Acoustic emission during the electrochemical corrosion of 304 stainless steel in H2SO4 solutions

Acoustic emission (AE) behaviour during the electrochemical corrosion of 304 stainless steel (304SS) in H₂SO₄ solutions was studied. AE signals which related to transpassive dissolution are detected in solutions with low pH, and are very slightly influenced by current density and pre-strain. During hydrogen bubble evolution, a weak correlation exists between the AE signal amplitude and the hydrogen bubble diameter. The concept of potential – pH – AE diagram is proposed and such a diagram is drawn based on AE activity and b-values. The main mechanisms of AE sources which are transpassive dissolution and hydrogen bubble evolution, are also discussed.     

AE response of α-brass during stress corrosion crack propagation

The present work was aimed at characterizing the acoustic emission (AE) response of α-brass during SCC propagation. For that purpose, slow strain rate tensile tests (SSRT) under potentiostatic control were performed in a 1 M NaNO₂ solution and in Mattsson’s solution. The differences between the AE signals generated by transgranular and intergranular stress corrosion cracking (SCC) were analysed. Results show that the AE activity during the propagation of TGSCC is various orders of magnitude higher than the AE activity during the IGSCC propagation. However, the mean amplitude and rise time of the AE signals registered during the propagation of TGSCC are similar to the AE parameters of the signals registered during the propagation of IGSCC. The measured AE activity was associated with the mechanical tearing of the metallic ligaments left behind by the propagating crack.     

AE response of type 304 stainless steel during stress corrosion crack propagation

Acoustic emission (AE) signals generated by transgranular stress corrosion cracking (TGSCC) and intergranular stress corrosion cracking (IGSCC) have been compared by means of slow strain rate tensile tests (SSRT) performed using both solubilised and sensitised type AISI 304 stainless steel in a 1 M NaCl + 1 M HCl solution. Results show that the AE activity during the propagation of TGSCC is much higher than the AE activity during the IGSCC propagation. However, no significant difference was found between the mean amplitude and rise-time of the AE signals registered during the propagation of TGSCC and those measured for IGSCC propagation.     

Effect of copper on metal dusting of austenitic stainless steels

Three steels, 304SS, 310SS and 800H, were alloyed with 5%, 10%, and 20% (by weight) copper, and then exposed to 68%CO-31%H₂-1%H₂O gas at 680 °C (a_C = 19 and p_O2=5.4×10^-25 atm) under thermal cycling conditions. Kinetic measurements showed that copper-free alloys all dusted, with 304SS experiencing the greatest metal wastage. Copper additions did not have any effect on metal wastage of 304SS, but reduced the attack on 310SS and 800H markedly at levels of 5% and 10%. However, increasing the copper content to 20% produced large copper-rich precipitates which accelerated dusting by promoting internal graphitisation.Dusting was associated with surface coking. When pitting occurred, on copper-free alloys and on copper containing 304SS, large coke structures grew above the pits. Internal grain boundary carburisation always took place, and intragranular carbides also precipitated when dusting occurred. A lamellar surface layer of internally precipitated spinel and austenite also developed in association with dusting. The copper effect is discussed in terms of its alloy solubility and its known beneficial effect in Ni-Cu binaries.     

Stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling saturated magnesium chloride solutions

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of the sensitized stainless steels of type 304, 310 and 316 were investigated as a function of test temperature in boiling saturated magnesium chloride (MgCl₂) solutions under a constant applied stress condition. The test temperature dependence of fracture appearance and three parameters of time to failure (t_f), steady-state elongation rate (l_ss) and transition time to time to failure ratio (t_ss/t_f) suggests that type 304 suffered SCC and HE, while type 316 suffered only HE and type 310 SCC. It was also found that the test temperature dependence of three parameters for the sensitized type 304 and 310 was almost similar to that of the solution annealed stainless steels, whereas that of type 316 showed a clear difference between sensitized and solution annealed specimens. The relationships between the logarithms of the time to failure and the steady-state elongation rate became a straight line for all stainless steels. However, its slope depended upon the fracture mode; −2.0 for SCC and −1.5 for HE. This showed that the steady-state elongation rate was the parameter for predicting the time to failure for the stainless steels in the MgCl₂ solutions. The results obtained were explained in terms of martensite transformation, hydrogen entry site, sensitization, and so on.     

