Anti-Corrosion

6.2 ANTI-CORROSION

Graphite, graphene, hybrid filler containing carbon nanotubes were used to improve the electrical conductivity and anti-corrosion properties of polyurethane coatings.²² At the same filler loading, the electrical conductivity of hybrid filler system was significantly higher than that of the single filler system (0.77 S/m at 5 wt% while single filler system was not conductive).²² Hybrid filler system had the best electrical conductivity and acceptable anti-corrosion capacity.²²

Multiwalled carbon nanotubes were decorated with TiO₂ nanoparticles to form a new hybrid structure of filler which was then used in the epoxy composite.²³ The blend of both fillers was sonicated in acetone followed by magnetic stirring and drying in vacuum oven.²³ The hybrid filler/epoxy nanocomposite exhibited superior anti-corrosion and mechanical performance as compared with the nanocomposite produced by loading of only MWCNTs, TiO₂ nanoparticles, or neat epoxy.²³ The composite coating reduced corrosion rate on mild steel to 0.87×10⁻³ from 16.81 mili-inches per year.²³

Titanium and its alloys are wildly and successfully used in producing implants for their good mechanical properties, bioactivity, and corrosion resistance.²⁴ To achieve good bioactivity and anti-corrosion properties, the surface of titanium often needs modifications, such as an alkali treatment, anodic oxidation of TiO₂ and coatings.²⁴ Graphene oxide and cross-linked gelatin were used in hydroxyapatite coatings preventing corrosion of titanium.²⁴ The coating acted as a barrier that prevented the electrolyte from reaching the metal surface.²⁴ These coatings had better bond strength and corrosion resistance than hydroxyapatite coatings.²⁴

Graphene can accelerate metal corrosion because of its thermodynamic stability and high conductivity.²⁵ A few-layer fluorographene was prepared by a liquid-phase exfoliation method.²⁵ Fluorographene was incorporated into poly(vinyl butyral) coatings to enhance its corrosion protection performances.²⁵ The coating had enhanced barrier property preventing the penetration of aggressive species.²⁵ Unlike graphene, fluorographene cannot promote metal corrosion. Because of its insulating nature, it impedes the formation of metal-filler galvanic corrosion cells.²⁵

The effects of carbon nanofillers morphology (namely carbon black, multiwall carbon nanotubes, and graphene) on the anticorrosive and physicomechanical properties of hyperbranched alkyd resin-based coatings were studied.²⁶ Graphene filler gave the best corrosion resistance.²⁶

An anticorrosive pigment is incorporated in the topcoat of an anticorrosion coating system which greatly reduces the corrosion rate of the substrate metal in the environments of aggressive ions.²⁹ The inorganic cation exchange pigment is selected from the group consisting of a metal ion-exchanged silica, metal ion exchanged alumina, synthesized zeolites, natural zeolites, and natural cation exchangers.²⁹

The coating composition for protecting iron and steel structures contains particulate zinc, conductive pigments, and hollow glass microspheres.³⁰ A conductive pigment is selected from the group consisting of graphite, carbon black, aluminium pigments, black iron oxide, antimony-doped tin oxide, mica coated with antimony-doped tin oxide, carbon nanotubes, and carbon fibers.³⁰ Zinc acts as a sacrificial anodic material and protects the steel substrate, which becomes the cathode.³⁰ Addition of microspheres and conductive pigments reduces microcracking.³⁰

A coating comprising functionalized graphene and polymer protects roll steel, galvanized roll steel, equipment, automobiles, ships, construction and marine structures from corrosion, fouling and UV deterioration.³¹ The functionalized graphene has 1-10 sheets.³¹ The functionalized graphene contains a chemical group selected from amino, cyano, carboxylic acid, hydroxyl, isocyanate, aldehyde, epoxide, urea, or anhydride.³¹ The suitable resin is a phenolic resin, a polyester resin, a polyurethane, or an epoxy resin.³¹

Concluding Remarks

Publisher Summary

This chapter presents that, because of the excellent biocompatibility, matching shade, good anti-corrosion ability, bioceramics are ever more widely being used in the biomedical field. Most of these bioceramic appliances need to bear forces during their lives. Therefore, the mechanical properties became the most important factors to rank and select proper material. With the rapid development of modern material science and technology, more and more high-strength, high-toughness bioceramics have been developed and introduced into biomedical usage. The ever-more critical evaluation of existing and new bioceramic appliances will greatly increase the security and confidence of the medical usage of these materials. From a mechanical point of view, a bioceramic and its appliances need to meet these standards: similar elastic modulus, hardness, and stress–strain relationship as the natural tissue; high fracture toughness and subcritical crack growth index; good wear resistance and minimal fatigue crack growth susceptibility; and all bioceramic appliances should undergo proper designed proof test to guarantee their quality.

