Corrosion atmospheric. See Atmospheric

Less importantly, ammonia is used (/) As a refrigerant in both compression and absorption systems (see Refrigeration and refrigerants). Ammonia s high latent heat, low vapor density, chemical stabiHty, and low corrosion to iron parts promote its use in large industrial installations. ( 2) In the pulp and paper industry for the pulping of wood and as a dispersant for casein in the coating of paper (see Paper Pulp). (J) In the metal industry for detinning of scrap metal, in the extraction of certain metals, eg, copper, nickel, molybdenum, and tungsten from their ores, in metal treating where cracked (dissociated) ammonia is used as a reducing atmosphere for the bright annealing of stainless steels, nickel and its alloys, for the reduction of metal oxides and for the nitriding of steels for case-hardening (see Metal treatments). (4) As a medium to reduce nitrogen oxides in stack gases. Both catalytic and noncatalytic systems have been developed. (3) As a modifying reagent in the flotation of phosphate ores, and as collecting reagents for the froth flotation  [c.358]

Porcelain enamels are used in modem mass transit faciUties because of inherent fire resistance as well as ease of maintenance and durabiUty in the face of highly corrosive atmospheres resulting from vehicular traffic. Newer appHcations of porcelain enamels include electronic substrates (see Electronic MATERLALs), pyrolytic self-cleaning oven interiors, microwave oven interiors, outdoor cookers, and fireplace liners as well as wood-stove exteriors, writing boards for erasable markers, institutional surfaces such as bathroom stalls and hospital operating room walls that need to be easily disinfected, and subway car interiors and elevator walls that are durable and easy to clean. Porcelain enamel surfaces are frequently used in food contact appHcations. The glassy nature of porcelain enamels has a significant effect on encapsulating and minimizing any solubiHty of constituents. Enamels can also be formulated to have specific acid, alkaH, and abrasion-resistant properties.  [c.207]

Operating conditions which affect belt-conveyor design include climate, surroundings, and hours of continuous service. Temperature and humidity extremes may dictate total enclosure of the belt surroundings which involve such conditions as high temperature or corrosive atmosphere can affect belt, machinery, and struc ture and continuous sei vice may require extremely hign-quahty components and even specially designed equipment for sei vicing while the belt is in operation. For example, idlers may be obtained with tilting stands which allow them to be tipped out of the way for sei vice while the belt is running.  [c.1917]

Mild steel, also low-alloy irons and steels 0 3 0 3 < 400 1 < 750 Wronglit, cast Good Good 67 6.7 Higli strengths obtainable by alloying, also improved atmospheric corrosion resistance. See ASTM specifications for particular grade  [c.2446]

Corrosion can range from the highly uniform (chemical or electrochemical polishing) to the highly localised such as occurs during pitting, intergranular attack and stress-corrosion cracking. Uniform, or near-uniform, corrosion without doubt accounts for the greater proportion of metal deterioration in terms of both mass of metal converted to corrosion products and cost. However, although detrimental it is at least predictable on the basis of laboratory and field testing, so that allowance can be made for it in the design of a structure see Section 9.2). In completely uniform corrosion the anodic and cathodic sites are physically inseparable, and although this type of corrosion occurs in many practical situations (e.g. chemical and electrochemical polishing, passivity) corrosion is more usually near-uniform, particularly in natural environments. Thus the appearance of the rust on steel that has been exposed to the atmosphere for some lime may give the impression that attack has been quite uniform, but removal of the rust with an inhibited acid will reveal that the surface is undulating, indicating that although the whole surface has corroded the rates at different areas of the surface are not uniform. This is due to a variety of factors associated with the structure of the metal, the nature of the surface, deposits of dirt and corrosion products, etc.  [c.151]

