Electrodeposition of sol–gel films on Al for corrosion protection

doi:10.1016/S0010-938X(03)00106-9

Corrosion Science

Volume 45, Issue 12, December 2003, Pages 2893-2904

https://doi.org/10.1016/S0010-938X(03)00106-9 Get rights and content

Abstract

Sol–gel films were electrodeposited on aluminum electrodes following the methodology we have developed and is based on applying negative potentials. This increases the pH at the surface, causing acceleration of the polymerization. Our process follows the “two step method”, in which the monomer is first hydrolyzed in acidic solution (pH ∼ 4) and only then the negative potential is applied, which consumes protons and releases hydroxyl ions, thus enhancing the condensation.

Films made of different monomers, i.e., tetraethoxysilane (TEOS), methyl trimethoxysilane and phenyl trimethoxysilane (PTMOS), were prepared, characterized and examined for their corrosion inhibition properties. Potentiodynamic polarization, electrochemical impedance spectroscopy, optical and scanning electron microscopy as well as atomic force microscopy have been used as a means of film characterization. Hydrophobic and steric silanes, such as PTMOS showed a considerable corrosion inhibition capacity as compared to the capacity exhibited by less hydrophobic and steric derivatives such as TEOS. The difference between the conventional dip-coating method and the electrodeposition approach for depositing sol–gel films was also examined, indicating a clear advantage of the latter.

Introduction

Aluminum, due to its widespread applications [1] is an obvious substrate for corrosion studies. Its low cost, lightweight, high thermal and electrical conductivity grant aluminum a remarkable industrial and economical importance. Many of its applications are practicable due to its natural tendency to form a passivating oxide layer, which can artificially be generated by anodizing the substrate. However, this passivating layer is deteriorated in aggressive media, such as chloride, which results in pitting corrosion [2], [3], [4], [5], [6], [7], [8], [9]. Several methods have been applied for protecting aluminum and aluminum alloys against corrosion. Electrochemical passivation, inhibitions by soluble or adsorbed species as well as formation of a protecting coating on the surface are among the numerous available methods. These have recently been reviewed and therefore are only briefly mentioned here [10], [11], [12], [13], [14].

One of the relatively new approaches, which has been reported, however, only scarcely applied for corrosion inhibition, is sol–gel coating [15], [16], [17], [18], [19], [20]. A sol–gel film is formed as a result of the hydrolysis and condensation of alkoxy-substituted transition metals, e.g., tetraalkoxysilanes [21] (Eqs. (1) and (2)). $Si (OCH_{3})_{4} → n H_{2} O Si (OCH_{3})_{4−n} (OH)_{n} +n CH_{3} OH$ $Si (OCH_{3})_{4−n} (OH)_{n} + Si (OCH_{3})_{4} → OH^{−} / H^{+} Si (OCH_{3})_{4−n} (OH)_{n−1} OSi (OCH_{3})_{3} + CH_{3} OH$

Such a film can provide protection against corrosion of metals by creating a physical and chemical barrier between the metal and its environment. Taking into account that a sol–gel film can bear different organic groups (using monomers with non-hydrolizable moieties) makes it a very appealing approach for designing films with superior protection capabilities. Moreover, procedures for incorporating inorganic, organic and biological substances in the course of the condensation process, have been developed and can be used for increasing corrosion inhibition [22], [23], [24], [25], [26], [27].

However, the conventional methods for depositing sol–gel films on metals, i.e., dip-coating, spin-coating and spraying, are suitable to flat surfaces only, produce relatively thick coating and do not distinguish between the Al and other non-metallic parts that need not be covered and protected. We have recently developed a novel approach for the electrochemical deposition of sol–gel films, based on electrochemically accelerating the condensation step by altering the pH on the surface [22]. It is well known that the hydrolysis of alkoxy metals is catalyzed by acid, while the condensation step is base-catalyzed. Our approach is derived from the “two-step” method, where the hydrolysis of the alkoxide metal monomer is carried out under acidic conditions followed by condensation at high pH levels. According to the methodology we have developed, application of negative potentials to the substrate to be coated consumes protons at the metal surface, causing a local increase of the pH. This allows the deposition of the sol–gel film on the conducting parts only and controlling the thickness of the deposited film by the potential of the substrate.

Here we report on the application of our approach for depositing well controlled thin silica based films [22]. We examine the corrosion inhibition of such films prepared from different alkoxy silane monomers, namely, tetraethoxysilane (TEOS), methyl trimethoxysilane (MTMOS) and phenyl trimethoxysilane (PTMOS). We find that the corrosion inhibition increases with the hydrophobicity of the film. The capability of the films to inhibit corrosion was studied by potentiodynamic polarization, scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS).

Section snippets

Experimental

Electrochemical experiments were performed using a VersaStat potentiostat (EG&G). The impedance measurements were carried out using an AutoLab system (PGSTAT 10, EcoChemie). Electron probe X-ray microanalysis (EPXMA) measurements were performed with a JEOL JSM-6400 SEM. AFM images were acquired with a NanoScope II (Digital Instruments, CA) operated in the contact mode using a 0.58 N m⁻¹ cantilever. The thickness of the deposited films was measured with a profilometer (Alpha Step 100, Tencor) and

Results and discussion

The electrochemical deposition of a sol–gel film [23] is driven by applying either negative or positive potentials that induce significant pH changes at the electrode surface due to the reduction or oxidation of water (Eqs. (3) and (4)). $2 H_{2} O +2 e^{−} → H_{2} +2 OH^{−}$ $2 H_{2} O → O_{2} +4 H^{+} +4 e^{−}$

The pH changes accelerate the condensation process, i.e., the polymerization, resulting in the deposition of an insoluble film on the electrode surface. We have been motivated by the large variety of available derivatives of

Conclusions

Sol–gel films have been electrochemically deposited on aluminum and tested as corrosion inhibitors. The deposition is based on altering the pH on the Al surface, which is driven by a cathodic reaction of water reduction. Three different monomers that differ primarily by their hydrophobicity have been used and showed remarkable differences in their protection. The more hydrophobic film, which was based on phenyl trimethoxysilane showed the best corrosion protection, while that made of

Acknowledgements

This work was supported by the Ministry of Science, Culture and Sports under the strategic research (contract 1095).

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Electrodeposition of sol–gel films on Al for corrosion protection

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Electrochim. Acta

Corros. Sci.

Prog. Org. Coat.

Surf. Coat. Technol.

Prog. Org. Coat.

Ceram. Int.

Non-Cryst. Sol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Int. J. Heat Mass Transfer

J. Electroanal. Chem.

Electrochim. Acta

Electrochim. Acta

Corros. Sci.

Langmuir