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The effect of molybdenum on the corrosion behaviour of some steel alloys
199
Desalination, 95 (1994) 199-209 Elsevier Science B.V., Amsterdam - Printed in The Netherlands
The effect of molybdenum on the corrosion behaviour of some steel alloys ML Hazza and M.E. El-Dahshan Department of Chemical Engineering, King Saud University, PO Box 800, Riyadh 11421 (Saudi Arabia) (Received July 20, 1993; in revised form October 10, 1993)
SUMMARY
There has been a recent increase in interest in MO-containing steels as potential materials in the construction of desalination plants. The general conclusions reached from these studies is that the alloys are superior in their corrosion behaviour to other stainless steels and are more resistant to pitting attack. A number of formulas have been given to express the pitting resistance of the steel alloys. All of the formulas consider molybdenum to be the “magic” element in improving the pitting and crevice corrosion resistance. This paper reports on the effect of adding 2-20% molybdenum to iron as a single alloying element on the corrosion behaviour of the alloys. The alloys were tested in three different solutions: saline water from the Arabian Gulf, 3.5 % NaCl, and 7.0% NaCl solutions. Two different types of tests were carried out: static and dynamic, both in rotation and in flow systems. The results obtained showed that molybdenum does not improve the pitting or crevice corrosion resistance as has been formulated but rather triggers the chromium and nickel content in the alloy to do the job.
Key wmk Molybdenum seawater, steel alloys
effect, corrosion in desalination,
sodium chloride,
Wll-9154/94/$07.00 @ 1994 Elsevier Science B.V. All rigI& resend SSDIOOll-9164(94)00014-F
200 INTRODUCTION
Selecting construction materials for desalination equipment has always been a problem. A wide variety of metallic materials has been tried and/or in use with each one presenting some problems. Mild steel is an unreliable material since it almost always suffers general corrosion. Copper alloys like brass and copper-nickel alloys are frequently used and to some extent have quite a good level of corrosion resistance. However, these alloys are sensitive to high water velocities and water contamination such as hydrogen sulphide. Titanium is another alternative which offers excellent corrosion resistance, but its high price and low value of Young’s modulus (modulus of elasticity) requiring additional support plates limit its wide use. Stainless steels are often used for desalination equipment. Generally, the inherent corrosion resistance of stainless steels increases with an increase in chromium content. Thus the steels with higher chromium content (18% and above) are generally more interesting since these types of steels can be passivated [l]. Their surfaces can exist in three different electrochemical states: active, passive and transpassive. In the active and transpassive states uniform corrosion attack takes place. In the passive state the corrosion rate of stainless steel is nearly zero. The widely used basic materials for desalination units were double-layer steels with stainless steel cladding, stainless steel and carbon steels, as well as copper-base alloys. The fraction of carbon steels made up about 45%, stainless steels 30% and copper alloys 25% of the total specified metal amount for a unit. Recent research in the USSR program on the problem of seawater desalination has been concentrating on investigation of corrosion resistance of relatively inexpensive low-alloyed steels of commercial grades with the aim of determining its possible use instead of stainless steel or double-layered steel with stainless steel cladding. Phelps et al. [l] have reviewed and collected a large amount of data on the corrosion performance of the steels in salt and brackish water. These data showed that steels have a good resistance in sea and brackish water at temperatures near ambient, provided that crevices and deposits which might lead to oxygen concentration cells can be avoided. However, stainless can suffer serious localized corrosion in the form of pitting or crevice attack. Resistance to these forms of corrosion is improved by increasing the molybdenum content of the stainless steels. The aim of this study is to investigate the effects of molybdenum in simple iron-molybdenum alloys on their resistance to corrosion in marine-type environments. We will also discuss the relationship of these results to the effect of molybdenum on stainless steels.
201
EXPERIMENTAL
Materials used were mild steel alloys as well as the several nominal alloys which were prepared from elements of very high purity. The alloys were prepared by melting in a high vacuum induction furnace under a vacuum of 10m4 to 10B5 torr. The immediate surfaces of approximately 1 mm of the cast blocks, which were either 2.5 cm square or 2.5 cm in diameter in section, were machined away to remove any contaminates. The obtained ingots were then forged to 10 mm thickness and hot rolled to 3 mm thickness at 1050°C. After hot rolling to 3 mm, the sheets were heated (normalized) in an ash evacuated quartz tube furnace for 1 h at 920°C. Table I shows the nominal composition of the studied alloys. Mild steel was used here to compare its behaviour with the corrosion characteristics of the suggested new alloys. The corrosion test specimens of 3 X20 X40 mm were cut from the steel sheet using a shaping machine. For the weight loss measurement tests, a hole was drilled in one end of the samples to allow for their easy suspension. Prior to the tests the standard specimens were progressively ground on a wet silicon carbide paper from 180,400 to 1000 grade papers. They were then washed and degreased with carbon tetrachloride. TABLE I Nominal composition of the new stainless steel alloys Alloy
1 2 3 4 5 6 7 8 9 10 11 12
Composition, % C
S
Cr
Ni
0.06 0.05 0.05 0.02 0.04 0.07 0.03 0.03 0.03 0.03 0.29 0.30
0.003 0.003 0.003 0.002 0.002 0.002 0.003 0.003 0.002 0.002 0.003 0.002
17.0 16.9 16.8 17.2 17.1 -
9.8 10.0 10.3 10.2 9.8 9.7 -
MO 1.89 3.96 5.93 5.0 10.0 15.0 27.0 5.0 14.0
Ti
Fe
-
Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.
1.33 2.32 4.13 -
202
Test solutions Various solutions were used including artificial seawater, seawater obtained from the Eastern Region, and Arabian Gulf seawater (Table II). Experiments were also carried out with the same waters after being diluted 10-100 times with distilled water. During measurements the electrolytes were stirred continuously with a rapid stream of air that also ensured their saturation with oxygen. TABLE II Characteristics of Arabian Gulf seawater Electrical conductance TDS clCa2’ Mg2+ HCO; COjPH
69,600 S 43,089 ppm 27,300 ppm 543 ppm
1,790 ppm 115 ppm 15.5 ppm 8.14
Static condition In this test the various specimens were exposed to the various testing solutions for periods of over 12 months under static conditions where there was no flow of solutions. The specimens were removed to determine how metals had been lost as well as to investigate any surface changes. The exposure time, weight loss, surface area exposed and the density of the metal were used to calculate the corrosion rate of the metal.
Rotating conditions is very important to study the effect of rotating the sample in the solution, thus accelerating the corrosion of the materials. Four rotation speeds (68, 112, 135 and 155 RPM) were applied. The rotating testing method is an easy and simple one, and at the same time produces corrosion behaviour and corrosion products similar to those obtained from a working desalination plant [2]. It
203
Twotechniquesusedfor determiningthe corrosionrate, weightloss and Tcsfelplot A Tafel plot for electrochemical testing was carried out with an automatic corrosion unit Model 350A, produced by Princeton Applied Research Company. These tests were made in a 1 1electrochemical cell containing two graphite counter electrodes, a saturated calomel reference electrode and the working electrode that is fabricated from the alloys under investigation. The potential was scanned in the range of -f250 mV with respect to the free corrosion potential with a scanning rate of 1 mV/s. RESULTS AND DISCUSSION
Corrosion rates were measured by weight loss under static and dynamic conditions. The reproducibility of the corrosion rates was good, these being calculated from data obtained from the average of four specimens. In the static testing conditions the various specimens were exposed to the testing solutions, and the test was carried out for over 18 months. The steel alloys containing chromium and nickel showed no weight loss apart from an experimental error, while the corrosion rates obtained from all the alloys were almost zero. This is compared to the value of 1.82 mm/y to about 2.55 mm/y for the steel alloy free of both chromium and nickel, and rate values of between 2.500 mm/y to about 3.0 mm/y for mild steel alloys. The corrosion rates for various alloys are summarized in Table III. The alloys containing chromium and nickel with various molybdenum or titanium contents had a very bright surface appearance, of the same color as that of the fresh samples before suspending them in the testing solutions. The alloy rich in molybdenum but chromium and nickel-free had a rather thick oxide film. This oxide could be a mixture of various iron oxides similar to those reported in mild steel