhydrophobicity is called Lotus state superhydrophobicity. When air

is stuck in only one of the structures (either the larger or the smaller

ones) another effect can be seen (Figure 5). A water droplet will still show

a very high advancing and static contact angle on such a surface, but if the

surface is turned up-side down, the droplet will adhere to the surface due

to a very large contact angle hysteresis created by a very low receding

contact angle. This effect is named the superhydrophobic rose state, since

water droplets on rose petals behave in this way [35].

Whether the adhesive effect of the rose state is caused by wetting of

the larger or the smaller structure is discussed. Traditionally, it has been

said that wetting into the larger structures causes the adhesion while air

in the smaller structure kept the contact angle high [26], but several

recent reports show the opposite, i.e. how water penetration into the

smaller structures (often promoted by surface structure shape) is

responsible for the high adhesion [43, 44]. Figure 5 shows four possible

wetting states of a surface exhibiting a hierarchical structure.

Figure 5. Schematic view of four possible wetting states on a surface exhibiting hierarchical

structure.

10 | BACKGROUND

It is clear that the wetting state depends on several surface properties

making it hard to predict. One of the properties is the pitch between the

protrusions and many attempts have been made to find the pitch size at

which the transition between different wetting states occurs. This varies,

however, from surface to surface depending on size, shape and

distribution of the asperities [29, 45, 46]. The shape of the surface

asperities has been given special attention by for example Teisala et al.

who propose that wetting of the smaller structure in a sample with

hierarchical structures is promoted by a rounded shape of said structures

[44]. It has also been shown that while vertical walls are sufficient to

create superhydrophobic structures, overhanging structures are needed

to create structures exhibiting high contact angles for non-polar liquids

[30, 45, 46].

2.3.2. Fabrication of superhydrophobic surfaces

It should be mentioned that the composite air/material layer needed

to create superhydrophobic surfaces can be achieved on surfaces not only

by protrusions but also by the presence of pores, the surfaces are then

termed positive or negative, respectively.

Superhydrophobic polymeric surfaces can be made in two steps where

one step focuses on lowering the surface energy and the other step

concerns addition of structure, alternatively in a one step process where

both these features are addressed simultaneously. Techniques to fabricate

superhydrophobic surfaces include: replication of natural surfaces

(directly or using a template) molding, roughening by introduction of

nanoparticles (often consisting of metal oxides or polymer), addition of

carbon nanotubes and surface modification by low surface energy

materials by plasma, electrospinning, and electron or laser treatment [36,

47-50].

Materials exhibiting low enough surface energy to make them

hydrophobic (CA > 90°) have been shown to reach superhydrophobicity

when roughened, regardless of the exact value of the surface energy, and

the conclusion that the roughness is the more critical property to achieve

superhydrophobicity can be drawn [29]. High contact angles, low

adhesion and good mechanical stability cannot be optimized concurrently

and the balance between them has to be optimized for each specific

application. In addition to this, due to the roughness features needed, it is

BACKGROUND | 11

even more challenging to produce superhydrophobic surfaces robust

enough to withstand a standard scratch test [32].

Durable and self-healing superhydrophobic surfaces

There have been a few reports on durable superhydrophobic surfaces,

and one example is the fabrication of a superhydrophobic surface by

Ebert and Bhushan. They spray coated silica particles onto an epoxy

substrate, and the resulting surface proved stable during wear

experiments using AFM, ball-on-flat tribometer and water jet apparatus

[47]. Another example is the durable superhydrophobic surface fabricated

by Deng et al. [51], using porous, raspberry-like, silica particles coated by

a fluorosilane. They tested the durability by looking at the impact of

falling sand on the superhydrophobicity and by an adhesion test using

double-sided adhesive tape. Ke et al. [52] also fabricated robust

superhydrophobic surfaces by spray coating silica particles onto the

substrate folloed by lowering of the surface energy using

polydimethylsiloxane. They then used a shear test to evaluate the

durability of the superhydrophobicity.

Lastly, there are a few reports on self-healing superhydrophobic

surfaces, for example Xue et al. [53] used an approach where particles

and low surface energy polymer are coated in a way that allows new

roughness structures to emerge if the top surface gets abraded. The same

approach was also used by Chen et al. [54], but instead facilitated by the

addition of fluorinated silica nanoparticles and photocatalytic TiO2

nanoparticles to polystyrene.

12 |

EXPERIMENTAL | 13

3. Experimental

Here follows a short description of the most important characteristics

of the four coatings featured in this thesis, and of the key techniques used

to evaluate them. A more thorough description of both coatings and

techniques can be found in Papers I-V.

3.1. Coatings

3.1.1. Alkyl ketene dimer wax

The alkyl ketene dimer (AKD) wax, used in Paper I, was spray coated

onto polished carbon steel substrates using the rapid expansion of

supercritical solutions (RESS) technique [55, 56]. Supercritical carbon

dioxide was used as solvent and different surface structures were

obtained by moving the spray nozzle at different speeds, and by varying

the number of layers applied. For reference, AKD-wax was also dissolved

in toluene and spin-coated onto a substrate. Figure 6 shows the molecular

structure for AKD where R1 and R2 are long carbon alkyl chains, in Paper

1 differential scanning calorimetry (DSC) showed a mixture of C16 and

C18 chains for the AKD used.

Figure 6. The structure of the AKD molecule where R1 and R2 represent long carbon alkyl

chains. Reprinted with permission. Copyright © Elsevier 2013.

14 | EXPERIMENTAL

3.1.2. Polydimethylsiloxane with hydrophobic silica particles

In Papers II, III and V, a model polydimethylsiloxane (PDMS) based

coating, containing varied amounts of hydrophobic silica nanoparticles,

was developed and evaluated. The formulation was based on as few

components as possible to allow fundamental understanding of the

system. PDMS was chosen as material due to its low surface energy, 20.4

mJ/m2, which renders a water contact angle of 107 ° on a flat surface

[36]. In Paper II, a range of particle sizes was evaluated, starting at

hydrophobic silica nanoparticles, with a primary particle size of 16 nm,

and continued by 60-70 nm, 250 nm and 500 nm silica particles which

were hydrophobized using dimethyl dichlorosilane before addition to the

matrix. In Papers III and V, only the 16 nm hydrophobic silica particles of

varied amounts were used. The mixtures were spin coated onto glass

slides, or polished carbon steel substrates, depending on the intended

evaluation method. A fluorosilane was used to crosslink the PDMS

coating through a condensation curing mechanism, see Figure 7. The

curing process was slow, and a steady state value for the receding contact

angle was found only after one week of curing for these surfaces. The

optimization of the PDMS-based coating is described in detail in Paper II,

in Paper III the corrosion protective properties of the coating are

evaluated and in Paper V the nanomechanical properties are studied.

Figure 7. Condensation curing of the model PDMS system, where in the first step the

fluoropolymer crosslinker is hydrated and in the second step the crosslinker binds to the

PDMS prepolymer in a condensation reaction.

EXPERIMENTAL | 15

3.1.3. Commercial polydimethylsiloxane

A commercial PDMS-formulation (PDMS 3-1953 Conformal, Dow

Corning, Ellsworth Adhesives AB) was evaluated for comparison. This

coating was chosen to match the model PDMS coating developed in Paper

II as closely as possible. It is also cures by the condensation curing

mechanism and exhibits a tack-free time of 8 min at ambient conditions.

