Model for Corrosion Inhibition Efficiency by Molecules with Systematically Changed Structure
I. Bako
E.Kalman
F.H. Karman
J.Telegdi
Central Research Institute for Chemistry of the Hungarian
Academy of Sciences
P.O. Box 17, H-1525 Budapest, Hungary
email: fanni@cric.chemres.hu
1. INTRODUCTION
Metals are effected by different forms of corrosion attack in
various environments. Corrosion inhibitors are usually used for
protection. Organic compounds can be interface or interphase inhibitors
in aqueous solution. Interface ones function primarily by retarding
surface reactions through direct adsorption while interphase inhibitors,
after an initial adsorption, alter the metal surface reactivity
through the formation of an extended phase at the metal-solution
interphase.
A broad range of corrosion inhibitors are used for mild steel under aqueous conditions. Organic chemicals commonly used against uniform corrosion have a little effect on pitting. Due to environmental restrictions for toxic inorganic inhibitors a wide scale of organic phosphorous compounds are generally applied. Organic polyphosphonic acids are effective corrosion inhibitors in cooling water systems (1,2,3). Some amino derivatives and certain sulphur analogous compounds were checked for steel by electrochemical method (4,5).
The inhibition efficiency of additives depends on many factors which include the number of adsorption sites and their charge density, molecular size and mode of interaction with the metal surface. The adsorption depends mainly on the electronic structure of the molecules.
Due to strict legislation laws we focused on chemicals with low toxicity, that's why the compounds substituted were derivatives of natural chemicals. An another factor which plaid important role in development of new chemicals was the biodegradability and the diminished phosphorous content.
Semiempirical calculations are useful tools which helps us in
finding more effective corrosion inhibitors (6,7). The approach
is based on correlations between the dependent variables (in our
case the corrosion rate) and the set of independent variables
( e.g. HOMO and LUMO energies, dipole moments etc.), which account
for some molecular properties.
2. EXPERIMENTAL
Chemicals under investigation were:
R-CH(NH(CH2)nX)Z, where n=0,1,2 and X=OH, COOH, PO3H2
The investigation involved electrochemical methods as well as weight loss measurements.
Weight loss measurements of mild steel specimen studied by gravimetric
method were taken in the model water with or without additives,
at room temperature.
Electrochemical experiments were performed in a cell of a SOLARTRON
1286 ECI apparatus driven by PC software. The anodic and cathodic
polarisation curves were obtained between -0.8V and -0.1V with
scan rate 10mV/min (auxiliary electrode: platinum; reference:
SCE; working electrode: cylindrically shaped corrosion test specimen
fabricated from mild steel). The model solution used with (100ppm)
or without inhibitors contained 0.47g CaSO4xH2O;
0.23g MgSO4xH2O; 0.11g NaHCO3;
0.13g CaCl2xH2O in one litre bidistilled
water, pH7.
The geometry of organic molecules were optimized by using the
AM1 method of the quantum chemical program package MOPAC 6.0.
3. RESULTS
The different character of molecules allowed us a prediction for the most effective substituent.
In these seria the influence of hydroxy-, carboxy- and phosphonomethylation was indicated by the efficiency parameters which proved that the most effective substitution is the phosphono one but the carboxymethylated chemicals gave good results, too.
Results of semiempirical calculation are reported in Table 1.
The EHOMO is the highest occupied molecular orbital,
ELUMO is the lowest unoccupied one and means the dipole
moment of molecules. Very good correlation was found between corrosion
rate (v) and quantum chemical indices, by applying the following
equation.
Table 1. Quantum chemical indices, calculated and measured corrosion
rate
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The correlation value 0.98 proves a significant correlation between the calculated and the measured corrosion rate as the Fig.1. shows.
4. CONCLUSION
Equation 1 shows that not only the dipole moment influences the
corrosion rate but other quantum chemical parameters of molecules
(e.g. HOMO or LUMO energies etc,) play important role in complex
corrosion inhibiting process. All of this results help us in planning
of new, effective inhibitor molecules.
5. REFERENCES
1. M. Duprat, Corrosion 37, (5) 262 [1989]
2. U.S.Patent 4,276,089
3. U.S.Patent 4,409,121
4. A. Viebeck et al., J. Electrochem. Soc. 131, 1853 [1984]
5. D.W. DeBerry and A. Viebeck, Corrosion 44, (5) 299-305) [1989].
6. F.B. Growcock, W.W. Freiner and A.Andreozzi, Corrosion, 45, 1007 [1989]
7. P.G.Abdul-Ahad and S.H.F. Al Madfai, Corrosion, 45, 978 [1989]