A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies

The roles of grain boundary character and crystallographic texture on the intergranular stress corrosion cracking (IGSCC) of API X-65 pipeline steel has been studied using scanning electron microscope (SEM) based electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that low angle and special coincident site lattice (CSL) boundaries, mainly Σ11 and Σ13b and, possibly Σ5, are crack-resistant while the CSL boundaries beyond Σ13b and the random high angle boundaries are prone to cracking. However, several cracks were found to have been arrested even when the random high angle grain boundaries were available for them, both at the crack-tips and areas immediately ahead of them, to continue propagating. Texture studies in the vicinities of these crack-arrest regions, as well as in the cracked areas, provided a new understanding of crystallographic orientation-dependent IGSCC resistance: the boundaries of {1 1 0}‖rolling plane (RP) and {1 1 1}‖RP textured grains, mainly associated with 〈1 1 0〉 and 〈1 1 1〉 rotation axes, respectively, were crack-resistant due to their low energy configurations, while the cracked boundaries were mainly linked to the {1 0 0}‖RP textured grains.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.     

Corrosion behaviour of sensitized and unsensitized Alloy 900 (UNS 1.4462) in concentrated aqueous lithium bromide solutions at different temperatures

Duplex stainless steels can undergo microstructural changes if they are heated improperly. When that happens, duplex stainless steels are sensitized and intermetallic phases appear. The high Chromium and Molybdenum content promotes the formation of secondary phases as a consequence of the heat treatment. These secondary phases, which are rich in alloying elements, such as Cr and Mo, deplete these elements from the neighbouring phases, leading to a reduction in corrosion resistance. In order to study the influence of the secondary phases on the corrosion parameters, samples of duplex stainless steel, Alloy 900 (UNS 1.4462), have been heated in argon atmosphere at 825 °C for 1 h. The corrosion behaviour of sensitized and unsensitized Alloy 900 has been analyzed in a concentrated aqueous lithium bromide (LiBr) solution of 992 g/L by means of cyclic potentiodynamic curves. Secondary phase presence reduces the pitting potential value of Alloy 900. Besides, the pitting potential decreases with temperature. On the other hand, the corrosion potential and open circuit potential values increase with temperature and sensitization.     

Corrosion resistance of HVOF-sprayed coatings for hard chrome replacement

HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) and electrolytic hard chrome (EHC) coatings corrosion resistances have been compared through electrochemical polarization tests (0.1 N HCl, 0.1 N HNO₃) and Corrodkote test. EHC coatings passivate in HNO₃, but undergo pitting corrosion in HCl and in Corrodkote test too. HVOF coatings do not passivate, but possess more noble corrosion potentials. Both in HNO₃ and HCl, they undergo more generalized corrosion, with similar i_corr; crevice corrosion along splat boundaries is sometimes detected after the HCl test. Their i_corr in 0.1 N HCl solution is lower than in several of EHC coatings. No visible damage in the HVOF coatings has occurred after the Corrodkote test.     

The polarization characteristics of Pb-free Sn-8.5Zn-XAg-0.1Al-0.05Ga alloy in 3.5% NaCl solution

The polarization characteristics of Pb-free Sn-8.5Zn-XAg-0.1Al-0.05Ga alloy with varying Ag content were investigated in 3.5% NaCl solution. The value of Ag content i.e X varied from 0.1 wt% to 2 wt%. An increase in the Ag content up to 2 wt% resulted in a progressive increase in the corrosion current density and shifted the corrosion potential (E_corr) towards more active values. These changes were also reflected in the corrosion rate of the solder alloy. However the linear polarization resistance values decreased with increase in the weight content of silver. Increasing the Ag content from 0.1 to 2 wt% did not show a significant improvement in the passivation behaviour of the solder alloy as reflected in the passivation current density (i_p) and critical current density (i_cc) values. XPS and SIMS depth profile results revealed that the oxides of Zn and Sn were present in high concentrations on the outer surface of the solder alloy along with very low concentration of Al₂O₃.     

