Ind. Eng. Chem. Res. 1993,32, 960-965
960
Effect of Trace Organic Compounds on the Corrosion of Cu/Ni Alloys in Sulfide Polluted Seawater Mahmoud R. Redat and Jamal N. Al-Hajji'94 Chemical Engineering Department and Mechanical Engineering Department, College of Engineering and Petroleum, Kuwait University, P.O. Box 5969,Safat, Kuwait
Trace organic complexing agents were investigated to check their ability to reduce the relatively high corrosion rates of Cu/Ni alloys in sulfide pollutedseawater. It is found that an organic complexing agent such as fuchsin in the concentration range of 5 ppm is an excellent inhibitor against uniform and localized attack for 70/30 Cu/Ni alloy in 2 ppm sulfide polluted seawater. Another metal complexing agent, SSA (5-sulfosalicylic acid), was found to be effective for the 90/10 Cu/Ni alloy against enhanced attack by sulfide polluted seawater while it was ineffective for 70/30 Cu/Ni alloy. EDTA (ethylene diaminetetraacetic acid disodium salt) was found to be ineffective for both Cu/Ni alloys when used by itself in the concentration range of 5 ppm. A mechanism is proposed to explain the effectiveness of the various selected trace organic complexing agents on the corrosiveness of sulfide polluted seawater. Introduction Sulfide,which is usually introduced in seawater in many ways such as from rotting vegetation and from industrial waste discharge (Richards, 1965),has been found to cause severe deterioration of copper-nickel alloys where they are normally very resistant to corrosion in unpolluted seawater (Baily, 1951;Niederberger et al., 1976;Sato, 1975; Hack, 1980). Most of the data indicated an acceleration of corrosion of Cu/Ni alloys in sulfide polluted seawater. Similar effects were even observed in aqueous solutions of sodium chloride contaminated with sulfide (Kato et al., 1984). It has been suggested that sulfide interferes with the normal growth of the protective oxide film that forms on the surface of copper-nickel alloys when exposed to seawater (Syrett, 1981). It has been reported that important factors that affect the behavior of the sulfide pollutant and its degrading ability on the copper-nickel alloys are (1) sulfide concentration (e.g., Al-Hajji and Reda, 1992a; Macdonald et al., 1979a,b),(2) hydrodynamic conditions of the seawater (e.g., Macdonald et al., 1979a,c),(3) oxidation products of dissolved sulfide (Syrett et al., 1979;Syrett and Mcdonald, 1979), (4) exposure time (Gudas and Hack, 1979), ( 5 ) preexposure to extraneous ions (e.g., Bostwick, 1961;North and Pryor, 1968;Adrian Roberta, 1980;Al-Hajjiand Reda, 1992b), (6) other deliberate pollutants such as Clz which are normally introduced in seawater to eliminate all living organisms in the seawater (e.g., Francis, 1985, 19861, and (7) trace organic compounds present in seawater, which is one of the main objectives of the present investigation. Macdonald et al. (1979a,b) investigated the effect of 0-55 ppm sulfide and concluded that the corrosion rate is very low for unpolluted and deaerated seawater, but for deaerated seawater polluted with sulfide,there is an active shift in corrosion potential which allows hydrogen reduction to become the cathodic reaction. However, the corrosion rate in this case is lower that that reported in naval ships (Niederberger et al., 1976) and in industry (Sato, 1975). It has been concluded (Syrett et al., 1979) that the concentrations of sulfide and adverse flow conditions are not the only causes of accelerated attack, but that the simultaneous presence of oxygen and sulfide +
Chemical Engineering Department.
