MSU RETURNING MATERIALS: PIace in book drop to ”saunas remove this checkout from —,—» your record. FINES wiII be charged if book is returned after the date stamped below. I‘ '{I - EFFECT OF SURFACTANTS UPON CORROSION INHIBITION OF HIGH-STRENGTH 7075-T6 ALUMINUM ALLOY By Freddy Castelianos A THESIS Submitted to Michigan State University in partial fulfiITment of the requirements for the degree of MASTER OF SCIENCE Department of MetaTTurgy. Mechanics and Materia] Science 1985 ABSTRACT EFFECT OF SURFACTANTS UPON CORROSION INHIBITION OF HIGH-STRENGTH 7075-T6 ALUMINUM ALLOY By Freddy Castellanos A potentiodynamic polarization technique has been used to investigate the effectiveness of various surfactants and borax-nitrite formulations upon the corrosion inhibition of 7075-T6 aluminum alloy in chloride-containing solutions. Small additions of surface-active agents provide effective protection against general corrosion to Al 7075-T6 in the presence of low chloride concentrations. The passive film formed by these surfactants and the borax-nitrite inhi- bitor, however, does not provide good protection against attack at high chloride concentrations. Most of the surface-active agents evaluated behaved similarly in the extent of protection to Al 7075-T6 against general corrosion, when used with and without inhibitor. Furthermore, these surfactants interacted synergistically with the borax-nitrite inhibitor formulations to give a better protection to this alloy. The effectiveness of these surfactants in the corrosion inhibition of high-strength 7075-T6 aluminum alloy is discussed from the results of the anodic polarization measurements. ACKNOWLEDGMENT The author wishes to express his apreciation to his major proffesor, Dr. Robert Summitt, for his counsel and encouragement during the investigation and for his helpful suggestions on the manuscript. ii TABLE OF CONTENTS LIST OF TABLES .................................... LIST OF FIGURES ................................... I. INTRODUCTION .......................... II. THEORETICAL BACKGROUND ................ III. APPARATUS AND EXPERIMENTAL PROCEDURE ... IV. EXPERIMENTAL RESULTS .................. V. DISCUSSION ............................ VI. CONCLUSIONS ........................... REFERENCES ........................................ LIST OF TABLES Table 1 Inhibitors Formulations ........... #0)“) Mechanical Properties of Al 7075-T6 iv Surface-Active Agents ............. Chemical Analysis of Al 7075-T6 ... Figure 10 11 12 LIST OF FIGURES Theoretical Electrochemical System with Two Oxidation-Reduction Reactions ............... Corrosion Cell for Polarization Measurements... Experimental Arrangement .................... Anodic Polarization of Al 7075-T6 in 1.0 wt. % NaCl Solution without Inhibitor ............. Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl Solution without Inhibitor ............. Effect of Increasing Chloride Concentration upon Anodic Polarization of Al 7075-T6 without Inhibitor ................................... Anodic Polarization of Al 7075-T6 in 0.58 wt. NaCl Solution with Inhibitor Formulation I Anodic Polarization of Al 7075-T6 in 0.02 wt. NaCl Solution with Inhibitor Formulation I Effect of Increasing Chloride Concentration upon Anodic Polarization of Al 7075-T6 with Inhibitor Formulation I ..................... Anodic Polarization of Al 7075-T6 in 0.58 wt. NaCl Solution with Inhibitor Formulation II Anodic Polarization of Al 7075-T6 in 0.02 wt. NaCl Solution with Inhibitor Formulation II Effect of Increasing Chloride Concentration upon Anodic Polarization of Al 7075-T6 with Inhibitor Formulation II ..................... Page 28 29 3O 32 33 Figure Page 13 Effect of Surfactant upon Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl Solution with Inhibitor Formulation II ........................ 35 14 Effect of Surfactant upon Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl Solution with Inhibitor Formulation III ....................... 36 15 Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl, 0.01 wt.% Zonyl FSA Solution without Inhibitor ....................................... 38 I6 Anodic Polarization of Al 7075-T6 in 0.02 wt.%‘ NaCl, 0.01 wt.% Hamposyl L-30 Solution without Inhibitor _ ............................... 39 17 Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl, 0.01 wt.% Polystep B-12 Solution without Inhibitor ............................... 40 518 Effect of Surfactant upon Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl Solution without Inhibitor ..... ' .......................... 41 19 Anodic Polarization of Al 7075-T6 in 1.0 wt.% NaCl, 0.01 wt.% Hamposyl C Solution without Inhibitor ........................ . .............. 43 20 Anodic Polarization of Al 7075-T6 in 1.0 wt.% NaCl, 0.01 wt.% Hamposyl 0 Solution without Inhibitor ....................................... 44 21 Anodic Polarization of Al 7075-T6 in 1.0 wt.% NaCl, 0.01 wt.% Hamposyl L-30 Solution with- out Inhibitor ............... - ................... 45 Figure Page 22 Anodic Polarization of Al 7075-T6 in 1.0 wt.% NaCl, 0.01 wt.% Zonyl FSA Solution without Inhibitor .................................. 46 23 Effect of Surfactant Concentration upon Anodic Polarization of Al 7075-T6 in 1.0 wt.% NaCl Solution without Inhibitor ............ 48 24 Effect of Surfactant upon Anodic Polarization of Al 7075-T6 in 0.02 wt.% NaCl Solution with Inhibitor Formulation IV ................... 49 vii I. INTRODUCTION A. General Introduction This study evaluates the effects of various surface- active agents on the corrosion inhibition by borax-nitrite in aqueous corrosion of aluminum alloys. A potentiodynamic polarization technique was used to investigate the effective- ness of the various surfactants and borax-nitrite formu- lations when a 7075-T6 high strength aluminum alloy was exposed to chloride-containing solutions. This electrochemi- cal technique accelerates the corrosion process, thus, the data may be obtained in a short time. Several surfactants were tested with relation to their effect on general corrosion. As a result, some additions have been found to provide protection to 7075-T6 aluminum alloy. 8. General Background Aluminum owes much of its wide range of applications to its good corrosion resistance. However, high strength aluminum alloys, like all other alloys, are subject to cor- rosion in chloride-containing solutions. Corrosion may be prevented in a number of ways. One way is to isolate the metal from the aggressive environment by coating the surface of the metal. Anodic and cathodic protection by external polarization also is used to reduce the corrosion rate of metals. Another way is to use inhibitors. Inhibitors may be grouped as anodic and cathodic types. The cathodic inhibitor retards the reaction at the cathode and permits the metal to stay at a negative potential (very close to the corrosion potential). The anodic inhibitor suppresses the reaction at the anode and promotes the formation of protective oxide films. Cathodic inhibitors are usually less effective than the anodic type in terms of reduction in corrosion rate. Anodic inhibitors, when used in sufficient quantity are very effec- tive.It is known that two or more inhibitors acting together can act synergistically, having a greater effect than would be expected from the effects observed when they act sepa- rately(1). Several studies have shown that a variety of inhibitors are effective in reducing the corrosion of aluminum and its alloys(2)'(8).Borax-nitrite based inhibitor formulations have been found to be effective as corrosion inhibitors for alu- minum alloys(9)'(11). The use of surface-active agents as corrosion inhibitors for aluminum and its alloys also has been studied(12)’(13). A progam was conducted to determine whether additon of small amounts of surfactants to a borax-nitrite based inhibitor formulation was effective in corrosion inhibition of 7075-T6 aluminum alloy. This study is similar to that performed by Khobaib(2) and others(12). II. THEORETICAL BACKGROUND A. The Basic Electrodic Equation: The Butler-Volmer Equation One of the most important trends in electrochemical kinetics consists of the development of equations which describe the potential-current relationships of an elec- trode. An understanding of the nature and shape of polar- ization curves is important fliprmflfical studies of cor- rosion phenomena. Since corroding systems are not at equilibrium our interest is in electrode kinetics. Excel- lent reviews of the electrode kinetics are given by <14).(15) (la-(23), Bockris and others 1. Electrode Reactions When an inert metal is immersed in a solution containing an oxidation-reduction system, electron-transfer reactions may occur. Electron transitions take place from the metal or electrode to the oxidized component of the system and from the reduced component to the metal. We will designate the forward reaction A“ + ne' 3.2-} A(a'")+ as the cathodic reaction, and the reverse reaction as the anodic reaction. The rate of the cathodic reaction can be written as v = KCC c 1 exp (- BFA¢ /RT). . (1) 3 4 and similarly, the rate of the anodic reaction can be written va = KaC2 exp ((1-8 )FA¢ /RT), (2) where Kc and Ka are the rate constants for the cathodic and anodic reactions, respectively, and are independent of poten- tial and concentration. The quantities C1 and 