Reduced corrosion rates

Boron, in the form of boric acid, is used in the PWR primary system water to compensate for fuel consumption and to control reactor power (3). The concentration is varied over the fuel cycle. Small amounts of the isotope lithium-7 are added in the form of lithium hydroxide to increase pH and to reduce corrosion rates of primary system materials (4). Primary-side corrosion problems are much less than those encountered on the secondary side of the steam generators. 

Nickel, while reducing corrosion rates a little, is not as important in its effect as the aforementioned elements. 

Most of the published evidence suggests that marine fouling cover— particularly where it is continuous and well established — reduces corrosion rates of steels . Indeed, 35%o seawater is by no means the most corrosive of saline environments towards steel. Brackish water, as found in estuarine or certain other coastal areas, is considerably more aggressive towards steel, and careful design measures should be taken to ensure that effective corrosion control is achieved in such circumstances. 

Anodic polarization also may occur. Typically, this begins with the formation of a thin, impervious oxide film, chemisorbed at the anode (as on the surface of stainless steels). However, for most metals used in boiler plant systems this chemisorption process must be aided by anodic corrosion inhibitors to reduce corrosion rates to tolerable levels. An example is the application of nitrite-based inhibitors, widely used in HW heating systems. 

The kinetics of the decay rates of both free and combined bromine is more favorable for bromine. Thus, bromine tends to be more environmentally acceptable. This effect can also help reduce corrosion rates. 

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

Forming the passive layer for an active metal and that can reduce corrosion rate. An increase in the flow speed permits to supply oxygen at the interface and attain the critical passive current for alloy in a given medium. 

Chemical additives are often added to the circulating cooling water to reduce corrosion rates, reduce scaling and fouling, and prevent growth of bacteria and algae. Plants often contract with specialty service companies to provide these additives and supervise their use. 

Generally these compositions contain an epoxy-novolac, a hardener, a catalyst, silica fillers, and an internal lubricant/mold release compound. Brom-inated epoxies and antimony trioxide are included to provide the required flame retardant characteristics. Other, unspecified additives are used to promote adhesion or to reduce corrosion rates. Because of their superior thermal capabilities and electrical properties, epoxidized novolacs are preferred over epoxy homopolymers. Near stoichiometric amounts of hardeners such as novolacs (Equation 1), anhydrides, and primary amines can be used to cure the resins in the presence of a catalyst. The linkages which are formed include ethers, esters, or secondary amines, respectively. 

In contrast to the above, precipitates that adhere to the metal surface as continuous, nonporous films greatly reduce corrosion rates because the controlling mechanism becomes the slow solid-state diffusion of ions through the films. Further, if the film is a poor conductor of electrons, then the oxidation (corrosion) reaction is retarded because electrons have difficulty reaching the solution interface to enter into the cathodic reaction. 

There is no commercially available controller on the market, but development costs for this relatively simple system are estimated to be 2,000 (easily justified by chemical additive savings and/or reduced corrosion rates). 

The resistivity of concrete is an important parameter used to describe, for example, the degree of water saturation, the resistance to chloride penetration or the corrosion rate. The resistivity of concrete may have values from a few tens to many thousands of n m (Table 2.3) as a function of the water content in the concrete (relative humidity), the type of cement used (Portland or blended cements), the iv/c, the presence of chloride ions or whether the concrete is carbonated or not At early ages, the resistivity of concrete is low and considerable increases occur due to hydration of the cement AU of these factors can be rationalised on the basis of ion migration in the porous and tortuous concrete microstructure a high relative humidity increases the amount of water-filled pores (decrease of resistivity), the iv/c ratio and type of cement determine the pore volume and pore-size distribution (less but more coarse pores with pure Portland cement more but finer pores and less interconnectivity of pores with blast furnace slag or fly ash) chloride ions increase the conductivity of the pore solution and carbonation decreases it. An increased resistivity is accompanied by a reduced corrosion rate [38]. Table 2.4 shows resistivities determined for mature concrete in various chmates [39-41]. 

The environment may be changed in the following ways in order to reduce corrosion rates ... 

By bacterial manipulation, what is meant is that certain bacteria are used to slow down corrosion. For example, it has been reported " that carbon steel corrosion in presence of the biofihn of aerobic bacterium Pseudomonas fragi or the facultative anaerobe Escherichia coli had been slowed down by 2-fold to 10-fold. Another example is a biofilm-producing baclerinm, identified as Bacillus sp., that was able to reduce corrosion rates of mild steel by establishment of antimicrobial agent-producing biofllm, which inhibited the activity of SRB. 

From the foregoing, the most corrosive concentration of sodium chloride occurs when the chlorides reach approximately 20,000 mg/L, and above this point, an increase in salinity may reduce corrosion rates due to lower oxygen solubilities (KMC Oiltools 2006). 

The available field data there is often poor, with no clear evidence of the amount of inhibitor applied, whether it reached the rebar and if it is reducing corrosion rates and extending time to cracking. 

Sheltering from direct rainfall may reduce corrosion rates, as is shown in many of these tests and in Fig. 2.5. 

The addition of nitric acid to hydrochloric or sulfuric acid significantly reduces corrosion rates. Titanium is essentially immune to corrosion by aqua regia (3 HCl 1 HNO,) at room temperature. ASTM grades 7 and 12 show respectable corrosion rates in boiling aqua regia. Corrosion rates in mixed acids will generally rise with increases in the reducing acid component concentration or temperature. 

In an attempt to reduce corrosion rates, additional elements have been included in antiscale formulations. 

To clean up amine, the rate of particulate generation must be slowed and the iron sulfide already in the amine filtered out. Reducing corrosion rates is the way to do this. 

HOBr and HOCl are equal in microbicidal activity. However, since HOBr dissociates at a higher pH range than HOCl, HOBr is often used in place of HOCl in water systems operated at pH values >7, e.g. pH 8.5. Another benefit of the application of HOBr instead of HOCl are reduced corrosion rates. In the presence of amino groups containing substances HOBr forms (as does HOCl) N-bromo-amines or amides which are more effective than corresponding chlorine releasing chloramines. A disadvantage of HOBr is that it is very susceptible to oxidant demand. 

Cathodic protection methods are useful for designing against corrosion, but theses methods require knowledge of electrochemical polarization. The main objective in protecting a metallic stmcture is to eliminate or reduce corrosion rate by supplying an electron flow to a stmcture to reduce or eliminate metal dissolution (oxidation). This implies that the anodic reactions is suppressed on the surface of the stmcture. This can be accomplished using secondary materials and appropriate instrumentation to supply electrons to the stmcture. 

Criterion This is a criterion that requires known Tafel anodic constant (slope) /3 of the metal to be protected cathodicaUy. According to Jones [138] and the overpotential required to reduce corrosion rate can be graphically determined as shown in Figure 8.4. However, the same results can be mathematically estimated using eq. (3.20a). 

Zinc alloys containing copper, iron, antimony, arsenic, or tin are known to increase the zinc corrosion rate while zinc alloyed with cadmium, aluminum, or lead will reduce corrosion rates. - If enough gas pressure is generated, the sealed button cell could leak or rupture. In commercial use, the high surface area zinc powder is amalgamated with small amounts of mercury (3 to 6%) to bring the corrosion rate within a tolerahle limit. 

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