* Mechanical Engineering Department. 0888-588519312632-0960$04.00/0
ions can also cause accelerated attack. Also, it has been concluded (Syrett and Macdonald, 1979)that the presence of sulfide and oxidation products from the reaction between sulfide and oxygen (e.g., sulfur and polysulfide) leads to the formation of thick, nonprotective, porous, cuprous sulfide which interferes with normal growth of protective oxide film, and thus, subsequent exposure to unpolluted seawater will lead to an increase in the corrosion rate. It has been concluded (Gudas and Hack, 1979)that the corrosion of 90/10 Cu-Ni alloy is very high at low concentrations, as low as 0.01 ppm sulfide and 1-day exposure time, while the corrosion rate of 70/30 Cu/Ni alloy is high only at higher sulfide concentrations and longer exposure times. It has been also demonstrated that the 1/R, values (Syrett et al., 1979) in aerated seawater are approximatelythe same as those obtained in deaerated seawater containing sulfur (4 ppm sulfide and 3 ppm sulfur),and hence their data cannot explain the premature accelerated corrosion rate of Cu/Ni alloy piping systems in naval ships (Niederberger et al., 1976) and condenser tubes in industry (Sato, 1975). It has been reported (Eiselstein et al., 1983) that the localized corrosion rate of Cu/Ni alloy in aerated seawater contaminated with 55 ppm sulfide was 155 times higher than that in unpolluted seawater, and this can explain the accelerated attack (19 mmlyear) observed in the 90/10 Cu/Ni alloy piping systems in naval ships (Niederberger et al., 1976). Another recent investigation (Habib and Hussain, 1989)measured the corrosion rate of 90/100 Cul Ni alloy in aerated and stirred seawater containing 0, 1, 2, 5, and 10 ppm sulfide. Their measurements showed that while the corrosion rates increased with sulfide concentration up to 5 ppm, they were followed by a decrease at higher concentrations. In contrast to this it was indicated (Cohn and Rice, 1967,1969)that sulfide did not affect the corrosion rate of Cu/Ni alloys. Thus, it can be concluded that the effect of sulfide pollutant is controversialand a logical explanation is needed to clarify these often reported discrepancies. An investigation was conducted to nullify this deleterious effect of sulfide. Previous attempts to reduce the effect of sulfide were to preexpose the alloys to low concentrations of some known steel inhibitors which might lead to their adsorptionlchemical reaction at the surface and thus a reduction of corrosion rate (Al-Hajjiand Reda, 1992b). It was found 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 961 Table I. Chemical Analysis of the Alloys Investigated alloy 90/10 Cu/Ni 70/30 Cu/Ni
% cu 88 69
composition % Ni 10 30
-.A- 2 ppm Sulfide
% Fe
1.4 0.6
- + - 2 ppm Sulfide + Fuchsm
% Mn 0.4 0.5
Table 11. Environmental Test Matrix for the Two Alloys under Investigation testing conditions unpolluted seawater, stagnant and aerated unpolluted seawater, stagnant and aerated polluted seawater, aerated and stagnant 2 ppm sulfide polluted seawater, aerated and stagnant
90/10 Cu/Ni clean
70/30 Cu/Ni clean
SSA + EDTA
SSA + EDTA
2 ppm S2-
2 ppm S210 ppm S2fuchsin fuchsin + EDTA fuchsin + SSA fuchsin + EDTA + SSA EDTA SSA
fuchsin fuchsin + EDTA fuchsin SSA fuchsin + EDTA + SSA EDTA SSA
+
that preexposure of Cu/Ni alloys to either chromate or nitrite solutions for 2 days followed by exposure to a stagnant solution of sulfide polluted seawater results in the formation of a protective layer which causes a reduction in the corrosivenessof seawater (Al-Hajjiand Reda, 199213). This method of protecting the Cu/Ni alloy though successful in stagnant solution does have its limitations, and it is very doubtful that it will be successful under erosive flow conditions. An new idea is attempted here to protect the copper-nickel alloy against severe corrosion by sulfide ions through the introduction of various trace organic compounds.
