Initiated corrosion

The crevice geometry and normally occurring corrosion combine to produce accelerated attack in the shielded region, a so-called autocat-alytic process. Initially, corrosion in oxygenated water of near neutral pH occurs by Reactions 2.1 and 2.2 ... 

The corrosion-product layer that forms due to oxygen corrosion is discussed in Chap. 3, Tuherculation. However, the initial corrosion product is ferrous hydroxide [Fe (OH)2l (Reaction 5.4) ... 

As corrosion proceeds, reaction by-products may form on the metal surfaces, creating a resistance to electrical exchanges at these surfaces. Consequently, the reaction rate diminishes correspondingly. If the corrosion reaction is stopped, a time-dependent recovery occurs if the reaction is restarted, the initial corrosion rate is reestablished. This effect is often observed in conventional dry cells and automobile batteries. 

Ancient iron structures sometimes show no sign of corrosion or at most, very little. The clean atmosphere of past centuries may be responsible in that it allowed a very thin adherent layer of oxide to develop on the surface [22], This layer very often protects against even today s increasingly aggressive industrial pollutants Very often the conditions of the initial corrosion are the ones that determine the lifespan of metals [23], A well-known example is the sacred pillar of Kutub in Delhi, which was hand forged from large iron blooms in 410 a.d. In the pure dry air, the pillar remains free of rust traces but shows pitting corrosion of the iron... 

Air vents are most effective when they are fitted at the end of a length of 300 mm or 450 mm of uninsulated pipe that can act as a collecting/cooling leg. Air is an excellent insulating material, having a thermal conductivity about 2200 times less than that of iron. The last place where it can be allowed to collect is in the steam space of heat exchangers. Further, as it contains oxygen or carbon dioxide, which dissolve readily in any subcooled condensate that may be present, the presence of air initiates corrosion of the plant and the condensate return system. 

Greenblatt, J. H., Initial Corrosion of A1 Alloys in High Temperature Water , Corrosion, 19, 295t(1963)... 

Weather conditions at the time of initial exposure of zinc and steel have a large influence on the protective nature of the initial corrosion products This can still be detected some months after initial exposure. Finally, rust on steel contains a proportion of ferrous sulphate which increases with increase in SO2 pollution of the atmosphere. The effect of this on corrosion rate is so strong that mild steel transferred from an industrial atmosphere to a rural one corrodes for some months as though it was still exposed to the industrial environment. ... 

The above catalogue of difficulties, in relating the aggressivity of natural waters to their chemical composition, arises precisely because of the low corrosion rates that are usually found with most metals. Under such circumstances, water composition is only one of many factors that determine the rate of attack. The other factors include flow regime, temperature and the conditions under which the initial corrosion product is laid down. The best summary of the behaviour of metals commonly used in natural waters is still that produced by Campbell for the Society of Water Treatment and Examination... 

Vaporous cavitation can remove protective films, such as oxides, from metals and so initiate corrosion . In addition, the very high local pressures and temperatures associated with the final stage of cavity collapse can induce chemical reactions that would not normally occur. Thus certain additives are damaged by cavitation and their decomposition products can be corrosive. 

Dust particles also play an important role (Fig. 3.1). They act as nuclei for the initial corrosion attack and as some particles are hygroscopic their presence tends to increase the periods of wetness of the steel surface. 

Before the silica film can form, some corrosion of the metal must necessarily take place, and it follows that initial corrosion rates are high. Fig. 3.62 illustrates this point and suggests that uniform rates of corrosion are not reached until at least 100 h after the onset of the attack. As a result, useful data on the corrosion of high silicon irons can be obtained only from tests of at least this duration. 

Attempts made to produce an alloy more resistant to hydrochloric acid have resulted in alloys containing 17-18% silicon or 14-5% silicon and chromium plus 3% molybdenum. The first is produced in Britain, and the second in the United States. The reason for the increase in resistance to hydrochloric acid of the Fe-18 Si alloy is thought to lie primarily in the increased density of the silica-rich film left on the metal by initial corrosion. The addition of 6% chromium with some molybdenum to Fe-14-5 Si causes the formation of extremely stable complex carbides with the consequent complete elimination of graphite plus the formation of a more penetration-resistant silica film, probably containing chromium in substantial quantity. 

In dry air the stability of zinc is remarkable. Once the protective layer of zinc oxide formed initially is complete, the attack ceases. Even under under normal urban conditions, such as those in London, zinc sheet 0 -8 mm thick has been found to have an effective life of 40 years or more when used as a roof covering and no repair has been needed except for mechanical damage. The presence of water does, of course, increase the rate of corrosion when water is present the initial corrosion product is zinc hydroxide, which is then converted by the action of carbon dioxide to a basic zinc carbonate, probably of composition similar to ZnCOj 3Zn(OH)2 . In very damp conditions unprotected zinc sometimes forms a loose and more conspicuous form of corrosion product known as wet storage stain or white rust (see p. 4.171). 

The atmospheric corrosion data in Table 4.34 (and also Table 13.8) is related to historic environments. Current use in the industrial areas listed with acidic pollution would show much lower corrosion rates as the corrosion of zinc in the atmosphere is essentially related to the SOj content (and the time of wetness) and in many countries the sulphurous pollution has been greatly reduced in the past 20 years. Zinc also benefits from rainwater washing to remove corrosive poultices thus, although initial corrosion rates are usually not very different on upper and lower surfaces, the latter tend —with time—to become encrusted with corrosion products and deposits and these are not always protective. 