In addition to the alloying ingredients which are added, certain other metals are usually present in small amounts. In the alloys which contain aluminium, for example, iron usually amounts to about 0-02-0 05%. By special techniques and care in melting this can be reduced to about one-tenth of the above figure. Many workers have shown that such high-purity alloys have a markedly better resistance to salt water than those of normal purity, but their behaviour towards industrial atmospheres is not greatly different. Furthermore, the practical value of the higher resistance to corrosion is largely offset when components are used in electrical contact with other more cathodic metals. The effect of a steel bolt for example, even when it has been zinc or cadmium plated, is much greater at the point of contact than that of the excess of local cathodes in the impure alloys. Galvanic corrosion at joints with other metals therefore is not markedly less in the case of the high-purity alloys. Nevertheless, such alloys have their place, and when they can be used without other metal attachments provide better intrinsic resistance to corrosion by sea-water than the alloys of normal purity.  [c.748]

Commercially pure nickel has good mechanical properties and good resistance to many corrosive environments and therefore finds application where this combination of properties is required. Of more importance, however, is the fact that nickel forms a wide range of alloys having desirable engineering and corrosion-resistant properties. With regard to corrosion resistance to aqueous solutions, among the most important of these alloying elements are Cr, Fe, Cu, Mo and Si. Since the range of corrosion-resistant nickel alloys includes some that owe their corrosion resistance to passivity and others that are resistant because they are sufficiently noble not to displace hydrogen from acidic solutions, the corrosive environments in which nickel alloys can be successfully used are very varied, embracing acids, salts and alkalis (both oxidising and non-oxidising in character) sea-water, natural waters and the atmosphere and combinations of these encountered industrially.  [c.760]

Silver-based low-temperature brazing alloys have been known for many years and are widely employed throughout the engineering and chemical industries. Their output is measured in many tonnes per year, and their utility in joining almost all the commonly used materials of construction has made them indispensable for the production of a very wide range of plant, equipment and structures of all kinds. They exhibit a high resistance to corrosion by industrial atmospheres and possess excellent strengths at temperatures considerably higher than may be employed for lead or tin-based solders (see Section 10.5).  [c.937]

It is not economically or technically feasible to use chromium in a fabricated form, but the high resistance of the metal to corrosion can be utilised by applying a thin coating of chromium to less resistant metals. Although the metal is base ( °cr /Cr = -0.74 V (SHE)) it is protected by a thin, stable, tenacious, refractory, self-sealing film of CrjOj. This is preserved by oxidising conditions, and the metal is very resistant to high-temperature oxidation and to atmospheric exposure in most natural environments. Unlike silver and copper, it is not tarnished by hydrogen sulphide, nor is it fogged like nickel by atmospheres containing sulphur dioxide.  [c.545]

The most common fonn of corrosion (in tenns of tons of materials lost) is electrochemical corrosion, which can occur for example in aqueous solutions, in tire atmosphere and in tire ground. Here, tire actual corrosion reaction is invariably tire anodic or oxidation reaction, whereby a metal dissolves while releasing electrons and ions. Thus one might say tliat corrosion is a negative way of looking at an electrochemical dissolution or oxidation reaction. The reason for separating tliis topic from otlier dissolution or oxidation reactions which are of economic benefit, e.g. oxidation of silicon to fonn semiconductor devices, is based on tlie historic roots of corrosion science and tlie tremendous economic significance of material destmction. In industrialized countries, tlie cost of corrosion is estimated to be about 3.5% of tlie GNP. Areas such as constmction materials, electronics and transportation are affected and tluis an extensive number of reference books is available [1, 2, 4, 5, 6, 7, 8, 9, K), H, 12,13,14,15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32 and 33]. Frequently, modes of corrosion are described according to tlie type of attack (e.g. unifonn corrosion, localized corrosion) or tlie topic is categorized according to tlie specific material involved.  [c.2714]