18.1 Introduction

The environment is the main cause of the decay of metallic elements from the cultural heritage. Art curators with the support of analytical science attempt to intervene to stop the deterioration. In many cases, the conservation efforts cannot stop the deterioration process or reverse the ensuing damage, but they can help slow the rate of degradation to ensure a longer existence for artefacts.

The interactions between the metal and its environment are responsible for the electrochemical reactions leading to corrosion. To avoid further degradation and in general the appearance of corrosion products, a more or less insulating barrier must be created on the metallic surface between the object and the atmosphere. Different barrier coatings have been developed, such as anodic oxides, ceramic and inorganic coatings as corrosion inhibitors, organic coatings and conversion coatings. Most of them were first developed for industrial applications. They generally require the removal of the corrosion products, and their application leads to important changes in the visual appearance of the metallic surface. These treatments have also been applied for the outdoor protection of large structures that are part of the cultural heritage, like bridges or buildings protected by French legislation. But for a large proportion of works belonging to the cultural heritage, such as patinated artefacts or sculptures and archaeological artefacts, these protection systems cannot be applied.

7.5 Coating adhesion tests

Adhesion of galvanized coating to the substrate does not usually need to be tested. Unlike organic coating materials, where the performance and durability of anti-corrosion protection is directly proportional to the adhesion of the protective layer to the substrate and its consistency, the anti-corrosion effect of a zinc coating is mainly related to cathodic protection. It only becomes efficient when both the electrically conductive metals are in contact, regardless of possible cracks and pores in the coating (see Section 8.1). A properly applied galvanized coating on steel that is suitable for hot-dip galvanizing usually exhibits sufficient adhesion to the substrate. However, what should be kept in mind is that parts with a thick coating require especially careful handling. Mechanical processing of hot-dip galvanized products is considered as non-standard handling of the product from the point of view of coating adhesion.

6.9 Anti-corrosion coating

Graphene has been used to make the thinnest coating known the world over and can be used for protecting metals against corrosion (Sharma, 2014). The potential use of graphene as anti-corrosion coating was discussed in the journal ACS Nano by D. Prasai and his colleagues. Rusting and other corrosion of metals is an important global problem. Contact of metal with air, water, or other substances can cause corrosion. Graphene is a single layer of carbon atoms. It is evaluated for use as anti-corrosion coating. An ounce of graphene arranged in a single layer and comprised of rows of benzene rings can fill the size of 28 football fields. Graphene, whether made directly on copper or nickel or transferred onto another metal, can be used to prevent corrosion. Copper coated with graphene by chemical vapor deposition (CVD) was found to corrode seven times faster than uncoated copper. Nickel coated with multiple layers of graphene was found to corrode at a rate of 20 times slower than uncoated Nickel. The same amount of corrosion protection can be obtained using a single layer of graphene that was obtained using five layers of organic coatings. Graphene coatings may be ideal for applications in industrial microelectronics. They can be used in aircraft, implants and as interconnects.

4.4 Smart anti-corrosion coatings

The most effective corrosion protection strategy requires various functionalities that among others may include corrosion identification with signaling effect, corrosion retardation with water/ion traps, and corrosion inhibition. Zhang and Frankel (1999) added the pH indicator phenolphthalein to an acrylic-based coating and could sense the corrosion onset detecting color change associated with the pH increase caused by the cathodic reaction in the corrosion process. Maia et al. (2013) encapsulated phenolphthalein in silica capsules and added it to water-based epoxy coating applied on AA2024 and magnesium alloy AZ31. In both cases, corrosion-sensing functionalities were developed and the local corrosion onset could be detected by eye due to the color change of phenolphthalein under alkaline pH. Water (Zheludkevich et al., 2005b) and chloride traps (Dias et al., 2013; Tedim et al., 2012) postpone corrosion onset because they delay the ingress of corrosive species to the metallic substrate.

Since corrosion is inevitable, the addition of anti-corrosion pigments, mainly Cr(VI)-containing compounds, has been one of the most effective corrosion-protection strategies in coated Al alloys (Bastos et al., 2005; Seth et al., 2007; Williams et al., 2012; Twite and Bierwagen, 1998; Bohm et al., 2001; Deyá et al., 2007). In spite of progressive restrictions on the use of corrosion inhibitors based on chromates, there are still several Cr(VI)-based compounds/processes used as pigments in anti-corrosion protective systems for aerospace applications. For example, chromic acid is used for anodizing, dichromium tris(chromate) is used as anti-corrosion in surface treatments, and strontium chromate, potassium hydroxyoctaoxodi-zincatedichromate, and pentazinc chromate octahydroxide are used as pigments in anti-corrosion coatings. All of these are carcinogenic and are being researched by REACH regulations. Therefore, the latest developments have been focused on the design of more advanced coatings for corrosion protection, free of Cr(VI)-based pigments. To succeed with this strategy, corrosion-protective coatings combining good barrier properties with functionalities providing effective corrosion protection have been one of the priorities in the aerospace sector. Among these, smart anti-corrosion coatings have been considered to have high potential.