Experimental Procedure Test Specimens. Copper-nickel specimens were supplied in the form of pipes of either 90/10 or 70/30Cu/Ni (C70600and C71500). The chemical analysis of the alloys investigated is shown in Table I. The pipes were cut and flattened followed by punching out of 5/8-in. disks for corrosionmeasurement and evaluation. This was followed by a grinding process of the surface of each specimen according to ASTM standards. Testing Conditions. Experiments of corrosion measurement were conducted utilizingstandard seawater. This was prepared with distilled water and standard seawater salt. Standard seawater salt (Marinemix + Bio-Elements from Wiegandt GMBH & Co., Federal Republic of Germany) was used to reduce the variability effects resulting from conducting measurements using natural seawater. Experiments were also conducted in sulfide polluted seawater. The sulfide was introduced using research grade sodium sulfide (NazS). The level of sulfide in the seawater was checked by the iodimetric method of analysis. Various experiments were performed as shown in Table 11. The specifications of organic complexing agents (analytical grades) used in this study are shown in Table 111.
1
I
I
I
I
I
I
I
I
0
5
10
15
20
25
30
35
40
EXPOSURE TIME (hours) Figure 1. Electrochemically determined corrosion rate measured as inverse polarization resistance of 90/10 Cu/Ni alloy in aerated and stagnant seawater containing various additives over a period of 24 h. Each organic compound is added in a concentration of 5 ppm.
Test Equipment and Procedures Electrochemical corrosion measurements were made at 20 "C for all the previously mentioned conditions, using a computer-controlledpotentiostat/galvanostat(EG&G). A standard electrochemicalcorrosion testing cell was used, and the experiments were performed by following the guidelines of the ASTM standards. Preliminary corrosion measurements were always performed after 4 h. Also, repeated linear-polarization measurements were performed on all the specimens over a period of 24 h, and the values of polarization resistance (R,) were determined over this period. Cyclic voltammetric sweeps were also performed to check for localized attack. The electrochemical tests were run with a saturated calomel reference electrode (SCE). The stability of the SCE was checked against a fresh SCE following each experiment to ensure the reliability of the experiments. The scan rate of the experiments was 0.166 mV/s. The experiments were conducted repeatedly under the same environmental conditions in order to obtain reproducible results. In unpolluted seawater, reproducibility was good. In polluted seawater, reproducibility of the Tafel slopes was poor, which is consistent with reported results in the literature (DeSanchez and Schifferin, 1982). Thus the corrosion rates are presented as 1/R, values from an hourly measurement of R,, and these values were independently checked by a single R, measurement after 4 h to ensure that the repeated R, scans around the open circuit potential did not affect the values afterward.
Results and Discussion Sulfide Corrosivenessand Inhibition. The corrosion rate (i.e., proportional to l/Rp) of 90/10 Cu/Ni alloy in sulfide polluted seawater versus clean seawater is shown in Figure 1. It is clear that sulfide presence can cause a
Table 111. Organic Complexing Agents Used in the Present Investigation abbreviation chemical name chemical formula molecular weight SSA 5-sulfosalicylicacid CsH3(OH)(COOH)S03H.2HzO 254.21 EDTA ethvlenediaminetetraacetic acid disodium salt ICH~N(CH2COOH)CH&OONal2.2H20 372.24 . . . .fuchsin fuchsin basic; magenta; rosaniline methanol, (4-amino-3-methylphenyl) bis(Caminopheny1) H2NC,Hs(H2NC6H4)2COH 282.66
962 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 ppm sulfide
-1.4
1o
' ~
.-&.2 ppm Sulfide +
2.5
.-&.2 ppm Sulfide 1o-4 n
9
-ci
'"
-2 ppm Sulfide Fuchsln EDTA
- 6 -2 ppm Sulfide + SSA
.- 0 . 2 ppm Sulfide + Fuchsln + EDTA + SSA - 0 . 2 ppm Sulfide + Fuchsln + SSA
. - 0 . 2 ppm Sulfide + EDTA
+-2ppmSulfide+SSA
F 0
Fuchsln
+ + b-v
-..-2ppmSulfide+EDTA
-e -2 ppm Sulfide + SSA + EDTA
7 5
10
15
20
I
I
I
I
25
30
35
40
EXPOSURE TIME (hours) Figure 2. Electrochemically determined corrosion rate measured as inverse polarization resistance of 90/10Cu/Ni alloy in aerated and stagnant seawater containing various additives over a period of 24 h. Each organic compound is added in a concentration of 5 ppm.
