Polarographic Studies for the Corrosion of Carbon Steel in Hydrochloric Acid Solutions

: The present work deals with the quantitative corrosion determination of carbon steel in hydrochloric acid by the use of polarographic and weight loss technique. The polarographic peak obtained at (-0.27 v) in 0.1 M hydrochloric acid was used for the determination of iron and a calibration was ranged (10 -5 –10 -3 ) M. To confirm the obtained results a weight loss measurement were carried out, its results are in good agreement with that of the polarographic measurements. The research included in addition to that a kinetic study of corrosion reactions of iron in hydrochloric acid. Reaction rates were calculated at different temperature, and apparent energies of activation have been calculated from corrosion rates and Arrhenius plots.


Part 1:
The carbon steel specimen (obtained from ministry of industry) had the following composition as revealed by emission spectroscopic analysis: Element C Si Mn Ni Fe % 0.05 0.4 0.61 0.04 98.9 The surface of the carbon steel were first polished then rinsed with distilled water before being immersed in the corrosive medium.
The net area of carbon steel exposed was 30 cm 2 and the average weight was 4 grams.
Solutions were prepared from twice distilled water and analar grade hydrochloric acid, the volume of the acid solutions used in each experiment was 75 ml. The temperature was adjusted electrically to ±0.01 o C by using "Haake -Fk Thermostat, Germany". The experiments were done uniformly for duration of 8 hours.
Corrosion kinetic studies of carbon steel in (0.5 -3) M where followed as weight loss measurements at regular time intervals of (1 -8) hours at a constant temperature 300 K and constant atmospheric pressure. A weight losses were determined using sensitive balance (± 10 -4 g) "Sartorius BP 301S, ISO 9001".
Similar experiments were done with all solutions at various temperatures 308, 313, and 318 K.

Part 2:
Differentialpulse polarograms were determined using a "174 A Polarographic analyzer of Princeton Applied Research, Princeton, New Jersey, USA". In conjunction with stand equipped with mechanical drop timer. A threeelectrode system were used, the working electrode being a droppingmercury electrode (DME), the reference electrode a saturated calomel electrode, and a Hg pool electrode as a counter electrode (14,15,16) .
The following conditions were applied: potential scan rate 10 mv./sec, voltage range 0.75 v., modulation amplitude 50 mv., drop time 0.5 sec., and current range (0.1 -1) µA. All polarographic measurements were carried out at room temperature (22 o C). The solution was deareated by passing through it a stream of nitrogen gas for 15 min.
A number of solutions, containing known concentrations of iron as were prepared, and the differential pulse polarogram were recorded in the presence of 0.1 M hydrocloric acid as supporting electrolyte. It is well known that Ferric ion gives a well defined polarographic peak appears at (-0.24) -(-0.34) volt versus saturated calomel electrode in different supporting electrolyte (17) . The differential pulse polarogram for in 0.1 M solution as supporting electrolyte gave one well-defined peak at a potential of -0.27 volt as shown in figure 1-a. A series of ferric ion solutions having the concentration ranging from (10 -3 to 10 -5 M) were prepared by the oxidation of Fe 2+ to Fe 3+ using nitric acid and heating (16) The obtained results for the variation of the measured peak current (Ip) with concentrations of Fe 3+ shows a linear relation. Table (1) tabulates all the results obtained that cover the calibration range. While figure (2) shows the calibration graph using simple equation of first degree, were constructed by a plot of peak current in (µA) versus concentration of Fe 3+ in mg/l, and the correlation coefficient (C.C.), coefficient of determination (C.O.D.) and the percentage linearity were summarization in table (2). Percentage linearity (r 2 %) The corrosion of iron in carbon steel specimens was determined after inserting the specimens in different concentration of (0.5 -3) M for eight hours, by the use of two methods: polarographic peak current with the same procedure as for standard, and the weight loss method. Table (3) indicates the peak current ( ) and the weight loss calculated according to the initial volume (75 ml).  Figure (3): peak current and weight loss against temperature using DP polarographic method.

Weight loss vs. time curve:
By the application of weight loss method, data are plotted for weight of metal dissolution against time in hours for 0.5 M to 3 M acid concentrations at various temperatures (300 -318) were presented in Table  (4) and figure (4).  As may be noted from these figures, all the graphs are linear, slops of such lines were calculated and taken as the rates of reactions and are expressed in g.cm -2 .hr -1 .
The calculated values of the rates against concentration are tabulated in table (5). Reaction rate data obtained from Figure (4) (ad) which were presented in table (4) are plotted as log rate vs. molar concentration of acid in Figure (5). It may be noted that all the graphs are straightline curves establishing a linear function relationship between log rate and concentration. Where the apparent energy of activation for the corrosion is process and is the pre-exponential factor.
A plot versus reciprocal temperature ( ) in the manner depicted in table (6)   Values of could thus be derived from the slopes of the plots in Figure (7). Table (7) shows an increase in the apparent energy of activation as the concentration of acid decrease resulting in the consequent decrease of the surface tendency for corrosion.

Conclusion:
Carbon steel has been extensively used under different conditions in all industries. An acid wash process is widely used in many industries.
Corrosion attracts the attention of researchers. Researchers all over the world are searching for some methods to avoid corrosion damage, and a method to test corrosion at its early stage. -68 -0202

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The differential pulse polarographic method presented for the quantitative determination of allows the accurate determination of corrosion of carbon steel in acidic medium.
The result obtained is very reproducible since with the use of a dropping mercury electrode the electrode surface is always new and the behavior of the electrode is independent of its history. This proposed method is simple, rapid, sensitive, inexpensive, and highly reproducible.