Zairyo-to-Kankyo, 52, 408-415 (2003) 論文 Proposal of a New Stability Index Taking Account of Corrosion Inhibiting Action of Silica and Derivation of Empirical Equations Containing the New Index for Corrosiveness and Scale Formation of Water Makoto Yuasa*,**, Toshiharu Wake***, Takashi Fujieda*, Nobuyuki Momozawa* ,**, Aritomo Yamaguchi*, Yoshiaki Shibata***, Shintaro Someya***, and Akira Takahashi**** Facultyof Science and Technology, Tokyo University of Science **Institute of Colloid and InterfaceScience , Tokyo University of Scienc ***Japan Organo , Corp. A new stability index [an improved stability index (SI')] has been proposed that introduces corrosion inhibiting ability of silica in a broad range of temperature and pH into the conventional index of the corrosive action and scale formation of water, and new empirical equations [Eqs. 1-3] that characterize water were derived using the new index. The derived SI' was effective in the range of SI>6. The SI' was balanced at 0, and then SI'=0 differed from SI=6. In addition, a possibility was found for prediction and inhibition of corrosion of ferrous metals based on the equations by controlling the concentration of silica ([SiO2]). y=K0{1-exp(-3.06•~10-1SI')} (1) K0=(2.09T+1.872•~102)exp{-(-1.08•~10-4T+7.62•~10-3)[SiO2]} (2) SI'=x-7.21{1-1.71•~10-1exp(-8.59•~10-3[SiO2])} (3) [y: corrosion rate (V) (mdd), K0: constant affecting saturation value of corrosion rate (y) (mdd), T: temperature (•Ž), [SiO2]: silica concentration (mg SiO2/dm3), x: stability index (SI) (-)] Key words: stability index, empirical equations, corrosion inhibition, mild steel, water quality factor, silica 1. Introduction Corrosion and scaling of ferrous plant materials caused by water have been problems in water conduction plants and water treatment systems including cooling water, waste water treatment, industrial water treatment, and water demineralizing systems. For instance, we encounter various problems including (1) corrosion of pipes due to corrosive factors such as dissolved oxygen and chloride ions caused by city or industrial water as circulating water and (2) scaling problems such as blocking of pipes and reduction in the thermal conduction of heat exchangers produced by concentration of dissolved substances and for- mation of insoluble salts due to evaporation of part of circu- lating water1)-7) in the most frequently used open cooling water systems ranging from those for small scale air-condi- tioning in buildings to those for large scale cooling in com- plexes. In order to overcome these problems, various anti- corrosion agents have so far been used in open cooling water systems. These agents include (1) chromate and phosphate salts against corrosion and (2) polyphosphate and phosphonate salts against scaling. In addition, they are used widely with a small amount of other agents in water conduction plants and water treatment systems so that the combined agents give additive and synergetic effects1)-9). Nevertheless, the demand for environmental protection in recent years has made the use of these agents unfavorable due to the toxicity of chromate salts and the eutrophication of water by phosphate salts, and hence the development of environment-friendly and less toxic anticorrosive agents and the reduction of added anti- corrosive agent concentration are being required1)-7). Many non-environment-pollutive oligomers and polymers with carboxylic acid groups are examined from these points of view1)-7),10)-28). Moreover, the reduction of added anticorrosive agent concentration is one of the important subjects to be examined and the elucidation of the relation between water quality and metal corrosion in conformity with the present situation is necessary. Calcium and silica have been known as possible anticor- rosive components of water in water conduction plant and water treatment systems including cooling water systems. These components have both advantages as corrosion inhibitor and disadvantages as scale former, and calcium carbonate has among others been regarded as the primary component that causes scale problems, in addition to its anticorrosive action. Then, Langelier index (denoted hereafter as saturation index)*1, Ryznar index [denoted hereafter as stability index (SI)]*1, corrected Ryznar index, and Larson index have been examined as an index of the corrosiveness or scale formability of water1),4),29)-38 Silica has long been used in one-through cooling water sys- tems because it is scarcely toxic and anticorrosive. Then, combined uses of silica with other agents are expected to exhibit a high corrosion inhibiting action at lower concen- trations. Control of silica concentration in water is neces- sary, however, because it deposits as scales. Moreover, *2641 , Yamazaki, Noda, 278-8510 Japan **1-3 , Kagurazaka, Shinjuku, Tokyo, 162-8601 Japan ***4-9 , Kawagishi 1-chome, Toda, 335-0015 Japan
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Zairyo-to-Kankyo, 52, 408-415 (2003)
論 文
Proposal of a New Stability Index Taking Account of
Corrosion Inhibiting Action of Silica and Derivation of
Faculty of Science and Technology, Tokyo University of Science**Institute of Colloid and Interface Science, Tokyo University of Scienc***Japan Organo, Corp.
