On the corrosion of binary magnesium-rare earth alloys

The corrosion properties of high-pressure die cast (HPDC) magnesium-rare earth (RE) based alloys have been studied. Binary additions of La, Ce and Nd to commercially pure Mg were made up to a nominal 6 wt.%. It was found that the intermetallic phases formed in the eutectic were Mg₁₂La, Mg₁₂Ce and Mg₃Nd, respectively. Results indicated that increasing RE alloying additions systematically increased corrosion rates. This was also described in the context of the electrochemical response of Mg-RE intermetallics - which were independently assessed by the electrochemical microcapillary technique.This study is a discrete effort towards revealing the electrochemical effect of carefully controlled binary alloying additions to magnesium in order to elucidate the microstructure-corrosion relationship more generally for HPDC Mg alloys. Such fundamental information is seen to not only be useful in understanding the corrosion of alloys which presently contain RE additions, but may be exploited in the design of magnesium alloys with more predictable corrosion behaviour. There is a special need to understand this relationship - particularly for magnesium that commonly displays poor corrosion resistance.     

Galvanic currents and corrosion rates of reinforcements measured in cells simulating different pitting areas caused by chloride attack in sodium hydroxide

Chlorides induce localized corrosion in the reinforcement. The pits formed a substantial reduction in the cross-sectional area. The smaller is the ratio between the size of the corrosion spot and the surrounding passive metal area acting as cathode, the higher is the corrosion rate or the progression in the pit depth. This, however, is not always the case. To understand well the macro or microcell behavior of the corrosion of reinforcements, this paper reports a study on the effect of decreasing sizes of anodic zones until relatively small, maintaining the size of the cathode constant. In addition, to enhance the pure anodic behavior of the corroding zone, an acidic solution was used as an anolyte. Measuring the galvanic current (I_g) and the corrosion rate (I_corr) by using zero resistance ammeter (ZRA) and polarization resistance (R_p) techniques, respectively, was crucial to differentiate the microcell contribution from that of the macrocell. The results indicate that while I_g increased as the anodic area decreased, I_corr presented a maximum for the area ratios, S_A/S_C = 1/3.     

Effect of rare earth on inclusion evolution and corrosion resistance of HRB400E steel

The effect of rare earth elements Ce and La on the evolution behavior of inclusions in HRB400E steel was studied through experimental observations and thermodynamic calculations. Neutral salt spray corrosion experiments were also conducted to investigate the effect of Ce–La on the corrosion resistance of steel. The results showed that the typical inclusions in steel without rare earth were MnS and MnO–SiO₂. A small amount of Mn–Si–O–S inclusions was also observed. After adding rare earth, the typical inclusions were transformed into isolated (Ce,La)₂O₂S, (Ce,La)₂O₃ + MnS, and (Ce,La)₂O₂S + MnS complex inclusions. The thermodynamic calculations indicated that the rare earth elements in molten steel preferentially reacted with MnO–SiO₂ inclusions and dissolved oxygen and sulfur to form (Ce,La)₂O₃ and (Ce,La)₂O₂S. Small amounts of [S] and [Mn] adhered to the surface of the nucleated rare earth inclusions to form complex inclusions. After Ce–La treatment, the corrosion rate of the steel decreased from 3.491 to 1.992 mm year⁻¹, and the corrosion resistance was improved. The change in corrosion behavior is due to the modification of the inclusions into rare earth inclusions with good compatibility with the steel matrix.     

Corrosion behaviour of zinc–cerium alloys: Role of intermetallic phases

New zinc–cerium alloys (up to 5 wt.%) was synthesized and their corrosion behaviour was studied in a reference corrosive media. Metallographic analyses show that cerium is exclusively present in a Zn₁₁Ce intermetallic phase, homogeneously dispersed in the zinc matrix. With an optimal concentration at around 1.5 wt.%, the Zn₁₁Ce phase acts as a tank of cerium to form a protective Ce-enriched passive layer. Nevertheless, at high Ce content, the benefit in terms of corrosion resistance obtained by the incorporation of Ce in the corrosion layer is shaded by the galvanic coupling between the intermetallic phase and the matrix.     