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

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 behavior of three iron-based model alloys in reducing atmospheres containing HCl and H2S at 600 °C

The corrosion of three Fe-xCr alloys (x = 8, 12, 18 wt.%) was examined at 600 °C in reducing atmospheres containing HCl and H₂S with two different H₂S contents and compared with the behavior of the same materials in H₂S-free H₂-CO₂ and H₂-HCl-CO₂ mixtures producing similar oxygen and chlorine pressures. Exposure to the low-H₂S gas mixture had only a reduced effect on the behavior of Fe-8Cr and Fe-18Cr, but increased significantly the corrosion rate of Fe-12Cr. Increasing the H₂S level accelerated the corrosion of all the alloys, but particularly that of Fe-18Cr. In both cases only minor amounts of sulfur and chlorine were present close to the alloy/scale interface. An increase of the Cr content reduced the corrosion rate in both H₂S gas mixtures, especially in the range 8-12 wt.% Cr, due to a larger volume fraction of Cr₂O₃ in the scale. The results have been discussed on the basis of the thermodynamic stability diagrams of the Fe-O-Cl-S and Cr-O-Cl-S systems.     

Temperature effect in the corrosion resistance of Ni-Fe-Cr alloy in chloride medium

The corrosion susceptibility of alloy 33 in 0.5 mol/L sodium sulphate solutions containing or not 0.1 mol/L sodium chloride was tested at three different temperatures: 22 °C, 40 °C and 60 °C. Electrochemical studies were performed using corrosion potential measurements (E_corr) as well as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion potential measurements showed that alloy 33 was passivated by a previously air formed film which was not destroyed during immersion in both solutions. No corrosion was observed during these tests although the temperature affected the film. Potentiodynamic polarization experiments showed that at high anodic potentials the previous film was broken up, and localized corrosion occurred in both solutions and at the three temperatures tested. Electrochemical impedance spectroscopy tests confirmed the presence of a stable passive film on the alloy surface at open circuit potential. Mott-Schottky analysis indicated that the passive film is an n-type semiconductor due to the presence of point defects of donor species, such as oxygen vacancies and interstitial metallic cations. As the potential increases the Cr(III) present in the barrier layer oxidizes producing Cr(VI) soluble species. The dissolution creates metallic cation vacancies that are acceptor species and the film changes from n-type to p-type semiconductor. The passive film rupture and the following localized attack are related to the drastic oxidative dissolution of the film at high anodic potentials, independent of its p-nature, chloride presence or increased temperature.     

Crevice corrosion damage function for grade-2 titanium of iron content 0.078 wt% at 95 °C

A crevice corrosion damage function has been developed for grade-2 titanium in 0.27 mol/dm³ NaCl at 95 °C. Crevice corrosion experiments were conducted for various durations using a galvanic coupling technique, and corrosion depth profiles subsequently measured using a combination of metallographic and image analysis techniques. The damage function was then determined by plotting the maximum penetration depth (d_max) in μm as a function of time (days). This function possessed two clear stages; an initial stage in which d_max = 89.4t^0.87, and a second stage in which corrosion spread laterally without an increase in d_max, before the crevice finally passivated. The initial high penetration rate was associated with an intergranular attack down Fe-containing grain boundaries. In the second stage, further increases in d_max were limited by the IR drop between the propagation sites deep within the grain boundaries and the external cathode.     

The effects of peak temperature and cooling rate on the susceptibility to intergranular corrosion of alloy 690 by laser beam and gas tungsten arc welding

This study investigates the effects of welding method, peak temperature, and cooling rate on the susceptibility to intergranular corrosion of alloy 690 weldments. The experimental results reveal that the laser beam welding process with cooling rate of around 212.6 °C/s can be produced with much less mass loss and a lower value of maximum reactivation current density/maximum anodic current density than with the gas tungsten arc welding process, where cooling rate is at around 17-20.6 °C/s. This is because the very rapid cooling rate during welding leads to an insufficient exposure time of around 2.1 s within the chromium (Cr)-carbide precipitation temperature range, suppressing Cr-carbide precipitation and Cr-depletion along grain boundaries in the weld decay region of the heat affected zone.     

An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing

A first systematic investigation was carried out to understand the corrosion of common Mg alloys (Pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt.% NaCl by measuring hydrogen evolution and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3 wt.% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)₂ surface film on Mg alloys is probably formed by a precipitation reaction when the Mg²⁺ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120-X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle. The electrochemical measurements of the corrosion rate, based on the “corrosion current” at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.     

The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R_sn) and the DC limit of the spectral noise resistance . The results from R_sn and also indicated higher corrosion rates at higher concentration and temperature. Also R_n and correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid.     