02 are the con- centrations of the reacting ions at the immediate surface of the electrode, A¢ is the potential difference between the electrode or metal and a reference electrode, e.g., hydrogen or saturated calomel electrode (SCE), 3 is a factor greater than zero but less than unit, called the symmetry factor, F and R are the Faraday constant and gas constant, respectively, and T is the absolute temperature. These rates may be expressed in terms of current density as ic = -Fvc, ' (3) and ' ia = Fva, (4) where v and F have the units moles/cmZ-sec and amperes times second per mole, respectively, then the units of i are amperes per square centimeter. The above equations also can be written as 1' c -FKCC1 exp (- B FA¢ /RT), (5) and ia = FKaC2 exp ((I-B )FA¢ /RT). (6) Anodic currents are positive and cathodic currents negative. The net current density for the reactions will be i=i+i. (7) 2. The Exchange-Current Density At equilibrium the rates of the cathodic and anodic reactions become equal, thus, there is no net current. The currents corresponding to these reactions are equal in magnitude and opposite in direction, i0 = FKCC1 exp (-8 FmfiVRT) = FKaCZ exp((I-8)FmfiVRT), (8) where i0 is defined as the exchange-current density and the equilibrium potential will be A¢°= RT/F(1-28) In KcCl/KaCz. (9) 3. The Polarized electrode If the potential difference between the electrode and the reference electrode is changed from the equilibrium value, the cathodic current density and anodic current density are un- equal to each other. Thus, there is a net current density and the magnitude of this will depend on the change in the poten- tial difference introduced. The net current density can be written i = i0 (exp ((1-B)F(A¢ -A¢0)/RT) - exp (~BF(A¢ -A¢°)/RT)), (10) where Apis different from A¢°and is defined as the non-equi- librium potential difference. The difference A¢-A¢° is de- fined as the overpotential n , and measures how much the potential difference has departed from the equilibrium poten- tial value, thus *1 = 4¢ - A¢°. (11) Equation (10) can be written 1 = i0 ( exp ((1-8)Fn /RT)- exp (-BFn /RT)). (12) Equation (12) is a fundamental equation in electrode kinetics known as the Butler-Volmer equation(15)’(22)’(23) . At positive overvoltage i is positive, so that the net current is anodic, and at negative overvoltage, i is negative, therefore, the net current is cathodic. If the electrochemical reaction occurs at large anodic overvoltage, i.e., atn>> RT/F, the second term of equation (12) can be neglected, then, it becomes i = i0 exp ((1-8)F n/RT).. (13) If the electrochemical reaction occurs at large cathodic . overvoltage, i.e., at n< *- A¢ 2)/RT)- 6XP(-82F(A¢ *- A¢2)/RT)). (22) where A¢ * is the corrosion potential (potential difference (21) between the metal and the reference electrode), and it lies between A¢1 and A¢ 2. The subscript 1 refers to the anodic reaction and the subscript 2 to the cathodic reaction. Given the values of i i andA¢ 1,A¢ 2, one can compute the oa’ 0C corrosion potential, Ad *. The net current density is given by i = icorr (9XP((1-81F(A¢ -A¢ *)/RT)- 8XP(-82F( A¢ - 11¢ *)/RT)). (23) This equation resembles the Butler-Volmer equation for a single reaction, where the corrosion potential and corrosion current density have replaced the equilibrium potential and the exchange-current density, respectively. For the anodic reaction, i.e., i >i , the current i is a c I = 1correxp((1-81)F(A¢ - A¢*)/RT). (24) and, if ic< ia(cathodic reaction), Equation (23) becomes 1 = icorrexp(-82F( A¢- A¢*)/RT). (25) Equations (24) and (25) can be written as A¢ - A¢*= RT/(1-31)F In i - RT/(1-81)F In i (26) corr’ 12 and mp- A¢*= -RT/32F ln i + RT/BZF ln icorr’ (27) Both, Equations (26) and (27) can be reduced to the form of the Tafel equation. Extrapolation of Tafel branches to their interaction at the corrosion potential gives the corrosion current density and is shown in Figure 1. Hence, the Tafel extrapolation technique may be used to determine anodic and cathodic Tafel slopes and the corrosion current density. III. APPARATUS AND EXPERIMENTAL PROCEDURE It is well known that corrosion processes can be ex- plained in terms of electrochemical reactions, for this reason, electrochemical techniques often are employed to evaluate general corrosion. One such technique is the polari- zation method which permits studies of the kinetics of cor- rosion phenomena and their reaction mechanisms. The measured current during polarization experiments is related to the reactions taking place. The procedure is to polarize a working electrode anodically or cathodically with respect to a reference electrode, and to measure the current associated with the potential change. Figure 2 shows a cell for polarization measurements. After sufficient data have been obtained a polarization curve relating current to po- tential is plotted. Information regarding to corrosion behavior of different materials in different solutions are obtained from the polarization curves. They also can be used to evaluate the effectiveness of corrosion inhibitors and surface-active agents, to calculate corrosion rates, and to detect changes of corrosion with time. A. Equipment The experimental arrangement used in this investigation is shown in Figure 3. the measurements were conducted by means of a potentiostat/galvanostat, corrosion cell, and electrome- 13 14 Working Electrode /’ Solution Bridge Auxiliary 77 7 Saturated Electrode ‘ A Calomel Electrode 'r—' .FIH’//’ -——-l —— ———-J 1 — L: h- Luggin } r’,I.2a1:!illary (“1 Solution Under Study 77,? l \ / Magnetic Stirrer FIGURE 2 CORROSION CELL FOR POLARIZATION MEASUREMENTS. 15 Potentiostat / AUX o w o E o REF I 3 Electrometer / Probe Solution Bridge r SCE Reference "’Electrode x, Polarization Cell HeFerence Cell FIGURE 3 EXPERIMENTAL ARRANGEMENT. 16 ter probe. 1. Cell The test cell used was a glass beaker containing the so- lution in which the working electrode, the auxiliary electrode, a Luggin capillary with salt bridge connection to the reference electrode and a thermometer were inserted. 2. Electrodes a. Working Electrodes The working electrodes for this investigation were rec- tangular pieces, with width and thickness of 0.31 mm and 0.16 mm, respectively. b. Auxiliary Electrode A platinum screen electrode was employed as the auxylia- ry electrode to transfer current to or from the working elec- trode. After using the platinum-screen counter electrode, no material is retained on its surface which might contaminate subsequent experiments. c. Reference Electrode A saturated calomel electrode (SCE) was used as the refer- ence electrode. A solution bridge from the Luggin capillary 17 to another beaker containing the reference electrode made the electrical connection between this electrode and the solution. This liquid junction was used to avoid contamination of the test solution. 3. Potentiostat The potentiostat maintains the working electrode at a constant potential with respect to the reference electrode. A Princeton Applied Research (PAR) Model 173 was the potential connmfller for this investigation. Additional details are pre- sented elsewhere(31). 4. Electrometer Probe In conflumtflNI with the PAR Model 173 potentiostat, a PAR Model 178 electrometer probe was utilized to monitor the po- tential at the reference electrode. B. Solutions All solutions were prepared using distilled water just before each experiment. Concentrations are expressed throughout on a weight percentage basis. All tests were conducted in unstirred solutions at room temperature. The specimens were tested in solutions with different concentrations of sodium chloride with and without inhibitorS' 18 and commercial surface-active agents. The inhibitors and surface-active agents are listed in Tables 1 and 2, respec- tively. C. Specimens 1. Material For all the measurements bare rectangular sheets of high strength 7075-T6 aluminum alloys were employed. The chemical analysis and mechanical analysis for this Al alloy are given in Tables 3 and 4, respectively. 2. Specimen Preparation The working electrodes were mechanically polished up to 600 emery paper followed by cleaning in acetone, degreasing throughly in petroleum ether, then rising in distilled water, and finally drying in a stream of air. 0. Experimental Technique Before the corrosion cell was assembled, it and its com- ponents were cleaned using detergent followed by rinsing in distilled water. The solution under study was poured into the glass vessel and the working electrode, counter electrode, thermometer and Luggin capillary were immersed. Then the so- 19 TABLE 1 Inhibitor Concentrations, weight per cent. Sodium Borate Sodium Nitrate Sodium Nitrite Sodium Metasilicate Pentahydrate Sodium Hexameta- phosphate Mercaptobenzo- thiazole (MBT) SUM I 0.35 0.1 0.05 0.01 0.002 0.001 0.513 II 0.35 0.22 0.11 0.01 0.002 0.001 0.693 III 0.35 0.22 0.11 0.01 0.002 0.692 0000 IV .198 .124 .061 .006 .0013 .0006 .3909 m Hamposyl C Hamposyl 0 Hamposyl L-30 Polystep 8-12 Zonyl FSA 4-(4-Methyl-1-Piperi- dinyl)-Pyridine 1,3-Di-(4-Piperidyl)- Propane 20 TABLE 2 Surface-Active Agents Class Anionic Anionic Anionic Anionic Anionic Cationic Cationic M’s. Sarcosinate Sarcosinate Sarcosinate Ethoxylate Sulfate Fluorosurfac- tant Tertiary Amine Alkyl Amine Form Liquid Liquid Liquid Liquid Liquid Liquid Flaked Solid