Studies on the initial stages of zinc atmospheric corrosion in the presence of chloride

A four-year exposure program was carried out in Taiwan in which 23 test sites with different climatic and pollution conditions were chosen and evaluated according to ISO standards 9223-9226. Examination of the results indicated that most of the tests sites were very corrosive to zinc specimens and there was a severe white rust problem for freshly galvanized items stored in high humidity outdoors environments. In addition, the initial stages of zinc atmospheric corrosion in the presence of chloride were studied quantitatively in a non-aqueous electrolyte (methanol) using ex situ electrochemical impedance spectroscopy (EIS) to determine polarization resistance (R_p). The samples were exposed to the synthetic atmospheres with careful controlled relative humidity, temperature, and contaminating salts. It was observed that a change of R_p was accompanied by a change in the corrosion product on the zinc surface, and that the R_p increased with relative humidity (RH) during pre-exposure. Furthermore, the corrosion products of zinc were analyzed qualitatively by grazing-angle X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Zinc hydroxycarbonate (Zn₅(CO₃)₂(OH)₆) and zinc oxide (ZnO) in this surface layer were found to provide protection against chloride contaminants.     

Effect of citrate ions on the electrochemical behaviour of low-carbon steel in borate buffer solutions

The electrochemical behaviour of cold-rolled low carbon steel was studied on both active and passive potential regions in borate buffer solutions with and without the addition of sodium citrate (NaCit). In the active region anodic charges increased significantly and R_CT values decreased with citrate, due to the formation of soluble complexes. In the passive potential region the film formed at +0.4 V in borate buffer solution with and without 0.010 M NaCit is probably enriched by Fe₃O₄ oxide, while films formed at +0.8 V are probably enriched by γ-Fe₂O₃. The equivalent circuit [R_s(R_CTQ)] fitted all experimental impedance data.     

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO₃ media. The results in 6 N HNO₃ show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO₃, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO₃ with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R_P) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest R_P value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO₃ revealed that the passive films were mainly composed of Cr₂O₃ and Fe₂O₃ for all the alloys. The corrosion resistance of different NAG SS to HNO₃ corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper.     

Corrosion behaviour of AM60 magnesium alloys containing Ce or La under thin electrolyte layers. Part 2: Corrosion product and characterization

The corrosion behaviour of AM60 containing Ce or La (AMRE1) under thin electrolyte layers (TEL) in 3.5 wt.% NaCl solution was investigated. The composition and structure of corrosion product was analyzed by XRD, FTIR, SEM and EDS. The results indicated crystal corrosion products were not affected by TEL and the amorphous ones are Mg(OH)₂ and compounds containing CO₃²⁻ in corroded section and only latter one is present in “unaffected” area. The smart map further confirmed the skeleton structure formed due to RE alloying. Moreover corrosion morphologies of AMRE1 alloys exhibited localized corrosion and pit initiation was inhibited by TEL.     

Electrochemical corrosion behavior of biomedical Ti–22Nb and Ti–22Nb–6Zr alloys in saline medium

The aims of this study were to investigate the effects of Zr addition and potentiodynamic polarization on the microstructure and corrosion resistance of Ti–22Nb and Ti–22Nb–6Zr alloy samples. The corrosion tests were carried out in 0.9% NaCl at 37 °C and neutral pH value, utilizing the OCP, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. The results of XRD and optical microscopy indicated that the addition of Zr stabilized the β phase, which plays a crucial role in the corrosion resistance improvement of the Ti–22Nb–6Zr alloy. From the polarization curves, it can be seen that the alloys exhibited a wide passive region without the breakdown of the passive films and also low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with the addition of 6 at% Zr into the Ti–22Nb alloy. The EIS results of these two alloy samples after 1‐h immersion in 0.9% NaCl solution, and being fitted by R_S(Q_PR_P) model, suggested that the corrosion resistance of the passive films improved with the addition of Zr and only a single passive film formed on the surfaces. However, two time constants were observed for the Ti–22Nb and Ti–22Nb–6Zr alloy samples after potentiodynamic polarization, the spectra of which can be fitted using the R_s(Q_o(R_o(Q_bR_b))) model. In addition, the corrosion resistance of the two alloy samples was reinforced significantly because of polarization when compared to the immersed samples. All these observations suggested a nobler electrochemical behavior of the titanium alloys with the addition of Zr element and after polarization.     

