Anti-Corrosion

Increasing the nickel content

A synergistic anti-corrosion effect of the two alloying elements, Cu (0.3 wt%) and Ni, was especially observed when the nickel content was raised from 1 to 4 wt%, providing better corrosion resistance than in the case of mild steel (Chen et al., 2009) Electrochemical Impedance Spectroscopy (EIS) measurements showed that Ni alloying facilitated steel passivation at the initial stage and its compactness and blocking ability in the later stage. Cu alloying promoted the formation of lepidocrocite while Ni alloying delayed the crystallisation of some rust phases and facilitated the formation of amorphous phases.

2.1 Anti-Corrosion Coatings: Mechanisms

There are many mechanisms by which anti-corrosion coatings operate, but generally these can be divided into three barriers: barrier creation between substrate metallic materials and the surrounding environments, the inhibition of the corrosion process, and the coating acting as a sacrificial material. However, one of the newest approaches to have emerged is called active-passive. In this approach, the protection coating acts as a barrier layer which prohibits the permeation of corrosive agents to the underlying metal surface (passive). In comparison, the active approach allows the formation of an effective passive layer which impedes corrosion half-reactions, thus leading to a Schottky barrier at the interface, and therefore, resulting in the depletion of electrons [1].

Corrosion science involves the study of electrochemical processes taking place at electrodes. An electrode is essentially the boundary between a solid phase (i.e., metal) and a liquid phase (i.e., aqueous environment), and the corrosion processes take place across the phase boundary. The basic wet corrosion cell consists of four essential components: an anode, a cathode, an electrolyte, and connections. The first concept of corrosion control is that removal of any of these four components of the simple wet corrosion cell will naturally stop the corrosion reaction [2].

From an engineering point of view, the major point of interest in corrosion science and engineering is the kinetics (or the rate) of corrosion reactions. The principal goal for studying corrosion reaction kinetics is to develop an empirical relationship that permits the prediction of corrosion rate under conditions that are different from those employed in the laboratory and to determine the mechanism of the overall corrosion process.

It has long been observed that corrosion is thermodynamically possible for most ambient conditions. Thus, it is of primary interest to know how fast corrosion occurs in a particular environment. Fortunately, most metal alloys only corrode slowly in many ambient environments. Moreover, corrosion in aqueous systems is governed primarily by electrochemical reactions, and the understanding of the fundamental laws of electrochemical reaction kinetics is, thus, essential to develop more corrosion-resistant metal alloys and to improve the methods of protection against metal substrate corrosion [3].

Electrochemical reactions either produce or consume electrons. The rate of an electrochemical reaction is limited by various physical and chemical factors, and the reaction is polarized or retarded by these factors. Polarization is defined as the displacement of electrode potential resulting from a net current. Its magnitude is frequently measured as an overvoltage and is usually reported in volts or millivolts.

Chromium-based compounds and zinc have historically been the most common coating materials, but due to stringent health, safety, and environmental rules and regulations by many governmental agencies around the world, the former usage had declined progressively in the last two to three decades, while the application of zinc as a coating material is significantly discouraged due to large price fluctuations [1]. Hence, alternative materials are being sought actively, and one such candidate is the hybrid material comprised of unfunctionalized graphene (UFG) and polyetherimides (i.e., PEI).

8.6 Self-healing anti-corrosion coatings using the layer-by-layer approach

9.2 Smart anti-corrosion nanocoatings

The application of nanoparticles in the synthesis of anti-corrosion coatings is one of the most important accomplishments of nanotechnology. It improves barrier properties, anodic and cathodic protection and enhances adhesive characteristics. Application as a corrosion inhibitor is also used in the synthesis of smart coatings. Because of their unique properties such as their lateral surfaces and high chemical reactivity, nanoparticles are able to carry a significant amount of corrosive inhibitor particles on their surfaces.

The main principle in using nanoparticles for smart coatings is the selection of a type which will create temporary bonds with the inhibitors, so that the by-products of corrosion are released when these bonds are broken, so delivering inhibitor into the homogeneous medium. In a group of smart anti-corrosion nanocoatings, the bonds developed between nanoparticles and inhibitors are sensitive to the hydroxide ions which are among the main by-products of the metallic corrosion process. As soon as hydroxide ions are released, the bonds are broken and the inhibitor moves toward the damaged area, which is reduced by reacting with the corrosive agents, generating insoluble oxides which are deposited on the metal surface, so preventing electrolytes from diffusing to the metal surface and deactivating it.

Among the most significant advantages of these coatings is their lack of chemical inhibitors such as chromates, which are strongly carcinogenic. The use of chromates in non-smart coatings may lead to excessive consumption and various environmental hazards. When selecting a nanoparticle type, consideration of high side area, low cost and the creation of an accessible surface for its appropriate distribution is very important. Smart nanocoatings produced by using small amounts of inhibitor (< 5%) compare well with coatings which use a large amount of inhibitor (20–30%) and exhibit better corrosive resistance. Anti-corrosion nanocoatings are particularly practical for the inner and outer surfaces of oil and gas transport lines, tanks and reservoirs, and for the inner surfaces of aircraft fuel tanks which are difficult to access (Agarwal et al. 2006; Morin et al. 2007; Qu et al. 2009; Simchi et al. 2011; Vasile et al. 2011).