alloys. The corrosion rate of the molybdenum-rich alloys, free from both chromium and nickel, showed no difference according to the corrosive media, i.e., the corrosion rates were more or less the same in the test solutions of seawater and 3,596 and 7.0% NaCl. These results indicate that in spite of the fact that seawater contains various anions, it contains the main effective part, i.e., the presence of both sodium chloride and oxygen. This is in.good agreement with the obtained results which are the major factor affecting corrosion and forming of deposited sodium chloride crystals [3,4]. All the specimens tested in both sodium chloride solutions and seawater showed low corrosion rates in a static condition compared with the actual
204 TABLEIII Corrosion rate of the tested alloys in both seawater and sodium chloride solutions Alloy
1 2 3 4 5 6 7 8 9 10 11 12
Corrosion rate, mm/y Seawater
3.5% NaCl
7.0% NaCl
0.0 0.0 0.0 0.0 0.0 0.0 2.4118 2.50465 2.0441 2.5339 1.9957 1.9838
0.0 0.0 0.0 0.0 0.0 0.0 2.5046 2.2262 2.2297 2.3967 2.5490 2.5152
0.0 0.0 0.0 0.0 0.0 0.0 2.1801 2.0026 1.9938 2.1877 1.8146 2.0885
working plants or with the rotating tests. The corrosion rate (measured by Tafel tests) increased rapidly with an increasing period of exposure up to 10 h and then the corrosion rate of the steel chromium and nickel-free alloys increased more gradually with increasing the exposure time, becoming almost constant after 100 h. Similar results were obtained for aluminum alloys tested in brackish and Arabian Gulf water [2]. It seems there is no relationship between the molybdenum content of the alloy and alloy corrosion rates, as shown in Table III. Apparently the static conditions do not represent the actual working conditions where the flow of solution or rotating the sample has the major effect on corrosion of the various alloys. For steel free of Cr and Ni, in a dynamic sodium chloride solution with 3.5 % concentration, considerably higher corrosion rates of between 25-50 fold over those observed under static conditions were observed (Table IV). The mild steel shows a very similar behaviour where the corrosion rate under dynamic conditions is about 20-30 times that of static. The behaviour of alloys 7-12 is different from the behaviour of the other six alloys containing chromium and nickel. The main factor which affected the corrosion rate in dynamic conditions of alloys 7-12 was the temperature, where the corrosion rate reached a drastic value of 412.57 mm/y, i.e., about fivefold that reached at ambient temperature and more than 200 fold for the corrosion rate of the same alloy under static conditions.
205
TABLEIS’ Corrosion rate of molybdenum-richalloys (chromium and nickel free) at various rotationspeeds in 3.5% NaCI and at 25°C MOY
7 8 9 10 11 12
Corrosionrate, mmy 68 rpm
112 rpm
135 rpm
155 rpm
74.99 72.58 99.27 67.32 41.05 65.15
82.35 67.97 108.43 91.70 74.14 79.40
86.98 75.53 85.35 104.27 78.87 93.77
73.08 143.34 99.54 103.79 75.59 89.61
Note: The corrosion rate of the 12th alloy tested in 3.5% NaCl at 7O’C was 412.57 mm/y.
The first six alloys containing both chromium and nickel tested in various solutions did not show any change in the surface appearance. The other six alloys showed a very thick oxide scale with some small areas of the surface having a rather thin scale. Careful inspection with a low-power microscope revealed a very interesting phenomenon with the deposition of a very clear crystalline material. Fig. 1 shows the nature of this deposit. The alloys were heavily pitted, and the mwrphology of the surface is illustrated in Fig. 2. The alloys containing molybdenum showed a great improvement in corrosion resistance, particularly to the pitting and crevice corrosion compared to mild steel alloys which suffered severely from these two types of corrosion. The effect of titanium on both the crevice and pitting is less pronounced than that of the molybdenum effect [5,6]. We can say that the passive film on stainless steel is greatly improved when thealloy contains molybdenum. In spite of the fact that molybdenum is an expensive alloying element, it is still cheaper than the cost of titanium additions. The addition of molybdenum (up to 2%) improves the crevice corrosion through the progressive increase in the temperature at which crevice corrosion is observed. The authors beheved that chromium and molybdenum raise the pitting potential in the noble direction, reducing film breakdown within the crevice. The higher crevice corrosion resistance tir the Inconel alloy 625 and the Hastelloy alloy C-276, containing only 2!5% and 16% chromium, respectively, was attributed to some degree to their contents of nickel and molybdenum [7,8]. Thereare numerous publications thatattribute the crevice corrosion resistance of stainless steel to the presence of
206
Fig. 1. NaCl crystals deposited on the specimen surface.
