Protective coatings

Protective coatings of paint or plastic resins are a familiar means of corrosion control by cutting off access of water, air, and electrolytes to the structure. The coating must, however, be complete and remain intact to be protective. Local penetration of the coating will generally create an 

Electroplated coatings are commonly applied to meet a variety of service requirements and also for decorative purposes. Service requirements include corrosion resistance, wear resistance and contact with chemicals and foodstuffs. The coatings described here are those used chiefly for their corrosion-resistant properties. 

Chrome plate is a widely used electroplated coating and its appearance can be bright, satin, matt or black. It combines resistance to corrosion, wear and heat, use in contact with foodstuffs and decorative qualities. By itself it does not give a high level of protection against corrosion. To obtain a high level of corrosion protection, the chromium is deposited over a coating of nickel. 

The deposit of nickel, which itself has good corrosion resistance, gives protection to the base metal and is itself protected from surface oxidation by the chromium. As a decorative finish, chrome plate can also be applied to plastics, usually acrylonitrile-butadiene-styrene (ABS). Hard chrome deposits, typically 150-500 m, are used to build up rollers, hydraulic rams, valves, etc., as a hard, abrasive-resistant wearing surface which is subsequently ground to give a highly accurate surface finish. 

Nickel deposits are most widely used as a base for chromium plating the thicker the deposit (typically 20 urn), the greater the corrosion resistance. 

Nickel plating is used in engineering where wear resistance, hardness and corrosion resistance are required, e.g. oil valves, rotors, drive shafts and in printed circuit board manufacturer. It is also used for its decorative properties, e.g. doorknobs. 

The protective nature of coatings with regard to the corrosion of steel is best illustrated by the reduced transport of water and oxygen to the steel coating interface. We have shown in Chap. 10 that the overall corrosion reaction is 

When ordinary unpainted steel is exposed to an industrial atmosphere, the corrosion rate is approximately 70 mg/cm /year. Based on (13.10), the water requirements are 34 mg/cm /year and the oxygen needed is 30 mg/cm /year. Thus, any protective coating must have low permeability for H2O and O2 if corrosion is to be reduced. 

Some permeability values for H2O and O2 by various coatings are given in Tables 13.3 and 13.4 and show that some coatings are not suitable as rust inhibitors. 

A Mossbauer spectroscopic study on mst formed under coated steel panels exposed to a marine environment for 18 months showed the presence of y-Fe203, aFeOOH, and yFeOOH next to the paint surface. The rust at the steel interface consisted primarily of yFeOOH. The coatings used for the different tests were the following  

In all cases, there was sufficient permeability of water and oxygen to corrode the steel resulting in a delamination of the paint film which detaches the corrosion products from the metal surface. 

Industrial protective coatings cover heavy duty, maintenance and marine systems including coatings for structural steel, ships, tanks, off shore installations, mining, oil and gas pipelines and refineries. 

In the case of topcoats, the two pack acrylic polyurethane almost invariably uses aliphatic isocyanates because of their excellent appearance characteristics and long term durability. The carbon carbon backbone and urethane linkage also impart excellent resistance to chemicals, fuels and environmental conditions. 

The acrylic topcoat is normally applied over an epoxy primer or rm inorganic/organic zinc rich primer, which maximises the anti-corrosion characteristics of the complete system. 

The type of OH functional acrylic polymer selected will be dependent on the level of resistance required, hardness/flexibility, ease of application and aesthetic requirements. 

The second component in the two pack is a polymeric multi functional isocyanate. The isocyanate group is extremely hydrogen acquisitive and will therefore abstract hydrogen from the hydroxy functional acrylic resin, forming a urethane link in the process. The multi functionality of the isocyanate ensures the formation of a network structure. Reactions take place at ambient temperatures or can be forced along at slightly above ambient temperatures, e.g. 60°C for 20 minutes as a typical cure cycle. 

A wide range of paints and other organic coatings is used for the protection of mild steel structures. Paints are used mainly for protection from atmospheric corrosion. Special chemically resistant paints have been developed for use on chemical process equipment. Chlorinated rubber paints and epoxy-based paints are used. In the application of paints and other coatings, good surface preparation is essential to ensure good adhesion of the paint film or coating. 

Brief reviews of the paints used to protect chemical plant are given by Ruff (1984) and Hullcoop (1984). 

The life of equipment subjected to corrosive environments can be increased by proper attention to design details. Equipment should be designed to drain freely 

Baines, D. (1984) Chem Engr, London No. 161 (July) 24. Glass reinforced plastics in the process industries. 

Bendall, K. and Guha, P. (1990) Process Industry Journal (March) 31. Balancing the cost of corrosion resistance. 

As already stated, corrosion can take place only if an electrolyte is present. The obvious method of controlling corrosion, then, is to prevent electrolyte from contacting the metal surfaces i.e. exclude the environment from the metal. One method of doing this is to provide a protective coating on the metal surfaces, the method chosen depending on the type of metal to be protected, the environment in which the metal will operate, and the coating material. The coating may consist of  

The normal thickness of coating is 5-25 jjim and has low frictional properties, making it an ideal surface on fasteners which reduces the tightening torque and prevents jamming. It also provides an effective barrier to prevent bimetallic reaction between steel fasteners and aluminium, e.g. where parts are fixed to an aluminium framework. Cadmium-plated surfaces can be easily soldered without the use of corrosive fluids. 