German and Portuguese researchers have invented a coating for aluminium alloys which possesses self-healing properties. If a part of the coating is damaged, a nanometre layer of gel fills the microscopic cracks and pores and prevents further invasion into the metal. This is a zirconia silica–gel in which benzotriazole-contained nano-tanks are dispersed. These nano-tanks contain silica particles coated with a thin layer of charged polymers containing polyethylenimine and polystyrene sulphonate. Benzotriazole acts as an inhibitor agent. The zirconia-bearing silica–gel matrix creates adhesion between the coating and aluminium. Investigations have shown that this alloy protects aluminium coatings in salty water and that defects with dimensions of less than 10 μm are repaired in less than 24 hours. The coating is also able to repair cracks with dimensions up to 100 μm in water and saltwater solutions (Nicholls et al. 2002; Andreeva et al. 2008; Lvov et al. 2008; He and Shi 2009; Boissiere et al. 2011).

1) The automobile industry

Composite materials can significantly lower fuel consumption, and have anti-corrosion and anti-vibration performances, so a large number of composite materials will be used in the automobile industry. At present, the application ratio of automotive steel has dropped to 14% ~ 15%, and it has been reported that in 2000, there were one in five cars which was manufactured using composite materials in the United States. Moreover, in order to reduce environmental pollution, many countries are vigorously developing the usage of natural gas to replace gasoline as motor fuel, and the cylinders of natural gas use FRP structure. The working pressure can go 20 ~ 100MPa, life reaches 10 ~ 20 years, and this cylinder has been in series and into practical application.

Composite materials used in automotive components are mainly glass fiber un-saturated polyester. In the past they were made by hand lay-up and spray-up molding, but in recent years, sheet molding compound (SMC), glass fiber mat reinforced thermoplastics (GMT) compression molding and resin transfer molding (RTM) have been used. The method of making sheet molding compound (SMC) is that chopped glass fiber roving or glass fiber mat is impregnated using the mixture (paste resin) of unsaturated resin system, fillers and other additives, and then wrapped with polyethylene or polypropylene film on both sides of itself to form the sheet type compound. SMC is widely used as a result of continuous production, easy usage and lower price.

In addition to glass fiber composite materials, the application of advanced composite materials in the automotive industry can also be found. The factor of high cost restricts the usage of advanced composite materials, but the good news is that the current price of carbon fiber has dropped to somewhere close to the price that automobile industry can accept.

3 Alloying

The addition of alloy elements into carbon steel to produce various alloy steel will improve its anti-corrosion, such as nickel, chrome, titanium, and copper.

The above method can be adopted to prevent the corrosion of the steel bars in concrete, but the most economic and effective way is to improve the density and the alkalinity of concrete and make sure that the steel bars are thick enough.

In the hydration products of the cement, there is about 1/5 Ca(OH)₂, and when the pH value of the media reaches to about 13, there is passive film on the surface of steel bars, so the bars in concrete are difficult to generate rust. But when CO₂ in the air diffuses into the concrete and reacts with Ca(OH)₂ to neutralize the concrete. When pH value falls to 11.5 or below, the passive film will be destroyed and the steel surface reveals active state; and if there is humid and oxygen condition, the electrochemical corrosion will start on the surface of steel bars; because the volume of rust is 2 ~ 4 times than steel, it will lead to the cracking of concrete along bars. CO₂ diffuses into the concrete and carries the carbonization, so the improvement of the density of concrete will effectively delay the carbonization process.

Because CL⁻ will destroy the passive film, the consumption of chloride should be controlled in the preparation of reinforced concrete.

Abstract

The main purpose of the zinc coating is to protect iron and steel components against corrosion; their decorative effect is secondary. In terms of anti-corrosion efficiency is essential the substrate coverage and the thickness of the coating. Higher roughness of the coating must not be detrimental to the function parts; sharp protrusions may not cause injury. For quality assessment, it is important to know which events are considered defects and what procedure is performed to acceptance testing. Some coatings defects are repairable, techniques of repair are governed by binding regulation.

4.5 Material testing and function screening

The application of filled capsules and particles in coatings aims to modify the coating properties in order to achieve additional functions (e.g. self-healing, anti-corrosion, anti-icing and anti-fouling properties) while retaining the original and fundamental characteristics and performance of the coatings such as a good adhesion, adequate drying behaviour and sufficient corrosion protection. Besides the necessary testing in accordance with well-known standard procedures such as the cross-cut or salt-spray test, several additional screenings are essential for the development of multifunctional and self-healing coatings in order to successfully develop novel formulations. The testing of host/guest systems and coatings filled with host/guest systems ought to be systematically planned in order to provide feedback for the material developer and to give technologically relevant hints for appliers of coatings. Examples of such procedures will be detailed in Section 4.7.

