Corrosion rate concentration

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. 

The reactor coolant pH is controlled using lithium-7 hydroxide [72255-97-17, LiOH. Reactor coolant pH at 300°C, as a function of boric acid and lithium hydroxide concentrations, is shown in Figure 3 (4). A pure boric acid solution is only slightly more acidic than pure water, 5.6 at 300°C, because of the relatively low ionisation of boric acid at operating primary temperatures (see Boron COMPOUNDS). Thus the presence of lithium hydroxide, which has a much higher ionisation, increases the pH ca 1—2 units above that of pure water at operating temperatures. This leads to a reduction in corrosion rates of system materials (see Hydrogen-ION activity). 

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Fig. 24. Corrosion rates of carbon steel as a function of oxygen concentration at cation conductivity values of ( ...

Carbon steel is not normally a suitable piping material for concentrated sulfuric acid because of high corrosion rates in flowing acid. However, where temperatures and flow rates are low, heavy waU steel pipe is sometimes used for transferring product acid. 

At neutral or higher pH, the concentration of ions is too low for this reaction to contribute significantly to the overall corrosion rate. However, as pH decreases, this reaction becomes more important until, at a pH of about 4, it becomes the predominant cathodic reaction. 

Sa.tura.tion Index. Materials of constmction used in pools are subject to the corrosive effects of water, eg, iron and copper equipment can corrode whereas concrete and plaster can undergo dissolution, ie, etching. The corrosion rate of metallic surfaces has been shown to be a function of the concentrations of Cl ,, dissolved O2, alkalinity, and Ca hardness as well as buffer intensity, time, and the calcium carbonate saturation index (35). 

Zirconium is completely resistant to sulfuric acid up to Foiling temperatures, at concentrations up to 70 wt %, except that the heat-affected zones at welds have lower resistance in >55 wt % concentration acid (Fig. 1). Fluoride ions must be excluded from the sulfuric acid. Cupric, ferric, or nitrate ions significantly increase the corrosion rate of zirconium in 65—75 wt % sulfuric acid. 

Materials of Construction. Glass has excellent corrosion-resistance to wet or dry bromine. Lead is very usefiil for bromine service if water is less than 70 ppm. The bromine corrosion rate increases with concentrations of water and organics. Tantalum and niobium have excellent corrosion-resistance to wet or dry bromine. Nickel has usefiil resistance for dry bromine but is rapidly attacked by wet bromine. The fluoropolymers Kynar, Halar, and Teflon are highly resistant to bromine but are somewhat permeable. The rate depends on temperature, pressure, and stmcture (density) of fluoropolymer (63). 

The concentration dependence of iron corrosion in potassium chloride [7447-40-7] sodium chloride [7647-14-5] and lithium chloride [7447-44-8] solutions is shown in Figure 5 (21). In all three cases there is a maximum in corrosion rate. For NaCl this maximum is at approximately 0.5 Ai (about 3 wt %). Oxygen solubiUty decreases with increasing salt concentration, thus the lower corrosion rate at higher salt concentrations. The initial iacrease in the iron corrosion rate is related to the action of the chloride ion in concert with oxygen. The corrosion rate of iron reaches a maximum at ca 70°C. As for salt concentration, the increased rate of chemical reaction achieved with increased temperature is balanced by a decrease in oxygen solubiUty. 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

Other Effects Stream concentration can have important effects on corrosion rates. Unfortunately, corrosion rates are seldom linear with concentration over wide ranges. In equipment such as distillation columns, reactors, and evaporators, concentration can change continuously, makiug prediction of corrosion rates rather difficult. Concentration is important during plant shutdown presence of moisture that collects during cooling can turn innocuous chemicals into dangerous corrosives. 

The composition of the test solution should be controlled to the billest extent possible and be described as thoroughly and as accurately as possible when the results are reported. Minor constituents should not be overlooked because they often affect corrosion rates. Chemical content should be reported as percentage by weight of the solution. Molarity and normality are also nelpbil in defining the concentration of chemicals in the test solution. The composition of the test solution should be checked by analysis at the end of the test to... 