The commercial PDMS-formulation was spin coated onto polished steel

substrates using the same settings as for the model coatings, 2000 rpm

for 30 s, and it was subsequently cured at 60 °C and 15 %RH for 1 hour.

Due to the clear color of the coating, it was concluded that it does not

contain particles with a diameter larger than 20 nm, but the coating does

contain adhesion promoters and other additives which were not present

in the experimental formulation.

3.1.4. Layered composite coating

The coating system presented in detail in Paper IV, was designed to

allow evaluation of each component with respect to the corrosion

protective properties achieved from the complete coating system. A

polyester acrylate (PEA) coating was chosen as base coat due to its

promising results regarding corrosion protection [57, 58] and its good

adhesion to the substrate. On top of that, a layer of TiO2 nanoparticles

was coated using a liquid flame spray (LFS) technique, resulting in

particles with a diameter < 100 nm [59-64]. Finally a hexametyl

disiloxane (HMDSO) coating was applied, by plasma polymerization [65],

to lower the surface energy of the coating system. These layers were then

applied one by one, two by two, and all three on top of each other, to

compile the sample matrix. A visualization of the buildup of the coatings

is provided in Figure 8, and in Figure 9 the chemical structures of PEA

and HMDSO are shown.

Figure 8. The buildup of individual samples constituting the evaluation matrix for the three

layered composite coating. Reprinted with permission. Copyright © Elsevier 2015.

16 | EXPERIMENTAL

Figure 9. Chemical structure of PEA and HMDSO, as well as the structure of HMDSO after

plasma coating.

3.1.5. Summary of coating properties

In table 1, a few important properties of the different coatings are

summarized to provide a base to the subsequent discussions. The main

coating method was spin coating because of its simplicity. The two spray

coating techniques (RESS and LFS) were very successful at creating the

coveted hierarchical structure, but the porosity of the applied coatings

posed a considerable drawback during the corrosion protection tests. The

coating thickness was approximately 11 ± 5 µm for all coatings included in

this thesis, which is considerably thinner than commercial polymeric

coatings used for corrosion protection.

Wetting states ranging from hydrophilic to superhydrophobic in the

Lotus state were realized in this work. Hydrophobicity was found to

greatly improve the corrosion protective properties of the coatings, and

focus was put on the development and evaluation of hydrophobic

coatings. The presence of particles will also enhance the corrosion

protective properties of a coating. The addition of 16 nm hydrophobic

silica to PDMS was investigated, as well as addition of a layer consisting

of TiO2 to a PEA base coat, with and without, subsequent

hydrophobization through addition of HMDSO plasma polymer.

EXPERIMENTAL | 17

Table 1. Summary of important properties for the four coating systems. Superhydrophobicity

is denoted SH.

AKD PDMS –

model

PDMS -

commercial

PEA-TiO2-

HMDSO

Application

method

Spray or

spin

coating

Spin coting Spin coating Spin coating,

LFS, plasma

Coating

thickness (µm)

6 - 8 ~13 13 - 16 ~11

Wetting states

achieved

SH Lotus,

SH rose,

Wenzel

SH Lotus,

SH rose,

Hydrophobic

Hydrophobic SH rose,

Hydrophobic,

Hydrophilic

Barrier particles None Hydrophobic

Silica

16 nm

None Hydrophilic

TiO2

<100 nm

18 | EXPERIMENTAL

3.2. Methods

3.2.1. Environmental scanning electron microscopy

The most important advantage with environmental scanning electron

microscopy (ESEM), compared to conventional scanning electron

microscopy (SEM), is that non-conductive samples can be imaged in the

presence of water vapor, or in another gaseous environment. This entails

that fragile samples can be imaged, without the need of first applying a

conductive coating onto it, which reduces the risk of misinterpretation of

the image due to coating artefacts.

The water vapor in the sample chamber fills two functions. Firstly, it

increases the pressure, up to ~10 Torr (1.33 kPa) is possible, which allows

imaging of sensitive samples. Secondly, water molecules hit by the

scattered electrons from the sample will become ionized, and the freed

electron can ionize another water molecule, creating a cascade effect

enhancing the signal from the sample on its way to the detector. This can

greatly improve the image quality when imaging non-conductive samples.

The ionized vapor molecules carry a positive charge and drift towards the

sample, which is gaining a negative charge from the electron beam, and

neutralizes the charging of the sample. This makes it possible to image

non-conductive sample without the negative effects encountered in

conventional SEM [36, 66].

3.2.2. Atomic force microscopy

Atomic force microscopy (AFM) is a powerful tool for imaging of

surface topography at the nanometer scale. AFM can be operated in

different modes, and for soft surface samples, such as polymers, the non-

contact mode is preferable since the cantilever tip of the AFM otherwise

could destroy the surface [36, 67-69]. In this work, two modes have been

of special importance, PeakForce QNM® (Quantitative Nanomechanical

Mapping) and ImAFM (intermodulation AFM) and they are therefore

shortly described here.

Using the AFM PeakForce QNM mode, nanomechanical properties of

the substrate can be mapped, see Figure 10. One force curve is captured

at each image pixel. From this force curve images showing local surface

deformation, energy dissipation and tip-surface adhesion are obtained.

By further fitting a contact mechanics model to a part of the force curve

EXPERIMENTAL | 19

(the Derjaguin-Muller-Toporov (DMT) model was used here) information

on the elastic modulus is obtained. For this model to be valid, careful

estimation of the tip end radius, and calibration of the spring constant

and the deflection sensitivity of the cantilever have to be performed

before the measurement [70-72].

Figure 10. Schematic illustration of which parts of the force curve are used to extract the

different sample properties in the PeakForce QNM mode.

ImAFM is a newly developed method to determine the local

mechanical properties of a sample through an alternative route. It works

by oscillating the cantilever with two frequencies close to the resonance

frequency of the cantilever. The nonlinear tip-surface interaction results

in interferences between these two frequencies, causing the appearance of

new frequencies, so called intermodulation products, in the response

signal. These are then analyzed to show conservative and dissipative

forces between the tip and the surface which in turn, using different

models, can be converted to represent the mechanical properties and the

viscous nature of the surface of a material. The sensitivity of this

technique allows accurate characterization of, for example, composites

with soft matrix and hard fillers [73, 74].

20 | EXPERIMENTAL

3.2.3. Confocal Raman microspectroscopy

Confocal Raman microspectroscopy is a combination of microscopy

and spectroscopy. The light source consists of a laser, and a pinhole

aperture is used to reduce the amount of scattered light which, in contrast

to conventional light microscopes, allows illumination of only a single

point on the sample. This leads to a good spatial resolution, and by using

a green laser (λ = 532 nm) and a numerical aperture (NA = 1) for the

objective, the best lateral resolution that can be obtained is ~200 nm,

while the vertical resolution is ~500 nm. The confocal Raman microscope

can be used to scan a sample both in lateral and vertical direction making

it possible to create 3D-images of the sample [69, 75].

The technique is based on the inelastic scattering of the laser beam

light which is absorbed by the sample and then re-emitted, only now

shifted in frequency compared to the incoming light. The frequency shift,

also called Raman shift, provides information about rotational and

vibrational energies of molecular bonds. The subtracted Raman spectra

usually show peaks characteristic of specific molecular bonds, and the

intensity of a given peak is proportional to concentration and can

therefore be used for quantitative analysis as well as qualitative.

Combined with the lateral and vertical scan possibilities, confocal Raman

microscopy makes a powerful tool to analyze the chemical composition of

a sample in the bulk near the surface. In Figure 11 this is illustrated by

showing how the scanning of a sample will return a full spectrum in every

individual pixel [69, 75].

Figure 11. Visualization of how a confocal Raman scan, containing local chemical

information about the sample, returns one spectrum in each individual pixel (three pixels

with corresponding spectra are highlighted).

EXPERIMENTAL | 21

3.2.4. Contact angle measurements

Measuring contact angles from sessile drops is a convenient method to

characterize surface wetting properties due to its simplicity. A droplet of

the chosen liquid is placed onto the sample surface from a syringe, and

the shape of the sessile droplet profile, during and after deposition, is

captured using a high speed camera. The contact angle is then evaluated

by measuring the angle of the droplet tangent (at the point where water,

surface and air meet) [32, 36, 76, 77]. Within some limits the contact

angle is independent of the droplet size. However, it is important to use a

droplet volume small enough to avoid shape distortion by gravity. At the

same time it is important to keep the droplet size larger than the

structural features present on the surface [29, 31]. In the work presented

here a droplet size of 4 µL was used when evaluating the static contact

angle.

To measure the advancing contact angle by the needle-in-droplet

method, the droplet volume has to be increased until the point where the

contact angle reaches a constant value, and the base line moves, resulting

in a continuous increase in the base diameter of the droplet. To measure

the receding contact angle, liquid is instead withdrawn from the droplet

and the contact angle found when the droplet base diameter continuously

decreases, is the receding contact angle. Surprisingly large droplet

volumes (i.e. >10 µL) can be needed to successfully evaluate the receding

contact angle, and for both advancing and receding contact angle it is

important to use a low rate for the addition/withdrawal of liquid to allow

the droplet to continuously find its equilibrium [77]. A volume

expansion/contraction rate of 0.1 µL/s was used throughout this work.

Even though the method is simple and robust, there are pitfalls,

especially when evaluating contact angles above 150°. The results

obtained can vary as much as 15° depending on the magnification used,

the applied contrast and the fitting model used, as well as, on if automatic

or manual fit of the drop contour was employed [32, 76, 77]. In Figure 12

the large variation of the contact angle depending on the automatic fitting

method chosen is shown. Clearly, both the Ellipse and Circle fitting

models result in poor fits when droplets exhibiting high contact angles

are evaluated, and should therefore not be used in these cases. Due to this

variation all contact angles presented in this thesis were evaluated by

hand using a protractor and the tangent method. When evaluated by

hand using a protractor, the droplet shown in Figure 12 exhibits a contact

22 | EXPERIMENTAL

angle of 163°. Thus it is of importance to report the method and

parameters used for contact angle evaluation, together with the reported

contact angles, to ensure legibility. Also, the static contact angle alone

does not provide enough information to draw unambiguous conclusions

regarding wetting, it is also important to report advancing and receding

contact angles to further describe the surface wetting characteristics [77].

Figure 12. Example of how the magnitude of the contact angle on a superhydrophobic

surface greatly depends on the choice of fitting model.

3.2.5. Electrochemical measurements

Corrosion starts and proceeds at the interface between the metal and

the coating and electrochemical measurements present a way of

investigating this process during exposure to corrosive electrolyte. The

measured results can then be fitted to models, including electric circuit,

allowing useful characteristics of the corrosion process to be extracted,

making this a suitable method for the study of corrosion protective

coatings [5, 19].

EXPERIMENTAL | 23

In this work, open circuit potential (OCP) and electrochemical

impedance spectroscopy (EIS) were employed to follow the corrosion

process of coated steel samples in real time during exposure to corrosive

electrolyte. The OCP is the potential measured if no external current is

applied to the circuit, and the EIS can be said to measure the resistance to

aqueous and ionic transport of an organic coating [5, 9, 78]. OCP was

measured versus a reference electrode of the Ag/AgCl type, and in

general, a higher measured OCP value corresponds to a more noble

material. EIS works by measuring the current response when applying a

small (10 – 20 mV) sinusoidal voltage to the system over a range of

frequencies. The applied voltage does not affect the corrosion since it is

small enough to keep the system near the steady state, which makes EIS a

reliable test method when evaluating corrosion in coated metal systems

[3, 9, 19].

Figure 13. In (a) a schematic view of the electrochemical cell and in (b) visualization of the

simplest equivalent circuit used to fit and evaluate EIS measurements from metals coated

by a one-layer coating.

To measure OCP and EIS, a coated metal sample was mounted in an

electrochemical cell, as shown in Figure 13a, where a circular area

(diameter 1 cm) of the sample constitutes the working electrode (WE),

and a platinum mesh was used as counter electrode (CE). A glass tube

was used to position the reference electrode (RE) close to the sample

surface. To evaluate the collected EIS measurements fitting to an

equivalent circuit can be done, and the simplest equivalent circuit

representing a one-layer coating on a metal is shown in Figure 13b. When

fitting the data to such a model circuit, the coating capacitance and

24 | EXPERIMENTAL

resistance can be evaluated. This does, however, require a good fit of the

data to the model, which can be hard to achieve due to the complexity of

polymer coatings. One, often used, way of addressing this problem is to

exchange the coating capacitance, Cp, in the equivalent circuit with a

constant phase element (CPE), which in a simplified manner takes into

account effects of surface heterogeneities [19]. The use of CPE, and the

coating parameters that can be extracted from such fits, is further

discussed in Paper IV. The coating capacitance can be used to calculate

water uptake in the coating, and also to predict apparent diffusion

coefficients through coatings [3, 9, 79, 80]. This is further discussed in

Paper III.

The most straightforward way to monitor the overall protective

properties of a coating is through the measured impedance at the lower

frequency limit, where the coating resistance is the dominating

contribution [9, 81]. In Figure 14, typical data achieved when measuring

OCP and EIS on a coated metal are shown. The OCP decreases when

corrosion starts and simultaneously a drop in the impedance, at the lower

frequency limit, is seen in the EIS spectrum. Commonly, the protective

properties of an organic coating are considered to be lost when the

impedance at the lower frequency limit falls below 106 Ωcm2 [6, 80].

Figure 14. Typical data collected from OCP and EIS measurements, the drop in OCP seen

at 40 days show the onset of corrosion, and simultaneously the EIS drops below 106 Ωcm

2

indicating that the barrier properties of the coating are lost.

RESULTS AND DISCUSSION | 25

4. Results and discussion

In this chapter the results obtained, and conclusions drawn, from each

coating system are first discussed. Then some interesting results from

Papers I - IV are summarized and discussed in relation to two unifying

concepts: superhydrophobicity and corrosion protection. In addition,

there are two further sections at the end of this chapter, one considering

the coatings made from a commercial PDMS (which was not included in

any of the papers) and one concerning the mechanical evaluation of the

PDMS coating, including the nanomechanical properties discussed in

Paper V.

4.1. The AKD system

In Paper I, AKD wax was used to fabricate model surface coatings

exhibiting different wetting states to allow evaluation of how said wetting

states affect the possible corrosion protection properties of the coating.

Through spin coating and spray coating, at varied speeds and number of

layers applied, samples exhibiting four different wetting states were made

and the measured water contact angles on these surfaces were used to

distinguish between their wetting states. Raman microspectroscopy was

used to ensure that the surface chemistry of the four samples was the

same, i.e. that the surface energy of the coatings was the same, and the

different wetting states achieved can hence be exclusively attributed to

differences in surface structure.

To relate the different wetting states to possible corrosion protection

of the underlying substrate, electrochemical measurements were

performed on samples immersed in 3 wt% NaCl water solution. The data

collected for the sample exhibiting the Lotus-like superhydrophobic

wetting state indicated a break in the electrical circuit, and after thorough

control of the equipment, it was concluded that the break had to be

caused within the evaluated system. The air layer present at the

superhydrophobic Lotus-state surface functions as a barrier and

26 | RESULTS AND DISCUSSION

decreases the transport of corrosive ions from solution to the metal

surface to the point where no electrical current can flow, which causes the

break in circuit. When no electrical current flows, corrosion cannot occur,

and this effect is highly desired when designing corrosion protective

coatings. The AKD wax coating showed this behavior for a period of 8

days, before the underlying substrate started corroding; indicating that

the long term stability of the air film may pose an obstacle if considering

engaging this effect in commercial coating systems.

The other wetting states evaluated, a rose-like superhydrophobic, a

mixed and a Wenzel state system, showed reduced corrosion protective

properties when compared to the Lotus-state one. The wetting Wenzel

state surface performed the worst, which was expected due to the

inherent wetting of the surface structures making it easier for the

corrosive electrolyte to reach pin holes and defects in the coating. In

Figure 15, the electrochemical data for the AKD coatings are shown, in (a)

the uncharacteristically low OCP value of circa -3 V can be seen for the

Lotus-like coating during the first 8 days. The sharp peak at 2 days was

explained by temporary contact through the air layer induced by for

example vibration. The dotted line represents the OCP for uncoated steel,

and when the samples reached this value, corrosion was occurring. In

Figure 15b the EIS for the Lotus-like coating is shown, and the break in

circuit can be seen in the very high barrier properties exhibited by this

coating, also lasting for approximately 8 days.

Figure 15. Electrochemical results achieved when investigating the different wetting states

of the AKD wax system. In (a) OCP for all four coatings, where a break in the circuit can be

clearly seen in the extremely low values measured for the Lotus-like sample. In (b) the EIS

for the Lotus-like sample is shown. Redrawn with permission. Copyright © Elsevier 2013.


4.2. The PDMS-hydrophobic silica nanoparticle system

In Paper II, the development of a one-pot superhydrophobic surface

coating formulation was addressed through the addition of hydrophobic

silica nanoparticles to a hydrophobic PDMS matrix. Particles of varied

sizes were used (16, 60-70, 250 and 500 nm), one at the time and in

combination two by two. Superhydrophobicity in the Lotus state was only

reached when high amounts of particles were added due to the formation

of cracks in the coating, which in turn added the extra roughness needed.

Superhydrophobicity in the rose state was reached for coatings with

slightly lower particle content, before, and at modest, crack formation.

The results correlate well with the ongoing discussion regarding how the

shape of the asperities on the surface promotes different wetting states,

sharp shapes such as the wedges of the cracks are known to promote

wetting in the superhydrophobic Lotus state, while rounded shapes, such

as those provided by the spherical particles, will promote wetting in the

superhydrophobic rose state.

Figure 16. ESEM images of PDMS coatings containing 40 wt% hydrophobic silica particles.

In (a) coating prepared using more solvent exhibiting wetting in superhydrophobic Lotus

state, and in (b) coating prepared using less solvent exhibiting wetting in the rose state. (b)

is reprinted with permission. Copyright © Elsevier 2015.

In Paper III four different PDMS coatings, with the addition of 0, 10,

20 or 40 wt% hydrophobic 16 nm silica particles, were chosen for

corrosion protection evaluation. In an attempt to minimize crack

formation, less solvent was used during fabrication of these batches of

PDMS coatings. This, however, also affected the contact angle and

superhydrophobicity in the rose state was instead achieved for the 40


wt% sample. In Figure 16, ESEM images of different surface structures

are shown, both containing cracks but their difference in morphology

results in different superhydrophobic wetting states for the two batches.

The 40 wt% coating (rose state) failed within two days of exposure to 3

wt% NaCl water solution, due to the formation of cracks, which allowed

the electrolyte to easily penetrate the coating and reach the underlying

metal. None of the other three coatings were subjected to particle

addition of high enough amount to change the contact angles but they all

still performed well during electrochemical corrosion protection

evaluation. The 20 wt% coating showed a long term stability of almost 80

days, which is not commonly seen for organic coatings this thin (10 – 20

µm). The synergistic effect of the hydrophobic matrix, and the enhanced

barrier properties, stemming from the addition of hydrophobic particle,

was concluded to be the reason for the promising results. The wetting

data for this system are shown in Table 2, together with the time to

failure, defined as the time until the electrochemical impedance

decreased below 106 Ωcm2 during exposure to electrolyte. Furthermore,

the coating without particles, and the one with 10 wt%, showed a large

increase in coating impedance during the first 24 hours. The increase was

so large, that passivation of the underlying metal was found to be the

most likely explanation. This effect is further discussed in section 4.6.3.

In Paper V, the nanomechanical properties of the PDMS without and

with addition of 20 wt% hydrophobic silica nanoparticles were evaluated,

the results obtained in that study are further discussed in section 4.8.1.

Table 2. Wetting data, and time to failure when immersed in electrolyte, are presented for

the four PDMS surfaces containing varied amounts of hydrophobic silica particles.

Sample SCA (°) ACA (°) RCA (°) Time to failure (days)

No added silica 121 ± 1 120 ± 1 69 ± 1 14 ± 7

10 wt% silica 120 ± 1 120 ± 1 70 ± 3 35 ± 12

20 wt% silica 122 ± 1 122 ± 1 65 ± 4 48 ± 33

40 wt% silica* 152 ± 3 161 ± 4 16 ± 3 1 ± 0.5

*The RCA depends largely on exact preparation conditions which affect the surface

structure, here data from Paper III, where less solvent was used during preparation, are

shown.


4.3. The three layered composite system

To better understand the results obtained in Paper III the coating

system presented in Paper IV was developed. This consisted of a base coat

of PEA, a layer of TiO2 particles (< 100 nm) and a hydrophobic HMDSO

top coat. Coating systems were fabricated exhibiting varied wetting

properties ranging from hydrophilic to supehydrophobic. The complete

three layer system showed excellent corrosion protective properties and

was stable in 3 wt% NaCl water solution for more than 100 days.

Furthermore, the three layered coating managed to quell the corrosion

that did start at some pin hole in the coating. The results obtained for the

layers combined two by two did however not give a straightforward

answer to which factor was the most important to achieve this effect. The

PEA coating, although being weakly hydrophilic with a SCA of 61 ± 1°,

showed fair corrosion protection on its own; the impedance was not as

high as for the three layer system, but the long term stability was good.

This was explained by good adhesion to the substrate. Adding particles on

top of the coating posed no significant change, which could be a result

from two opposing effects which were expected to occur, plugging of pin

holes in the PEA coating (improvement) and enhancement of coating

wetting due to the hydrophilic nature of the TiO2 particles (deterioration).

Finally, results achieved when combining the PEA with only the HMDSO

top coating showed a small improvement, but suggested that the thin

plasma coating may not reach full coverage. The conclusion is that the

excellent corrosion protective properties seen in the three layer coating

system, once more, is a combination of several factors. These factors are:

the good adhesion of the base coat, the elongated diffusion path caused by

particle addition and the lowered surface energy acquired from the

HMDSO top coat.


4.5. Unifying concept 1: Superhydrophobicity

4.5.1. Shape, size and distribution of protrusions

As was touched upon in the introduction, wetting of hydrophobic

structured surfaces is by no means a simple matter, but depends on the

size, shape and geometrical distribution of the protrusions and

depressions creating the surface roughness. In the research presented

here, superhydrophobicity was reached for at least one sample of each

coating system. Four of these surfaces are presented here to provide a

platform for a discussion on the effect of shape, size and distribution of

protrusions on the superhydrophobic state. From Paper I, the AKD-wax

coated through a RESS procedure that formed a flake like structure

which, when added to the inherent hydrophobicity of the wax, rendered

the surface superhydrophobic in the Lotus state was chosen. In Papers II

and III the hydrophobicity of a PDMS coating was tuned by the addition

of silica nanoparticles of different sizes and in different ratios. For

example, the addition of 40 wt% 16 nm silica particles and the addition of

40 wt% 16/500 nm silica particles with a ratio of 10:1 (both from Paper

II) resulted in superhydrophobic surfaces, and data from these two

surfaces will be discussed in this section. Lastly, the three-layered

composite coating system, described in Paper IV, consisting of PEA-TiO2-

HMDSO also exhibited superhydrophobicity, and will also be included

here.

The water contact angles measured for these surfaces are summarized

in Table 3. Looking at the receding contact angle, it is seen that

superhydrophobicity of two different states was achieved, the AKD wax

and the PDMS 40 wt% 16 nm silica samples (from Paper II) exhibit

superhydrophobicity in the Lotus state, while the PDMS 40 wt% 15/500

nm (ratio 10:1) and the PEA-TiO2-HMDSO exhibit superhydrophobicity

in the adhesive rose state.

This behavior can be explained by the shape and size of the features

making up the roughness of the surface. In Figure 17, ESEM images of the

four surfaces are shown, and parallels can be drawn between the sharper,

more flake-like, features seen in the AKD-wax, and in the 40 wt% 16 nm

silica in the PDMS coating where sharp edges are created through crack

formation. It has been proposed that widely spaced, narrow, features

promote a high receding contact angle [32]. This is consistent with the

first two coatings showing high receding contact angles while the latter


two results in pinned droplets on the surface. When looking at the smaller

scale structures, all surfaces except the AKD-wax exhibit a structure of the

same magnitude (≤ 1 µm) and with the same rounded shape of the

protrusions. As discussed in Paper II, and earlier by Teisala et al. [43], the

rounded shape promotes wetting in the adhesive, superhydrophobic,

rose state, which is concluded also here. To reach superhydrophobicity, in

any state, it has been stated that the smaller scale structures need to be

below 100 nm [52, 82-85]. Considering the width of the wax flakes, and

the size of nanoparticles (16 nm silica particles and < 100 nm TiO2

particles, respectively) added to the other coatings, this requirement is

met for all four surfaces exhibiting superhydrophobicity discussed in this

section.

The surface energy of these surfaces can be assumed to be comparable,

since the low surface energy is induced mainly by CH2 – groups in all

coatings, and as was stated earlier, as long as the surface energy is low

enough, the structure determines if superhydrophobicity is reached [29].

Therefore only surface roughness is discussed when comparing these

coatings.

Table 3. Water contact angles for four different surfaces, the first two exhibiting the

superhydrophobic Lotus state, and the last two exhibiting the adhesive superhydrophobic

rose state.

* Wetting data for the sample that showed Lotus-like behavior

** Sample from Paper II where more solvent was used during preparation

Sample SCA (°) ACA (°) RCA (°)

AKD wax* 152 ± 2 152 ± 2 134 ± 9

PDMS 40 wt% silica** 159 ± 3 169 ± 1 154 ± 1

PDMS 40 wt% 16/500 nm – 10:1 153 ± 3 158 ± 9 Pinned

PEA-TiO2-HMDSO 151 ± 11 162 ± 1 Pinned


Figure 17. ESEM images of the four different surfaces exhibiting superhydrophobicity, left

column showing the larger surface structure and the right column showing the smaller

surface structure. (c and d are images from the 40 wt% 16 nm silica system prepared using

more solvent, presented in Paper II)


Relating wetting to structure has been found difficult since the shape,

height and pitch of the structures all contribute to the final wetting state.

Several studies have focused on investigating this relation, and some

conclusions have been drawn. Athauda et al. [86] showed how the water

contact angle on top of two differently sized nano particles, layered on top

of each other, greatly increased when the larger particles were coated on

top of the smaller instead of the opposite. Tuteja et al. [45, 46] showed

how high re-entrant or overhang structures increase the water contact

angle of a surface significantly. However, neither of the surfaces

developed in this work can be expected to exhibit such overhang surface

structures.

Regarding width, height and spacing of the protrusions no generally

applicable relation that predicts the contact angle from such features has

yet been found, but some interesting studies have been made.

Nosonovsky and Bhushan investigated how the pitch of 30 µm high

cylindrical pillars, covered with a nanostructure similar to that seen in

Figure 17b, affected the wetting state of the sample with spacing ranging

from 23 – 210 µm. They found that a smaller spacing promotes

superhydrophobicity in the Lotus state while a larger spacing promotes

the rose state, but the boundary at which the transition occurs also

depends on the density of the nanostructure, and was in this case

approximately 100 – 110 µm [35]. Although wetting is highly dependent

on several factors, and the exact distance between features, stated above,

is valid only for cylindrical pillars of this type, the length scales discussed

agree well with the results presented in Figure 17, where the spacing

between the larger structures (left column) is below 100 µm for the two

samples displaying wetting in the superhydrophobic Lotus state and

above 100 µm for wetting in the rose state.


4.5.2. Air film stability under water

One important factor when considering superhydrophobic surfaces for

the application of corrosion protection, which has not been discussed in

the included papers, is the stability of the superhydrophobic effect when

the surfaces are immersed in water. By time there is an inevitable

transition from the partly wetted Cassie-Baxer state to the fully wetted

Wenzel state, and if no external factors affect the air layer the dynamic

transition will primarily be mediated by the hydrostatic pressure and gas

diffusion [87, 88].

At increasing pressure the water meniscus will advance into the valley

between protrusions, resulting in an increase of the advancing contact

angle along the side of the protrusion, and when the contact angle is large

enough the wetting front will depin and water will advance down the side

of the protrusion [32, 89]. The structure may, however, not be fully

wetted immediately since several metastable states can exist, in which the

wetting front pins at a new site of the protrusion. The metastable state

can exist since the air pressure in the void is increased when the

advancing liquid compresses it [88]. However, the air will start to diffuse

into the water bulk and the meniscus will once more advance until

depinning occurs and a new metastable state, or the bottom of the valley,

is reached. In Figure 18 (upper row), a schematic drawing of water

advancing down the protrusions of a surface is shown.

The presence of hierarchical structures can significantly prolong the

stability of submerged structures since the structured side walls enhance

the pinning of the contact line in the metastable states [88]. An example

of this is depicted in Figure 18 (lower row). Water will first wet the

microstructure and subsequently the nanostructure of a hierarchical

surface [28], and smaller features will maximize the impalement pressure

[32]. The air layer formed on a submerged surface exhibiting only one

level of structure may last for a few tens of minutes while the air layer

formed on a surface exhibiting an hierarchical structure can be stable for

several tens of hours [89]. That is, however, still a very short time

stability when commercial applications are considered.

Poetes et al. describe how the stability of the air film present at a

superhydrophobic surface depending on immersion depth varied between

0.3 and 1.4 m. They find a maximal stability of circa 16 days at the

immersion depth of 0.3 m, and an exponential decay of the stability time

with increasing immersion depth [90]. The AKD coating displaying


superhydrophobicity in the Lotus state, described in Paper I, was

immersed at a depth of 3 cm and the electrochemical results obtained

demonstrated the presence of an air layer for 8 days. According to the

results reported by Poetes et al. [90] much longer stability of the air layer

cannot be expected, which is a considerable limitation in any commercial

use utilizing the presence of an air layer at a submerged structure.

Figure 18. Top row shows how increased hydrostatic pressure, in combination with diffusion

of air from the pocket into the water bulk, cause the meniscus to first sag and then, when

the advancing angle is reached, to depin and either find a new metastable state or

completely wet the structure. Lower row show how hierarchical structures prolong air pocket

stability by adding more possible pinning sites.


4.6. Unifying concept 2: Corrosion protection

4.6.1. Effect of wetting state on corrosion protection

Paper I demonstrates how a superhydrophobic coating, in the Lotus

state, can serve as very efficient corrosion protection by breaking the

electrical circuit. This effect arises from the severely decreased transport

of ions from solution to the metallic substrate due to the insulating air

film formed on the superhydrophobic surface. Similar effects have also

been reported earlier [12, 91, 92]. Almost complete insulation of the

corrosion current by a layer of air is a very effective and also an

environmentally friendly way of achieving corrosion protection, which

should make superhydrophobic surfaces wetting in the Lotus state highly

coveted as corrosion protective coatings. However, as discussed earlier in

section 4.1 and 4.5.2, the air layer found on this type of surface is not

stable over time, and thereby not suitable for long term corrosion

protective applications of surfaces immersed in aqueous solutions [92].

In addition, the issue of wear of the surface coating has to be addressed.

Since superhydrophobicity in the Lotus state is easiest to achieve using

sharp, thin, protrusions it is difficult to make such surfaces mechanically

robust. The contact angle data and results from the electrochemical

evaluation of the AKD wax coating wetting in the Lotus state are shown in

Table 4, and Figure 19a and b, respectively.

Two other coatings that performed well during the electrochemical

evaluation of corrosion protective properties was the PEA-TiO2-HMDSO

coating presented in Paper IV, and the PDMS coating containing 20 wt%

hydrophobic silica nanoparticles presented in Paper III. Contact angles

and electrochemical results obtained from these coatings are also

included in Table 4, and in Figure 19c-f. The PEA-TiO2-HMDSO coating

exhibit wetting in the superhydrophobic rose state and the characteristic

data suggesting a break in the circuit is not seen (Figure 19c and d).

Hence, in this case there is not enough air to insulate the substrate from

the electrolyte. However, instead, this coating shows a considerably long

stability over time compared to other thin coatings. This is due to the

combined effect of a particle layer, a hydrophobic top coat and good

adhesion to the metal.

In comparison, the PDMS coating containing 20 wt% hydrophobic

silica nanoparticles did not exhibit superhydrophobicity but showed a

hydrophobic behavior (Table 4). However, this coating still performed


well during electrochemical testing. In Figure 19e and f, it can be seen

how the impedance reaches values at the lower frequency limit (0.01 Hz)

even higher than those seen for the PEA-TiO2-HMDSO coating, although

it also shows a slightly shorter long-term stability. From these results, the

conclusion can be drawn that the exact contact angle value is not the most

important characteristic affecting the corrosion protection. Instead it

seems that, excluding superhydrophobicity in the Lotus state that

provides an insulating air layer, superhydrophobicity is generally not

needed to receive the enhancements in coating performance which

instead could be associated with hydrophobicity combined with long

diffusion times caused by particle addition.

Table 4. Contact angles for three different coatings performing well during the immersed

corrosion protection evaluation.

* Data for the sample wetting in the Lotus state

Sample SCA (°) ACA (°) RCA (°)

AKD wax* 152 ± 2 152 ± 2 134 ± 9

PEA-TiO2-HMDSO 151 ± 11 162 ± 1 Pinned

PDMS 20 wt% hydrophobic

silica nanoparticles

122 ± 1 122 ± 1 65 ± 4


Figure 19. Electrochemical results for three coatings representing different wetting states, in

a and b) OCP and EIS (respectively) for the superhydrophobic Lotus-like AKD wax, in c and

d) OCP and EIS (respectively) for the PEA-TiO2-HMDSO layered coating showing

superhydrophibic rose-state, and in e and f) OCP and EIS (respectively) for the hydrophobic

PDMS coating containing 20 wt% hydrophobic silica particles. Redrawn with permission.

Copyright © Elsevier 2013 and 2015.


4.6.2. Barrier effect from particles

The barrier properties of a coating can be improved by the addition of

particles to a polymeric matrix [48, 93-96]. For corrosion protective

applications an important reason is the elongation of the diffusion path,

which delays the onset of corrosion by slowing down the diffusion of

corrosive species through the coating to the substrate. Another reason is

that the particle can fill cavities or act as a bridge to connect molecules in

the coating, resulting in a denser coating [97]. It has also been reported

that the addition of nanoparticles improves the barrier properties much

more than addition of micrometer sized particles due to their higher

surface to volume ratio [98, 99]. In Paper III, addition of hydrophobic

silica nanoparticles to the PDMS matrix was found to enhance and

prolong the corrosion protective properties of the coating. However, also

the PDMS coating without any particles performed well and the coating

system used in Paper IV was designed to better understand the

contributions arising from particle addition and from hydrophobicity to

the combined coating.

The three layered PEA-TiO2-HMDSO coating evaluated in Paper IV

show how the addition of particles on top of a base coat may block pores

in the coating by making it denser, but in the case of TiO2 addition, this

effect is counteracted by the hydrophilicity of the particles which instead

promote wetting of the coating. The addition of a very thin hydrophobic

coating, HMDSO, to the base coat enhanced the corrosion protective

properties, but not for all three replicates. The main conclusion in Paper

IV was that both particles and hydrophobicity are needed, and that their

properties can act synergistically, to greatly enhance the corrosion

protective properties of the coating. Also the positive effects can be

expected to be greater when the particles are distributed through the

coating as in Paper III, instead of being present in a confined layer as in

Paper IV, both due to a further elongated diffusion path and due to less

porosity when the particles are surrounded by the matrix polymer.

Another positive effect from particle addition is that not only the

diffusion paths for corrosive species are prolonged, but also the diffusion

path for corrosion products from the surface out into the electrolyte. It

has been seen in several reports how dense corrosion products can get

stuck in a coating, blocking holes and defects, and thereby enhance the

protective properties of the coating [3, 19]. In Figure 20, an example of

this effect, seen for the full PEA-TiO2-HMDSO coating system in Paper


IV, is shown. This coating showed an onset of corrosion already at day 20,

which proceeded for 9 days, but then stayed stable for 100 days.

Figure 20. One of the PEA-TiO2-HMDSO samples showing how corrosion is initiated after

20 days of exposure to 3 wt% NaCl water solution, proceeds for 9 days and then stops,

allowing the surface coating to stay stable for 100 days. Reprinted with permission.

Copyright © Elsevier 2015.


4.6.3. Possible passivation of underlying substrate

An interesting effect was noted in Paper III, where, in the beginning of

exposure to electrolyte a large increase in impedance was measured

during the first 24 hours. The effect is shown in Figure 21, and although

an improvement in the barrier properties can be seen due to corrosion

products getting stuck in the coating, and thereby blocking pores, the

increase of impedance associated with that effect is normally much

smaller than what is seen here, typically in the order of only one

magnitude [100].

Figure 21. Electrochemical data collected from the PDMS coating containing 10 wt% silica

particles showing a large increase in impedance during the first 24 hours of exposure.

Reprinted with permission. Copyright © Elsevier 2015.

The most likely explanation to this large increase in impedance is that

the coating can promote passivation of the underlying metal surface. Iron

can form conductive oxides that are stable at high pH, and the major

reaction product of the cathodic reaction is OH- through the reduction of

O2 [5]. It has also been suggested that accumulation of OH- under surface

coatings can take place since organic coatings often exhibit selective

permeability [78].


Iron oxides are not stable in an environment containing Cl- ions,

making the electrolyte containing 3 wt% NaCl used here, a very corrosive

environment. However, because of selective permeability the diffusion of

Cl- through a polymer coating is much slower than that of water [78], and

by combining the slow diffusion of Cl- and local increase in pH,

passivation of the substrate is possible. This may explain the large

increase in measured impedance. However, at some point, the Cl- ions

will penetrate through the coating and cause the passive oxide layer to

break.

Another factor indicating passivation is the color of the corrosion

products. For example, in Paper IV, for the PEA-TiO2-HMDSO coating

where the corrosion was seen to stop (Figure 20) the corrosion products

were black or dark grey, which is characteristic of the more stable iron

oxide Fe3O4, while corrosion causing failure of the coatings has been

noted to be red, characteristic of the porous Fe2O3 iron oxide.

Furthermore, there have been reports showing how stable Si-Fe oxides

can form and enhance the corrosion protection achieved when silica

nanoparticles are present in the coating under a mildly alkaline

environment. These Si containing oxides also resist Cl- ion penetration

allowing a Cl- free environment to be created near the metal/coating

interface which can stabilize other iron oxides [97, 99, 101, 102]. This

effect could help explain the surprisingly good corrosion protection seen

for the PDMS system with added hydrophobic silica particles, studied in

Paper III.


4.7. Commercial PDMS

PDMS coatings without added nano-particles were also prepared from

a commercially available PDMS formulation (results not included in

papers) for comparison. A condensation cure PDMS was chosen to match

the curing mechanism of the model PDMS described in Papers II and III.

The formulation was also coated, and subsequently evaluated, in the same

way as the model one to facilitate comparison. The water contact angles

and coating thicknesses for the two PDMS coatings are shown in Table 5.

Static and advancing contact angles match very well, while a discrepancy

can be seen for the receding contact angle. Since an increase in receding

contact angle was noted for the experimental PDMS coating in Paper II

during the first week of curing, the difference here indicates that the

coating used in Papers II and III presents a less homogenous surface

layer. When measuring coating thickness for the commercial PDMS a

difference was noted between different samples, and in Table 5, the

thicknesses for two samples are shown. This difference is most likely due

to the short tack-free time of the formulation. When preparing the last

coatings the formulation had already begun to cure and was therefore

more difficult to spin coat onto the samples.

Table 5. Water contact angles, and coating thicknesses, for the commercial and model PDMS

coatings.

* the two thicknesses for commercial PDMS are measured on two samples made from the

same batch, one made early during the coating process and one late, showing an increase

in thickness due to the short tack free time of the formulation causing the viscosity to

increase during the coating process.

The commercial PDMS coatings performed well during

electrochemical testing, showing results early after exposure that indicate

the presence of air at or in the surface and with a very good long term

stability of more than 100 days. The electrochemical data collected for the

commercial PDMS are shown in Figure 22. As for the model PDMS three

replicates were tested also for the commercial one and the best obtained

Sample SCA (°) ACA (°) RCA (°) Thickness (µm)

PDMS

commercial

120 ± 1 120 ± 0 103 ± 3 16 ± 4,

13 ± 2*

PDMS model 121 ± 1 120 ± 1 69 ± 1 13 ± 3


results are shown here. The OCP exhibit some turbulent behavior in the

beginning, and in correlation, the EIS at the lower frequency limit (0.01

Hz) is very high, on the border of what can be measured with the

instrument, and also somewhat scattered. Papers I and III explain this by

the presence of air at or in the coating obstructing the flow of the

electrical circuit. Both the indication of air present at the beginning of

exposure, as well as the high stable impedance modulus (at the lower

frequency limit), are different compared to the results obtained in Paper

III where the experimental PDMS coating exhibit a impedance of just

above 108 Ωcm2 at the beginning that then continuously deteriorate until

the coating fails after approximately 20 days. The mean time to failure for

the model coating was 14 ± 7 days. For the commercial PDMS one

replicate failed after 32 days, while the other two stayed stable for the

whole exposure time of 100 days.

Figure 22. Electrochemical data collected for the commercial PDMS coating during

exposure to 3 wt% NaCl water solution, in a) OCP vs. time and in b) impedance modulus

vs. frequency at different times.

The receding contact angle is the only parameter evaluated here which

is significantly different between the model and commercial PDMS

samples, but another important difference is that the commercial coating

contains adhesion promoters. Thus the model coating will have a greater

tendency to locally delaminate than the commercial PDMS samples,

which at least partly explains why the commercial coating performs better

in the corrosion protection test. However, since the content of the


commercial formulation is not fully known and may, most likely, contain

additives which enhance its performance, not too much weight should be

put on the differences seen here at this stage.

4.8. Mechanical properties of PDMS coating

Evaluation and improvement of the coatings regarding wear and

friction control clearly needs further development. Macroscopic wear test

of the model PDMS based coatings was performed using a minitraction

machine, and the results showed that the coating adhesion to the

underlying substrate is an issue that needs to be further addressed.

Furthermore, evaluation of the nanomechanical properties of the coating

is ongoing and some results are presented in this section.

4.8.1. Nanomechanical properties of PDMS coating

The model PDMS coating without, and with, the addition of 20 wt%

hydrophobic nanoparticles (16 nm) was evaluated using three different

AFM modes; tapping mode, PeakForce QNM and Intermodulation AFM

(ImAFM). The results show that both PeakForce QNM and ImAFM can

be used to investigate the nanomechanical surface properties of the

coatings.

Using the ImAFM mode, effective elastic modulus and adhesion were

extracted for the PDMS without added particles using the DMT model,

and the results are shown in Figure 23a. Red areas in the map showing

effective elastic modulus (left hand side) represent areas that are less

deformable. One possible explanation to why these patches occur is that

the PDMS coating has not cured in a homogenous way but instead

contains areas exhibiting higher crosslinking density, making those areas

less deformable.

The 16 nm hydrophobic silica particles were clearly visible in contrast

to the surrounding soft matrix using all three AFM modes when

evaluating the PDMS with 20 wt% particles. In topography

measurements of this sample it can be clearly seen that the particles

aggregate to form large secondary, and tertiary, structures as proposed

earlier by others [103]. The dimension of the particle aggregates observed

was estimated to measure between 20 and 110 nm.


Furthermore, a discussion has been raised regarding the rounded

shapes also seen in the lower right corner of Figure 23c. These rounded

shapes were seen also when using the PeakForce QNM mode and the

mechanical properties extracted for these rounded shapes are different

compared to those exhibited by particle aggregates with smaller

deformation depth, lower adhesion and lower energy dissipation. Due to

this, these rounded shapes were assigned to nanobubbles of air trapped in

the polymer matrix due to the increase in viscosity caused by the addition

of particles. This is an interesting, yet speculative, notion that could help

understand the excellent corrosion protective properties of these

coatings.

Figure 23. ImAFM mode scans of the PDMS without and with 20 wt% silica, the DMT model

was used for evaluation. In (a) effective elastic modulus and in (b) adhesion for the PDMS

coating without particles, in (c) effective elastic modulus and (d) adhesion for the PDMS

coating with addition of 20 wt% silica nanoparticles. All images are 1 x 1 µm.

CONCLUSION | 47

5. Conclusions

Three different coating systems, with focus on hydrophobicity and

superhydrophobicity, have been used to investigate which properties a

top coat layer in a corrosion protective coating system would benefit

from.

Through the use of AKD wax coatings, exhibiting the same surface

chemistry but different wetting states due to different surface structures,

it was possible to relate corrosion protective properties to the wetting

state. The superhydrophobic Lotus state can provide corrosion protection

to underlying substrates through the insulating air layer that forms on top

of such surface when it is immersed in the electrolyte, the air layer

obstructs the transport of corrosive ions to the surface to the point where

the electrical circuit is broken and corrosion cannot occur. This effect was

found to be efficient as corrosion protection during short exposure times,

in 3 wt% NaCl water solution, of less than ten days.

The aim of developing an easy-to-use, one-pot, superhydrophobic

coating was undertaken using nanoparticle containing PDMS coatings.

The conclusion is that high enough particle load to achieve

superhydrophobicity is hard to reach without causing the coating to

crack. It was also seen how the shape of the protrusions is an important

factor deciding which state of superhydrophobicity is formed; the Lotus

state is promoted by sharp, needle-like, structures, while rounded shapes

will promote the rose state.

Although not superhydrophobic, the hydrophobic PDMS coatings

containing silica nanoparticles were evaluated with respect to corrosion

protective properties and were found to perform better than expected for

coatings this thin (10 - 20 µm). The corrosion protective properties were

found to increase with increase in amount of added particles up to 20

wt%, where the coating provided corrosion protection to the underlying

substrate for almost 80 days when immersed in 3 wt% NaCl. The

properties of this coating system were concluded to be a synergistic effect

from the hydrophobic matrix and the addition of hydrophobic

nanoparticles.

48 | CONCLUSION

To further understand the corrosion protective mechanism shown by

the PDMS coating, a three layer composite coating system was developed.

Investigations of this coating system lead to the conclusion that several

factors are needed in combination to achieve the enhancement in

corrosion protection seen; good coating adhesion, blocking of pores and

an elongated diffusion path from addition of nanoparticles, and

hydrophobicity to further slow down the transport of water containing

corrosive ions. It was also noted that the hydrophobic top coat was

needed to exploit a positive effect from particle addition, if no

hydrophobic coating was applied; the hydrophilic particles instead

quicken the transport of water through the coating.

An additional conclusion, reached through comparison of the different

coating systems evaluated, is that if the insulating property of a

superhydrophobic surface in the Lotus state is not reached, the exact

wetting state is of less importance, as long as the surface is hydrophobic.

For example, the hydrophobic PDMS with 20 wt% hydrophobic

nanoparticles showed corrosion protective properties comparable to the

three layered composite coating wetting in the superhydrophobic rose

state. Therefore, when Lotus state superhydrophobicity is excluded, it was

concluded that other factors, such as elongation of diffusion path, and

blocking of pores by particles addition, were more important to the

resulting corrosion protection than the question of whether the coating

was hydrophobic or superhydrophobic.

FUTURE WORK | 49

6. Future work

There are several promising ways to continue research in this area.

One would be to start adapting either the PDMS or the PEA-TiO2-

HMDSO coating to commercial standards. To do this, coating adhesion to

the substrate, wear resistance and facilitation of the application process

have to be addressed. Another important factor of commercial corrosion

protection is the addition of active inhibitors; it would be interesting to

evaluate addition of such inhibitors to either of the coatings mentioned

above. However, care must be taken since most inhibitors are designed to

function in the presence of water, which makes them unsuitable to

combine with water repelling coatings.

When discussing commercial applications the concept of particle

volume concentration and also of critical particle volume concentration of

filler particles would be interesting to apply to facilitate understanding of

significant changes in properties in relation to filler concentration. This

calculation is, however, not entirely straight forward to perform for the

system investigated here since the fumed silica nanoparticles exhibit an

uneven surface, and it is not known whether the polymer can access all of

these structures, leading to uncertainty in the effective density of the

silica particles. One way of elucidating this is through oil absorption

measurements, and another one could be through further AFM studies of

the interphase between the polymer and the particle.

Another interesting path would be working on understanding the

corrosion protective mechanism of these coatings. One interesting

approach regarding the PDMS coating would be to look at enhancing the

wetting properties by patterning the surface, and then comparing the

corrosion protection with or without the addition of particles to evaluate

if wetting or elongated diffusion path is most important. It would also be

interesting to evaluate the diffusion of oxygen through the coatings. This

could be done using free films of the coatings, or by coating them onto

glass substrates of known porosity.

Furthermore, the addition of particles to the commercially available

PDMS formulation would be of great interest since the initial testing of

50 | FUTURE WORK

this PDMS gave promising results. This is however, not without

challenges since the formulation cures by humidity, making particle

addition in ambient conditions impossible. This could be overcome by

working in a controlled environment, such as N2 gas. Another alternative

would be to try yet another commercial PDMS formulation that does not

cure by humidity, but instead by, for example, heat.

ACKNOWLEDGEMENTS | 51

7. Acknowledgements

I would like to thank my supervisors, professor Per Claesson and

adjunct professor Agne Swerin, for giving me the opportunity to perform

this work, for countless interesting discussions, invaluable guidance and

keeping me on the right track (or at least one track). I would also like to

thank professor Jinshan Pan for sharing his expertise on electrochemistry

with us.

Thanks to all my co-authors, I enjoyed working with you and I am

proud of the publications we produced together. I am also grateful for the

funding provided by the SSF (stiftelsen för strategisk forskning) and RISE

Research Institutes of Sweden that made it possible for me to carry out

this work.

I have spent some great years at the division of Surface and Corrosion

Science, and at SP Chemistry, Materials and Surfaces, and a lot of the joy

during the work was due to truly wonderful colleges, both current and

past, thank you! Special thanks go to Petra, Josefina and Hanna who have

been there during both ups and downs to encourage me, both

educationally and privately.

Finally, I would like to thank my family. My parents who have always

believed in me and throughout the years always encouraged me to

“plugga så länge du bara orkar Lina!”, and my sister who has been there

to cheer me on whenever that has felt tough, and who always

congratulates me on every smallest progress.

Jesper, thank you for understanding how it feels to write a thesis, and

for taking care of our life while I did. Thank you for your endless support,

and for always being able to make me laugh, even at times when I do not

want to. We certainly are an effective team.

Henry, thank you for bringing perspective into all this, you will always

remain the most important thing in my life.

52 |

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Hydrophobic and superhydrophobic coatings for …860828/...Hydrophobic and superhydrophobic coatings for corrosion protection of steel LINA EJENSTAM Doctoral Thesis, 2015 KTH Royal