Atmospheric Emissions. The hydrogen sulfide found in many hydrothermal fluids is extremely toxic and has an unpleasant odor (25). In The Geysers area of California, special equipment has been installed at geothermal installations to remove hydrogen sulfide from the waste stream by the Stretford process. This technology utilizes a vanadium catalyst to reduce about 95% of the hydrogen sulfide to elemental sulfur. The product would be salable except that it is usually contaminated with vanadium and other traces of heavy metals which make it a hazardous material (see Sulfur removal and recovery). Hydrogen chloride, found in some geothermal steam, has proven to be more of a problem from a corrosion standpoint than as an atmospheric contaminant. Other gases are present in relatively small quantities. Geothermal plants release only about 5% as much carbon dioxide, and less than 1% of the nitrous and sulfur oxides emitted from fossil power plants in generating an equivalent amount of electricity (see Airpollution Airpollution control PffiTHODS).  [c.267]

Oxidation. Immense progress in technology has imposed ever-increasing demands on the mechanical and chemical properties, in particular the oxidation and scaling resistance, of metallic materials. The terms metal oxidation and scaling can be used to describe the attack of a metal or an alloy by aggressive gases such as oxygen, sulfur, the halogens, or water vapor. This attack can take place under a variety of circumstances, varying from the mild oxidizing conditions which may exist at room temperature in air to the severe conditions imposed by hot furnace gases contacting metallic surfaces. Especially stringent requirements are placed on the scaling resistance of components where condensed molten salts and oxidizing atmospheres simultaneously exist. The attack under such conditions is commonly referred to as hot corrosion, ie, a particularly severe form of corrosion (see Corrosion AND CORROSION CONTHOL).  [c.115]

A solution of sulfur trioxide [7446-11-9] dissolved in chlorosulfonic acid [7990-94-5] CISO H, has been used as a smoke (U.S. designation FS) but it is not a U.S. standard agent (see Chlorosulfuric acid Sulfuric acid and sulfur trioxide). When FS is atomized in air, the sulfur trioxide evaporates from the small droplets and reacts with atmospheric moisture to form sulfuric acid vapor. This vapor condenses into minute droplets that form a dense white cloud. FS produces its effect almost instantaneously upon mechanical atomization into the atmosphere, except at very low temperatures. At such temperatures, the small amount of moisture normally present in the atmosphere, requires that FS be thermally generated with the addition of steam to be effective. FS can be used as a fill for artillery and mortar shells and bombs and can be effectively dispersed from low performance aircraft spray tanks. FS is both corrosive and toxic in the presence of moisture, which imposes limitations on its storage, handling, and use.  [c.402]

The stainless quality is conferred on steels that contain enough chromium to form a protective surface film. About 12 wt % chromium is requited for protection in mild atmospheres or in steam. At 18—20 wt % chromium, sufficient protection is achieved for satisfactory performance in a wide variety of more destmctive environments, including those occurring in the chemical, petrochemical, and the power-generating industries. Stainless grades having 25 wt % chromium or more and containing other alloying elements such as molybdenum provide even higher corrosion resistance. In certain stainless steels, the chromium depresses the martensite transformation below room temperature. By thus stabilizing austenite the chromium permits achievement of desired mechanical properties without loss of corrosion resistance (42—48) (see Steel).  [c.121]

Elemental copper is resistant to aerated alkaline solutions except in the presence of ammonia. Copper does not displace hydrogen from acid but dissolves readily in oxidising acids such as nitric acid (qv) or in acid solutions that contain an oxidising agent, such as sulfuric acid solution containing ferric sulfate. Because of corrosion resistance to salt solutions, copper is used in marine appHcations (see Coatings, marine Corrosion and corrosion control). Resistance to oxidation by water vapor at high temperatures has made copper a material of choice in cooling systems. Although the surface of copper oxidizes on exposure to the atmosphere, further corrosion is inhibited by the formation of a tightly adherent protective coating of corrosion products. In many instances this takes the form of a green patina that imparts a rich appearance to architectural and artistic uses. Studies of the chemistry of atmospheric corrosion have shown that the initial attack on copper metal involves the formation of sulfides and oxides. Upon further oxidation and reaction with water, basic copper sulfates, such as CuSO Cu(OH)2 and CuSO 3Cu(OH)2, are formed. The latter compound is found in nature as the mineral brochantite. A basic carbonate, CuCO Cu(OH)2, may also be present in some deposits (15).  [c.195]