drastic increase in the corrosion rate of 90/10 Cu/Ni alloy in seawater by as much as a factor of 1000 in stagnant and aerated seawater solutions. It is obvious that this increase is due to the reaction of copper ions with sulfide and the subsequent formation of a nonadherent, nonprotective, and brittle copper sulfide layer (Syrett et al., 1979; Syrett and Mcdonald, 1979). Fuchsin, a common complexing agent for sulfur(I1) and sulfur(IV), when added in the concentration range of 5 ppm has resulted in a drastic decrease in the corrosion rate to a level lower than even the corrosion rate in unpolluted seawater. This is true for 24-h exposure time. Thus it appears that fuchsin is a successful complexing agent for sulfide and is able to prevent the formation of copper sulfide (inhibit the heterogeneous surface reaction) because of favorable reaction for the formation of a complex between fuchsin and sulfide rather than reaction between sulfide and copper ions. This could be due to the higher stability constant of sulfide with fuchsin. In support of this mechanism is the results shown for in Figure 1,where fuchsin and EDTA (a common complexing agent for metal ions) are added to the 2 ppm sulfide polluted seawater. This combination of complexing agents in the concentration range of 5 ppm each results in a drastic reduction in the corrosion rate to a level which is much lower than even that in unpolluted seawater. This is due to the favorable formation of complexes between copper and nickel ions with EDTA. Thus in this case fuchsin complexes the sulfide ions while EDTA complexes the metal ions. The addition of another common metal complexing agent such as SSA when added to a solution containing fuchsin and sulfide is not as effective as EDTA, and the corrosion rate is higher by an order of magnitude with respect to the fuchsin inhibited solutions of sulfide polluted seawater. In fact when both 5 ppm EDTA and 5 ppm SSA are added to a seawater solution containing 5 ppm fuchsin and 2 ppm sulfide, the effect of fuchsin is nullified. In support of this it is shown in Figure 2 that when fuchsin is absent, it is observed that SSA alone is an effective inhibitor for 2 ppm sulfide polluted seawater. However, in the absence of fuchsin, it appears from Figure 2 that EDTA has no beneficial effect whatsoever. An interesting point is that in the absence of sulfide and fuchsin, 5 ppm EDTA or 5 ppm SSA or a combination is an excellent inhibitor for high corrosiveness of unpolluted seawater. This is again true for an exposure time of 24 h and duplicate runs. This can be easily explained due to the formation of complex between dissolved metal ions through the pores of oxide with either of these complexing agents, and this complex will remain
0
20
10
40
30
50
EXPOSURE TIME (hours) Figure 3. Sulfide concentration with time in the polarization cell. Each organic compound is added in a concentration of 5 ppm. I
I
I
I
1
ppm Sulfide
-0 70-44
.-A.2 ppm Sulfide 0 . 2 ppm Sulfide + Fucshvl
..
1
. 2 ppm Sulfide+ Fucshin + EDTA
+ -2 ppm Sulfide + Fucghin +EDTA + SSA
h
- 3e-2 ppm S a &
-
I
+ Fucshin + SSA
e+
10"
1
0
0
5
10
15
8 20
25
30
8 35
40
EXPOSURE TIME (hours) Figure 4. Electrochemically determined corrosion rate measured as inverse polarization resistance of 70/30Cu/Ni alloy in aerated and stagnant seawater containing various additives over a period of 24 h. Each organic compound is added in a concentration of 5 ppm.
