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Levko Elena, associate professor, candidate of technical sciences, associate professor
National Metallurgical Academy of Ukraine, Ukraine
Championship participant: the National Research Analytics Championship - "Ukraine";
the Open European-Asian Research Analytics Championship;
The paper provides systematic investigation of SO42-, Fe 2+, Н+(main components of etch solution based on sulphuric acid) influence on processes, occurred in iron and steel (steel 10кп, steel 45) electrodes in the region of transition from active to passive condition.
It was stated that slow of anodic iron and carbon steel dissolution is probably caused by salt film formation:
At the presence of Fe2+ ions the film formation becomes easier:.
Keywords: passivity, anode dissolution, iron, carbon steel, kinetic equations, process mechanism
В статье систематически изучено влияние концентрации SO42-, Fe2+, Н+(основные компоненты травильных растворов на основе серной кислоты) на процессы, протекающие на железном и стальных (ст 10 кп, ст 45) электродах в области перехода от активного к пассивному состоянию. Установлено, что замедление анодного растворения железа и углеродистых сталей, вероятно, вызвано образованием солевой пленки по схеме:
В присутствии ионов Fe2+процесс образования пленки облегчается
Ключевые слова: пассивность, анодное растворение, железо, углеродистые стали, кинетические уравнения, механизм процесса
At rather high anodic polarization dependence current – potential departs from linear, where limiting current occurs. This event, which has been countless discussed [1 – 5], was been slightly investigated in concentrated acid sulfate electrolytes, used while chemical and electrochemical steel bar processing. Scientific and practical value of these papers decreased due to non-consideration of anions and its concentrations. Evidences about SO42- anion influence on iron passivation process are contradictory: papers [6 – 8] provide active effect of sulfate-anions, but paper  shows strong SO42-interaction with iron, thus SO42- anions take part in passivation process. There is no evidence of Fe2+effects.
The purpose of this paper is to provide systematic investigation of SO42-, Fe 2+, Н+(main components of etch solution based on sulphuric acid) influence on processes, occurred in iron and steel (steel 10кп, 45) electrodes in the region of transition from active to passive condition.
Polarization curves were investigated in solution of H2SO4+ Na2SO4, H2SO4+ Na2SO4 + FeSO4 with constant ionic force µ=10-11 in standard three-electron cell at potential sweep speed 4 mV/s. Comparison electrode – saturated silver-chloride. Auxiliary electrode – platinum wire.
Characteristic form of curves current-potential is shown on fig.1 (pronounced passivation maximum, which suddenly pass into plateau, is seen). Full passivation occurs only when ? > 0.6 V. The presence of Fe2+ ions in the solution slows the anodic process both in active region, and in the region of transition from active to passive condition.
I-current (mA), ? – potential (V)
Solution 1.58 m. H2 SO4+ 1.92 m. Na2 SO4; temperature 25°
Fig. 1. Iron anodic polarizing curves (1), steel 45 (2), steel 10кп(3)
To clarify the observed events the influence of electrolyte composition and temperature on current Imax and potential ?max of first maximum.
The concentration of hydrogen ions, in sulphuric acid without additions of Fe2+ ions, was changed at interval 1 – 5 mol/l at [SO42-] = 3.5 mol/l, sulphate ions – 1.58 – 3.5 mol/l at pH=0. The measurements were performed at 25±0.1°.
Maximum current changes symbatly to concentration H+ (fig. 2, a), and potential ? max (V) moves to cathode region.
Iron ? max = 0.106 – 0.035 lg [H+], (1)
Steel 10КП? max= 0.006 – 0.032 lg [H+], (2)
Steel 45 ?max= 0.038 – 0.032 lg [H+]. (3)
As is seen from equation (4) – (6) and fig. 2, b, ? max and Imax decreases with concentration increase of SO42-, i.e. sulphate-ions provides passivation.
Iron ? max = 0.115 – 0.050 lg [SO42-], (4)
Steel 10КП?max= 0.023-0.055 lg [SO42-], (5)
Steel 45 ? max = 0.058 – 0.060 lg [SO42-], (6)
In sulphuric acid, contained Fe2+ ions, the dependence ? maxand Imax form [H+], [Fe2+], t is obtained using methods of mathematic modeling. The investigation conditions are provided in table 1. Experimental values ? maxand Imax in each plan point – is an average values, obtained from 2 – 3 polarizing curves. After experiment is done, and regression coefficient evaluation and model adequacy are done the following equations, provided in the table 2, are obtained.