Carbon steel is easily the most commonly used material in process plants despite its somewhat limited corrosion resistance. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. It is also used routinely for the storage of concentrated sulfuric acid and caustic soda [up to 50 percent and 55°C (I30°F)]. Because of its availability, low cost, and ease of fabrication steel is frequently used in services with corrosion rates of 0.13 to 0.5 mm/y (5 to 20 mils/y), with added thickness (corrosion allowance) to assure the achievement of desired service life. Product quahty requirements must be considered in such cases. 

After increasing coolant corrosion-inhibitor concentration, coupon corrosion rates decreased by almost 70%. 

The corrosion rate is controlled mainly hy cathodic reaction rates. Cathodic Reactions 5.2 and 5.3 are usually much slower than anodic Reaction 5.1. The slower reaction controls the corrosion rate. If water pH is depressed. Reaction 5.3 is favored, speeding attack. If oxygen concentration is high. Reaction 5.2 is aided, also increasing wastage hy a process called depolarization. Depolarization is simply hydrogen-ion removal from solution near the cathode. 

Oxygen corrosion of steel doubles for every 35-55°F (20-30°C) rise in temperature, beginning near room temperature. Corrosion is nearly proportional to temperature up to about 180°F (80°C) in systems open to the air. Although reaction rates increase with temperature, dissolved oxygen is driven from solution as temperatures increase. As temperatures approach boiling, corrosion rates fall to very low values, since dissolved-oxygen concentration also decreases as water temperature rises (Fig. 5.4). 

Above a pH of about 10 near room temperature, surface alkalinity increases and corrosion rates fall sharply. When sodium-hydroxide concentration rises to several percent, the corrosion rate drops to almost zero. Even at very high sodium-hydroxide concentrations, the corrosion rate increases only slightly. 

Differences in alloy carbon concentration, heat treatment, and mechanical forming usually produce only small differences in corrosion rate in a pH range of 4—10. It is less certain how corrosion rates vary at high and low pH due to these factors. Cast irons containing graphite particles may experience a unique form of attack called graphitic corrosion (see Chap. 17, Graphitic Corrosion ). 

Aluminum corrodes at a fairly low rate between a pH of 5.5 and 8.5 at room temperature. At concentrations between 50% and 95%, sulfuric acid causes rapid attack below 10%, corrosion is much less. Hydrochloric acid is quite corrosive in all but dilute concentrations. The corrosion rate in hydrochloric acid increases 100-fold as temperature increases from 50°F (10°C) to 176°F (80°C) in a 10% hydrochloric acid solution. 

Aluminum is resistant to nitric acid at concentrations above 80%. At 50% nitric acid concentration at room temperature, corrosion rates are as high as 0.08 in. (0.20 cm) per year. 

Hence, copper heat exchanger tubes handling acetic acid can he more seriously corroded at low temperatures than at high temperatures. Sulfuric acid at room temperature is handled routinely in carbon steel drums and tanks when water concentration is low, but it becomes extremely corrosive as water concentration increases. As ferric-ion concentration increases during acid cleaning of industrial systems, the corrosion rate of steel increases rapidly. 

Copper-alloy corrosion behavior depends on the alloying elements added. Alloying copper with zinc increases corrosion rates in caustic solutions whereas nickel additions decrease corrosion rates. Silicon bronzes containing between 95% and 98% copper have corrosion rates as low as 2 mil/y (0.051 mm/y) at 140°F (60°C) in 30% caustic solutions. Figure 8.2 shows the corrosion rate in a 50% caustic soda evaporator as a function of nickel content. As is obvious, the corrosion rate falls to even lower values as nickel concentration increases. Caustic solutions attack zinc brasses at rates of 2 to 20 mil/y (0.051 to 0.51 mm/y). 

Zinc brasses are corroded much more rapidly by ammonium hydroxide than by caustic solutions. Corrosion rates approaching 240 mil/y (6.1 mm/y) have been measured at room temperature in two normal ammonium-hydroxide solutions. Corrosion rates in hot, concentrated caustic solutions may be as high as 70 mil/y (1.8 mm/y). 

Copper alloys are attacked hy ammonium hydroxide, but corrosion rates increase only moderately with caustic concentration. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

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