Fig. 2. Higher magnification of Fig. 1.
207
molybdenum irrespective of whether the alloys are austenitic or ferritic [9]. For austenitic stainless steels, significant improvement in seawater corrosion resistance was obtained by adding molybdenum, as low as 2-4% for alloys 316 and 317 [lo]. Molybdenum was found to be like nickel and chromium raises the pitting potential of alloy in the noble direction [ 111, so a likely explanation for the beneficial effect of molybdenum can be proposed in terms of increased difficulty in breaking down the passive film. However, we believe that the corrosion attack of mild steel in dynamic testing is a combination of both corrosion and erosion, and at the same time each of these two phenomena helps each other to take place as well as to increase the corrosion rate generally. The term “erosion corrosion” applied here does not imply purely mechanical removal of the metal but an actual corrosion process which is intensified by the influence of high-flow veh&y. As the present results have shown, two important phenomena take place with a mild steel alloy. First, the formation and deposition of sodium chloride crystals in association with the corrosion products as well as on the top of the test coupons after the removal of corrosion products (Fig. 3). Second, these sodium chloride crystals have not been detected by any technique with the tested model alloy steels. The Fe-Cr-Ni-Mo or Ti alloys have not shown any failure of materials either through pitting or crevice corrosion or with
Fig. 3. EDAX analysis of the crystals in Fig. 1, showing the presence of NaCI.
208
the formation of any corrosion products. This is another factor proving that pitting or crevice corrosion of the material is at least associated with the deposition of sodium chloride crystals. The improvement of pitting resistance of Fe-Cr-Ni-Mo alloys reported in the present work could be due to the formation of austenitic steels which is similar to several studies in different countries. However, the results obtained in the present work and those obtained elsewhere raise the question of the mechanism by which molybdenum, and to a lesser extent titanium, affects the corrosion resistance of Fe-Cr-Ni alloys, i.e., stainless steels. The presence of both nickel and molybdenum in the Fe-Cr alloys is extremely beneficial in improving resistance to corrosion in general and to pitting in particular. The stainless steel alloy without nickel suffered from pitting. It was found that the pitting severity is reciprocal to the increased molybdenum content.
CONCLUSIONS
The stainless steel alloys containing molybdenum showed extremely low weight loss under various conditions, and the corrosion rates for these alloys for times as long as 24 months were nil. The alloys free of nickel and chromium but containing a higher level of molybdenum were drastically attacked, i.e., the molybdenum is beneficial in the presence of both chromium and nickel. The role of molybdenum is not very clear, and this point needs further investigation and research.
ACKNOWLEDGEMENT
The authors wish to thank King Abdul Aziz City for Science and Technology for financial support of Research Project AR-7-22.
REFERENCES 1 2 3 4 5
E.H. Phelps, R.T. Jones and H.P. Leckie, I. Electrochem. Sot., 116 (1969) 813. Z. Ahmed and S. Rashidi, Desalination, 44 (1983) 265. F.C. Wood, H.J. Woodthorpe and Y.N. Wu, Desalination, 20 (1977) 3 19. V.E. Heitz and R. Manner, Werkstoffe und Korrosion, 29 (1978) 783. M.I. Hazza and M.E. El-Dahshan, unpublished.
6 7 8 9 10 11
M.E. El-Dahshan, A.A. AlSahybani, F.M. Al-Habdan and M.I. Hazza, Corrosion and corrosion protection in desalination plants, KACST Project No. AR-7-22, 1988. HF. Ebling and M.A. Scheil, Advances in the technology of stainless steels and related alloys, ASTM STP 369, Amer. Sot. for Testing and Materials, 1963, p. 275. K. Osozawa, K. Bohnenkamp and H.J. Engell, Corr. Sci., 6 (1966) 421. R.M. Kain, Crevice corrosion resistance of austenitic stainless steels in ambient and elevated temperature seawater. Paper presented at NACE Annual Conf., Atlanta, 1979. M. Henthorne, Corrosion, 30 (1974) 39. M.I. Hazza, J. Eng. Sci., in press.