A new stability index [an improved stability index (SI')] has been proposed that introduces corrosion inhibiting ability of silica in a broad range of temperature and pH into the conventional index of the corrosive action and scale formation of water, and new empirical equations [Eqs. 1-3] that characterize water were derived using the new index. The derived SI' was effective in the range of SI>6. The SI' was balanced at 0, and then SI'=0 differed from SI=6. In addition, a possibility was found for prediction and inhibition of corrosion of ferrous metals based on the equations by controlling the concentration of silica ([SiO2]).
Corrosion and scaling of ferrous plant materials caused
by water have been problems in water conduction plants
and water treatment systems including cooling water,
waste water treatment, industrial water treatment, and water demineralizing systems. For instance, we encounter
various problems including (1) corrosion of pipes due to
corrosive factors such as dissolved oxygen and chloride
ions caused by city or industrial water as circulating water
and (2) scaling problems such as blocking of pipes and reduction in the thermal conduction of heat exchangers
produced by concentration of dissolved substances and for-mation of insoluble salts due to evaporation of part of circu-
lating water1)-7) in the most frequently used open cooling
water systems ranging from those for small scale air-condi-
tioning in buildings to those for large scale cooling in com-
plexes. In order to overcome these problems, various anti-corrosion agents have so far been used in open cooling
water systems. These agents include (1) chromate and
phosphate salts against corrosion and (2) polyphosphate and phosphonate salts against scaling. In addition, they
are used widely with a small amount of other agents in
water conduction plants and water treatment systems so
that the combined agents give additive and synergetic
effects1)-9). Nevertheless, the demand for environmental
protection in recent years has made the use of these agents unfavorable due to the toxicity of chromate salts and the eutrophication of water by phosphate salts, and
hence the development of environment-friendly and less toxic anticorrosive agents and the reduction of added anti-
corrosive agent concentration are being required1)-7).
Many non-environment-pollutive oligomers and polymers with carboxylic acid groups are examined from these
points of view1)-7),10)-28). Moreover, the reduction of added anticorrosive agent concentration is one of the important
subjects to be examined and the elucidation of the relation
between water quality and metal corrosion in conformity with the present situation is necessary.
Calcium and silica have been known as possible anticor-
rosive components of water in water conduction plant and
water treatment systems including cooling water systems. These components have both advantages as corrosion
inhibitor and disadvantages as scale former, and calcium
carbonate has among others been regarded as the primary
component that causes scale problems, in addition to its anticorrosive action. Then, Langelier index (denoted
hereafter as saturation index)*1, Ryznar index [denoted
hereafter as stability index (SI)]*1, corrected Ryznar
index, and Larson index have been examined as an index
of the corrosiveness or scale formability of water1),4),29)-38
Silica has long been used in one-through cooling water sys-
tems because it is scarcely toxic and anticorrosive. Then, combined uses of silica with other agents are expected to
exhibit a high corrosion inhibiting action at lower concen-trations. Control of silica concentration in water is neces-
sary, however, because it deposits as scales. Moreover,
*2641, Yamazaki, Noda, 278-8510 Japan
**1-3, Kagurazaka, Shinjuku, Tokyo, 162-8601 Japan
***4-9, Kawagishi 1-chome, Toda, 335-0015 Japan
Vol.52, No.8 409
neither of the above-mentioned saturation index and stabil-
ity index (SI) takes into consideration the effect of silica.
