EFFECT OF THE POTENTIAL ON THE FILM COMPOSITION AND THE STRESS CORROSION CRACKING OF MILD STEEL IN AMMONIUM NITRATE SOLUTIONS

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1 Journal Journal of Chemical of Chemical Technology Technology and Metallurgy, and Metallurgy, 53, 4, 53, 2018, 4, EFFECT OF THE POTENTIAL ON THE FILM COMPOSITION AND THE STRESS CORROSION CRACKING OF MILD STEEL IN AMMONIUM NITRATE SOLUTIONS University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofia, Bulgaria fachikov@uctm.edu Received 15 March 2018 Accepted 30 April 2018 ABSTRACT The relation between the film composition and the susceptibility of mild steel (0.17% C) to stress corrosion cracking (SCC) in ammonium nitrate solutions at different potentials has been studied. The susceptibility to SCC is evaluated by constant slow strain rate tests under potentiostatic conditions (-0.5 V V, SCE). The composition of the surface film is identified by Mossbauer spectroscopy. It is found that in the region of the highest susceptibility to SCC (0 V V, SCE) the film is composed mainly of fine particles of FeOOH as well as some quantity of γ-fe 2. However at potentials outside this region, where the resistance of the steel to SCC increases considerably, the film is composed exclusively by oxides with a spinel structure Fe 3 and γ-fe 2. It is suggested that the conditions which favour the formation of a spinel structure film may lead to an increased resistance of the steel to SCC. Keywords: stress corrosion cracking, mild steel, nitrate solutions, Mossbauer spectroscopy. INTRODUCTION In current practice the most dangerous and widespread type of а local corrosion is the stress corrosion cracking (SCC) of metals. The latter occurs in almost all areas of the industry and the difficult solution of this corrosion problem refers to the need to take into account a large number of simultaneously acting factors related to the metal, the environment and the service conditions of the metal products. Therefore, the identification of the influence of the different factors is an important and necessary condition for studying SCC, as well as for development of methods for a successful protection from this phenomenon [1, 2]. In the research practice, the Mossbauer spectroscopy has become a reliable method for a qualitative and quantitative determination of the corrosion products of iron and its alloys. Its identical sensitivity to both crystalline and amorphous iron compounds [3, 4] is an important advantage of this method. The general corrosion rate of iron and carbon steels in concentrated ammonium nitrate solutions is high as this medium contains two aggressive ions, N - - oxidation ion and the complex NH ion. The first of these ions allows the cathodic process to be carried out in the absence of dissolved oxygen as a depolarizer, while the second one facilitates Fe ions passing into the solution as complex ions. It has been found that the general corrosion rate of low carbon steel is the highest in 6N NH 4 N. Depending on the electrolyte composition, the cathodic polarization may decrease or increase the general corrosion rate of iron and low carbon-containing steels in ammonium nitrate solutions. In the presence of air, the rate of corrosion decreases during cathodic polarization. But it starts to increase significantly at fixed potential values in deaerated solutions containing Fe 2+ - ions. Similar effects are obviously due to depassivation of the metal surface under these conditions [5, 6]. According to ref. [6] the iron surface reaction in a concentrated ammonium nitrate solution releases 740

2 initially Fe 2 + -ions, which are eventually oxidized to Fe 3 or γ-feooh. Magnetite is formed at 50 o C and 80 o C in oxygen absence. The atmospheric air leads to the formation of non-passivating γ-feooh and complete dissolution of samples. The same authors [6] have found that Fe 3+ /Fe 2+ ratio in the magnetite increases with the solution temperature increase. The corrosion products near the metal surface and in the bulk of the solution, as well as the conditions of formation, growth and transformation of the products are also studied [7]. Some authors [8, 9] have studied the relationship between the susceptibility to SCC of iron and low carbon steel in nitrate environments and the spontaneous formation of a passive layer in the temperature range of 25 C C. The established correlation between the corrosion cracks propagation rate and the tendency of the system to reach Fe 2 region (according to the Pourbe diagram) provides the assumption that the influence of the temperature on SCC as a whole is connected with the formation of a passive film of γ-fe 2. The effect of the ph, temperature and concentration of ammonium nitrate solutions on the susceptibility of mild steel to SCC and general corrosion are investigated in [10, 11]. It is shown that the increase of temperature and concentration of the solutions causes a decrease in the stress corrosion life. At very low ph values, stress corrosion cracking is associated with very high rate of general corrosion. The present study is focused on the effect of the potential on the composition of the corrosion products and the sensitivity to stress corrosion cracking (SCC) of low carbon steel in hot ammonium nitrate solutions. air (ΔF air ), i.e. ΔF medium. In fact ΔF = (F b -F f ) /F b 100 (%), where F b was the initial cross-sectional area, while F f was the area after the sample destruction. The character of the corrosion failure was determined by a visual and microscopic observation and analysis of the destroyed samples. A Mossbauer analysis was performed to the composition of the corrosion products formed on the surface of the low carbon steel in ammonium nitrate medium. A standard Mossbauer spectrometer operating at a constant acceleration mode in the transmittance regime was used. The source was Co 57 (Pd). The speed scale for all spectra was given in relation to the reference zero of α-fe. The spectra were recorded at room temperature (25 ± 2 C). The average sample thickness was 10 mg cm -2. The examinations were carried out with samples of a standard low carbon steel of the following chemical composition (%): C ; Si ; Mn ; P ; S-0.029; Cr ; Ni ; Cu A hot ammonium nitrate solution (5N NH 4 N, 90 o C) was used for a corrosive environment where the steel studied reveals a susceptibility to SCC [13, 14]. RESULTS AND DISCUSSION The influence of the potential, E, on the tensile strength R m is illustrated in Fig. 1. It can be seen that R m values decrease in the area of 500 mv to mv (SCE). The decrease of this quantitative index is well outlined in the range from 0 mv to +800 mv (SCE), where R m decreases about 2 times when compared to its value at the corrosion potential (-370 mv, SCE), and EXPERIMENTAL Stress corrosion cracking susceptibility tests of a typical representative of the low-carbon steels, widely applied in the practice, were conducted using a slow strain rate technique with a constant rate of open 2.34x10-5 mm s -1 in the corrosion solutions [12]. The alteration of the tensile strength R m (Nm -2 ) and the ductility was used for a quantitative assessment of the tendency to corrosion-mechanical failure. The ductility was determined on the ground of the ratio of the relative change of the cross-sectional area of the sample due to failure (ΔF medium ) in the corrosion medium and that in the Fig. 1. Influence of potential on the tensile strength, Rm of the test steel in 5 M NH 4 N, 90 C. 741