Desalination, 95 (1994) 199-209 Elsevier Science B.V., Amsterdam - Printed in The Netherlands
The effect of molybdenum on the corrosion behaviour of some steel alloys ML Hazza and M.E. El-Dahshan Department of Chemical Engineering, King Saud University, PO Box 800, Riyadh 11421 (Saudi Arabia) (Received July 20, 1993; in revised form October 10, 1993)
SUMMARY
There has been a recent increase in interest in MO-containing steels as potential materials in the construction of desalination plants. The general conclusions reached from these studies is that the alloys are superior in their corrosion behaviour to other stainless steels and are more resistant to pitting attack. A number of formulas have been given to express the pitting resistance of the steel alloys. All of the formulas consider molybdenum to be the “magic” element in improving the pitting and crevice corrosion resistance. This paper reports on the effect of adding 2-20% molybdenum to iron as a single alloying element on the corrosion behaviour of the alloys. The alloys were tested in three different solutions: saline water from the Arabian Gulf, 3.5 % NaCl, and 7.0% NaCl solutions. Two different types of tests were carried out: static and dynamic, both in rotation and in flow systems. The results obtained showed that molybdenum does not improve the pitting or crevice corrosion resistance as has been formulated but rather triggers the chromium and nickel content in the alloy to do the job.
Key wmk Molybdenum seawater, steel alloys
effect, corrosion in desalination,
sodium chloride,
Wll-9154/94/$07.00 @ 1994 Elsevier Science B.V. All rigI& resend SSDIOOll-9164(94)00014-F
200 INTRODUCTION
Selecting construction materials for desalination equipment has always been a problem. A wide variety of metallic materials has been tried and/or in use with each one presenting some problems. Mild steel is an unreliable material since it almost always suffers general corrosion. Copper alloys like brass and copper-nickel alloys are frequently used and to some extent have quite a good level of corrosion resistance. However, these alloys are sensitive to high water velocities and water contamination such as hydrogen sulphide. Titanium is another alternative which offers excellent corrosion resistance, but its high price and low value of Young’s modulus (modulus of elasticity) requiring additional support plates limit its wide use. Stainless steels are often used for desalination equipment. Generally, the inherent corrosion resistance of stainless steels increases with an increase in chromium content. Thus the steels with higher chromium content (18% and above) are generally more interesting since these types of steels can be passivated [l]. Their surfaces can exist in three different electrochemical states: active, passive and transpassive. In the active and transpassive states uniform corrosion attack takes place. In the passive state the corrosion rate of stainless steel is nearly zero. The widely used basic materials for desalination units were double-layer steels with stainless steel cladding, stainless steel and carbon steels, as well as copper-base alloys. The fraction of carbon steels made up about 45%, stainless steels 30% and copper alloys 25% of the total specified metal amount for a unit. Recent research in the USSR program on the problem of seawater desalination has been concentrating on investigation of corrosion resistance of relatively inexpensive low-alloyed steels of commercial grades with the aim of determining its possible use instead of stainless steel or double-layered steel with stainless steel cladding. Phelps et al. [l] have reviewed and collected a large amount of data on the corrosion performance of the steels in salt and brackish water. These data showed that steels have a good resistance in sea and brackish water at temperatures near ambient, provided that crevices and deposits which might lead to oxygen concentration cells can be avoided. However, stainless can suffer serious localized corrosion in the form of pitting or crevice attack. Resistance to these forms of corrosion is improved by increasing the molybdenum content of the stainless steels. The aim of this study is to investigate the effects of molybdenum in simple iron-molybdenum alloys on their resistance to corrosion in marine-type environments. We will also discuss the relationship of these results to the effect of molybdenum on stainless steels.
