Design of Corrosion Inhibitors Use of the Hard and Soft Acid-Base (HSAB) Theory Frederick H. Walters University of Southwestern Louisiana, Lafayette, LA 70504 There are three themes that stretch over the breadth of all -areas of chemistry. They are (1) the concept of the periodic table and neriodic trends. (2) the concept of acids and bases, and (3) t i e concept of oxidation-reduction. Corrosion is intimately tied to oxidation-reduction, but until recently little work has been done utilizing the first two concepts. Wulfsberg (I) in his text on descriptive inorganic chemistry does an excellent job of leading the reader through the periodic table, explaining periodic trends and covering aqueous chemistry, redox chemistry (including Pourbaix diagrams) as well as the chemistry of the oxides, halide, sulfides, nitrides, and hydrides. Many graduate inorganic texts such as Huheey (2) also contain chapters on HSAB theory. Pearson has a text that renrints kev -napers . . (3). . . . and Ho (4) has a text applying HSAB to organic chemistry. Of muchhelp are the DaDerS hv Pearson (5-9) and others in the chemical literaElectronegatlvlty and the HSAB Theory The origins of HSAB lie in the studies by inorganic coordination chemists such as Ahrland, Chatt and Davies (15) or Schwartzenbach (16) who observed that metal complexes fell into two cateeories. Metals in Class A were most stable with ligands conraining 0 , N, or F, whereas Class B metals nrefer lieands withP. Se. I. and S. From this the cateeories of hard an; soft acids and bases were developed by Pearson. Reoresentative tables of hard and soft acids and bases can he foind in ref 6. Class A or hard acids are donor atoms of small size and high positive charge with no unshared electrons in the valence shell. They occupy a great deal of the periodic table and have electronegat&itiesbetween 0.7 to 1.6 and are thus the most electropositive metals. Class B or soft acids have a larger size and alower positive charge and are often neutral. They have unshared electron pairs in their valence shell. They are characterized by high electronegativities for metals (1.9 to 2.54) and are the electronegative metals. Simply put, they form a group of metals around gold, which is the most electronegative of all metals (Pt, Pd, Au, Ag, Hg, Ti, Ph). Some metals are on the borderline (Ni, Zn, Cd, Sb, Bi, and Ru), and some metals depend on the oxidation state (Fe3+, C03+, In3+,and Sn4+are hard acids, whereas Fez+,Cozf, In+, and Sn2+ are borderline acids; Rh+ and Ir+ are soft acids, whereas Rh3+ and Ir3+ are borderline acids). Hard bases are those containing oxygen or fluorine. These are elements with high electronegativity and small size. Borderline hases are those of N and C1. Soft bases are those containing S and Br. The softest bases contain C, P , As, Se, Te. and I. Hvdroeen. when it occurs as the hvdride ion (H-), . isalsoa soft hase. soft bases have~aulin~elt&ronegativities of 2.1 to 2.96 and have larae anionic radii. The hard and soft acii-base principle simply stated is that "Hard Lewis Acids tend to combine with Hard Lewis Bases and Soft acids prefer Soft hases." This can be modified slightly ro include.'Less soft acids tend tocombine with less sofr bases and softer acids preier softer bases" ( 1 ) . The influence of oxidation state can be seen in the cases of sultur or haloeens. Hieh oxidation states such as S(+6) in S o l - .--~~~~~ n~ are hard, whereas lower oxidation states such as s(-2) in s2~~
~
-
~
-
are soft. The Pauling electronegativity increases with increasing oxidation n;mher, b u t t h e change is not enough (less than 0.2) to merit assigning separate electronegativities to each oxidation state. The two exceptions to this rule are thallium and lead. Pauling electronegativities also vary with suhstituent groups, but available data are sketchy. In general, electron-donating suhstituents lower the electronegativitv of an atom. and electron-withdrawing substituents on an atom increase'the electronegativity (I). As is noted above the Pauline electronegativity is a key parameter used to classify the different categories. Other electronegativity scales have been proposed (such as Allred and Rochow, Mulliken, Sanderson) and are documented in Huheey (2). Recently electronegativity has undergone changes in definition and calculation, and it has been suggested that electronegativity is the third dimension to the periodic table (17). Leland Allen's work (18)redefines electronegativity as valence shell energy and has translated valence shell energy into a quantum mechanical operator that can he employed in computational chemistry programs. Furthermore, he proposes a simple equation to correct for the affect of electronegativity in formal charge calculations. Parr (19) has developed a model that says electronegativity is the electronic chkmical potential. His model has been adopted bv Pearson into his hard and soft acid-base theory. he-se changes have not been incorporated into the corrosion papers surveyed here. HSAB Scales There have been several attempts to place the hard and soft acid-base theory on a quantitative-basis. Pearson (3) provides reprints and comments on these attempts. Problems arise because of solvent effects and because of the lack of data. There need to he a t least twoscales, one dealing with hardness and softness and the other related to acid-base strength. One of the first attempts was by Edwards PO), who developed the following equation:
K is the stability constant, a and i3 are constants, En is the redox potential, and H is related to the Lewis acidity. This work is done in a polar solvent (H20). Drago and co-workers (21) in studying gas phase and low polarity solvents came up with a four-oarameter eouation. one factor dealing the electrostatic contribution o i a n acid ( E d or hase (E;) and the other factor beine the covalent contribution of an acid (Ca) or base (CB).H i;the enthalpy of the reaction -AH = E,Eb + C,C, (2) Pearson (7, 9) suggests another four-parameter equation that should accurately describe the situation but cautions that absolute values and scales are not available yet and many models are not applicable under all conditions. Appllcatlons of HSAB to Corroslon Since Thomas's review paper (22) presented to the Fifth European Symposium on Corrosion Inhibitors on HSAB in 1980, several European (23) Russian (24) and Japanese Volume 68
Number 1 January 1991
29
workers (25-33) have applied HSAB concepts to corrosion orohlems. Work since the late 1970's by Kunitsugu Aramaki and his co-workers (25-33) a t Keio University has focused on the study of corrosion inhibitors in acidjc solutions and the HSAB principle. All the papers cited but one are in English and quite readable. His work has resulted in a better understanding of the design of corrosion inhibitors and will be discussed below. See the original papers for full details. Polar organic compounds are chemically adsorbed on a metal surface by formation of a donor-acceptor hond between a polar atom of the compound and the metal. Since the c o m ~ o u n dand metal act as a base and acid. resoectivelv. the stagility of the adsorption hond is related to Pearson's HSAB principle. Bulk metals are soft acids, and so soft base inhibitors are most effective for metals corroding in an acid solution. Oxidized metal surfaces are hard acids (metallic ions in the oxide), and thus a hard base is more easily adsorbed on the oxide surface [i.e., studies of anodically polarized nickel (27,31,33) and iron (28,30,32,33)]. The inhibition efficiency I is defined hy: I=*--- i corr (4)
i eorro
where i corrO and i corrare corrosion current densities for the uninhibited and inhibited metal electrodes. The following equation: log(N1- O = p'x
+n
(5)
where x , the electronegativity of the polar atomin theinhibitor molecule, is the means of evaluating the HSAB principle. p', the slope, is a function of the metal and is negative for effective inhibitors. a is the intercept. AC impedance and polarization methods have been used to characterize and study the corrosion inhibitors. Work dealing with corrosion inhibitors applies electronegativities and the HSAB theory to the heteroatoms involved in structurally similar inhibitors and to the hardness or softness of the metal surface. Studies hv Aramaki have concentrated on alkyl derivative8 of group 4A, 5A, 6A, and 7A metals (25-29). Plotsof theelectroneaativitv of the oolar atom versus logtl/l - 0 were linear forhroupi 4A, SA, 6A, and 7A inhibitors thus validatinr the HSAB orinciple. This work was done in perchloric acid.-Group 4A c&pouids were very poor inhibitors, whereas 5A and 6A compounds were good inhihitors. Initial studies on a series of metals in this system concludes that the relative order of softness is Al< V < Cr < Fe < Co < Ni > Cu < Zn. Nontransition metals are harder acids than transition metals, and some studies of main erouo elements indicate a relative order of Me < A1 < In < i n , although some changes in the relationskp were observed for Mg and Al where n-propyl bromide is better than hoth n-propyl chloride and iodide. Indium and tin are softer than maenesium or aluminum. and mouo 4A inhihitors that have iacant d orbitals can possibiy f i r m n bonds with these metals. Studies on the inhibition efficiency of group 6 and 7A inhibitors on Ti and Mn (29) enable one t o place titanium before vanadium and manganese before iron. The slopes of these plots can be related to the relative softness of the metal. The table lists some relative p' values. Values ol p' taken from Aramakl(28)