3 Journal of Chemical Technology and Metallurgy, 53, 4, 2018 Fig. 2. Comparison of the relationships (E-lg i), curve 1, and (ΔFmedium / ΔFair - E), curve 2, obtained in 5M NH 4 N, 90 o C. Fig. 3. Intercrystalline corrosion crack in the studied steel, 5 M NH 4 N, 90 C. 742 Fig. 4. Mossbauer spectra of corrosion products formed in 5M NH 4 N, 90 o C, at оpen circuit potential: a - on the sample surface; b - in the solution. more than 4 times when compared to its value in presence of tensile destruction in air. The cathodic polarization (with respect to E corr ) increases the tensile strength reaching values specific for a destruction in air at potentials of -700 mv (SCE). That indicates a decreased susceptibility to a corrosionmechanical failure. This result also shows the possibility of a cathodic protection at potentials -700 mv (SCE). At the same time, the area of a reliable cathodic protection is relatively narrow (200 mv). The parameter R m decreases again at more negative potentials, which is possibly related to an influence of hydrogenation on the steel strength at these potentials. The comparison of the potentiostatic polarization curve obtained for an unloaded steel specimen and the curve illustrating the ductility change with the potential in the course of the stress corrosion test (Fig. 2) shows that the corrosion-mechanical index ΔF medium has the lowest values in the transition area (at potentials more positive than the critical potential of passivation) as well as in a greater part of the passive zone, i.e. the steel susceptibility to SCC is the most exhibited in the area of the active-passive zone and in the area of a total passivity. This character of ΔF medium vs. E relationship is related to the change of the ratio of the rates of the mechanical breakdown and the passive state regeneration depending on the value of the potential. The microscopic observations and the surface fracture analysis of the destroyed samples show that the

4 intercrystalline stress corrosion cracking is the cause for the test samples failure in the potential range from ca -500 mv to mv (SCE). The corrosion crack propagation observed on a not completely destroyed sample follows the grain boundaries. It is shown in Fig. 3. Fig. 4 and Fig. 5 show the Mossbauer spectra of corrosion products formed at the open circuit potential and at a potential of +500 mv (SCE), i.e. in the area of the most intensive stress corrosion cracking. The (a) spectra refer to the products on the sample surface, while the (b) spectra - to those released in the solution. It is evident that there is no significant difference between the corrosion products in the solution and on the surface of the steel (at both potential values). The composition of the corrosion product layer formed in the most hazardous potential area of the corrosion cracking (0 mv mv, SCE) differs from that of the products formed at the corrosion potential of the tested steel (-370 mv, SCE) under identical environmental conditions. The content of Fe 2 + compounds in this case is less than that in the case of the open circuit potential. The majority of the products are in a paramagnetic state, i.e. these are fine dispersive compounds of Fe 3 + (mainly FeOOH and a negligible amount of γ-fe 2 ) of a content of 68 %. The rest (32 %) are in an unordered magnetic phase consisting mainly of γ-fe 2, a negligible quantity of α-feooh and traces of magnetite. The major part of the substance in the product spectra at +500 mv (SCE) gives a double-like spectrum. The magnetic fraction of those in the solution is less due to the presence of α-fe 2 and FeOOH in the super magnetic state. The composition of corrosion products at the open circuit potential consists almost entirely of a magnetic phase of a spinel structure (Fe 3 and γ-fe 2 ), while those on the surface of the steel contain more Fe 3. CONCLUSIONS The change of the composition and the structure of the corrosion products film appear to be related to the observed powerful influence of the potential on the susceptibility of the investigated steel to stress corrosion cracking in an ammonium nitrate environment. It is to be expected that the films composed of spinel-structured oxides have better physical-mechanical properties and a higher corrosion-mechanical resistance. The sensibility Fig. 5. Mossbauer spectra of corrosion products formed in 5M NH 4 N, 90 o C, at mv, SCE. a - on the sample surface; b - in the solution. of the investigated steel to stress corrosion cracking is less at potentials of the corrosion potential area when compared to that at potentials in the passive region. It can be expected that all conditions which favor the formation of a spinеl structure film will increase the steel resistance to stress corrosion cracking. REFERENCES 1. H.L. Logan, The stress corrosion cracking of metals, N.Y., J. Wiley, I.I. Vasilenko, R.K. Melehov, Stress corrosion cracking of steels, Naukova dumka, Kiev, 1977, p.143, (in Russian). 3. A. Vértes, I. Czakó-Nagi, Mӧssbauer Spectroscopy and Its Application to Corrosion Studies, Electrochimica Acta, 34, 6, 1989,

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