In rusting, the initial corrosion product of iron is ferrous hydroxide. Reacting with oxygen and water, it forms higher oxides, mainly hydrated ferric oxide and magnetite. Rust formed in industrial or marine environments contains corrosion-promoting salts and is particularly dangerous. Rust is not considered a satisfactory base over which to paint and it too must be removed. 

Both iron- and copper-based alloys are corroded more easily on either side of the neutral pH band. In low pH conditions e.g. due to carbon dioxide, the acidic environments attack the alloys readily, causing damage both at the points of initial corrosion and perhaps, consequentially, further along the system, by screening the surface with corrosion products and permitting the development of differential aeration cells. 

It is therefore not surprising to find that where boiler systems operate under these constraints, they suffer rapid and extensive oxygen-initiated corrosion to the internal surfaces of the FW system, the feed lines, the boiler shell, and all internal heating surfaces. With poor oxygen scavenging, heavy pitting corrosion and tuberculation is found, especially on the tubes and at the waterline of the FW tank and boiler shell. 

Passivation is a necessary and natural initial corrosion process that occurs on all hot waterside surfaces. It is the conversion of a reactive metal surface into a lower energy state that does not readily further react or corrode, and it involves the development of a passive oxide film on a clean surface. 

The mechanism for the initial corrosion of steel in neutral or alkaline solutions is generally accepted as the oxidation of iron from the metallic state to the ferrous ion ... 

Single-stage and multistage desalting processes are utilized to remove as much of the salt as possible. Any remaining salts which carry into the distillation unit could initiate corrosion. 

Corrosion inhibitor Two inhibitors could be used a. Copper corrosion inhibitor to help prevent sulfur, hydrogen sulfide, and mercaptan attack on copper b. Ferrous metal corrosion inhibitor to prevent water/oxygen initiated corrosion of iron and steel system components... 

Polar organic compounds present in fuel can provide a protective, film-like layer on iron and steel surfaces. This film helps prevent water from reaching the metal surface to initiate corrosion in fuel storage and transportation equipment. 

Ultra-low sulfur fuel has little ability to prevent water-initiated corrosion of metal since most of the naturally occurring, film-forming inhibitors have been removed by hydroprocessing. Water-initiated corrosion can occur readily in ultra-low sulfur fuel systems. The addition of a traditional film-forming corrosion inhibitor will provide good protection against water-initiated corrosion. 

In fuel systems containing water, certain microorganisms can survive. Typical species are the bacterial anaerobe Disulfovibrio and the fungal species Cladosporium resinae. These microorganisms are called hydrocarbon utilizing microbes and often referred to as HUM or HUM-bugs. They can initiate corrosion in fuel systems. 

The term varnish is used to describe the hard, amber-colored coating of fuel oxidation products adhering to engine components. The term sludge is used to describe the heavy, dark-colored deposits which settle from solution out of the fuel. Sludge can accumulate in areas of low turbulence and act as a prime site for initiating corrosion. 

In fuel combustion systems, S02 and S03 can form upon the burning of fuel sulfur. When sulfur oxides combine with water vapor, acids form. This problem of acid formation and accumulation is a known phenomena and usually occurs under low-speed and load operating conditions. The acids which condense on fuel system components can initiate corrosion of valves, piston rings, and fuel injector nozzles. 

Sulfur readily attacks copper and its alloys, bronze and brass, to form copper sulfide, CuS. Elemental sulfur, mercaptan sulfur, or hydrogen sulfide can all attack copper bearing parts. Fuel storage tanks and piping systems can contain copper heating coils, and cooling coils, as well as brass or bronze valves and fittings. These parts are all susceptible to sulfur-initiated corrosion. 

Together, water and SO can combine to form sulfur-bearing acids. These acids can accumulate to initiate corrosive wear, oxidation of lubricating oil, and the formation of piston lacquer deposits within the combustion chamber. Engine deposits can result in operability problems such as preignition knock, dieseling, and wear. 

Most fuel system corrosion inhibitors function by strongly adsorbing onto the exposed surfaces of metal to form a protective filmlike layer. This inhibitor layer acts as a barrier to prevent water from contacting the surface of metal system components and initiating corrosion. 

Control acid-initiated corrosion and erosion of fuel injector nozzle orifices... 

Severe hydroprocessing required to refine ultra-low sulfur fuel grades generates hydrogen sulfide as a by-product. If low levels of hydrogen sulfide remain soluble in the finished fuel, the possibility exists for hydrogen sulfide-initiated corrosion of copper and the resulting failure of the ASTM D-130 specification for finished fuel. 

Water contamination of fuel occurs. Water can originate from fuel processing, atmospheric condensation, or external sources. Water may contain dissolved salts, may be acidic or basic, or may contain solubilized organic compounds. Water-initiated corrosion can result. 

Sulfuric acid and hydrofluoric acid are used as catalysts in the production of gasoline alkylate. After processing, this acid must be removed from the finished alkylate. This is typically accomplished by water washing or caustic washing the alkylate. However, if residual sulfuric acid or hydrofluoric acid remains in the fuel or alkylate, the acid can initiate corrosion. The acid is very aggressive toward initiating corrosion of ferrous metal. It is difficult for filming-type corrosion inhibitors to overcome acid attack of metal. 

Aluminum is a soft, ductile, and relatively inexpensive metal. The surface of aluminum readily oxidizes in the air and water to form a highly resistant oxide film. This oxide film serves to make aluminum resistant to attack when used in environments containing sulfides, sulfur dioxide, carbon dioxide, and other corrosive gases. It is highly resistant to water-initiated corrosion, but is susceptible to galvanic corrosion by trace amounts of copper, tin, lead, nickel, or carbon steel. The reaction of aluminum in water to form Bayerite is shown in FIGURE 9-2. 

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