Corrosion. Copper and selected copper aHoys perform admirably in many hostile environments. Copper aHoys with the appropriate corrosion resistance characteristics are recommended for atmospheric exposure (architectural and builder s hardware), for use in fresh water supply (plumbing lines and fittings), in marine appHcations (desalination equipment and biofouling avoidance), for industrial and chemical plant equipment (heat exchangers and condensers), and for electrical/electronic appHcations (coimectors and semiconductor package lead-frames) (30) (see Packaging).  [c.226]

Atmospheric exposure, fresh and salt waters, and many types of soil can cause uniform corrosion of copper aHoys. The relative ranking of aHoys for resistance to general corrosion depends strongly on environment and is relatively independent of temper. Atmospheric corrosion, the least damaging of the various forms of corrosion, is generaHy predictable from weight loss data obtained from exposure to various environments (31) (see Corrosion and CORROSION CONTKOL).  [c.226]

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted.  [c.274]

Contact Finishes. Base metal substrates quickly develop oxides, tarnish, or corrosion films, in humid, polluted atmospheres. Because these films may prevent adequate metal-to-metal contact when the connector or connector—conductor surfaces are mated, coatings of other metals commonly are used to obtain corrosion resistance, to provide conductivity, or to faciHtate termination to conductors by soldering, wire wrapping, or by other means. AppHcation of finishes is achieved by electroplating (qv), cladding, and by hot-dipping when low melting metals such as tin are used (see Metal SURFACE treatments). Selective appHcation of a contact finish to portions of the substrate can be accompHshed using any of these coating techniques. The principal noble contact finishes are gold, palladium, rhodium, and alloys having a high gold or palladium content. Palladium-based finishes usually have a thin (0.05—0.1 l-lm) top coating of gold. The non-noble finishes are tin, silver, and nickel. Alloys of these metals, such as 50 wt % Sn, 50 wt % Pb, also are widely used.  [c.30]

Lead and Lead—Tin Alloys. Lead is plated from fluoborate solutions, as shown in Table 13. Lead is used in battery parts for its good resistance to sulfuric acid (see Batteries). Low tin alloys of lead are used in bearings 93% lead—7% tin was specified for bearings on piston aircraft engines. Lead—tin alloys of 3—15% tin are called teme for hot-dip coatings, temeplate can be 20% tin. Temeplate has been used to protect steel in gasoline tanks. Solder deposits are also plated. Lead—tin baths are similar to lead baths. Staimous fluoborate is added to supply the tin. Sulfamate solutions have been described for lead plating (108,109), but have found Httle industrial use. Requirements for lead and lead—tin deposits on steel are specified (110). These deposits have outstanding resistance to atmospheric corrosion, especially with a 0.25 pm copper strike undercoat. Lead coatings (19 pm) show an expected life of more than nine years in an industrial atmosphere. Up to 15% tin does not decrease the corrosion resistance. Newer baths based on methane sulfonic acid are gaining in use both for lead (qv) and lead—alloy (see Lead alloys) baths. Lead is under strict regulation in the environment and waste streams. The future of lead plating is uncertain.  [c.160]

Reinforced-Thermosetting-Resin (RTR) Pipe Glass-reinforced epoxy resin has good resistance to nonoxidizing acids, alkahes, salt water, and corrosive gases. The glass reinforcement is many times stronger at room temperature than plastics, does not lose strength with increasing temperature, and reinforces the resin effectively up to 149°C (300°fX (See Table 10-17 for temperature hmits.) The glass reinforcement is located near the outside wall, protected from the contents by a thick wall of resin and protected from the atmosphere by a thin wall of resin. Stock sizes are 2 through 12 in.  [c.979]

How does galvanising work As Fig. 24.4 shows, the galvanising process leaves a thin layer of zinc on the surface of the steel. This acts as a barrier between the steel and the atmosphere and although the driving voltage for the corrosion of zinc is greater than that for steel (see Fig. 23.3) in fact zinc corrodes quite slowly in a normal urban atmosphere because of the barrier effect of its oxide film. The loss in thickness is typically 0.1 mm in 20 years.  [c.234]

