1. Technical Field
The embodiments herein generally relate to a surface pre-treatment process and particularly to metallic substrates. The embodiments herein more particularly relates to a surface pre-treatment film for anti corrosion of metallic substrates and a method of manufacturing the same.
2. Description of the Related Art
Metallic engineering materials are usually alloys that are designed to meet high demands with respect to mechanical strength and physical properties such as high strength to weight ratios and stiffness but they are highly susceptible to corrosion in corrosive environments.
Steels and stainless steels are widely used in different industrial fields because of their mechanical properties. However, they tend to corrode in different media especially in the presence of halide ions. In order to provide additional long term corrosion protection and improve adhesion of the polymer to the metal, the metallic substrates are pre-treated before applying of an organic paint. The metallic coatings are one of the most widely used methods not only to change the surface properties of the construction elements such as hardness, wearability, solderability, brightness, etc., but also to provide improved protection against corrosion.
A complete coating system consists of three individual layers. The first layer is the conversion coating which is a surface pre-treatment step. These layers protect the underlying metal from corrosion and also give improved adhesion to the substrate. The second layer is the primer and the final layer is the top coat. The primary focus of the embodiments herein is the first layer of the coating system, i.e. the surface pre-treatment step.
One of the traditional methods of protecting the metal substrates from corrosion is a surface passivation treatment, wherein a hard non-reactive surface film is formed spontaneously to inhibit further corrosion. This layer is usually an oxide or nitride that is a few molecules thick. The method is effective for steel and aluminum metals, but has the disadvantages of poor reproducibility and susceptibility to chemical contamination.
Another method of protecting the metal substrates from corrosion is the use of chromate-based conversion coatings, which have been successfully used as a surface pre-treatment process for different alloys for many decades. But, these hexavalent chromium containing compounds are known to be carcinogenic and generally regarded as very hazardous soil and ground water pollutant.
Moreover the chromium compounds have been used as effective pretreatments for 100 years but strong toxic and carcinogenic properties of Cr (VI) lead to consideration of these pretreatments as a potential lung carcinogen responsible for the DNA damage and make them environmentally hostile so that their application is limited.
The use of inorganic oxide coatings provide a good protection to metallic substrates by changing the immediate surface layer of metal into a film of metallic oxide or compound, having better corrosion resistance. But they have some drawbacks which are as follows. They are brittle. The thicker coatings (□ 1 μm) are difficult to achieve without cracking and the relatively high temperatures (400-800° C.) are often required to achieve good properties.
To overcome this limitation, much work has been done to introduce organic component into the inorganic sol-gel to form the inorganic-organic hybrid sol-gel coatings.
The hybrid materials are formed through the hydrolysis and condensation of organically modified silicates with traditional alkoxide precursors. Introduction of covalently bonded Si—R groups allows chemical modification of the resulting material properties. But, the pure inorganic coatings from TEOS (tetra-ethyl-orthosilane) had apparent cracks on the surface. The hybrid sol-gel films perfectly fit with the requirements of a pre-treatment. However, they cannot provide any active corrosion protection and cannot stop the development of corrosion processes when the defect appear, as they contain micro-pores, cracks and areas of low cross-link density that provides pathways for diffusion of corrosive species to the coating/metal interface.
Hence there is a need for an anti-corrosion pre-treatment process that is more workable in overcoming the drawbacks.
The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.