A study on the passive state stability of steel embedded in activated fly ash mortars

The main reason for the premature failure of reinforced concrete structures (RCS) is corrosion of the reinforcements. The use of new mortars based on activated fly ash, an alternative to conventional portland cement, requires extensive research in order to check its passivating properties for reinforcements and the instability or permanence of the passive state achieved. The present paper studies this objective, analysing the passivating capacity of specimens manufactured with three different fly ash mortars treated with three activators and the stability of the passive state under the effect of 0%, 0.2%, 0.4% and 2% chloride additions in relation to the binder material, in an environment of apparently constant humidity and in humidity cycles. The evolution with time of corrosion potential (E_corr) and polarisation resistance (R_p) was studied. Polarisation curves and time constants (τ) were also assessed for some specimens using short duration galvanostatic pulses.     

Microbially influenced corrosion and inhibition of nickel–zinc and nickel–copper coatings by Pseudomonas aeruginosa

The present study describes the effect of Pseudomonas aeruginosa on the corrosion rate of nickel–zinc and nickel–copper alloy coatings. The presence of bacteria was associated with decreases in R_ct values, suggesting that P. aeruginosa promoted the corrosion of nickel–copper alloy coatings. However, R_ct values of nickel–zinc coatings increased in response to bacterial inoculation, corresponding to a decrease in corrosion rate for nickel–zinc alloy coatings. Our results suggest that the activity of P. aeruginosa facilitated the corrosion of nickel–copper alloy, while serving a protective function for the nickel–zinc alloy.     

Corrosion inhibition of Armco iron by 2-mercaptobenzimidazole in sodium chloride 3% media

The effect of 2-mercaptobenzimidazole (2MBI) on the corrosion of Armco iron in NaCl media has been investigated in relation to the concentration of the inhibitor by various corrosion monitoring techniques. Surface morphology was studied by scanning electron microscopy (SEM). Results obtained revealed that 2MBI is a good anodic inhibitor. The addition of increasing concentrations of 2MBI moves the corrosion potential towards positive values and reduces the corrosion rate. EIS results show that the changes in the impedance parameters (R_t and C_dl) with concentrations of 2MBI is indicative of the adsorption of these molecules leading to the formation of a protective layer on iron surface. The adsorption of this compound is also found to obey Langmuir’s adsorption isotherm in NaCl.     

Inhibitory effect of some amino acids on corrosion of Pb-Ca-Sn alloy in sulfuric acid solution

The present article describes the inhibition effect of amino acids cysteine (Cys), methionine (Met) and alanine (Ala), towards the corrosion of lead-alloy (Pb-Ca-Sn) in H₂SO₄ solution by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurement and scanning electron microscopy (SEM) methods. The influence of inhibitor concentration, temperature and time on inhibitory behavior of the amino acids was investigated. The corrosion data including corrosion current density (I_corr), corrosion potential (E_corr) and charge transfer resistance (R_ct) were determined from Tafel plots and EIS. Recording impedance spectra showed that the charge transfer resistance is increased by increasing adsorption time. The SEM micrographs revealed that the corroded surface area is decreased in the presence of amino acids. Meanwhile, the inhibition efficiency (IE) was found to be depending on the type of amino acid and its concentration. The IE for 0.1 M Cys in 0.5 M H₂SO₄ is greater than 96%. Adsorption isotherms were fitted by Langmuir isotherm.     

Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate

Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc electrode, RDE, in 3.5% NaCl and its corrosion inhibition using K₂HPO₄ have been studied. Results showed that by rotating the electrode in blank and inhibited solutions, corrosion current density, i_corr, increased, corrosion potential, E_corr, shifted toward more positive values and charge transfer resistance, R_ct, decreased. The inhibition efficiencies increased with electrode rotation rate. This increase was attributed to the enhanced mass transport of inhibitor molecules toward the metal surface and formation of more protective films. Little decrease of efficiencies at higher rotation speeds was probably because of the separation of protective films due to high shear stresses.     

Effect of cerium addition on microstructure and corrosion resistance of die cast AZ91 magnesium alloy

A novel AZ91 Ce containing magnesium alloy characterized by excellent corrosion resistance is fabricated by adding rare earth Ce (cerium) in the form of a Mg‐Ce master alloy. The metallographic investigation shows that Ce added to AZ91 can obviously decrease the size of β‐Mg₁₇Al₁₂ and forms Al₁₁Ce₃ intermetallic compounds in the shape of fine needles. The corrosion tests and electrochemical measurements indicate that the corrosion resistance of AZ91 Ce containing magnesium alloy is obviously higher than that of AZ91. Furthermore, increasing the content of Ce in the magnesium alloy can further enhance the corrosion resistance. X‐ray photoelectron spectroscopy (XPS) reveals that Ce can be incorporated into corrosion products in the form of CeO₂ in the course of corrosion. Based on the preliminary analysis, the addition of Ce can improve the corrosion resistance of AZ91 by decreasing the size of β‐Mg₁₇Al₁₂ and enhancing the protective effectiveness of corrosion products.     