Source W.R. Grace & Co. W.R. Grace & Co. W.R. Grace & Co. Stepan Chemical Co. Du Pont Reilly Tar & Chemical Co. Reilly Tar & Chemical Co. 21 TABLE 3 Chemical Analysis of Al 7075-T431) Element Zn Mg cu Cr Mn WT- % 5.52 2.76 1.41 0.23 0 22 TABLE 4 Mechanical Properties of Al 7075-T6(31) Tensile Strength Yield Strength Hardness (psi) (psi) (Rockwell) Specimen 72,000 66,000 76RB Handbook 83,000 73,000 BSRB 23 lution bridge junction from the Luggin capillary to the re- ference electrode, which was immersed in another beaker con- taining the same type of solution under investigation, was connected. The Luggin capillary was employed in order to minimize errors in the measurements of potentials caused by IR drop through the electrolyte. The experimental instrumentation set-up is shown in Figure 3. All experiments were carried out in accordance with the ASTM Standard 65-82, "Standard Recommended Practice for Standard Reference.Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements." Specimens were immersed in the solution for three hours before the commencement of the polarization. The potential attained by the working electrode after this equilibration period was taken as the corrosion potential. Then, the cor- rosion potential was changed anodically (the potential was increased) at a rate of 10 mv per minute for about three hours, and the corresponding current, betWeen the working electrode and the counter electrode, recorded during this time. All potentials are reported with reference to the sat- urated calomel electrode (SCE) as specified. IV. EXPERIMENTAL RESULTS A. Effect of Chloride Concentration Figures 4 and 5 show the anodic polarization of 7075-T6 Aluminum alloy for concentrations of 1.0 wt. % and 0.02 wt. % of NaCl without inhibitor, respectively. Figure 6 shows the effect of increasing chloride concentration upon the anodic polarization of Al 7075-T6. From these figures it can be seen that the corrosion potential of Al 7075-T6 depends on the solution concentration, i.e., the higher the concentration of chloride ions, the lower the corrosion potential, Additionally, the current density is dependent upon the chloride concen- tration; increasing current densities corresponds to increasing concentration of chloride ions. These results agree with those obtained before(31) . The corrosion current density for the anodic profile corresponding to 1.0 wt. % NaCl may be obtained by means of the Tafel extrapolation technique; it was found to be about 10,000 uA/cmz. The same cannot be said for the anodic polarization of Al 7075-T6 in 0.02 wt. % NaCl so- lution due to the absence of a long enough linear region. Figures 7 and 8 show the anodic polarization behavior of Al 7075-T6 in 0.58 wt. % NaCl and 0.02 wt. % NaCl solutions, both solutions containing inhibitor formulation 1. The difference in the amount of passivation is significant. In fact, a complete breakdown of passivity occurs at high chloride concentrations. In Figure 9, it can be seen the 24 D) 25 FIGURE 4 10 103 ( uA/cmz) 10 10 ANODIC POLARIZATION 0F AL 7075-T6 IN 1.0 WT.% NACL SOLUTION WITHOUT INHIBITOR (V vs SCE) 26 )- " 1 + t f" L— 1- f 4 .1, .. f: l .1 _4 it If; . c [,1 I )- _Mw—r—Jrr’rd“ 101 102 103 10 ( u A/ cmZ) FIGURE 5 ANODIC POLARIZATION 0F AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITHOUT INHIBITOR (V vs SCE) Ls] fa) In) («I 27 X-- 1.0 wt.% NaCl, no inhibitor D-- 0.02 wt.% NaCl, no inhibitor #43” ..... _.—.—-—--"'”‘—"” _..BF'H'H—F‘ I EF’“—T—d 4 101 102 103 104 10 ( llA/sz) FIGURE 6 EFFECT OF INCREASING CHLORIDE CONCENTRATION UPON ANODIC POLARIZATION 0F AL 7075-T6 WITHOUT INHIBITOR (V vs SCE) 28 SOLUTION WITH INHIBITOR FORMULATION I . 8 . 7 1 .e . . 5 _ . 4 _ . ... 11:. .1 . 2 1; i .1 4F .1 ~ 0 '£ 31 ‘ '.1 '4: f _T '. 2 V. f ' . 3 [5: 4 ,1“ n4 1 ay’ . .5 (1,247! -. a ’f -+ '-7, --------------- _ 7.8 ‘ 1 1 4 1 101 102 103 104 105 106 ( uA/cmzl FIGURE 7 ANODIC POLARIZATION 0F AL 7075-T6 IN 0.58 WT.% NACL (V vs SCE) 29 FIGURE 8 71- EA: 8838 7) ,cl- .. .41“ -1 IF .4 .1 ..li*' a I I‘ll)— _ .1 *- .1 .21 . .31» .1 .4” “FiL .1 I»: .51- )? _1 .e» .fif . 7L Er’fi’fl,fiEY' fi F 2 10,1 1 . _ - 101 ( uA/cmz) ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITH INHIBITOR FORMULATION I (V vs SCE) 30 (v vs SCE) (TIA/cmz) FIGURE 9 EFFECT OF INCREASING CHLORIDE CONCENTRATION UPON ANODIC POLARIZATION OF AL 7075-T6 WITH INHIBITOR FORMULATION I C3 -- 0.02 wt.% NaCl,inhibitor I X -- 0.58 wt.% NaCl,inhibitor I 1: .1- .1 .1- -$ 1: .1- .1- .1' .1- 1r .1- ,. i‘ g/‘ .cuef/T 102 103 104 10 31 effect of increasing chloride concentration upon the break- down of the passivity. It has been found(12) that a loss of passivity, using a borax-nitrate based inhibitor formulation, occurs above about 4.8 wt. % of NaCl. Figures 10 and 11 show the anodic profiles of this alloy for the same concentrations of chloride ions (0.02 wt. % and In 1.0 wt. % , respectively), but in solutions containing inhi- bitor formulation II. In this inhibitor formulation the con- centration of nitrate and nitrite was increased with respect to the inhibitor formulation I. As it is known, nitrate is (32) used to prevent corrosion of aluminum and aluminum (12)’(33), in the presence of chloride-containing so- alloys lutions. From Figure 12 it can be observed that there is a very small change in the degree of passivity, even though, the increase in nitrate and nitrite concentrations was small. B. Effect of Inhibitors and Surfactants Figures 13 and 14 show the effect of small additions of Polystep B-12 surfactant upon the anodic polarization of 7075-T6 aluminum alloy. As can be noticed from Figure 13, the corrosion potential has been moved in the active direction, by the addition of surfactant, from -0.689 volts to -0.726 volts. The corrosion potential was somewhat altered by the addition of Polystep B-12 to the inhibitor formulation II. The same cannot be concluded for the addition of this surfactant to (.0 («I M 32 I .{i - j . .Z .T ..F/ -aw” . J ________ 4 101 102 103 104 105 106 ( uA/CmZ) 1 FIGURE 10 ANODIC POLARIZATION 0F AL 7075-T6 IN 0.58 WT.% NACL SOLUTION WITH INHIBITOR FORMULATION II (V vs SCE) 33 Ul 1 Ln: b} T L FIGURE 11 1 ( uA/cmz) ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITH INHIBITOR FORMULATION II 10 (V vs SCE) 34 c: -- 0.02 wt.%NaCl,inhibitor II X -- 0.58 wt.%NaCl,inhibitor II ~0-n -.-‘-‘ _ —-' .- - F‘. .- ‘- - -- .--- ...-o —-' ._.- --- -- --- -- 103 10 2 ( uA/cm ) FIGURE 12 EFFECT OF INCREASING CHLORIDE CONCENTRATION UPON ANODIC POLARIZATION 0F AL 7075-T6 WITH INHIBITOR FORMULATION II 10 (V vs SCE) 35 X--0.01 wt.% Polystep B-12 c:-- no surfactant ,r' [3§:§;;r”' Lu T ob “ L 10‘ 1 O L__....J (u A/cmz) FIGURE 13 EFFECT OF SURFACTANT UPON THE ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITH INHIBITOR FORMULATION II (V vs SCE) 36 3’13" 0.01 wt.% Polystep 3-12 .8 h X-- no surfactant FIGURE 14 ( uA/cmz) EFFECT OF SURFACTANT UPON ANODIC POLARIZATION 0F AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITH INHIBITOR FORMULATION III (V vs SCE) 37 inhibitor formulation III. In this case, from the anodic profiles, it is evident that the corrosion potential is almost the same for both solutions, it is about -0.525 volts. It must be pointed out that formulation III does not have MBT like the other inhibitor formulations. However, the addition of Polystep B-12 demonstrated the ability for a little increase in passivation against chloride attack at low concentrations of chloride ions. Figures 15,—16 and 17 show the anodic polarization of Al 7075-T6 in 0.02 wt. % NaCl solution without inhibitor for different surfactants. Zonyl FSA, Hamposyl L-30 and Polystep B-12, respectively. In Figure 18 is shown the effect of small additions of surfactants (Zonyl FSA), in anodic polarization behavior of 7075-T6 aluminum alloy, to 0.02 wt. % NaCl so- lution with no inhibitor. The anodic profiles shown in Figures 15, 16 and 17 contain no discontinuities. It is evident that a film was formed but no passivation occurred as a result of absence of inhibitor formulation. This suggests that the surfactants have a synergistic effect. The cor- rosion potential of the corroding Al alloys was independent of the addition of different surfactants to the solution, as well as, the corrosion current density. The corrosion current density corresponding to these anodic profiles was found to be about 150 uA/sz. Comparing Figures 15, 16 and 17, one can say that these surfactants have the same effect on the anodic polarization behavior of Al 7075-T6 in solutions with FIJI“ IIIi. .9: .. .. . . ... .k: IIITIL. ll. . » .5 III. 'llv Lil Ill? .... .1111... III I. L. 15" kill}- LIPII IL ‘ ~ . . 5 e s a v ~ \I U! Cl M 38 FIGURE 15 _f‘ )— (I .1 I. t {ff 1 F 11 _ L If _, _ ...! _1 . ,4 y?” '- .: [I ‘l .. 24/ ' _ . _d_.,u———#””’TTTI’ a __ M‘T_ — _{ I V 1 101 102 103 104 (llA/sz) ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL, 0.01 WT.% ZONYL FSA SOLUTION WITHOUT INHIBITOR (V vs SCE) ...a IF) («I LO 0. 39 _ 4 1:: 4 1; I .j 4 F- 2 o d L I 4 '5 E I ‘ -4 l- .f .j d I g _ ._.(,/' " F ruff” . q ..... . _ ..j;é..r":’"_ 1y; ............................... .1 1 101 102 103 10 ( PA/cmz) FIGURE 16 ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL, 0.01 WT.% HAMPOSYL L-3O SOLUTION WITHOUT INHIBITOR (V vs SCE) 4O _l ' i .