An anodically polarized nickel electrode was studied (27) in 3 M perchloricacid withgroupP7A inhibitors, Theeffect of these inhibitors on current densities in the active passive and transoassive reeion was discussed in terms of-HSAB theory. Gioup 5 inGbitors suppressed the passive current densitvand were concluded t o chemisorh on the oassive film of the nickel electrode. Group 6 and 7 inhibitors idsorbed on the metal hindered the formation of the passive film and were chemically adsorbed. No significant effect of the group 4 inhibitors on the anodic polarization curves was found. AC impedance studies (31)have also been done. Recently, chloride ion has been introduced to compete with the perchlorate ion (33).The surface coverage o? the compound, esperially if it was a hard base, decreased with an increase in chloride concentration. The inhibition of a soft hase. however, increased with chloride concentration. Since the softness of a metal as an acid is increased by the adsorption of chloride, the compound classified as the soft base is readily adsorbed on the surface where chloride adsorbs. This work is of great interest to the chemist or corrosion specialist, as i t enables further understandina of the inhibitorv effect of chemically adsorbed organic inhibitors. Work on oxidized iron surfaces (28,30,32) shows that the surface behaves as a hard acid. The effect of group 5A inhihitors (R3N, R3P, RaAS, R3Sb; n = n butyl) on the current densities of iron electrodes in active and passive regions of anodic polarization curves and the amount of the comoounds adsorbed onto y - FegOn were related to the HSAB principle. The studies were carkid out in 3 M perchloric acid and in borate buffer (pH 8.45). The passive current density of the iron electrode in the huffer solution gained with the addition of R3N and R3P. These results suggest that hard acid-hard hase interactions occur between the oxidized surface (y - Fe203) and the inhibitors. The adsorption behavior of the polar organic compounds on the oxidized surface of y - Fe203were studied in methanol by means of high-performance liquid chromatography. The iesults showed that the hard hase was more adsorbable on the powder supporting the relationship between the adsorptive behavior and the HSAB principle. Impedance measurements were used to study the corrosion rate of iron in 1M HC104 in the presence of nine different anions (Cl-, Br-, I-, s042-, S2032-,SH-, SCN-. NO?-. Nz-) and/or the oreanic cation tetra n-butvlammonium ion (32). The corrosio> process was either inhibited or stimulated by the addition of these anions. Addition of tetra-n-butylammonium ion suppresses corrosion. Most of the anions were adsorbed onto the electrode at the corrosion potential. Surface coverage of the anions obtained from double-layer capacitance was confirmed hoth in the presence and absence of tetrs-n-butylammonium ion to be closely related to the HSAR principle. The adsorption ahility of > anions on the iron surface follows the order SH- > S,O? * " I- > SCN- > N3- > Br- > Cl- S O I - ~> CIOa-. ConclUSlonS This series of papers applies HSAB principles to corrosion, enabling one to understand better the factors that influence the design of a corrosion inhibitor such as metal, polar atoms, and environment or competing ions. Pearson's simple HSAB theory is able to provide a tool for understanding the influence of these factors. Literature Cited
P' Metal
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.lournal of Chemical Education
I . Wulfsberg,G.Principleaa/DeacripliueChemisfry:B?aokslCole:Montcrey,CA, 1987. 2. Huheey, James E. Inorganir Chemistry, 3rd ed.; Harper and Row: Nea York, NY, 1983. 3. Pearson. R. G. Hard ond Soft Aeida ond Baaes: Dowdan, Hutchinson and Ross: Straudsberg, PA, 1973. 4. Ho, Tse-Lok. Hard and Solt Acids and Bases Principle in Orgonie Chemistry; Auldemie: New Yolk, NY, 1977. 5. Paraan. R. G. J.Am. Chem.Soc. 1969.85.3533-5519. 6. ~ e a r ~ o nG.. ~science . 196fi.i~i.172-177.' 7. Poarsan, R. 0.J . Chem. Edue. l968,45,581-587. 8. Peam0n.R. C.J . Chem. Edur. 1968,45,643-643. 9. Pesmon,R.G. J. ChamEdur. 1987,61,651457.
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22 T ~ J ~ S . cJ I\ ,inn I ' ~ . ,~ ~ r r o r o s s:, S.PC 1980. I..III.C-~ 2 3 1lnrner.L:l'lclXe b. W , r l r t Korri. 1¶91,3j,9?-1t.' 21 Kuznrl..ov. 4. \I .I)~p.nsdre. H H l r l a n l 5 v . r 5% Ser
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