in the anodic and active regions and thus act as a barrier for further dissolution. In a separate experiment, which is shown in Figure 3 for the decay of sulfide ions with time for two solutions of 2 ppm sulfide polluted seawater, one containing 5 ppm SSA and the other 5 ppm EDTA, it is found that both EDTA and SSA show similar effectiveness in removing sulfide ions with time. It is suggested that a metal complex is formed similar to the behavior of Fe"EDTAcomplexforsulfur(IV)(Sadaetal., 1984,1986,1988). A detailed mechanism of the role of the metal complex on the corrosion rates will be presented shortly. In the case of 70/30 Cu/Ni alloy it is clear from Figure 4 that 5 ppm fuchsin is an excellent inhibitor for polluted seawater. It can be seen from Figure 4 that when 5 ppm fuchsin is added to sulfide polluted seawater, the corrosion rate of 70/30 decreased by an order of magnitude over a period of 24 h and it is observed that the corrosion rate even decreases further with time. Similar results were obtained when a combination of EDTA and SSA were added to the fuchsin inhibited solution containing sulfide polluted seawater, and thus SSA and EDTA together do not alter the beneficial effect of fuchsin. However, adding SSA alone to the fuchsin inhibited solution containing 2 ppm sulfide polluted seawater drastically increased the corrosion rate as shown in Figure 4. This is similar to the effect on 90/10 Cu/Ni alloy which is shown in Figure 1. EDTA when added to sulfide polluted seawater nullified the beneficial effects of fuchsin. It appears that metal complexing agents such as EDTA or SSA or a combination of both and a sulfide complexing agent such as fuchsin have different effects on 70130 and 90/10copper-nickel
10'~
3
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 963
E
Sulfide + SSA
--O-ZgpmSulfide+EDTA
. 0 . S S A + EDTA
Gi p
3e 3J c1
a" 5
0
10
15
20
25
30
35
40
EXPOSURE TIME (hours) Figure 5. Electrochemically determined corrosion rate measured as inverse polarizationresistance of 70/30 Cu/Ni alloy in aerated and stagnant seawater containing various additives over a period of 24 h. Each organic compound is added in a concentration of 5 ppm.
I
I
I
I
I
i
I
I
I
I
I
I
-10
-9
-8
-7
-6
-5
-4
I
L
- .b.- 2 ppm sulfide -+-2ppm
;
1.2
toppmsulfidc
1
--
0.8
-.
0.6
-1
-.
0.4
-:
0.2 0
-_
-0.2
-_
-0.4
I
log (I (pA/cm2>) Figure 7. Cyclic voltammetric behavior of 70/30 Cu/Ni alloy in unpolluted stagnant and aerated seawater.
0.5 0.2 0.1 0
-0.1
5e Y
-0.2
B
a"
-0.3 -0.5
-0.4 -10
-9
-8
-7
-6
-5
-4
-14
log (I (pA/cm2>> Figure 6. Cyclic voltammetric behavior of 90/10Cu/Ni alloy in unpolluted stagnant and aerated seawater.
alloys. This is due to the different corrosion mechanisms of the two alloys (Al-Hajji and Reda, 1992a). From Figure 5, it is interesting to note that 5 ppm EDTA or 5 ppm SSA when added to 2 ppm sulfide ponuted seawater is extremely corrosive for 70/30 Cu/Ni alloy and that in the absence of fuchsin and sulfide both 5 ppm SSA and 5 ppm EDTA or a combination of both are extremely effective in reducing the high corrosiveness of natural seawater. This is consistent with the effect of the above two complexing agents on 90/10 Cu/Ni alloy. From the above results a mechanism is proposed for the function of sulfide complexing agent and metal complexing agent on the corrosion rate of both Cu/Ni alloys in polluted and unpolluted seawater. Localized Attack. Figures 6 and 7 show cyclic voltammetric behavior of 90/10 and 70/30 Cu/Ni alloys, respectively. It is clear that both Cu/Ni alloys are susceptible to localized attack in aerated, stagnant, and unpolluted seawater. This is consistent with the results of many investigators (e.g., Fontana and Green, 1989).In the presence of 2 ppm sulfide and in stagnant seawater, Figures 8 and 9 confirm the fact that 90/10 is less susceptible to localized attack than 70/30 Cu/Ni alloy. This is in agreement with previous conclusions of many investigators (Al-Hajjiand Reda, 1992b). This difference is due to the different protective mechanism of the two Cu/Ni alloys. Figure 10clearly shows that, in the presence of 5 ppm fuchsin, the 90/10 Cu/Ni alloy will suffer localized attack, while in the absence of fuchsin, sulfide alone was found to enhance uniform corrosion. This is obviously
-10
-12
-8
-6
-4
log (I @A/cm2)) Figure 8. Cyclic voltammetric behavior of 90/10Cu/Ni alloy in 2 ppm sulfide polluted stagnant and aerated seawater. 1.5
1
9
5c U
B 0 a
0
I
I
I
I
I
I
I
-10
-9
-8
-7
-6
-5
-4
-0.5
I
log (I (pA/cm2) Figure 9. Cyclic voltammetric behavior of 70/30 Cu/Ni alloy in 2 ppm sulfide polluted stagnant and aerated seawater.