‘u' J ,1 T 'L- 1'1 I L 1! l _ 1 I. A I _ ,4 _1 - .1 l g ' _ {,1 . _ ff: g f’ " E 3/ ~ 1 "'6'”,— T dfpdiewr’flfl' 4 ~~_flfld__fim. war“ % 1 101 102 103 10 ( pA/sz) FIGURE 17 ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL, 0.01 WT.% POLYSTEP B-12 SOLUTION WITHOUT INHIBITOR (V vs SCE) 41 FIGURE 18 EFFECT OF SURFACTANT UPON ANODIC POLARIZATION 0F AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITHOUT INHIBITOR F X-- 0.01 wt.% Zonyl FSA I' F D-- no surfactant ' f *- .1? 1? b £- . g. 17 I f ‘ .- ‘o’ _‘ :f ' i ?- --( >- J I- -1 l- .. 1 ‘ 101 102 103 10- (u A/cmz) (V vs SCE) 42 low concentrations of chloride ions without inhibitor. The anodic profiles in Figure 18 show that the corrosion current density is decreased when small amounts of these surfactants were added to the solution. By means of the Tafel extrapo- lation technique it was found that the corrosion current density was reduced from about 1500 pA/cm2 to about 150 uA/cm2 The corrosion potential was independent of the addition of these surfactants. Figures 19, 20, 21 and 22 show the anodic polarization behavior of Al 7075-T6 in 1.0 wt. % NaCl solution without inhibitor for different surfactants. The anodic polarization behavior of the corroding aluminum alloys in these solutions containing small addition of surfactants is similar to that with low concentration of chloride ions. Again, the anodic profiles show no discontinuities associated with film for- mation and no passivation. The corrosion potential seems to be independent of additions of different surfactants to so- lutions without inhibitor. Comparing these figures to Figure 4, it may be also noticed that at high concentrations of chlorides ions, the corrosion current density is independent of the addition of these surfactants. However, the addition of Hamposyl C slightly reduced the corrosion current density, and it may be suggested that this surfactant provides better protection to 7075-T6 Al alloy than that rendered by the other at the same 01' concentration. Obviously, at this con- centration of Cl' ions, the values of the corrosion current Ln (.4. h) 43 FIGURE 19 ANODIC POLARIZATION OF AL 7075-T6 IN 1.0 WT.% NACL, 0.01 WT.% HAMPOSYL C SOLUTION WITHOUT INHIBITOR +- u- P- .. .1;- .. 1.; . .1. L 1 _ p 1:- L 1:1 .1. l... 1. .. . I.» 1.. 1.. I .1 _ .1 t: q .é' - .1, _ if _ ,3; - " 2/ - .4 24/ ' _ {a —l ..... :3/ fi—e——-4 A 1 101 102 103 104 105 106 ( uA/CmZ) (V vs SCE) 44 I" . I + ...!5 I“ _ .1: .1: _ 1 1 4 .1 1 I .g . I ._ "’2’, ' .. ---"--"-11‘é""fl' ;+;; — -------------------- ‘1 101 102 103 104 105 106 ( ll A/cmz) FIGURE 20 ANODIC POLARIZATION 0F AL 7075-T6 IN 1.0 WT.% NACL, 0.01 WT.% HAMPOSYL 0 SOLUTION WITHOUT INHIBITOR (V vs SCE) LU r_n (.1 t-J 45 FIGURE 21 ANODIC POLARIZATION OF AL 7075-T6 IN 1.0 WT.% NACL, 0.01% HAMPOSYL L-30 SOLUTION WITHOUT INHIBITOR . 1 I t I 4 ~ 1 I u' I l 1 1 .11 I- ! “I I t’ 4 if +- If ‘1 ” I: - I +- I-1 " 1; r- ' - i ‘4 1% .— .1 J "1 r ,r/ " X/‘J » _____ "436/ .. + E—P—P—" """ ‘ 1 101 102 103 1o4 105 1o ( uA/sz) (V vs SCE) 46 0.01 WT.% ZONYL FSA SOLUTION WITHOUT INHIBITOR ~ I 1. H I. 1 '1 _ _.I I >- .5 J I I .1: - I: " 3% 1 L' .1; .4 1‘47 I 1? ~{ {/13 1 101 102 103 104 1o5 106 ( uA/cmz) FIGURE 22 ANODIC POLARIZATION 0F AL 7075-T6 IN 1.0 WT.% NACL, (V vs SCE) 47 densities are greater than those with 0.02 wt. % of NaCl. In Figure 23 are shown the anodic polarization curves of 7075-T6 Al alloy in 1.0 wt. % NaCl solution without inhi- bitor for different concentrations of Hamposyl O surfactant. In these experiments there was no change in corrosion poten- tial, nevertheless, there was a slight reduction in corrosion current density when the concentration of surfactant was doubled. It suggests that by increasing the surfactant concen- tration, better protection could be achieved. Figure 24 shows the anodic profiles of Al 7075-T6 in 0.02 wt. % NaCl solution with inhibitor formulation IV for different surfactants. One can notice that when the surfac- tants were used with inhibitor, the inhibition was very good. In these tests the corrosion potential was dependent on the added surfactant. The shape of the curves suggests passiva- tion against chloride attack. All the surfactants tested exhibited similar effects but did not increase the amount of passivation against chloride attack, with the exception of I1,3-Di-(4-Piperidyl)Propane, which caused a decrease in the passivation. It is suggested that this surfactant may have a synergistic effect that is negative with respect to passiva- tion against chloride attack. U! '24 M 48 (1)-- 0.005 wt.% Hamposyl 0 (2)-- 0.01 wt.% Hamposyl 0 i D l l L 103 IO4 105 ( uA/cmz) FIGURE 23 EFFECT OF SURFACTANT CONCENTRATION UPON ANODIC POLARIZATION 0F AL 7075-T6 IN 1.0 WT.% NACL SOLUTION WITHOUT INHIBITORS IO1 10 10 (V vs SCE) 49 .9 - o--0.01 Wt.