201
EXPERIMENTAL
Materials used were mild steel alloys as well as the several nominal alloys which were prepared from elements of very high purity. The alloys were prepared by melting in a high vacuum induction furnace under a vacuum of 10m4 to 10B5 torr. The immediate surfaces of approximately 1 mm of the cast blocks, which were either 2.5 cm square or 2.5 cm in diameter in section, were machined away to remove any contaminates. The obtained ingots were then forged to 10 mm thickness and hot rolled to 3 mm thickness at 1050°C. After hot rolling to 3 mm, the sheets were heated (normalized) in an ash evacuated quartz tube furnace for 1 h at 920°C. Table I shows the nominal composition of the studied alloys. Mild steel was used here to compare its behaviour with the corrosion characteristics of the suggested new alloys. The corrosion test specimens of 3 X20 X40 mm were cut from the steel sheet using a shaping machine. For the weight loss measurement tests, a hole was drilled in one end of the samples to allow for their easy suspension. Prior to the tests the standard specimens were progressively ground on a wet silicon carbide paper from 180,400 to 1000 grade papers. They were then washed and degreased with carbon tetrachloride. TABLE I Nominal composition of the new stainless steel alloys Alloy
1 2 3 4 5 6 7 8 9 10 11 12
Composition, % C
S
Cr
Ni
0.06 0.05 0.05 0.02 0.04 0.07 0.03 0.03 0.03 0.03 0.29 0.30
0.003 0.003 0.003 0.002 0.002 0.002 0.003 0.003 0.002 0.002 0.003 0.002
17.0 16.9 16.8 17.2 17.1 -
9.8 10.0 10.3 10.2 9.8 9.7 -
MO 1.89 3.96 5.93 5.0 10.0 15.0 27.0 5.0 14.0
Ti
Fe
-
Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.
1.33 2.32 4.13 -
202
Test solutions Various solutions were used including artificial seawater, seawater obtained from the Eastern Region, and Arabian Gulf seawater (Table II). Experiments were also carried out with the same waters after being diluted 10-100 times with distilled water. During measurements the electrolytes were stirred continuously with a rapid stream of air that also ensured their saturation with oxygen. TABLE II Characteristics of Arabian Gulf seawater Electrical conductance TDS clCa2’ Mg2+ HCO; COjPH
69,600 S 43,089 ppm 27,300 ppm 543 ppm
1,790 ppm 115 ppm 15.5 ppm 8.14
Static condition In this test the various specimens were exposed to the various testing solutions for periods of over 12 months under static conditions where there was no flow of solutions. The specimens were removed to determine how metals had been lost as well as to investigate any surface changes. The exposure time, weight loss, surface area exposed and the density of the metal were used to calculate the corrosion rate of the metal.
Rotating conditions is very important to study the effect of rotating the sample in the solution, thus accelerating the corrosion of the materials. Four rotation speeds (68, 112, 135 and 155 RPM) were applied. The rotating testing method is an easy and simple one, and at the same time produces corrosion behaviour and corrosion products similar to those obtained from a working desalination plant [2]. It
203
Twotechniquesusedfor determiningthe corrosionrate, weightloss and Tcsfelplot A Tafel plot for electrochemical testing was carried out with an automatic corrosion unit Model 350A, produced by Princeton Applied Research Company. These tests were made in a 1 1electrochemical cell containing two graphite counter electrodes, a saturated calomel reference electrode and the working electrode that is fabricated from the alloys under investigation. The potential was scanned in the range of -f250 mV with respect to the free corrosion potential with a scanning rate of 1 mV/s. RESULTS AND DISCUSSION
Corrosion rates were measured by weight loss under static and dynamic conditions. The reproducibility of the corrosion rates was good, these being calculated from data obtained from the average of four specimens. In the static testing conditions the various specimens were exposed to the testing solutions, and the test was carried out for over 18 months. The steel alloys containing chromium and nickel showed no weight loss apart from an experimental error, while the corrosion rates obtained from all the alloys were almost zero. This is compared to the value of 1.82 mm/y to about 2.55 mm/y for the steel alloy free of both chromium and nickel, and rate values of between 2.500 mm/y to about 3.0 mm/y for mild steel alloys. The corrosion rates for various alloys are summarized in Table III. The alloys containing chromium and nickel with various molybdenum or titanium contents had a very bright surface appearance, of the same color as that of the fresh samples before suspending them in the testing solutions. The alloy rich in molybdenum but chromium and nickel-free had a rather thick oxide film. This oxide could be a mixture of various iron oxides similar to those reported in mild steel alloys. The corrosion rate of the molybdenum-rich alloys, free from both chromium and nickel, showed no difference according to the corrosive media, i.e., the corrosion rates were more or less the same in the test solutions of seawater and 3,596 and 7.0% NaCl. These results indicate that in spite of the fact that seawater contains various anions, it contains the main effective part, i.e., the presence of both sodium chloride and oxygen. This is in.good agreement with the obtained results which are the major factor affecting corrosion and forming of deposited sodium chloride crystals [3,4]. All the specimens tested in both sodium chloride solutions and seawater showed low corrosion rates in a static condition compared with the actual
204 TABLEIII Corrosion rate of the tested alloys in both seawater and sodium chloride solutions Alloy
1 2 3 4 5 6 7 8 9 10 11 12
Corrosion rate, mm/y Seawater
3.5% NaCl
7.0% NaCl
0.0 0.0 0.0 0.0 0.0 0.0 2.4118 2.50465 2.0441 2.5339 1.9957 1.9838
0.0 0.0 0.0 0.0 0.0 0.0 2.5046 2.2262 2.2297 2.3967 2.5490 2.5152
0.0 0.0 0.0 0.0 0.0 0.0 2.1801 2.0026 1.9938 2.1877 1.8146 2.0885
working plants or with the rotating tests. The corrosion rate (measured by Tafel tests) increased rapidly with an increasing period of exposure up to 10 h and then the corrosion rate of the steel chromium and nickel-free alloys increased more gradually with increasing the exposure time, becoming almost constant after 100 h. Similar results were obtained for aluminum alloys tested in brackish and Arabian Gulf water [2]. It seems there is no relationship between the molybdenum content of the alloy and alloy corrosion rates, as shown in Table III. Apparently the static conditions do not represent the actual working conditions where the flow of solution or rotating the sample has the major effect on corrosion of the various alloys. For steel free of Cr and Ni, in a dynamic sodium chloride solution with 3.5 % concentration, considerably higher corrosion rates of between 25-50 fold over those observed under static conditions were observed (Table IV). The mild steel shows a very similar behaviour where the corrosion rate under dynamic conditions is about 20-30 times that of static. The behaviour of alloys 7-12 is different from the behaviour of the other six alloys containing chromium and nickel. The main factor which affected the corrosion rate in dynamic conditions of alloys 7-12 was the temperature, where the corrosion rate reached a drastic value of 412.57 mm/y, i.e., about fivefold that reached at ambient temperature and more than 200 fold for the corrosion rate of the same alloy under static conditions.
205
TABLEIS’ Corrosion rate of molybdenum-richalloys (chromium and nickel free) at various rotationspeeds in 3.5% NaCI and at 25°C MOY
7 8 9 10 11 12
Corrosionrate, mmy 68 rpm
112 rpm
135 rpm
155 rpm
74.99 72.58 99.27 67.32 41.05 65.15
82.35 67.97 108.43 91.70 74.14 79.40
86.98 75.53 85.35 104.27 78.87 93.77
73.08 143.34 99.54 103.79 75.59 89.61
Note: The corrosion rate of the 12th alloy tested in 3.5% NaCl at 7O’C was 412.57 mm/y.
The first six alloys containing both chromium and nickel tested in various solutions did not show any change in the surface appearance. The other six alloys showed a very thick oxide scale with some small areas of the surface having a rather thin scale. Careful inspection with a low-power microscope revealed a very interesting phenomenon with the deposition of a very clear crystalline material. Fig. 1 shows the nature of this deposit. The alloys were heavily pitted, and the mwrphology of the surface is illustrated in Fig. 2. The alloys containing molybdenum showed a great improvement in corrosion resistance, particularly to the pitting and crevice corrosion compared to mild steel alloys which suffered severely from these two types of corrosion. The effect of titanium on both the crevice and pitting is less pronounced than that of the molybdenum effect [5,6]. We can say that the passive film on stainless steel is greatly improved when thealloy contains molybdenum. In spite of the fact that molybdenum is an expensive alloying element, it is still cheaper than the cost of titanium additions. The addition of molybdenum (up to 2%) improves the crevice corrosion through the progressive increase in the temperature at which crevice corrosion is observed. The authors beheved that chromium and molybdenum raise the pitting potential in the noble direction, reducing film breakdown within the crevice. The higher crevice corrosion resistance tir the Inconel alloy 625 and the Hastelloy alloy C-276, containing only 2!5% and 16% chromium, respectively, was attributed to some degree to their contents of nickel and molybdenum [7,8]. Thereare numerous publications thatattribute the crevice corrosion resistance of stainless steel to the presence of
206
Fig. 1. NaCl crystals deposited on the specimen surface.
Fig. 2. Higher magnification of Fig. 1.
207
molybdenum irrespective of whether the alloys are austenitic or ferritic [9]. For austenitic stainless steels, significant improvement in seawater corrosion resistance was obtained by adding molybdenum, as low as 2-4% for alloys 316 and 317 [lo]. Molybdenum was found to be like nickel and chromium raises the pitting potential of alloy in the noble direction [ 111, so a likely explanation for the beneficial effect of molybdenum can be proposed in terms of increased difficulty in breaking down the passive film. However, we believe that the corrosion attack of mild steel in dynamic testing is a combination of both corrosion and erosion, and at the same time each of these two phenomena helps each other to take place as well as to increase the corrosion rate generally. The term “erosion corrosion” applied here does not imply purely mechanical removal of the metal but an actual corrosion process which is intensified by the influence of high-flow veh&y. As the present results have shown, two important phenomena take place with a mild steel alloy. First, the formation and deposition of sodium chloride crystals in association with the corrosion products as well as on the top of the test coupons after the removal of corrosion products (Fig. 3). Second, these sodium chloride crystals have not been detected by any technique with the tested model alloy steels. The Fe-Cr-Ni-Mo or Ti alloys have not shown any failure of materials either through pitting or crevice corrosion or with
Fig. 3. EDAX analysis of the crystals in Fig. 1, showing the presence of NaCI.
208
the formation of any corrosion products. This is another factor proving that pitting or crevice corrosion of the material is at least associated with the deposition of sodium chloride crystals. The improvement of pitting resistance of Fe-Cr-Ni-Mo alloys reported in the present work could be due to the formation of austenitic steels which is similar to several studies in different countries. However, the results obtained in the present work and those obtained elsewhere raise the question of the mechanism by which molybdenum, and to a lesser extent titanium, affects the corrosion resistance of Fe-Cr-Ni alloys, i.e., stainless steels. The presence of both nickel and molybdenum in the Fe-Cr alloys is extremely beneficial in improving resistance to corrosion in general and to pitting in particular. The stainless steel alloy without nickel suffered from pitting. It was found that the pitting severity is reciprocal to the increased molybdenum content.
CONCLUSIONS
The stainless steel alloys containing molybdenum showed extremely low weight loss under various conditions, and the corrosion rates for these alloys for times as long as 24 months were nil. The alloys free of nickel and chromium but containing a higher level of molybdenum were drastically attacked, i.e., the molybdenum is beneficial in the presence of both chromium and nickel. The role of molybdenum is not very clear, and this point needs further investigation and research.
ACKNOWLEDGEMENT
The authors wish to thank King Abdul Aziz City for Science and Technology for financial support of Research Project AR-7-22.
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E.H. Phelps, R.T. Jones and H.P. Leckie, I. Electrochem. Sot., 116 (1969) 813. Z. Ahmed and S. Rashidi, Desalination, 44 (1983) 265. F.C. Wood, H.J. Woodthorpe and Y.N. Wu, Desalination, 20 (1977) 3 19. V.E. Heitz and R. Manner, Werkstoffe und Korrosion, 29 (1978) 783. M.I. Hazza and M.E. El-Dahshan, unpublished.
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M.E. El-Dahshan, A.A. AlSahybani, F.M. Al-Habdan and M.I. Hazza, Corrosion and corrosion protection in desalination plants, KACST Project No. AR-7-22, 1988. HF. Ebling and M.A. Scheil, Advances in the technology of stainless steels and related alloys, ASTM STP 369, Amer. Sot. for Testing and Materials, 1963, p. 275. K. Osozawa, K. Bohnenkamp and H.J. Engell, Corr. Sci., 6 (1966) 421. R.M. Kain, Crevice corrosion resistance of austenitic stainless steels in ambient and elevated temperature seawater. Paper presented at NACE Annual Conf., Atlanta, 1979. M. Henthorne, Corrosion, 30 (1974) 39. M.I. Hazza, J. Eng. Sci., in press.