It is then doubtful if these indexes can cope with the pre-
sent diversified water conditions.
We have investigated from the view point described
above the effects of water quality factors such as concen-
trations of silica, [SiO2] and chloride ion, [Cl-] on the sta-
bility index (SI), an index for the corrosiveness or scale
forming action of water, and the corrosion rate of mild
steel to gain the fundamental knowledge necessary to
construct a highly anticorrosive system with use of a
reduced concentration of anticorrosive agent39),40). We
proposed a new stability index [an improved stability index
(SI')] that combines the corrosion inhibiting action of sili-
ca in a broad range of temperature and pH of water with SI
and derived new empirical formulas (1)-(3) containing
the new index for the properties of water. We also made a
proposal for the way of inhibiting corrosion of ferrous met-
als in water conduction plants and water treatment systems
by controlling the concentration of silica added ([SiO2])
pHs=(9.3+A+B)-(C+D) (6)where pH is measured pH value of solution and pHs is the pH value at which the solution is saturated with CaCO3, that is, the critical pH value at which scales begin to deposit, and is calculated using Eq. 6. A, B, C, and D are respectively the total solid matter coefficient, temperature coefficient, calcium hardness coefficient, and M-alkalinity coefficient (A
and D are obtained from the existing saturation index nomograms). Water quality is judged using the calculated saturation index and stabili-ty index as follows1),4),29)-38(stability index)<6[(saturation index)>0] (scaling tendency)(stability index)=6[(saturation index)=0] (7)(stability index)>6[(saturation index)<0] (corroding tendency)
*2 M-alkalinity is expressed as the mg/dm3 equivalent of acid needed to
neutralize the pH of sample to pH 4.8 or mg/dm3 of CaCO3 correspond-ing to the acid. M-alkalinity gives the total amount of alkaline sub-stances including hydroxides, carbonates, phosphates (2/3 amount), and silicates. Note here that M-alkalinity was altered to the acid con-sumption (pH 4.8) in 1989 JIS amendments41).
410 Zairyo-to-Kankyo
Table 1 Conditions of test soutions.
which indicate that the relationship [SiO2]-[Cl-]-V (three-
dimensional curved surface) shifts upward in the z-axis
direction. Thus, an increase in T caused an increase in V.
The other results were similar to those obtained at 25•Ž.
The above findings demonstrate that SiO2 affects V while
Cl- shows no effect on V at scale forming water quality.
In other words, only SiO2 affects V and the anticorrosive
action of SiO2 alone works in the experimental conditions.
The fact that V is unaffected by SI but depends on [SiO2]
(and T) would be due to missing the effect of anticorrosive
SiO2 on SI (due also to the previous disregard of SiO2
because of sufficient corrosion inhibition by CaCO3 scaling
alone in the experimental conditions where CaCO3 scales
Fig. 1 Relation of corrosion rate (V) of mild steel with water
quality factors as SiO2 and Cl- at various stability index (
SI) values and at 25•Ž using the scale forming water
quality. SI (-): 4.4 (a), 4.8 (b), 5.2 (c), 5.4 (d) and 5.8 (
e).
Fig. 2 Relationship between SI and V at 25•Ž using the scale
forming water quality in test solutions with Cl- {[Cl-]
(mg/dm3): 10 (a) and 500 (b)}. [SiO2] (mg/dm3): 10
(•¡), 50 (•œ), 100 (•£), 200 (•Ÿ) and 400 (•¥).
Fig. 3 Relation of V with water quality factors as SiO2 and Cl- at
various SI values and at 40•Ž using the scale forming
water quality. SI (-): 4.4 (a), 4.8 (b), 5.2 (c), 5.4 (d) and
5.8 (e).
Vol.52, No.8 411
Fig. 4 Relation of V with water quality factors as SiO2 and Cl- at
various SI values and at 55•Ž using the corrosive water
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