Corrosion behaviour of corrugated lean duplex stainless steels in simulated concrete pore solutions

This work studies the corrosion behaviour of two corrugated lean duplex stainless steels (SAF 2001 and 2304 grades) in eight alkaline solutions (carbonated and non-carbonated, saturated Ca(OH)₂ solutions with different chloride contents). 2001 stainless steel is a new grade in market because of its composition. 2304 is a grade previously studied under different conditions. However, its use as reinforcement in concrete is new. Studies are carried out by polarization curves following scanning electronic microscopy (SEM) and optical observations. Results are compared to those of carbon steel and austenitic AISI 304 and duplex SAF 2205 under similar conditions. After corrosion tests in alkaline media with chloride, ferrite tends to corrode selectively in 2304 duplex, while austenite corrodes selectively in 2001 under the same conditions. The influence of the duplex microstructure on attack development and morphology is analyzed. The electrochemical parameters obtained from the polarization curves suggest 2001 could replace 304 keeping the structure its corrosion performance (and with clear economical advantages). 2304 shows better corrosion behaviour than the more expensive 304, but somewhat lower than the excellent behaviour shown by 2205.     

Pitting corrosion of intermetallic compound Ni3(Si,Ti) in sodium chloride solutions

The pitting corrosion of intermetallic compound Ni₃(Si,Ti) was investigated as functions of test temperature and chloride concentration in sodium chloride solutions by using a potential step method. In addition, the pitting corrosion of solution-annealed austenitic stainless steel type 304 and pure nickel was also studied under the same experimental condition for comparison. The pitting potential obtained for the intermetallic compound decreased with increasing chloride concentration and test temperature. A critical chloride concentration below which no pitting corrosion took place was found to exist and to decrease with increasing test temperature. The specific pitting potential at the critical chloride concentration also decreased with increasing test temperature. In addition, the pitting potential at various constant chloride concentrations above the critical chloride concentration decreased with increasing test temperature. The pitting potential of Ni₃(Si,Ti) was higher than pure nickel, but lower than that of type 304.     

Effects of dithionite ion on corrosion of type 304 stainless steel in sulphuric acid solution

Corrosion behaviour of type 304 stainless steel was investigated, with particular attention to additive effects of hydrosulphite (Na₂S₂O₄) on corrosion in 0.1 mol/l H₂SO₄ solution with various amounts of Na₂S₂O₄ up to 60 mmol/l.Corrosion of SUS304 occurred below pH 3.0 at 30 °C in a 0.1 mol/l H₂SO₄ solution in which Na₂S₂O₄ was added to 0.1-20 mmol/l. The maximum corrosion rate at 30 °C was measured as 7.2 g/m² h (7.9 mm/y) in 0.1 mol/l H₂SO₄-10 mmol/l Na₂S₂O₄ at pH 1.2. Microscopic surface observation revealed that active dissolution was accompanied by intergranular corrosion at the metal surface.The SUS304 was easily passivated in 0.1 mol/l H₂SO₄ solution with more than 30 mmol/l Na₂S₂O₄. NiS was detected in the passivated film.     

On the stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling saturated magnesium chloride solutions: Effect of applied stress

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of sensitized stainless steels of types 304, 310 and 316 were investigated as a function of applied stress at different test temperatures in boiling saturated magnesium chloride (MgCl₂) solutions under a constant applied stress condition. The stress dependence of fracture appearance and three parameters of time to failure (t_f), steady-state elongation rate (l_ss) and transition time to time to failure ratio suggests that types 304 and 310 suffered SCC, while type 316 suffered only HE. It was also found that the applied stress dependence of three parameters for the sensitized types 304 and 310 was almost similar to that of the solution-annealed stainless steels, whereas that of type 316 showed a clear difference between sensitized and solution-annealed specimens. The relationships between the logarithms of the time to failure and the steady-state elongation rate became a straight line for all stainless steels. However, its slope depended upon the fracture mode: −2.0 for SCC and −1.5 for HE. This showed that the steady-state elongation rate was a good parameter for predicting the time to failure for the stainless steels in the MgCl₂ solutions. The results obtained were explained in terms of martensite transformation, hydrogen entry site, and sensitization.     

Influence of Zn injection on characteristics of oxide film on 304 stainless steel in borated and lithiated high temperature water

The oxide film on 304 stainless steel exposed to the hydrothermal environments at 573.15 K up to 20 days without/with 10 ppb Zn injection has been investigated ex situ by X-ray photoelectron spectroscopy (XPS). ZnFe₂O₄ and ZnCr₂O₄ were found to be formed in the oxide film at the initial stage of immersion by substitution reaction between Zn²⁺ and Fe²⁺, and ZnCr₂O₄ became dominant after long-term immersion. The calculations of potential-pH diagrams, solubilities and crystallographic features of spinels have been done to evaluate the oxide film structure and the inhibition mechanism caused by Zn injection.     