Table 1. Experiment Planning Conditions
Note: [H+] and [Fe2+]– concentration (mol/l)
Table 2. Experimental values of kinetic parameter in equations or Imax and ?max
(solution of sulfuric with ferrous ion addition)
Imax = b0 [H+] b1 [Fe2+] b2 10-b3/T
Extension table 2
?max= b0 + b1 lg [H+] + b2 lg [Fe2+] + b3/T
Consider the possible causes which slow anodic dissolution of investigated metals in transition region at the given experimental conditions.
Active Iron dissolution starts with OH- absorption, proved experimentally by Housler K. B., and with FeOHадсcomplex formation using the reaction
Fe + OH- ↔ FeOHадс+e- (7)
Transitional monolayer FeOHадсcannot exist for a long time, since iron does not form a stable compound at oxidation level +1.
Potentiostatic polarization either removes the product form the surface or provided reversible oxidation to FeO.
FeOHадс↔ FeOадс+H++e-. (8)
FeOадсtransforms into phase oxide FeO. As is shown in paper , monolayer FeOадсappears at reversible potential of phase oxide formation FeO.
[SO42-] = 3.5 mol/l (a). pH = 0 (b), t = 25°.
Metal: 1 – iron, 2 – steel 10кп, 3 – steel 45.
Fig. 2. The dependence of current of first maximum Imax1 (mA) form ion [H+] (a) and [SO42-] (b) concentration (mol/l)
The analysis of thermodynamic formation conditions covered the layers on active and passive iron [13, 14] shows that at pH < 2 iron oxide unstable. At the same time if passive layer consists only of iron oxide, then the passivation possibility should not depend upon ion nature, but only of pH values. At low pH values, salt covering layer may occur, which consists, for example, from Fe2(SO4)3, which in turns stable in strongly acidic solution at high SO42- concentrations . The formation of iron sulphate may be at this reaction
2FeO + 4 H++ 3 SO42- ↔Fe2(SO4)3 + 2H2O + 2e-. (9)
?p = – 0.326 – 0.118 lg [H+] – 0.089 lg [SO42-]. (10)
Table 3 shows calculated values of electrochemical affinity A, of overstresses ??, of equilibrium potential ?p of reaction (9) for [H+] = 2.0 mol/l and [H+] = 5.0 mol/l at experimentally found potentials of first maximum (25°, [SO42-] = 3.5 mol/l).
Table 3. Thermodynamic parameter of reaction (9) и(13)
Thermodynamic reaction (9) parameters shows that at maximum potential for iron and carbon steel overstresses ?? <0. In the given conditions, reaction (9) proceeds in anodic direction, i.e. forms Fe2(SO4)3. This is also proved by electrochemical affinity values, equals to + 79.1 kJ. Decreasing pH value increases electrochemical affinity of equation (9).
Comparing coefficients at [H+] and [SO42-] in equation (10) and in equations (1) – (3), one can see, that the latter is a bit lass. Considering reaction (8) the salt film formation process can be provided as:
Fe2(SO4)3 + H2O + 4e- ↔ 2FeOH + 2H+ + 3SO42-, (11)
?p = ?0 – 0.030 lg [H+] – 0.045 lg [SO42-]. (12)
The values of theoretical coefficients in equation (12) reasonably coincide with experimentally proved values of coefficients in equations (1) – (3). Thus, it is expected that electrochemical process, resulting at slowing of metal anodic dissolution in transition region, occurs in accordance to equation (11).
If electrolyte contains Fe2+ ions, than maximum potential and current change symbatly in accordance with Fe2+ concentration change. Increased pH value leads to decrease first maximum current and to potential shift in anodic direction (table 2). The observed influence effect of Fe2+, H+ and SO42- on anodic processes in transition region, is probably connected with the reaction:
Fe2(SO4)3 + H2O + 2 e- ↔ FeO + Fe2+ + 3SO42- + 2H+ (13)
?p = – 0.537 – 0.059 lg [H+] – 0.030 lg [Fe2+] – 0.089 lg [SO42-]. (14)
Taking into account the reaction (8):
Fe2(SO4)3 + H2O + 3e- ↔ FeOH + Fe2+ + 3SO42- + H+ (15)
?p = ?0 – 0.020 lg [H+] – 0.020 lg [Fe2+] – 0.060 lg [SO42-] (16)
As is seen, the coefficients at lg [H+] and lg [Fe2+], experimentally obtained for iron and steel (equation in table 2), are close to calculated coefficients.
The slow of anodic iron and carbon steel dissolution in the region of transition from active to passive condition in acidic sulphate electrolyte, is probably caused by salt film formation:
At the presence of Fe2+ ions the film formation becomes easier: