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Transcript of Anticorrosive Coatings on Metal Substrate by Sol ... Chapter 9 Anticorrosive Coatings on Metal...

  • CHAPTER 9

    Anticorrosive Coatings on Metal

    Substrate by Sol-Gel Dip Coating Method

  • Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

    Dip Coating Method

    201

    Chapter 9

    Anticorrosive Coatings on Metal Substrate by Sol-Gel Dip Coating

    Method

    9.1 Introduction

    There is a current need for alternative coatings that can provide

    corrosion resistance to metals or alloy surfaces due to the environmental

    hazards posed by conventional coatings. The basic concept of chemical

    conversion of metal surfaces is based on deposition of a hydrophobic sol–gel

    barrier layer for surface protection and corrosion prevention. The properties of

    these organosilica coatings can be tuned by varying the composition of

    precursors. The evaluation of hydrophobicity, adhesive strength, and

    anticorrosion properties of organically-modified sol–gel derived coatings

    suggests their potential utility as technologically-compatible alternatives to

    conventional coatings. The deposition of sol-gel coatings on metals is relatively

    recent and has been not sufficiently investigated, in spite of its potential

    technological interest. Sol–gel-derived coatings have been found to be useful

    for several applications mainly due to the ease of solution based processing and

    the synthesis flexibility which can be used for forming a wide range of thin

    films and coatings [1, 2]. Using the sol–gel process, it is possible to deposit

    films with variable thickness from 100 A° to several µm. In addition, the use of

    organically-modified precursors provides unique opportunities to tailor the

    physical and chemical properties of the final materials. Due to the presence of

    an organic component, the organosilica coatings dry evenly and are more

    uniform and crack-free as compared to pure silica coatings. While there has

    been significant research activity in the use of sol–gel coatings for corrosion

    protection [3–5], efficient coating formulations that provide significant

    protection as a viable alternative to conventional coatings. One of the critical

    issues with sol–gel-derived coatings has been their poor adhesion to the metal

    surface due to weak non-covalent binding to the substrate. An additional

    concern is their porous nature, which makes them permeable to ions, moisture,

  • Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

    Dip Coating Method

    202

    and other corrosive species. In this context, organosilica sol–gel materials

    furnish unique advantages [6]. The properties of sol–gel-derived coatings can

    be engineered at the molecular level [7] for optimum physical and chemical

    properties such as better adhesion, improved hydrophobicity, low permeability,

    as well as texture, morphology, optical properties, and other characteristics.

    These materials can also be easily processed in the form of a coating using

    inexpensive, environmentally-friendly, and technologically-compatible

    methods.

    Herein, we used the organosilica sol–gel materials for coating metallic

    substrate. Methyltriethoxysilane (MTES) precursor is used to prepare

    hydrophobic coatings on copper substrate which not only provide improved

    adhesion but also act as a barrier protection layer for minimizing the

    permeability of corrosive species. It is found that the coatings are effective at

    preventing corrosion of metal substrate. These films are more elastic as

    compared to TMOS-derived silica coatings and therefore do not undergo

    cracking. These coatings act as barrier layers for metal surfaces for preventing

    corrosion. The presence of organic groups also renders these materials

    hydrophobic [8] making them impermeable to ions, moisture, and other

    hydrophilic species as compared to pristine sol–gel-derived silica coatings.

    Thus, by a judicious choice of the precursor, it is possible to impart desired

    properties to the final material such as adhesion, water-repellency, and

    hydrophobicity. Overall, the strategy presented herein may provide a generic

    approach for fabrication of protective coatings on different metallic surfaces.

    9.2 Experimental

    9.2.1 Preparation of silica films

    The hydrophobic silica coating on copper substrates have been prepared

    by sol-gel process using dip coating method from an alcoholic solution

    containing silica precursor Methyltriethoxysilane (MTES), methanol (MeOH),

    and ammonium hydroxide (NH4OH). The chemicals used were

    methyltriethoxysilane, (Sigma-Aldrich Chemie, Germany), methanol (s.d.fine-

  • Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

    Dip Coating Method

    203

    chem limited, Mumbai), and ammonia (NH3, sp.gr.0.91, Qualigens fine

    chemicals, Mumbai). Double distilled water was used for all the experiments.

    All the reagents were used as received.

    Prior to the deposition of the hydrophobic films on copper substrates,

    the substrates were cleaned in order to get uniform deposition. Pieces of 1 cm ×

    5 cm were cut from copper sheet and used as substrates. These substrates were

    mechanically polished using zero grade polish paper as an abrasive. This

    practice removed the grease and the native oxide layer from the surface of the

    copper plate. The coating solution was prepared under basic condition from the

    MTES, CH3OH, and H2O in molar ratio of 1:19.1:4.31 respectively with 7M

    NH4OH. The MeOH/MTES (M) molar ratio was varied from 9.5 to 19.1. The

    coating solution was stirred for approximately 15 minutes.

    After substrate preparation and sol preparation, film deposition on the

    copper substrates utilized a simple dip-coating process. The substrates were

    dipped in the sol at a constant rate of 6 mm/min, immersed in the sol for

    approximately 40 minutes, withdrawn at the same constant rate, and then air-

    dried for approximately 30 minutes. Following deposition, the substrates were

    sintered at 250°C for 3 hours at a heating rate of 2°C/min to ensure

    densification of the gel network.

    9.3 Results and discussion

    9.3.1 Reaction Mechanism

    The MTES silicon alkoxide contains one non-hydrolysable methyl

    group and three hydrolysable ethoxy groups. Therefore three hydrolysable

    ethoxy groups undergo hydrolysis and lead to the formation of monomeric

    units of the - Si(OH)3 which are responsible for the formation of silica network.

    The hydrolysis and condensation reactions of the MTES are as

    per the following chemical reactions:

  • Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

    Dip Coating Method

    204

    Hydrolysis:

    (9.1)

    Condensation:

    (9.2)

    9.3.2 Surface Morphological Studies

    The two-dimensional morphological study of the water repellent silica

    films have been carried out using the SEM micrographs. Figure 9.1 (a) and (b)

    shows the surface morphology of the silica films prepared with M = 12.7 and

    M = 19.1, respectively. Figure 9.1 (a) shows the irregular shaped silica particles

    which are non-homogeneously spread on the copper substrate. In the case of

    silica film prepared with M = 19.1 (figure 9.1 (b)), the SEM micrograph shows

    spherical silica particles distributed on the copper substrate. The high

    magnified SEM micrograph of this film (figure 9.1 (c)) shows very well

    spherical shaped silica particles with each having diameter typically ranges

    from 11 to 15 µm, distributed on the copper substrate.

    + 3H2O

    OC2H5

    OC2H5

    H3C OC2H5

    Si + 3C2H5OH

    OH

    OH

    H3C OH

    Si

    + 4H2O

    OH

    CH3 Si

    O

    OH

    H3C O

    Si

    O

    H3C O

    Si

    OH

    CH3 Si

    OH

    2

    OH

    H3C OH

    Si

    OH

    OH

    CH3 HO

    Si

    OH

    +

  • Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

    Dip Coating Method

    205

    Figure 9.1(a): SEM image of the silica film prepared with M = 12.7.

    Figure 9.1(b): SEM image of the silica film prepared with M = 19.1.

  • Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

    Dip Coating Method

    206

    This is due to the fact that the lower dilution of MTES (less M value)

    has high catalyst concentration in the sol during the hydrolysis and

    condensation reactions. Therefore, there is a rapid clusterification of siloxanes

    which give rise to dense and irregular network structure. However, an increase

    in the dilution of MTES in me