After plating, a chromate conversion coating is usually applied which gives the coating its iridescent green/brown appearance and adds to its corrosion resistance. 

The application of paints to metallic objects for corrosion control has been known for a long time. The mechanism of protection by paint films was viewed as a source of insulation of the metal from the corrosive environment such as oxygen and water and inhibiting the cathodic reaction. The idea of protective action of paint films by providing insulation of the metal from oxygen and water was questioned, based on the data given in the literature. 

Water/oxygen to given corrosion rate of 70 mg Fe/cm2/year  

Paint or coating systems consists of many layers which are known as coats. The first coat applied on a structure is the primer and the last is known as finish or top coat. The coats between the primer and finish coat are intermediate coats. 

Pigments used in barrier coatings should also be hydrophobic in nature, and possess good adhesion across pigment-binder interfaces. Some typical pigments used are flat platey aluminum flakes, glass flakes, stailness steel flakes and micaceous iron oxide. 

Barrier film allows penetration of water but restricts the access of ionic material and in some cases oxygen. 

Cellulose acetate has offered little in fulfilling the requirements of a protective coating material. Although stable to light, it is restricted in use by its limited solubility in solvents, and its poor compatibility with resins. It has been used only for special purposes as a lacquer material, and to a limited extent for surface coatings on paper and cloth. Cellulose mixed esters appear more promising for th se uses. Cellulose acetate 

Artificial leather coatings may be made from cellulose acetate butyrate compositions of high butyryl content. Large amounts of plasticizers of low volatility are employed. These coatings offer improved flexibility at low temperatures over those made from cellulose nitrate compositions. 

Films for X-ray and portrait purposes require good rigidity for handling and high resistance to moisture in order to prevent unevenness at varying humidities. Cellulose acetate with comparatively large amounts of plasticizer has been widely used for this purpose. Cellulose 

Cine Films must withstand severe mechanical wear and must be of good dimensional stability for projection. Cellulose nitrate has qualities which are very desirable for this purpose, and remains the standard product for the professional moving picture industry. Amateur and educational Cine Films must be made of less inflammable material. Cellulose acetate has been used for this purpose but is limited in its moisture resistance, its strength and its tendency toward brittleness at low humidities. Improvements have been obtained by the employment of cellulose mixed esters possessing a moderate higher acyl content. Such products are widely used for narrow width Cine Films. 

Cellulose acetate became of interest as a molding material because of its lower flammability as compared with celluloid. For the past ten 

Naphthalate polyester articles can be coated with polymerizable and cross-linkable compositions. The composition is made from the matrix and a 

Vogel et al [223] describes the successful development by Dornier of a cost-effective technique for producing fiber reinforced ceramics and are able to manufacture complex shapes and integrated structures. Carbon fiber reinforced SiC has the required damage tolerant fracture behavior and sufficient reliability with marked weight reduction and 

Katzman [224] burnt the size off the carbon fiber and applied an organometallic solution of Si(OC2H5)4 agitated ultrasonically, followed by pyrolysis at 1000°C in an atmosphere of NH3 to give a coating of SisN4. 

Carbon fiber can be coated with copper phosphate prepared from CuO and H3PO4 [225], which after curing for some 4 h forms Cu (H2P04)2 and 24 h gives CUHPO4. 

Liu et al [226] prepared C/C-SiC nanomatrix composites by chemical vapor infiltration. 

Slosarczyk et al [229] used PAN based carbon fibers, both uncoated and coated, with calcium phosphate applied by a sol gel technique. Carbon fiber reinforced hydroxyapatite composites were then prepared by hot pressing hydroxyapatite powder and carbon fibers at 1100°C and 25 MPa for 15 min in an Ar atmosphere. The best strength properties were obtained with the eoated fibers and were attributed to the —OH groups on the fiber surface bonding with the ealeium phosphate layer. 

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The naturally occurring isomer, a-licanic acid, has m.p. 74-75" C. It is unstable, and is readily isomerized to the / -form, m.p. 99-5 C. It occurs in oiticica and other oils which were previously used in the protective-coating industry. 

The importance of polymer composites arises largely from the fact that such low density materials can have unusually high elastic modulus and tensile strength. Polymers have extensive applications in various fields of industry and agriculture. They are used as constructional materials or protective coatings. Exploitation of polymers is of special importance for products that may be exposed to the radiation or temperature, since the use of polymers make it possible to decrease the consumption of expensive (and, sometimes, deficient) metals and alloys, and to extent the lifetime of the whole product. 

The application of fundamentally new ECT (Russia patent Jf 2063025) has made it possible to provide high-efficiency defect control accompanied by detecting both small surface defects and more rough under-surface defects under non-magnetic metal layer of 7 mm thick, or surface defects under protection coatings, dye, corrosion, hermetic and other type of layer of 10 mm thick. 

