Corrosion products system

Oxygen is a prime factor in the corrosion of system materials and the release, activation, and redeposition of activated corrosion products. 

S. M. Ah, "An Updated Version of Computer Code CORA II for Estimation of Corrosion Product Mass and Activity Migration ia PWR Primary Circuits and Related Experimental Loops," Eourth International Conference on Water Chemistry of Nuclear Systems, Bournemouth, U.K., Oct. 1986, pp. 107-109. 

Precipitate formation can occur upon contact of iajection water ions and counterions ia formation fluids. Soflds initially preseat ia the iajectioa fluid, bacterial corrosioa products, and corrosion products from metal surfaces ia the iajectioa system can all reduce near-weUbore permeability. Injectivity may also be reduced by bacterial slime that can grow on polymer deposits left ia the wellbore and adjacent rock. Strong oxidising agents such as hydrogen peroxide, sodium perborate, and occasionally sodium hypochlorite can be used to remove these bacterial deposits (16—18). 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

When antifreeze becomes unsuitable for use, either because of depletion of inhibitors, presence of corrosion products or corrosive ions, or degradation of the fluid, recycling and reuse of the antifreeze, rather than disposal, may be considered. Although ethylene glycol is readily biodegraded in typical municipal waste treatment faciHties, antifreeze disposal becomes problematic because the coolant may contain hazardous quantities of heavy metals picked up from the cooling system. Recycling may be economically preferred over coolant disposal and reduces the concern for environmental impact. 

Ideally, a system for recycling spent antifreeze consists first of the removal of the deleterious contaminants such as the corrosion products, corrosive ions, degradation products, and remaining inhibitors. Then the clean fluid could be reinhibited to a known concentration of both inhibitors and glycol. 

Attack always occurs beneath a deposit. Cooling water system deposits are ubiquitous. Deposits can be generated internally as precipitates, laid down as transported corrosion products, or brought into the system from external sources. Hence, underdeposit corrosion can be found in virtually any cooling water system at any location. Especially troubled... 

Figure 8.3 General wastage of an aluminum water manifold from a diesel engine cooling system. Note the generally wasted internal surface due to concentrated caustic and the presence of white deposits and corrosion products.

Metal surfaces in a well-designed, well-operated cooling water system will establish an equilibrium with the environment by forming a coating of protective corrosion product. This covering effectively isolates the metal from the environment, thereby stifling additional corrosion. Any mechanical, chemical, or chemical and mechanical condition that affects the ability of the metal to form and maintain this protective coating can lead to metal deterioration. Erosion-corrosion is a classic example of a chemical and mechanical condition of this type. A typical sequence of events is ... 

Several of the welded junctions were removed from the system for metallographic examination (Fig. 15.20). As can be seen from Fig. 15.20, the internal surface was covered with reddish and tan deposits and corrosion products. The metal surface itself retained a bright, metallic luster. 

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

A large number of parameters are involved in the choice of the corrosion protection system and the provision of the proteetion eurrent these are deseribed elsewhere (see Chapters 6 and 17). In partieular, for new locations of fixed production platforms, a knowledge of, for example, water temperature, oxygen content, conductivity, flow rate, chemical composition, biological activity, and abrasion by sand is useful. Measurements must be carried out at the sea location over a long period, so that an increased margin of safety can be calculated. 

Scaling—the formation of thick corrosion products as layers on a metal surface in piping systems it is usually the deposition of water-insoluble constituents on a metal surface. 

While a desalter costs more to install than the flash drum system, it has the advantage of removing up to 95% of the salt from the oil permanently. Because less salt reaches the fractionating tower in a unit equipped with a desalter, a smaller quantity of corrosion products is formed due to high temperature breakdown and hydrolysis, and the salt content of the residual fuel oil is much lower. 

Continuous releases of concentrated HjS streams must be segregated in a separate flare system to limit the extent of fouling and plugging problems. Releases of HjS such as diversion of sour gas product to flares during shutdown or upset of a downstream sulfur recovery unit are considered to be continuous, but safety valve releases are not included in this category. However, if a special HjS flare system is provided for continuous releases, the concentrated HjS safety valve releases should be tied into it rather than into the regular flare system. Due to the nature of HjS one should plan on frequent inspection and flushing of HjS flares to remove scale and corrosion products. 

Figure 4-469 shows the effect on corrosion rates of 1020 steel in different water systems with dissolved hydrogen sulfide. The difference in corrosion rates is due to different corrosion products formed in different solutions. In solution I, kansite forms. Kansite is widely protective as the pyrrhotite coats the surface giving slightly more protection until a very protective pyrite scale is formed. In solution II, only kansite scale forms, resulting in continued increase in the corrosion rate. Finally, in solution 111, pyrite scale is formed as in solution I however, continued corrosion may be due to the presence of carbon dioxide. 

The limitation of these instruments is that they only indicate overall corrosion rate. Their sensitivity is affected by deposition of corrosion products, mineral scales or accumulation of hydrocarbons. Corrosivity of a system can be measured only if the continuous component of the system is an electrolyte. 

By substituting the appropriate values for viscosity and diffusion at various temperatures, they found that corrosion rates could be calculated which were confirmed by experiment. The corrosion rates represent maxima, and in real systems, corrosion products, scale and fouling would reduce these values often by 50%. The equation was useful in predicting the worst effects of changing the flow and temperature. The method assumes that the corrosion rate is the same as the limiting diffusion of oxygen at least initially this seems correct. 

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