The smart functionality of an anti-corrosion coating for AA2024 can be based on the addition of extra functional additives (corrosion inhibitors) (Feng et al., 2007). Various smart solutions including the ability of self-healing have been reviewed recently (Fedrizzi et al., 2011; Ferreira et al., 2011; Montemor, 2014; Yasakau et al., 2014). The proposed solutions cover various bare materials (Hager et al., 2010; Fischer, 2010), different functionalities for self-repair of polymeric matrices (Feng et al., 2007; Kumar et al., 2006; Murphy and Wudl, 2010), additives for corrosion protection, and coatings with enhanced barrier properties (Hughes et al., 2010; Montemor, 2014), encapsulation of liquid healing agents (Samadzadeh et al., 2010; Murphy and Wudl, 2010), and advanced polymers and composites (Williams et al., 2008; Wu et al., 2008; Bergman and Wudl, 2008; Murphy and Wudl, 2010; Syrett et al., 2010; Burattini et al., 2010; Mauldin and Kessler, 2010; Blaiszik et al., 2010; Shchukin and Möhwald, 2007a).

The strategies that show potential for self-healing corrosion protection, and that have been studied in detail, can be classified according to the respective mechanisms of action. A strategy that has attracted interest among scientists is the encapsulation of functional agents, with healing ability, in micro or nanostructured hosting systems that are then dispersed in the protective-coating matrices. Attempts have been made at the development of systems with multiple protection mechanisms, in which different functionalities for repair of the polymer, water trap, and corrosion inhibition are combined in the same coating.

1.2 Thermal spraying of zinc (metallizing)

For reliable adhesion of a zinc coating applied by metallizing the surface requires pre-treatment by sand-blasting with the use of sharp-edged particles.

After the application of sprayed zinc it is necessary to apply an organic painting material on the deposited coating that will adhere very well in its profile. Combination of zinc coating with organic paint (referred to as a duplex system, see Section 8.3) offers a very good corrosion resistance with regard to efficient synergy of both the materials.

8.2 Atmospheric corrosion

In the course of time, the coating thickness is reduced at a variable rate depending on the corrosivity of the environment. Its useful life is indirectly proportional to the corrosion rate, expressed usually as the amount of corrosion loss in micrometers or grams per unit area. The anti-corrosion protection system of a hot-dip galvanized coating is very efficient if the galvanized part is exposed in a clean atmospheric environment. This means an environment where free access of air to the part’s surface is ensured and its possible wetting is temporary and short. The concentration of acidic pollutants in the atmosphere must also be low.

A sufficiently thick patina layer is created on a zinc coating in the course of a few weeks after its application. Until then the coating has reduced ability to resist corrosive influences of the environment and being moist. During this time it can be covered by corrosion products – white rust. In normal climatic conditions, effects on the zinc coating by white rust show a temporary character. Climatic influences gradually cause removal of corrosion products from the surface and their gradual replacement by a layer of patina (zinc carbonate). After several months’ exposure, the surface that has been affected by white rust acquires the same patina appearance as the remaining coating parts that have not been attacked by corrosion. As with any other product, galvanized coatings are subject to certain restrictive conditions limiting the suitability of their application and way of use. On condition the user respects the guarantee conditions and eliminates possible harmful local influences; galvanized coatings are able to provide steel products with reliable anti-corrosion protection for a very long time. In most applications they represent the most suitable, efficient, and reliable anti-corrosion system.

With knowledge of the corrosion rate of zinc in a particular environment, it is relatively easy to estimate the service life of a galvanized coating. With knowledge of the minimum coating thickness (Tables 7.2 or 7.3Table 7.2Table 7.3) and the corrosion rate of zinc in the particular environment, the expected service life is easy to determine. EN ISO 1461 sets minimum coating thickness values; however, it does not prevent the purchaser agreeing different values with the galvanizing plant. The application of a coating with a required thickness can be influenced by selecting the thickness of the semi-product or by using silicon-killed steel. A thicker coating may also be achieved by blasting the part’s surface with a fine sharp-edged abrasive material (see Section 5.4.3).