%4-(4-Methyl-1-Piperidi- I nyl)-pyridine 1F .3 F X--0.01 wt.%1,3-Di-(4-Piperidyl) g— . Propane 3;: .7 - 0--0.01 wt.%Zonyl FSA g} i ?--0.01 wt.%Polystep B-12 .1 .br-45--0.01 wt.%Hamposyl L-30 ,u’ - +-- no surfactant ,I' 1 .4 - "Q' I .3»— I .1 " 4. C) r- .4 .1 *- '1 .3 - _. .4 ~ 1 b I‘ .1 . I j .7 A 10‘1 1 101 ( uA/sz) FIGURE 24 EFFECT OF SURFACTANT UPON ANODIC POLARIZATION OF AL 7075-T6 IN 0.02 WT.% NACL SOLUTION WITH INHIBITOR FORMULATION IV (V vs SCE) V. DISCUSSION Since the corrosion rates are so much smaller for alumi- num and its alloys in neutral and near-neutral salt solutions than in markedly acid or basic solutions, it is not easy to evaluate the usefulness of the surface-active agents used in this study After evaluating the anodic polarization behavior of corroding 7075-T6 Al alloys for different combinations of in- hibitor formulations, surfactant type and concentration, and chloride concentration, it was found that almost all of the surfactants used in this investigation showed similar effects in the extent of protection of Al 7075-T6 against chloride attack. These surfactants when used with the inhibitor formula- tions provided good protection to this Al alloy in chloride- containing solutions at low concentrations of 01' ions. However, when the chloride concentration was increased the addition of surfactants did not provide sufficient protection to Al 7075-T6 to general corrosion. There was evidence of film formation but no passivation. This suggests that the passive film formed, by the different tested surfactants combined wiht the borax-nitrite inhibitor, is ineffective to protect this alloy when the chloride ions are present in high concentrations. The effectiveness of these surfactants when used without inhibitor formulation was very poor at low and high concen- SO 51 trations of chloride ions. It can be suggested that the surfactants evaluated in this study have a synergistic effect. Similar results were obtained by Khobaib(12) with other surface-active agents. OAlthough, the effectiveness rendered by these surfactants was very poor in the absence of the borax-nitrite inhibitor, it could be suggested, from the anaIysis of Figure 23, that this weak protection can be improved by increasing the concen- tration of surfactant. From this figure one can say that the increase in surfactant concentration did not passivate the Al alloy but slightly reduced the corrosion current density, i.e., a film was formed on the specimen surface. Then, it can be expected that when used in higher concentrations and combined with the inhibitor formulation better protection could be achieved. The study of the inhibition mechanism of the surfactants was not within the scope of this research. However, some studies have been done in this field. A mechanism‘of inhibition by anodic surfactants for aluminum is suggested by Vermilyea(13). He suggests that anodic surface-active agents become attached to the A1203 surface by the inorganic group, forming a hydrophobic surface, preventing access of water to the surface and hence preventing dissolution of the Al203. According to Khobaibuz), it seems that surfactants interfere in the dissolution 52 reaction by interacting synergistically with the passive film provided by the borax-nitrite formulation resulting in a stronger protective film. VI. CONCLUSIONS (1) Almost all the surfactants evaluated in this study showed similar effects in the extent of protection against chloride attack to Al 7075-T6. Most of them behaved similar- ly when used with and without inhibitor formulation and at low and high chloride concentrations. (2) The protection rendered by these surface-active agents when used alone was very poor. It suggests that the surface- active agents used in this work interact synergistically with the borax-nitrite inhibitor formulation to give a better protection to 7075-T6 Al alloy against general corrosion. (3) The passive film formed by these surfactants and the borax-nitrite inhibitor is still weak and does not provide good protection to this Al alloy against chloride attack at high chloride concentrations. (4) Small additions of surface-active agents to the inhi- bitor formulation have been found to provide effective pro- tection against general corrosion to Al 7075-T6 in the presence of low 01' concentrations. (5) According to Figure 23, one can suggest that increasing the concentration of surfactant and combining them with the inhibitor formulations, an improvement in protection of this alloy in chloride-containing solutions could be achieved. 53 2. 3. 4. 8. IO. 11. 12. 13. VII. REFERENCES U.R. 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