Correlation between corrosion rate and AE signal in an acidic environment for mild steel

The AE method is an effective technique for inspecting corrosion damage of mild steel, such as tank bottom floors. However, the correlation between AE signals and corrosion behaviour for mild steel has not yet been fully clarified. In the present study, the authors considered that the corrosion regions of bottom floors become a strong acid environment by Cl⁻, as reported in a previous study. Thus, the polarization resistance for the test pieces of mild steel was measured with an AC impedance method under a strong acid environment. It was clear that the polarization resistance indicated the corrosion rate for a test piece of mild steel in the experiments. While measuring the AE signals, the corrosion rate was monitored with a test piece. As a result, the AE signal showed good correlation with the corrosion rates of the test pieces. The corrosion behaviour of the test pieces was then discussed with the corrosion potential measured during the experiments. Furthermore, the cathode current was changed to control the generated hydrogen gas volume. The volume of the hydrogen gas generated from the cathode reaction was correlated to the AE signals.     

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 pair in aqueous LiBr solutions

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 stainless steel pair in a concentrated lithium bromide solution was investigated according to the mixed potential theory. A hydraulic circuit was designed to study dynamic corrosion processes in situ. A potential relation between corrosion current density (i_corr) and Reynolds number (Re) was found for copper, showing a mixed control of a chemical step and mass transport through the corrosion products film with the predominance of the former. No dependence of i_corr on Re could be established for AISI 304, showing a chemical step control. Moreover, under stagnant conditions, partial passivation may occur in AISI 304; however, under flowing conditions passivation is not possible. Copper is the anodic element of the pair under all flowing conditions analysed. The galvanic phenomenon is more important as Re increases, but the results show compatibility of both materials at all Re values analysed. Similarly, a potential relation between galvanic current density (i_G) and Re was found, showing a mixed control of a chemical step and mass transport with the predominance of the latter. Copper corrosion resistance decreases more rapidly as Re increases due to the AISI 304 galvanic effect: there is a synergy between the galvanic effect and the hydrodynamic conditions. Under stagnant conditions, the galvanic behaviour of the materials is close to the compatibility limit and an inversion of the anodic element of the galvanic pair takes place.     

KCl-induced high temperature corrosion of the austenitic Fe-Cr-Ni alloys 304L and Sanicro 28 at 600 °C

The influence of KCl(s) on the high temperature oxidation of the austenitic alloys 304L and Sanicro 28 at 600 °C in O₂ + H₂O environment is reported. 0.10 mg/cm² KCl(s) was added before exposure. The samples are investigated by grazing angle XRD, SEM/EDX, and AES. In the absence of KCl, both alloys show protective behaviour in dry O₂. In O₂ + H₂O environment, alloy 304L suffers local breakaway corrosion while Sanicro 28 still shows protective behaviour. The oxidation of both alloys is strongly accelerated by KCl. KCl reacts with chromium in the normally protective corundum-type oxide, forming K₂CrO₄. This depletes the scale in chromia and leads to the formation of a non-protective, iron-rich scale. The significance of KCl-induced corrosion in real applications is discussed and the oxidation behaviour of the two steels is compared.     

A comparative study on the corrosion behaviour of 304 austenitic stainless steel in sulfamic and sulfuric acid solutions

Potentiodynamic polarization and EIS have been employed to compare the corrosion behaviour of 304 stainless steel in NH₂SO₃H and H₂SO₄ solutions. Corrosion tests were carried out as a function of the acid’s concentration (0.1-0.5 M) and solution temperature (20-60 ^oC). The corrosion rate is higher in H₂SO₄ than in H₂NSO₃H in all concentrations and temperatures. Values of the activation energy (E_a) revealed that the corrosion process is faster in H₂SO₄ than in NH₂SO₃H solution. EIS data showed that the display of Nyquist plots, and hence the mechanism of corrosion, depends not only on the acid concentration but also on the solution temperature. In 0.1 M concentration, the equivalent circuits R_e(R_ctQ_dl) and R_e(R_ctQ_dl)(RQ)_ads describe the corrosion systems in H₂NSO₃H and H₂SO₄ solutions respectively. At concentrations ?0.2 M, the equivalent circuit R_e(R_ctQ_dl)Q_diff is applicable. Adsorption of the counter ion of the acid on the steel surface and the stability of the surface complex may explain the observed corrosion rates.     

