Sol–gel coatings on metals for corrosion protection

Abstract

Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates. Further, the sol–gel method is an environmentally friendly technique of surface protection and had showed the potential for the replacement of toxic pretreatments and coatings which have traditionally been used for increasing corrosion resistance of metals. This review covers the recent developments and applications of sol–gel protective coatings on different metal substrates, such as steel, aluminum, copper, magnesium and their alloys. The challenges for industrial productions and future research on sol–gel corrosion protective coatings are also briefly discussed.

Introduction

Metals, such as iron, aluminum, copper and magnesium and their alloys are used in a myriad of structural, marine, aircraft applications and cultural heritage, etc. While these metals are useful because of their physical characteristics, such as stiffness and high strength to weight ratios, they are highly susceptible to corrosion in aggressive environments. Corrosion is always the major reason of energy and material loss. It was reported that 1/5 of energy globally and average 4.2% of gross national product (GNP) is lost each year due to corrosion [1] and the economic impact of corrosion is estimated to be greater than $100,000,000,000 per year in the United States alone [2]. This cost includes the application of protective coatings (paint, surface treatment, etc.), inspection and repair of corroded surfaces and structures, and disposal of hazardous waste materials. A generic way to protect metals from corrosion is to apply protective films or coatings, which also permit the desired properties of the substrate to be coated through the chemical modification of the coatings [3], [4], such as mechanical strength, optical appearance, bioactivity, etc.

There are several techniques for the deposition of coatings on metals, including physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition, plasma spraying and sol–gel process. There are many advantages using sol–gel coatings, several most important features are listed as follows [5], [6]:

• Sol–gel processing temperature generally is low, frequently close to room temperature. Thus thermal volatilization and degradation of entrapped species, such as organic inhibitors, is minimized.

• Since liquid precursors are used it is possible to cast coatings in complex shapes and to produce thin films without the need for machining or melting.

• The sol–gel films are formed by “green” coating technologies: It uses compounds that do not introduce impurities into the end product as initial substances, this method is waste-free and excludes the stage of washing.

Ten year ago, Guglielmi [7] has already discussed the potential of sol–gel coatings as a corrosion inhibiting system for metal substrates. Since then, a great deal of work has been done to make various sol–gel based protective coatings. This review will introduce the basic chemistry involved in sol–gel processes, then the progress and development of sol–gel protective coatings on metal substrate, such as steel, aluminum, etc. Finally some problems and future work on sol–gel coatings will be summarized briefly.