In structural engineering, metals are the most demandable material due to their superior mechanical properties and ease of availability. However, metals constructed structures sooner or later get damaged by environmental attacks such as moisture, temperature, rain and wind, causing corrosion and erosion. This results in structural losses that lead to direct or indirect losses as much as 5-10% of the Gross National Production system. Indeed, countries like the United States, Great Britain, Germany, China, European States have dedicated budgets to control the corrosion-related problems.
Corrosion is material deterioration resulting from an electrochemical or chemical reaction with an environment. It causes significant material loss, which leads to productivity losses, maintenance, repair, replacement expenses, and restoration costs. Corrosion impacts every aspect of modern life; nevertheless, not everyone is aware of the impacts that are costing around them. It is a never-ending and all-encompassing activity and has several economic, security, and environmental effects. There is a possibility of direct or indirect economic damage. Examples of direct losses are repairing the damaged structures, applying protective coatings, maintenance and monitoring and labour costs. Indirect cost includes plant shutdown for repairing and maintenance. This shutting down causes a decrease in production and income and the daily wages required for the repairing work. Repairing and maintaining the corrosion effected structures is not only costly but, the restored structures are at high safety risk in a long-term scenario. Their deterioration can lead to structural collapses, some of which can be tragic, resulting in severe damage that ultimately leads to the company’s economic losses.
Surface modification to get the desired results have been known for many years. Since ancient times, aiming to improve particular responses of materials, especially when dealing with an open environment. The surface modification of materials can be done by applying coatings, and the heritage of mankind drives the origin. The familiar examples are the protection of metallic parts like iron, brass, silver etc., with clay minerals, vegetable oils, gelatins, beeswax, and animal fats. Examples of such coating applications are corrosion protection, gloss, brightness, wear protection, water repellence and lubrication.
Protection of materials from environmental attacks can be done by surface modification, and it has become the most attractive and demanding research area in recent times. Demand for modified and well-defined surface properties using advanced and conventional technological applications is desirable to fulfil the particular requirements of valorizing the material and advancing the related technology. Several routes define surface modification, but it can be simplified as a route to introduce a new behaviour of material’s surface, aiming to fulfil desirable results. These modified surfaces comprise enhanced morphological features that define the interaction with their surrounding environment.
Many researchers are working on the concept of protective coatings. The advancement in protective coatings promoted a new era in coatings technology, and the trend of surface modification by applying protective coatings will continue rising in the coming years. Coating, either polymeric, inorganic or hybrid, is a class of tailored materials that perform well-defined functions for many applications such as electronic appliances, home furniture, cars, solar cell panels, and advanced applications like orthopaedic implants, medical devices, radars, pipelines, tanks, aircraft, satellites etc.
The standard ingredients of protective coatings included film-forming polymers, either organic or inorganic, pigments, extenders, fillers, additives and volatile components (solvents). To achieve the optimum properties of coatings, the proper selection of raw materials is necessary. The coatings’ performance profile depends on the required quality, surrounding conditions, application method, curing mechanism, feature designing, and coating substrates.
The film-forming polymers can be categorized into two main groups. First is physical drying polymers that evaporate solvents without any chemical change during the curing process, such as cellulose acetobutyrate, thermoplastic acrylics, and cellulose nitrate copolymers. The second category belongs to chemically reactive polymers. The curing process is completed by chemical crosslinking with the aid of some sources like catalyst or heat, for example, poly(urethane), alkyd-amino, acrylic-amino and phenolic polymers combinations. A volatile component of the coating system contains a solvent or a combination of solvents to achieve good flow and processing of liquid coating materials. The typical solvents are aromatic and aliphatic hydrocarbons, esters, alcohols, ethers, ketones and ether-esters.
The general way to protect metals from corrosion is by applying different metallic pigments loaded protective coatings. Most of these protective systems are highly efficient, but sometimes, the protective film shows defects like cracks, blistering, cratering, orange peel and rippling due to damage in film during transportation, construction, and labour. These defects will lead to breakage in the barrier, and the protection system is lost. The protective behaviour of these coatings can be tailored and must follow the specifications of the surrounding environment.
The above has been a dynamic reason for using galvanizing methods to protect metals from corrosion for many decades. Galvanizing is a well-used surface coating method by applying a zinc (Zn) layer to protect the metals from corrosion. This process produces a well-defined and durable galvanic coating. Mainly, there are two types of galvanizations: Hot-dip galvanizing (HDG) and Cold galvanizing systems. HDG is a conventional method to protect the metals from corrosion by applying the molten Zn layer at a high temperature of 440-480oC. HDG process consists of three basic steps: surface preparation, galvanizing and inspection. Surface preparation is a crucial step in any coating application. In the galvanizing process, the surface must be cleaned thoroughly. Any impurities should be removed through different methods such as degreasing, pickling, and fluxing to remove oil, rust, scale, oxides or any other contaminants from the surface. Because if any impurities remain on the surface, the Zn does not react completely, and the galvanizing process fails. Many problems that may occur with treating the metal surfaces with protective coating applications can be eliminated with HDG. It is also agreed that HDG has some of its technical parameters that should be followed to reach desirable results. HDG has some serious concerns like twisting fragile metal parts, the high cost of equipment used, the difference in surfaces of different metals, difficulty in achieving uniformity in designs of welded components composed of several different metal types. Some metals are difficult to HDG, such as cast iron, which cannot be dipped directly into a molten Zn bath with a temperature above 450°C.