NFPA [34] contains extensive descriptions of Bleves (also see [82]) and describes them in summary as paraphrased here with permission liquefied gases stored in containers at temperatures above their boiling points at NTP will remain under pressure only as long as the container remains closed to the atmosphere. If the pressure is suddenly released to atmosphere due to failure from metal overstress by external fire or heat, corrosion penetration, or external impact (for examples), the heat stored in the liquid generates very rapid vaporization of a portion of the liquid proportional to the temperature difference between that of the liquid at the instant the container fails and the normal boiling point of the liquid. Often this can generate vapor from about one-third to one-half of the liquid in the container. The liquid vaporization is accompanied by a large liquid to vapor expansion, which provides the energy for propagation of vessel cracks, propulsion of pieces of the container, rapid mixing of the air and vapor resulting in a characteristic fireball upon ignition by the external fire or other source that caused the Bleve to develop in the first place, with atomization of the remaining cold liquid. Often the cold liquid from the vessel is broken into droplets that can burn as they fly out of the vessel. Often this cold liquid can escape ignition and may be propelled one-half mile or more from the initial site. In most Bleves, the failure originates in the vapor space above the liquid, and it is this space that is most subject to external overheating of the metal.  [c.504]

Contaminated atmospheres create additional corrosion problems. In marine environments (e.g. coastal power stations, offshore oil platforms and above the splash zone on ships) the chief corrosive agents is sea salt. The chloride content of sea salt precludes the use of stainless steel and bare steel or aluminum in these areas. Steel requires to be coated as described in the previous section. Any coating scheme used for the protection of steel must be properly selected and applied. BS 5493 contains guidelines for coating systems for a variety of environments. The most common reasons for coating failure are incorrect application and inadequate surface preparation. The use of an independent coating inspector, as discussed in Section 53.7, is recommended. Aluminum alloys can be coated with plastics, such as U-PVC, epoxy finishes or conventional paints, with the correct etch primer where applicable to ensure adhesion. Some alloys are suitable for anodizing, but the corrosion protection afforded by anodizing is variable and requires tight specification and control to ensure correct anodized film thickness and sealing efficiency. The lower grades of stainless steels are not suitable for external exposure in marine atmospheric conditions unless they are cleaned regularly with fresh water. Higher grades of stainless steel, nickel alloys and most copper alloys do not normally benefit from additional protection. The development of the colored patina on copper alloys can, however, be avoided by coating with inhibited lacquer.  [c.902]

For example, in a dry atmosphere a reactive metal such as aluminium may carry a natural protective oxide film of only some 3 nm thickness, while for increased corrosion resistance aluminium may be anodised to give a coating 10 times thicker see Section 15.1). However, thickness alone does not provide a criterion of protection and although a thick protective layer of millscale is formed on iron and steel during processing it is not continuous owing to spalling, and the attack on the exposed substrate at the discontinuities is far greater than if the surface was bare. Thus the kinetics of attack  [c.22]

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6).  [c.55]

Most aqueous solutions (ranging from bulk natural water and chemical solutions to thin condensed films of moisture) will be in contact with the atmosphere and will contain dissolved oxygen, which can act as a cathode reactant. The saturated solubility of oxygen in pure water at 25°C is only about 10 mol dm, and the solubility decreases significantly with increase in temperature and slightly with concentration of dissolved salts see Table 21.20 in Section 21 for oxygen solubilities). On the other hand, the concentration of H30 in acid solutions, which is given by the pH, is high, and since this ion has a high rate of diffusion its rate of reduction is normally controlled by the activation energy for electron transfer. Furthermore, the vigorous evolution of hydrogen that occurs during corrosion facilitates transport, so that diffusion is not a significant factor in controlling the rate of the reaction except at very high current densities. As the pH in  [c.99]