Oxidation of green rust suspensions containing different chromium ion species

The oxidation of hydrosulphate green rust (GR2(SO₄²⁻)) suspension containing different chromium ion species was investigated by X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy. The pH, oxidation-reduction potential and amount of dissolved oxygen in aqueous solutions were measured during the reactions. The results show that the addition of Cr(III)₂(SO₄)₃ solution suppresses the transformation of GR2(SO₄²⁻) into iron oxyhydroxides and oxides in aqueous solution, while the addition of Na₂Cr(VI)O₄ solution promotes the transformation of GR2(SO₄²⁻) in which Cr(VI) is reduced to Cr(III); α-FeOOH particles were refined by the addition of the chromium ions.     

Designing a toxic-element-free Ti-based amorphous alloy with remarkable supercooled liquid region for biomedical application

A series of toxic-element-free Ti–Zr–Ta–Si amorphous alloy ribbons have been successfully prepared by melt-spinning. The differential scanning carlorimetry (DSC), X-ray diffraction analysis, bending test and microhardness test are conducted for studying the thermal and mechanical properties. The results show that the Ti₄₂Zr₄₀Ta₃Si₁₅ metallic glass ribbon present excellent ductile behavior by the bending testing, without any fracture cracking after bending over 180 degree. In addition, this amorphous alloy possesses a very high glass transition and crystallization temperature of 799 and 898 K, respectively, as well as a very wide supercooled liquid region of 99 K. This amorphous alloy exhibits promising thermal stability during isothermal annealing at the middle temperature of its supercooled region, with more than 3000 s incubation time for isothermal annealing at 823 K (550 °C). This amorphous alloy also shows much lower value of corrosion current density (2.27 × 10⁻⁹ A/m²) than the 304 stainless steel in the 0.3 mass% sodium cloride solutions. This Ti₄₂Zr₄₀Ta₃Si₁₅ alloy is believed to be a promising based alloy for fabricating the bulk metallic glass foam by the spacer technique in the application of biomedical implants.     

Corrosion behaviour of aluminium in copper containing environment

Corrosion behaviour of pure aluminium galvanically connected to metallic copper or in the presence of Cu²⁺ ions was investigated by electrochemical measurements in Na₂SO₄ and Na₂SO₄ + NaCl test solutions. It has been found that in aerated Cl⁻ ion containing solutions pitting corrosion of aluminium emerged immediately, while in the absence of oxygen this process was less violent. Effect of passivating pre-treatment of aluminium surface on corrosion behaviour Cu-Al bimetallic system is also demonstrated.     

Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions

The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H₂SO₄ solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/l HCl and 0.37 mol/l H₂SO₄ solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al³⁺, Mg²⁺, Ca²⁺, Na⁺ Si⁴⁺ and Fe³⁺ ions eluted in different concentrations of HCl and H₂SO₄ solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H₂SO₄ solutions for the CIP alumina specimens at room temperature decreases.     

Pourbaix diagrams for chromium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Cr-Br⁻-H₂O system at 25 °C were developed in 400, 700, 850, and 992 g/L (4.61, 8.06, 9.79, and 11.42 M) LiBr solutions, common concentrations in different parts of absorption devices. The diagrams were compared with the simple Cr-H₂O system at 25 °C. Equilibria for the Cr-Br⁻-H₂O system at 25 °C were determined for bromide ion activities of 15.61, 194.77, 650.06, and 2042.65, which corresponded to the 400, 700, 850, and 992 g/L LiBr solutions, respectively. Activities of all the dissolved species containing chromium were plotted for 10⁻⁶, 10⁻⁴, 10⁻², and 10⁰. Comparison of the simple Cr-H₂O system at 25 °C with the diagrams for Cr-Br⁻-H₂O system at 25 °C showed that the dominant aqueous Cr(III) species in acid solutions was Cr⁺³ for Br⁻ activities of 15.61, 194.77, and 650.06, whereas it was CrBr⁺² for Br^- activity of 2042.65. Aqueous CrBr⁺² formed at a Br⁻ activity higher than 943.05. The chromium solubility range in the acid area of the diagrams extended slightly to higher pH values with increasing Br⁻ activity and decreasing water activity, as a result of destabilization of Cr₂O₃.     

Improving the intergranular corrosion resistance of 304 stainless steel by grain boundary network control

The grain boundary network (GBN) was controlled by grain boundary engineering (GBE) in a 304 stainless steel. The total length proportion of Σ3ⁿ coincidence site lattice (CSL) boundaries was increased to more than 70% associating with the formation of large size highly twinned grain-cluster microstructure. Only coherent twin boundaries (Σ3_c) were found to be resistant to intergranular corrosion (IGC) and only such boundaries could be termed “special” ones. The improvement of resistance to IGC of the GBE specimen can be attributed to the large size grain-clusters associated with high proportion of the Σ3ⁿ boundaries and the interconnected Σ3ⁿ-type triple junctions.