Corrosive defects detection under the airframe skin and protective coatings. 

Detection of corrosion and defects under a layer of protective coatings, rust and foulings to 8-10 mm in thickness. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Unprotected steel corrodes at a rate which is generally assumed to be 0.1 to 0.2mm per annum. Factors that influence the actual rate of corrosion include the maintenance program applied by the owner - particularly preservation of protective coatings, efficiency of cathodic protection systems in ballast tanks, corrosive properties of the cargo carried and environmental factors such as temperature and humidity. Under extreme conditions it has been known for the annual rate of corrosion on unprotected steel exposed on both surfaces to approach 1mm. 

Small molecules can penetrate and penneate tlirough polymers. Because of this property, polymers have found widespread use in separation teclmology, protection coating, and controlled delivery [53]. The key issue in these applications is the selective penneability of the polymer, which is detennined by the diffusivity and the solubility of a given set of low-molecular-weight compounds. The diffusion of a small penetrant occurs as a series of jumps... 

More recently, alternative chemistries have been employed to coat oxide surfaces with SAMs. These have included carboxylic 1129, 1301, hydroxamic 11311, phosphonic 1124, 1321 and phosphoric acids 11331. Potential applications of SAMs on oxide surfaces range from protective coatings and adhesive layers to biosensors. 

Munger C G 1984 Corrosion Prevention by Protective Coatings (Houston, TX National Association of Corrosion Engineers)... 

Cadmium is a soft metal, which forms a protective coating in air, and burns only on strong heating to give the brown oxide CdO. It dissolves in acids with evolution of hydrogen ... 

When considering how the evolution of life could have come about, the seeding of terrestrial life by extraterrestrial bacterial spores traveling through space (panspermia) deserves mention. Much is said about the possibility of some form of life on other planets, including Mars or more distant celestial bodies. Is it possible for some remnants of bacterial life, enclosed in a protective coat of rock dust, to have traveled enormous distances, staying dormant at the extremely low temperature of space and even surviving deadly radiation The spore may be neither alive nor completely dead, and even after billions of years it could have an infinitesimal chance to reach a planet where liquid water could restart its life. Is this science fiction or a real possibility We don t know. Around the turn of the twentieth century Svante Arrhenius (Nobel Prize in chemistry 1903) developed this theory in more detail. There was much recent excitement about claimed fossil bacterial remains in a Martian meteorite recovered from Antarctica (not since... 

Waxes are water repelling solids that are part of the protective coatings of a number of living things including the leaves of plants the fur of animals and the feathers of birds They are usually mixtures of esters m which both the alkyl and acyl group are unbranched and contain a dozen or more carbon atoms Beeswax for example contains the ester triacontyl hexadecanoate as one component of a complex mixture of hydrocar bons alcohols and esters... 

Wax (Section 26 5) A mixture of water repellent substances that form a protective coating on the leaves of plants the fur of animals and the feathers of birds among other things A principal component of a wax is often an ester in which both the acyl portion and the alkyl portion are characterized by long carbon chains... 

Better detection limits are obtained using fluorescence, particularly when using a laser as an excitation source. When using fluorescence detection, a small portion of the capillary s protective coating is removed and the laser beam is focused on the inner portion of the capillary tubing. Emission is measured at an angle of 90° to the laser. Because the laser provides an intense source of radiation that can be focused to a narrow spot, detection limits are as low as 10 M. 

Many seed oils, especially sunflower and linseed, contain waxes which serve as a protective coating for the seed. These waxes soHdify at colder temperatures and impart turbidity to the oil and interfere with subsequent processing. They are commonly removed from the cmde oil by refrigeration followed by filtration, a process commonly known as winterization. 

A varnish is often appHed on top of the paint layers. A varnish serves two purposes as a protective coating and also for an optical effect that enriches the colors of the painting. A traditional varnish consists of a natural plant resin dissolved or fused in a Hquid for appHcation to the surface (see Resins, natural). There are two types of varnish resins hard ones, the most important of which is copal, and soft ones, notably dammar and mastic. The hard resins are fossil, and to convert these to a fluid state, they are fused in oil at high temperature. The soft resins dissolve in organic solvents, eg, turpentine. The natural resin varnishes discolor over time and also become less soluble, making removal in case of failure more difficult (see Paint and FINNISH removers). Thus the use of more stable synthetic resins, such as certain methacrylates and cycHc ketone resins, has become quite common, especially in conservation practice. 

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Protective Coatings. The primary function of interior can coatings is to prevent interaction between the can and its contents. Exterior can coatings maybe used to provide protection against the environment, or as decoration to give product identity as well as protection. 

Two basic methods are used for the appHcation of protective coatings to metal containers, ie, roUer coating and spraying. RoUer coating is used if physical contact is possible, eg, coating of metal in sheet and coil form. Spraying techniques are used if physical contact is not possible, eg, to coat the inside surface of two-piece drawn and ironed can bodies (see Coating processes). 

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