Natural and Synthetic Rubber. Fluorination of natural or synthetic mbber creates a fluorocarbon coating (29,75,76) which is very smooth and water repeUent (see Water, waterproofing and water/oilrepellency). Rubber articles such as surgical gloves, O-rings, gaskets, and windshield wiper blades can be fluorinated on the surface while the interior retains the elastic, flexible properties of the natural mbber. Fluorinated O-rings can be used without extra lubricant in corrosive atmospheres since the fluorocarbon is unreactive. In food-processing equipment, grease or lubricants are eliminated and do not contaminate the food products. Fluorinated O-rings have smooth surfaces, very low frictional coefficients, and enhanced thermal stabiHties. Fluorinated windshield wiper blades have a very low coefficient of friction, mn smoother with less squeak, their surface is more resistant to the sun s uv radiation and attack by ozone, and they require less electrical energy for operation.  [c.279]

Carbon (qv) reacts with most elements of the periodic table to form a diverse group of compounds known as carbides, some of which are extremely important in technology. For example, calcium carbide, CaC2, is a source of acetjdene siUcon carbide, SiC, and boron carbide, B C, are used as abrasives (qv) tungsten carbide, WC, titanium carbide, TiC, and tantalum (niobium) carbide, TaC(NbC), find use as stmctural materials at extremely high temperatures or in corrosive atmospheres. Cementite, Fe C, and the multimetalhc complexes (Co, W) C, (Cr,Fe,Mo)23C, and (Cr,Fe)2C3, are the components in tool steels and SteUite-type alloys responsible for their hardness, wear, resistance, and excellent cutting performance. Numerous compounds also have potential appHcations as catalysts (see Boron compounds Tungsten and tungsten alloys Tantalum and tantalum compounds Niobium and NIOBIUM compounds Titaniumand titanium compounds).  [c.438]

Vane anemometers can be used for gas-velocity measurements in the range of 0.3 to 45 m/s (about 1 to 150 ft/s), although a given instrument generally has about a twentyfold velocity range. Bearing friction has to be minimized in instruments designed for ac-curacy at the low end of the range, while ample rotor and vane rigidity must be provided for measurements at the higher velocities. Vane anemometers are sensitive to shock and cannot oe used in corrosive atmospheres. Therefore, accuracy is questionable unless a recent cahbration has been made and the history of the instrument subsequent to calibration is known. For additional information, see Ower et al., op. cit., chap. T11.  [c.888]

Metallic coatings have been widely and successfully used as a means of alleviating many bimetallic corrosion problems both under conditions of total immersion and in corrosive atmospheres. If, for instance, aluminium and steel must be jointed together in sea-water, the galvanic corrosion can be largely eliminated by aluminising the steel either by hot dipping or by flame spraying, as is more popular in Europe. Both zinc and cadmium are also fairly compatible with aluminium and so the steel may be protected with thin coatings of these metals without incurring the risk of aggravated galvanic corrosion cadmium plating has even been applied to stainless steel for this purpose. The use of dissimilar metallic coatings eliminates bimetallic corrosion only if the coating is initially free from voids and remains so in service, a circumstance seldom realised in practice. Metallic coatings on the steel that contain or develop voids still reduce the galvanic corrosion rate (because of the smaller area of steel exposed) provided that they are anodic to the substrate.  [c.235]

Wetness of a metal surface The lime of wetness of the metal surface is an exceedingly complex, composite variable. It determines the duration of the electrochemical corrosion process. Firstly it involves a consideration of all the means by which an electrolyte solution can form in contact with the metal surface. Secondly, the conditions under which this solution is stable with respect to the ambient atmosphere must be considered, and finally the rate of evaporation of the solution when atmospheric conditions change to make its existence unstable. Attempts have been made to measure directly the time of wetness , but these have tended to use metals forming non-bulky corrosion products (see Section 20.1). The literature is very sparse on the r61e of insoluble corrosion products in extending the time of wetness, but considerable differences in moisture desorption rates are found for rusted steels of slightly differing alloy content, e.g. mild steel and Cor-Ten.  [c.340]

