Atmospheric corrosion water

Nickel is a silvery white and lustrous metal. It is resistant to atmospheric corrosion, water, alkali, and many organic compounds, but is slowly soluble in diluted nonoxidizing mineral acids and quite rapidly in oxidizing acids such as HNO3. It has ferromagnetic properties and is both malleable and ductile. 

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). 

With some important exceptions, gray-iron castings generally have corrosion resistance similar to that of carbon steel. They do resist atmospheric corrosion as well as attack by natural or neutral waters and neutral soils. However, dilute acids and acid-salt solutions will attack this material. 

Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, fresh water, and organic exposures). 

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. 

Corrosive species in the atmospheres include water, salts and gases. Clean atmospheres contain little other than oxygen, nitrogen, water vapor and a small quantity of carbon dioxide. These species are virtually non-corrosive to any of the common constructional materials for plant at normal temperatures. Steel is susceptible to corrosion in even fairly clean air where water can exist as liquid. For plant operating at temperatures up to approximately 100°C coatings are employed to protect steel if required. In clean air corrosion rates are low, and corrosion is primarily a cosmetic problem, although it may be necessary to prevent mst staining of nearby materials. 

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). 

The composition given in Table 2.8 is global and, for most components, is reasonably constant for all locations, but the water vapour content will obviously vary according to the climatic region, season of the year, time of the day, etc. However, only oxygen, carbon dioxide and water vapour need to be considered in the context of atmospheric corrosion. 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Sulphates and chlorides are present in industrial and marine atmospheres. In water they accelerate the corrosion of steel. Avoiding lodgement areas for water and dirt reduces the risk of the latter acting as a poultice in which the corrosive salts can build up. 

The unfortunate action of the compound layer is observed only rarely, usually in hot water. In cooking vessels (domestic or industrial) the copper is protected satisfactorily at some sacrifice of tin, and occasional re-tinning ensures long service. In atmospheric corrosion the arrival of compounds at the surface of the coating results in some darkening and in loss of solderability. 

The testing of vapour phase inhibitors, usually referred to as volatile corrosion inhibitors, is essentially a matter of placing a test specimen in the vapour space of a closed vessel containing an aggressive atmosphere — frequently water vapour, perhaps with SO2 present—and a quantity of the inhibitor. Variations on the basic technique include provision for circulation of the vapour, the use of paper impregnated with inhibitor, provision for temperature cycling, etc. 

Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to ISO-9223 standard, TOW is approximately the time when relative humidity exceeds 80% and temperature is higher than 0°C. No upper limit for temperature is established. In tropical climates, when temperature reaches values over 25°C, evaporation of water plays an important role and the possibility to establish an upper limit respecting temperature should be analyzed. The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

Results of outdoor and indoor corrosion rate and corrosion aggressivity in tropical corrosion test stations of Cuba and Mexico are reported. The results mainly concern to natural atmospheric corrosion tests obtained in the western side of the Isle of Cuba and in the Campeche State located at the Yucatan Peninsula in Mexico. The two regions are located in the tropical climate and receive the influences of the waters of the Atlantic Ocean and the Mexican Gulf. Data processed in this paper correspond to atmospheric corrosion tests carried out during a long period of time, about the last 20 years in Cuba and the last 10 years at Campeche up to the present. 

The presence of water does not only create conditions for the existence of an electrolyte, but it acts as a solvent for the dissolution of contaminants [10], Oxygen plays an important role as oxidant element in the atmospheric corrosion process. The thickness of the water layer determines the oxygen diffusion toward the metallic surface and also the diffusion of the reaction products to the outside interface limited by the atmosphere. Another aspect of ISO definition is that a metallic surface is covered by adsorptive and/or liquid films of electrolyte . According to new results, the presence of adsorptive or liquid films of electrolyte perhaps could be not in the entire metallic surface, but in places where there is formed a central anodic drop due to the existence of hygroscopic particles or substances surrounded by microdrops where the cathodic process takes place. This phenomenon is particularly possible in indoor conditions [15-18],... 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

Under indoor conditions, in the same way than outdoors, it is necessary the presence of surface humidity for corrosion to occur due to the electrochemical nature of the atmospheric corrosion process however, in indoor conditions there are no precipitations and the presence of surface water depends mainly on water content in the air and changes in temperature on the surface, as well as the presence of hygroscopic substances on the metallic surface. 

Recent reports about the microdroplets formation in the starting periods of atmospheric corrosion [15-18] show that the idea of a thin uniform water layers is not completely in accordance with the reality. It has been observed that when a water drop is on the metallic surface, formed in the place where a salt deposit existed before, microdroplets are formed around this central drop. The cathodic process takes place in these surrounding microdroplets, meanwhile the anodic process takes place in the central drop. This idea is not consistent with the proposal of an uniform water layer on the surface and it is very probable that this situation could be obtained under indoor conditions. It has been determined that microdrops (about 1 micron diameter) clusters are formed around a central drop. An important influence of air relative humidity is reported on microdrops formation. There is a critical value of relative humidity for the formation of microdroplets. Under this value no microdroplets are formed. This value could be considered as the critical relative humidity. This situation is very similar to the process of indoor atmospheric corrosion presence of humid air, deposition of hygroscopic contaminants in the surface, formation of microdrops. Water is necessary for corrosion reaction to occur, but the reaction rate depends on the deposition rate and nature of contaminants. 

The adsorption isotherms for metallic surfaces are reported in the literature however, an important part of the atmospheric corrosion process takes place under rust layers, which play a decisive role in the long-term course of corrosion because of its sorption capacity for water. The influence of the chloride and sulfate anions has a real effect only when the corrosion products layer is already formed. Thus, the adsorption isotherms of the steel corrosion products formed in different atmospheres were determined. 

The superficial characteristics of atmospheric corrosion products of steel depend on the type of atmosphere where the sample has been exposed. The way of adsorption of the corrosion products obtained in the coastal atmosphere is polymolecular due to a higher content of salts. This makes easier the presence of water in the metal-corrosion products interface and determines a high corrosion rate. The adsorption of water of a corrosion product formed in a rural zone obeys a Langmuir isotherm, i.e. a monomolecular adsorption takes place. It causes a lower corrosion rate. 

It has no sense to calculate TOW-ISO for coastal tropical atmospheres, because in those conditions corrosion process occurs at relative humidity lower than 80%. It has been determined that water adsorption by corrosion products is polymolecular in these conditions. As analogy, in highly polluted atmospheres, corrosion process should proceed at RH lower than 80%, so it has no sense to use TOW-ISO. 

The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

Dust (especially from industrial activities) and salt spray will also exacerbate atmospheric corrosion (Section 16.4). In enclosed industrial premises, atmospheric corrosion could be minimized by preventing noxious emissions, filtering the air to remove particulate matter, and scrubbing the air with water to remove SO2 and other objectionable gases, although the humidity should itself be kept as low as possible (e.g., steam leaks should not be tolerated). On the global scale, however, the cost to the public of atmospheric corrosion could be substantially reduced by sharply limiting SO2 and, to a lesser extent, NO. emissions from power plants, smelters, automobiles, and other industrial functions. This is an aspect of the acid rain threat (Chapter 8) that is usually overlooked. 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

Water is also used with oil and other additives in the form of an emulsion as a coolant and lubricant in cutting processes. These fluids must be nonflammable, as a spark from the cutting tool or electrical equipment would cause a fire, and they should not cause irritation to the skin or produce harmful vapours. Furthermore, for purposes of protection from corrosion, they should not cause corrosion of the cutting equipment or the material being cut and should give short-term protection from atmospheric corrosion. 

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