It was mentioned before that some corrosion inhibitors and self-healing agents can only be incorporated into selected coatings because interactions with ingredients of the coatings and therefore a weakening of the matrix can occur. This can become apparent in the form of undesirable effects such as pinholes, craters and loss of adhesion. Introducing such weak points into a coating can reduce its stability to mechanical stress and degradation processes. The encapsulation of active agents may help to overcome these limitations. Although the amount of inhibitor released from capsules accidentally ruptured during the dispersion and application processes is usually very small, it is advisable to ensure good compatibility of the guest with the coating matrix. The compatibility can be investigated by mixing a comparatively low concentration of pure guest into the coating formulation and evaluating the dry film for any defects.

A crucial point when developing multi-functional coatings with filled particles is the effective amount of functional agent required to accomplish the desired functionality. This amount varies depending on the specific effectiveness of the agent which is related to its chemical nature. In general, the maximum amount is determined by the occurrence of inadmissible changes to the characteristics of the coating caused by interactions between the surfaces of filler particles and the components of the formulation. A phenomenon that is usually observed is the increase in viscosity caused by rheological effects following the adsorption of fluid components such as film binders or solvents which influence the packing density of the dry coating. Finally, a ‘critical’ packing density is observed which is defined as the state in which the particles in the dry coating are in actual contact with each other and are barely covered by a thin film of the binder. Coatings with packing densities above the ‘critical’ one can show dramatic variations in their characteristics and properties (Brock et al., 2000). However, in most cases the concentration of filled capsules and loaded particles required to achieve the desired coating functionality is within the range below the ‘critical’ packing density.

18.4 Conclusions

For more than 40 years, many scientists involved in the conservation of the cultural heritage have carried out research programmes based on the evaluation of the performance of anti-corrosion systems that are commonly used for the protection of metallic artefacts. Through these different studies, it appears that because of the variety of conservation conditions, most of the systems tested (waxes, varnish, inhibitors) cannot satisfy the various requirements encountered in the field of cultural heritage conservation. For that reason, the development of new protective systems for preserving metallic artefacts is of great interest.

In this context, for 10 years we have been studying a new corrosion inhibitor family for conservation purposes. This new family, based on sodium carboxylates with different carbon chain lengths extracted from vegetable oil, fulfils the main conditions of application laid down by the rules of conservation ethic. In fact, carboxylates are easily removable with solvents such as ethanol, allowing complete reversibility of the treatment. No visual aspect modification is observed on the samples and the artefacts treated in this work. Moreover, the non-toxic character of these corrosion inhibitors makes their handling easier in comparison with benzotriazol. Through our work, it appears that these inhibitors exhibit good anti-corrosion performance on a wide range of metallic surfaces.

Phenolic Resins in Coating Applications ^[2][4][25]

The very good properties and characteristics that make phenolic resins good adhesives and molding compound as also make them a very good protective, environmental, high temperature, and anti-corrosion coating for a variety of materials, such as aluminum, bronze, iron, and magnesium.

Phenolic coating resins have good wetting and adhesive properties, and very good chemical and abrasion resistance. The baking step in coating production involves a crosslinking process. Crosslinking makes the coating insoluble, strong, and resistant to exposure to chemicals, solvents (except alkalies), and hot water. It also makes phenolic coating resins tasteless and odorless.

Phenolic coating resins are good electrical insulators. Dielectric strength for phenolic coating resins is about 500 V/mm; dissipation factor and water absorption are very low.

Phenolic coating resins have good thermal resistance with a continuous-use temperature of 145°C and can withstand high temperatures up to 350 °C for short periods.

Phenolic coating resins exhibit flexibility and compatibility with other resins, such as polyurethanes, epoxies, alkyds, and polyvinyl butyryl, and can be easily modified to suit various applications. Also, phenolic resins are sterilizable and can be used for food applications where sterilization is a Food and Drug Administration requirement.

Major coaling applications are as protective coatings, undercoats, and primers for automotives; metal containers and pipes; and industrial equipment. Examples of specific applications of phenolic resins, such as coatings, are in heat exchangers, pipelines, boiler pipes, reaction vessels, storage tanks, brine tanks, solvent containers, food containers, railroad cars, beer and wine tanks, beer cans, pail and drum linings, water cans, rotors, blower fans and ducts in heating and air conditioning systems, boats, ship, wood finishes, and paper.

Because of their versatility, phenolic coating resins, can be applied by most available coating technologies, such as dip and spray (pneumatic and electrostatic) coating in solutions, high solids, and powder forms. Georgia Pacific Resins, Inc. and other plastics companies offer a variety of grades of coating resins. A particular coaling application can have more than one resin type, for example, a rail car could have an epoxy primer, a modified phenolic undercoat, and a polyurethane finish.