Gases such as hydrogen sulphide and carbon dioxide do not increase the corrosivity of the atmosphere towards aluminiumService experience extends over 70 years and includes such well-known examples as Eros, Piccadilly Circus, London, which is in excellent condition, although cast in a low purity (98%) aluminium, and a cupola of San Gioacchino, a church in Rome which was covered in 1897 with sheet 1-25 mm thick and now shows attack to a depth of less than 0-13 mm. Twenty-year tests at selected marine, industrial and rural sites in the U.S.A. have shown that the greater part of the attack takes place in the first year or two and that thereafter the rate of attack maintains a low value. Results from typical environments are shown in Fig. 4.2, and it is apparent that clad alloys give the best results. The relatively high percentage strength losses are due to the extremely thin test specimens. After 20 years the average measured depth of attack for an aluminium-copper alloy at a sea coast test site did not exceed 015 mm. The falling-off in rate of pitting with time is in sharp contrast to the behaviour of the older-established structural metals which have a fairly uniform corrosion rate throughout their life, and indicates that the relative merit of aluminium increases with scheduled life.  [c.663]

The metal melts at 1356 K and oxidises at red heat in air to give the black 4-2 oxide CuO at higher temperatures the red-yellow -I-1 oxide CU2O is obtained. In dry air, little corrosion occurs, but in the ordinary atmosphere a green film slowly forms, and this protects the metal from further corrosion (hence its use in roofing). The composition of the green film varies normally it is a basic carbonate of copper, but near the sea basic chloride is also a component and in industrial areas a basic sulphate is found. Copper is readily attacked by halogens and by sulphur on heating. Since  [c.409]

The double fluorides decompose at even higher temperatures to form the metal fluoride and volatile NH and HF. This reaction produces pure salts less likely to be contaminated with oxyfluorides. Beryllium fluoride [7787-49-7] from which beryllium metal is made, is produced this way (18) (see Beryllium AND BERYLLIUM alloys). In pickling of stainless steel and titanium, NH4HF2 is used with high concentrations of nitric acid to avoid hydrogen embrittlement. Ammonium bifluoride is used in acid dips for steel (qv) prior to phosphating and galvanizing, and for activation of metals before nickel plating (19,20). Ammonium bifluoride also is used in aluminum anodizing formulations. Ammonium bifluoride is used in treatments to provide corrosion resistance on magnesium and its alloys (21). Such treatment provides an excellent base for painting and good abrasion resistance, heat resistance, and protection from atmospheric corrosion. A minor use for ammonium bifluoride is in the preservation of wood (qv) (22).  [c.149]

Molybdenum. Molybdenum is the most readily available and widely utili2ed refractory metal. Most engineering appHcations of this metal utili2e the high melting temperature, high strength and stiffness, resistance to corrosion in many environments, or high thermal and electrical conductivity. However, molybdenum must be coated if exposed to air above 535°C. The melting point of 2610°C, over 1000°C higher than for most high temperature superaHoys, permits molybdenum to be used in inert atmosphere furnace equipment. Furnace hardware and heat shields also perform well under extreme temperature conditions. The high thermal and electrical conductivity of molybdenum, as well as its inertness to molten glasses, permits it to be used for electrical heating or heat booster electrodes in commercial continuous glass making operations. Molybdenum also is used in a wide range of electronic and thermionic devices as well as crystal growing devices, x-ray tubes, magnetism and thyristors, and resistance weld electrodes. Other characteristics of molybdenum are its low thermal expansion, high stiffness, and the abiUty to take a high surface finish. Molybdenum is usehil for high temperature laser mirror components such as those to be used in fusion power systems (see Fusionenergy Lasers).  [c.127]

When moisture films are formed, water vapor can accelerate the corrosion rate. Hence, it is necessary to maintain the temperature above the dew point of the gas mixture by at least 20°C, to prevent the formation of moisture films. A temperature of 130°C or above, at atmospheric pressure, can be used for all mixtures of HCl gas and water vapor because the a2eotropic boiling point is 108.6°C. The boiling point of a2eotropic mixtures can be used as a guide at other pressures (see Table 6).  [c.446]