EIS monitoring study of atmospheric corrosion under variable relative humidity

Electrochemical impedance spectroscopy (EIS) technique was used to investigate atmospheric corrosion in laboratory simulated environments with variable relative humidity (RH) and fixed Cl⁻ content. The results show the suitability of EIS for analyzing electrochemical corrosion behaviour at 5-100% RH. At 5-30% RH, EIS spectra reflected the character of the electrode, whereas at 40-100% RH, the model of EIS spectra was established with the help of surface analysis. From 70% RH, the film resistance (R_r) reflects the degree of corrosion and the charge transfer resistance (R_t) provides quantitative representation of the corrosion rate, which were verified by weight loss tests.     

The role of Ce ions in electrochemical corrosion of 304 SS in Ce(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>.6H<Subscript>2</Subscript>O

Effects of temperature and potential on the electrochemical corrosion behavior of alloy AISI 304 (UNS S30400) Stainless steel were investigated in 3 wt.% cerium nitrate (Ce[NO₃]₃.6H₂O) solution. With an increase in electrolyte temperature from ambient temperature to 90°C, the corrosion potential of the alloy shifted towards the noble direction, and the resistance to polarization increased due to the formation of Ce-oxide on the electrode surface. The oxide films formed at the open circuit potential (OCP) and a passive potential of 0.4 V_SCE were examined by x-ray photoelectron spectroscopy (XPS). The oxide film formed at 50°C and a passive potentialof 0.4 V_SCE consists of mixed oxides of Ce and Cr, whereas that at OCP consists of only Cr oxide. The formation of Cr oxides on the electrode surface was primarily due to the nitrate (NO₃ ⁻) ions in Ce(NO₃)₃.6H₂O electrolyte.     

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.     

Semiconducting and passive film properties of nitrogen-containing type 316LN stainless steels

The semiconducting property of passive films of nitrogen-containing type 316LN stainless steels in different electrolytic media (0.5 M NaCl, borate buffer and borate buffer + 0.5 M NaCl) was investigated by electrochemical impedance spectroscopy (EIS). The nitrogen effect on the chemical composition of the passive films was investigated using X-ray photoelectron spectroscopy, (XPS). Based on capacitance results, the semiconducting parameters obtained from the Mott-Schottky plots indicated a decrease in the donor and acceptor density (N_D and N_A) with increase in nitrogen content, and variation in the flat band potential (E_FB), depending on the electrolytic media. Thus indicating that the oxide layers of the passive film are modified by nitrogen addition. The presence of nitrogen and in the passive film was confirmed by the XPS analysis of the passive film. Cyclic polarization for pitting and repassivation corrosion studies indicated a decrease in hysteresis loop with increase in nitrogen content in 0.5 M NaCl solution. In the highest nitrogen-containing alloy (0.556 wt.% N), the hysteresis loop was small and negligible indicating that the pit initiation is minimum in this alloy. Based on the results obtained, an attempt was made to correlate the semiconducting nature of the passive films with pitting corrosion resistance.     

Improvement of corrosion resistance of Cu-based bulk metallic glasses by the microalloying of Mo

(Cu₄₇Zr₁₁Ti₃₄Ni₈)_100−xMo_x bulk metallic glasses (BMGs) with x = 0, 1 and 2 at% were prepared by water-cooled copper mold casting. The effect of the addition of a small amount of Mo on the corrosion resistance of the Cu-based BMGs was examined by electrochemical polarization and weight loss measurement in 1 mol/L H₂SO₄ open to air. It is found that the corrosion resistance of Cu-based BMGs enhanced with increasing Mo content, as indicated by the decrease in passive current density and corrosion rate with the increase in Mo content. X-ray photoelectron spectroscopy (XPS) was employed to study the composition and the state of elements in the passive film, which revealed that the improvement of corrosion resistance of Cu-based BMGs microalloyed with Mo originated from the enrichment of ZrO₂ and TiO₂, but depletion in Cu- or Ni-oxide in the passive films formed during electrochemical polarization. In addition, the galvanostatic-step measurement was performed to investigate the formation kinetics of the passive films. It is demonstrated that the addition of an appropriate amount of Mo can promote the formation of the passive film on the surface.