due to the fact that fuchsin had already complexed all the sulfide ions and thus deprived the seawater of the corrosive sulfide pollutant. The situation is entirely different for 70/30 Cu/Ni alloy as shown in Figure 11,where sulfide ion complexation inside the pores of the passive layer acta as a barrier to the active regions in the passive layer. Thus it can be concluded from Figures 4 and 11that fuchsin is an effective inhibitor for 70/30 Cu/Ni alloy in aerated stagnant sulfide polluted seawater. In the case of 90/10 Cu/Ni alloy an effective inhibitor against localized and
964 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 I
0.2
0.4
i
0.3 0.2
I
g>
-9
-7
-8
-6
- 0 .O1
0.1
-
O
a
-0.05
-0.1 -10
-9.5
-9
-8
-8.5
-7.5
-7
log( I (pA/cm2)) Figure 11. Cyclic voltammetric behavior of 70/30 CuiNi alloy in 2 ppm sulfide polluted stagnant and aerated seawater with 5 ppm fuchsin added.
;
1.2
I
I
I
I
I
1
3
g > .-a Y
C
22
L 2
I
I
/I
/ f
-10
-9.5
-9
-8.5
-8
-7.5
-7
-6.5
log (1 (pA/cm2>)
0.1 5
3 E s0
I
I
log (I (pA/cm2))
0.05
I
L
Figure 10. Cyclic voltammetric behavior of 90/10 CuiNi alloy in 2 ppm sulfide polluted stagnant and aerated seawater with 5 ppm fuchsin added.
g >
I
0.1
-14 -13 -12 -11 -10
is
I
0.8
0.6
0.4
Figure 13. Cyclic voltammetric behavior of 70/30 Cu/Ni alloy in 2 ppm sulfide polluted stagnant and aerated seawater with 5 ppm fuchsin and 5 ppm EDTA added.
T h e Mechanism. The difference in the behavior of these complex ions on 90/10 and 70/30 Cu/Ni alloys is due to the difference in nature of the corrosion mechanism of these alloys is seawater. The difference in the corrosion mechanism of these alloys has been asserted by many investigators (Al-Hajjiand Reda, 1992a; Uhlig and Revie, 1985). Their finding is that 90/10 corrodes uniformly thus avoiding pitting attack and biofoulingwhile 70/30 protects itself by the formation of a passive film. Thus in unpolluted seawater SSA and EDTA or a combination of both can reduce the corrosion rate by forming an adsorbed metal ion complex in the case of 90/10 as shown in Figure 2 and by plugging the active sites of the passive layer in the 70/30 Cu/Ni alloy as shown in Figure 5. While in the presence of sulfide these complexing agents must compete with sulfide ions for the metal ions and since copper sulfide has a low solubility product constant, there will be a higher tendency for the formation of copper sulfide and thus these complexing agents are essentially ineffective. This is particularly true for 70/30 since these complexing agenta must diffuse into the passive layer and this is severely hindered due to the large size of these complexing agents. For the case of 90/10 Cu/Ni alloy SSA is able to protect the 90/10 Cu/Ni alloy and protect the surface from the action of sulfide. At the same time EDTA was found to be ineffective probably due to the instantaneous reaction between sulfide and EDTA metal complex. In a separate experiment in the present investigation it was found that EDTA metal complex has a high catalytic activity for oxidation of sulfide to sulfur through the reaction
-
0.2
2Cu"EDTA + S" 2Cu'EDTA + S (1) Since it is shown in Figure 14 that there is a decrease in the pH of the solutions with time, an alternative reaction is suggested here as follows:
0
-0.2 -0.4 -11
-10
-9
-8
-7
-6
log (I (pA/cm2>> Figure 12. Cyclic voltammetric behavior of 90110 CuiNi alloy in 2 ppm sulfide polluted stagnant and aerated seawater with 5 ppm fuchsin and 5 ppm EDTA added.