The corrosion resistance of niobium and its high electrical conductivity and ductihty make it a valuable stmctural material for chemical and metallurgical appHcations. The heat-transfer coefficient of niobium is more than twice that of titanium and three times higher than zirconium and stainless steels. Niobium is corrosion-resistant to most media, with the exception of hydrofluoric acid and hot concentrated hydrochloric and sulfuric acids. The pickling solution for removal of normal surface oxides is one part nitric acid, one part sulfuric acid, two parts hydrofluoric acid, and four parts water (by volume) (see Metal surface treatments). Niobium also shows good corrosion resistance to sulftdizing atmospheres of low oxygen potential, which maybe used in the production of substitute natural gas from sulfur-containing materials (67). Liquid sodium, potassium, sodium—potassium alloys, or lithium have htfle effect on niobium up to 1000°C, and its resistance to many other Hquid metals is good.  [c.26]

Of the nonmetals, phosphorus, arsenic, siUcon, sulfur, selenium, tellurium, and carbon attack some or all of the metals at red heat, forming alloys or low melting point phases. Rhodium and iridium are resistant to fused lead oxide, siUcates, molten copper, and iron at temperatures up to 1500°C. The PGMs are unaffected by most organic compounds, although the catalytic reactions that can occur on the surface of platinum and palladium result in an etched appearance of these metals. Palladium is much less resistant to chemical attack, especially by strong oxidising acids at elevated temperatures (see Table 3). Palladium is stable in air, even at elevated temperatures, and shows no corrosion or tarnishing in hydrogen sulfide atmospheres. However, it has been reported that some discoloration owing to sulfide film formation may take place in industrial atmospheres containing sulfur dioxide (3).  [c.164]

A further cause of depassivation is a reduction in the alkalinity of the concrete as in Fig. 2-2 (i.e., a reduction in the pH of the absorbed water). This occurs with carbonizing of the concrete by reaction with COj in the atmosphere. In structures with a sufficiently thick concrete covering over the steel inserts, especially with dense, low-porosity concrete of good quality, carbonization is unimportant. With poorer quality concrete and/or with too little concrete covering, carbonization penetrates to the steel reinforcement, which then loses its passivity. With depassivation due to chloride ions or carbonization, there is a danger of corrosion in damp concrete only if there is access to oxygen. If the concrete is thoroughly soaked on all sides, access is severely restricted, so that the cathodic partial reaction according to Eq. (2-17) cannot take place at any point on the reinforcing steel (see Section 5.3.2). Then, however, the anodic reaction according to Eq. (2-8) also cannot occur, i.e., the depassivated steel will not corrode.  [c.428]

Ruthenium and Os are stable to atmospheric attack though if Os is very finely divided it gives off the characteristic smell of OSO4. By contrast, iron is subject to corrosion in the form of rusting which, because of its great economic importance, has received much attention (see Panel above).  [c.1076]

The main febricated parts of the units are carbon steel, with suitable corrosion allowance for the conditions of the chilled and condensing water. When brackish or sea water is used in a barometric condenser, steel construction with a V4 -in. to -in. corrosion allowance is suggested, and minimum wall plates of V2 -in. to -in. may be justified. Internal splash plates should be V2 -in. to -in. minimum, because the atmosphere of water vapor-air is very corrosive. Alloy construction is not justified except in exceptional cases.  [c.291]

The main factor in causing filiform corrosion is the relative humidity of the atmosphere, and if this is below 65% (the critical relative humidity for the atmospheric corrosion of most metals, see Section 2.2) it will not occur. As the relative humidity increases the thickness of the filaments increases at 65-80% relative humidity they are very thin, at 80-95% relative humidity they are much wider and at approximately 95% relative humidity they broaden sufficiehtly to form blisters.  [c.170]

See pages that mention the term Corrosion atmospheric. See Atmospheric : [c.231]    [c.130]    [c.62]    [c.147]   
Corrosion, Volume 2 (2000) -- [ c.0 ]