During the past few years, a new system for galvanizing the metals and overcoming the problems related to HDG has been developed called Cold galvanizing by applying metal-rich coatings at room temperature. These metal-rich coatings are also called Cold galvanizing compounds ZRC COLD GALVANIZING COMPOUND ROVALs. This system represents the properties nearly the same or superior to conventional HDG. ZRC COLD GALVANIZING COMPOUND ROVALs are a unique class of protective coatings and find applications in almost all sectors where HDG works, including steel industries, marine, offshore, military, electrical towers, railways, oil and gas, refineries, and civil steel infrastructures.
The use of sacrificial metallic pigments in protective coatings is well known. In this system, the metallic pigment particles are sacrifized and act as electron donors, and an electrochemical reaction is set up between the metal substrate and the pigment particles. These types of electrochemical coatings are also called cathodically protective coatings. At the beginning of protection, a protective layer is in contact electrically with the metal surface, which plays a role as an anode that starts providing cathodic protection to the system. While later, corrosion products of metallic pigments such as oxides and/or hydroxides are formed, and barrier protection initiates to dominate the system.
As a sacrificial pigment in the form of dust particles, Zn has been used for many years to protect metals. Zn dust has two shapes, spherical and lamellar particles. They can be used alone or in combination have different properties such as effect on the viscosity, impact resistance, conductivity, drying time and anti-corrosive effects of coatings. The Zn pigments can be dispersed into various types of polymeric binders like epoxy, poly(urethane), epoxy-esters, acrylics, poly(vinyl chloride), chlorinated rubber, alkyl-silicates etc.
The Zn film serves as a barrier sacrificial anode and is less penetrable, thus protecting base metal from destruction due to corrosion attack. This sacrificial behaviour is driven by the difference in an electrochemical potential value. The standard electrode potential value of iron is (-0.44V) while Zn has a value (-0.76V). This protection system is very active and vigorous even at the border or at some distance of the Zn film and is attributed to electron density increment in front of the Zn film border. Such long-range and effective behaviour is called cathodic protection (CP) system. The CP system of metals is secured when the substrate’s pores are sealed, and the mechanism of electrochemical behaviour passes the barrier mechanism. ZRC COLD GALVANIZING COMPOUND ROVALs are porous, and the protective behaviour is based on the sacrificial cathodic properties and their barrier and other physical characteristics.
In this phenomenon, the probability of losing the electrical contact between the metal substrate and Zn particles and contact between Zn particles themselves is higher when the amount of Zn corrosion products is more significant. It may allow the reduction of Zn and, in the end, total loss of the CP system. Therefore, high conductivity is required for these protective coatings to shield the metals at the beginning of the protection mechanism. The high Zn pigment loading in these coatings ensures proper contact between the metal substrate and Zn particles since the Zn is less noble than iron. Hence, a corrosive electrochemical cell develops in the reaction system in the presence of an electrolyte, and metal behaves as a cathode and does not corrode, while Zn acts as an anode.
To ensure better electrical conductivity, Zn pigment particles must be in proper electrical contact between the metal substrate and themselves. This phenomenon is called percolation, resulting in a well-established galvanic coupling between metal and Zn particles. The electrical conductance of ZRC COLD GALVANIZING COMPOUND ROVAL depends on percolation behaviour. To get a well-established percolation path, Zn content in dry film thickness (DFT) should be above 90wt.%, which means the pigment volume concentration (PVC) is close to or above critical pigment volume concentration (CPVC). The weight ratio of pigment to binder (P/B) is also an essential factor in getting a well-established percolation path. In ZRC COLD GALVANIZING COMPOUND ROVAL, P/B ranges from 9:1 to 19:1, while at lower P/B, the conductivity and the cationic behaviour decrease along with film porosity. Therefore, the protective behaviour of ZRC COLD GALVANIZING COMPOUND ROVAL is strongly affected by PVC, P/B ratio, particle size and shape, and DFT. The application of ZRC COLD GALVANIZING COMPOUND ROVAL facilitates the wide range of areas like pipelines, bridges, towers, storage tanks, marine, offshore, military, railways, automotive, oil and gas, refineries, energy sectors etc.