uniform attack in aerated stagnant sulfide polluted seawater is a mixture of 5 ppm EDTA and 5 ppm fuchsin. This is clearly demonstrated in Figures 1 and 12. This combination of fuchsin and EDTA is also an effective inhibitor against localizedattack for the 70/30 Cu/Ni alloy in aerated stagnant sulfide polluted seawater as demonstrated in Figures 4 and 13.
2Cu"EDTA
+ S"
-
+
2Cu'EDTA sulfide oxidation products + H+ (2) The type of sulfide oxidation products that formed could not be determined, but it can be suggested according to the Latimer oxidation-potential diagram of sulfide in seawater that the products of reaction are mainly sulfur with some SIv and Svl. For both copper-nickel alloys fuchsin is an excellent inhibitor due to its ability to complex sulfide in the homogeneous phase and thus deprive the solution of the free sulfide ions. In support of the concept of different corrosion mechanisms of 70/30 and 90/10 Cu/Ni alloy is
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 961 JC 1.4 ppm
Sulfide
Acknowledgment
-.A- 2 ppm Sulfide t Fuchsin - - 2 ppm Sulfide t Fuchsin + EDTA
-.a-Zppm Sulfide t Fuchsin + EDTA + SSA . 0 . Z ppm Sulfide t Fuchsin t SSA -B
- 2 ppm Sulfide t SSA
This work was financially supported by Kuwait University Research Unit under Contract No. EM-067.This support is gratefully acknowledged.
- . o - 2 ppm Sulfide + EDTA Literature Cited
0
5
10
15
20
25
EXPOSURE TIME (hours) Figure 14. pH of the stagnant and aerated seawater containing various additives with time in the polarization cell. Each organic compound is added in a concentration of 5 ppm.
the effect of EDTA in the presence of fuchsin and sulfide. For 90/10Cu/Ni alloy fuchsin has already complexed all the sulfide ions in the homogeneous phase and EDTA complexes all the metal ions resulting in an excellent combinationfor protecting the alloy, while for 70/30 where there is a passive layer present fuchsin complexationwith sulfide and EDTA have a minor role in protecting the alloy. From the above mechanisms on the effect of EDTA, SSA, and fuchsin in the corrosion of copper-nickel alloys in polluted and unpolluted seawater, a new explanation has become possible to explain the effect of sulfide on the corrosion of copper-nickel alloys and the associated discrepancies which are often reported in the literature on the deleterious sulfide concentration. The discrepancies can be attributed to the presence of trace organic compounds which might be present. In this investigation it can be asserted that these organic compounds can significantly affect the corrosiveness of the sulfide pollutant in the seawater.
Summary It is concluded in the present investigation that 5 ppm fuchsin is an excellent inhibitor for 70/30 Cu/Ni alloy against localized and uniform corrosion in stagnant and aerated sulfide polluted seawater. Furthermore it is concluded that for 90/10Cu/Ni alloy EDTA will improve the inhibitive ability of fuchsin against localized and uniform attack. For the 70/30 Cu/Ni alloy EDTA will hve no effect. This difference is due to the different corrosion mechanisms of the two alloys. In the absence of fuchsin it is found that EDTA and SSA are ineffective inhibitors for the sulfide polluted seawater even though they are found to be successful in reducing the corrosiveness of unpolluted seawater. It can ascertained that trace organic compounds can significantly affect the corrosiveness of the sulfide polluted seawater.
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Received f o r review September 21, 1992 Revised manuscript received February 16, 1993 Accepted February 23, 1993