Corrosion behaviour of zinc deposits obtained under pulse current electrodeposition: Effects of coumarin as additive

The corrosion behaviour of zinc deposits obtained under pulsed current electrodeposition from an acidic chloride bath in the presence and absence of coumarin has been investigated. The effects of pulse peak current density (J_p) on the morphology of zinc deposits were studied by scanning electron microscopy. An increase in J_p from 40 to 280 A dm⁻² yields deposits with a finer grain size. The refinement of the grain size was more considerable in the presence of coumarin (J_p = 280 A dm⁻²). The preferred orientation of zinc deposits was studied by X-ray diffraction. At J_p = 40 A dm⁻², the preferred orientation of zinc deposits was (1 0 3) and changed to (0 0 2) at J_p = 80 A dm⁻². An increase in J_p to 280 A dm⁻² did not change the preferred crystallographic orientations except for an increase in the peak intensity of the (0 0 2) plane. In the presence of coumarin, the preferred crystallographic orientations changed at J_p = 280 A dm⁻² from the (0 0 2) plane to the (1 0 3) plane. The corrosion behaviour was investigated in an aerated 3.5% NaCl solution; the anodic polarization and electrochemical impedance spectroscopy curves were performed. The corrosion resistance of zinc deposits was improved by increasing the pulse peak current density (J_p); whereas, the presence of coumarin did not improve the corrosion resistance.     

Initial NaCl-particle induced atmospheric corrosion of zinc—Effect of CO2 and SO2

Initial corrosion and secondary spreading effects during NaCl particle induced corrosion on zinc was explored using in situ and ex situ FTIR microspectroscopy, optical microscopy, and SEM/EDAX. The secondary spreading effect which occurs upon introduction of humid air on NaCl deposited zinc surfaces was strongly dependent on the CO₂ and SO₂ content of the introduced air. Ambient level of CO₂ (350 ppm) resulted in a relatively low spreading effect, whereas the lower level of CO₂ (<5 ppm) caused a much faster spreading over a larger area. In the presence of SO₂, the secondary spreading effect was absent which could limit the cathodic process in this case. At <5 ppm CO₂, the corrosion is more localized, with the formation of simonkolleite (Zn₅(OH)₈Cl₂ · H₂O), zincite (ZnO) and sodium carbonate (Na₂CO₃), and a larger effective cathodic area. At 350 ppm CO₂, the corrosion is more general and formation of simonkolleite, hydrozincite (Zn₅(OH)₆(CO₃)₂) and sodium carbonate was observed. Sodium carbonate was mainly formed in more alkaline areas, in the inner edge of the electrolyte droplet and in the secondary spreading area. Oxidation of sulphur and concomitant sulphate formation was enhanced in the presence of NaCl particles, due to the formation of a droplet, the separation of the anodic and cathodic areas and the accompanying differences in chemical composition and pH in the surface electrolyte.     

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm⁻¹) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm⁻¹) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field.     

氮对高纯奥氏体不锈钢耐晶间腐蚀性能的影响

采用电化学、化学浸渍、俄歇电子能谱分析、物理化学相分析等方法研究了氮对高纯奥氏体不锈钢000Cr19Ni14耐敏化态晶间腐蚀和非敏化态晶间腐蚀性能的影响,并探讨了其作用机理.结果显示,高纯奥氏体不锈钢加氮合金化(≤0.20%)基本上消除敏化态晶间腐蚀,敏化处理不会引起晶界贫铬;当氮含量不超过0.087%时,对非敏化态晶间腐蚀影响很小,而超过0.087%时,由于在晶界氮元素的偏聚以及氮化铬的析出加速非敏化态晶间腐蚀.     

Effect of ZrO2 on corrosion behaviour of chromium coatings

To study the effect of ZrO₂ particles on corrosion behaviour of Cr coating, steel samples were plated in Cr(VI) baths without and with ZrO₂. The corrosion behaviour of plated samples was studied at different exposure times in a solution containing 0.01 mol l⁻¹ H₂SO₄ + 0.5 mol l⁻¹ Na₂SO₄ using cyclic voltammetry and impedance spectroscopy. The equivalent circuit model R_e(Q_cR_pore)(Q_s[OR_s]) was proposed to fit the corrosion process and the parameters Y₀(Q_c),Y₀(Q_s) and R_pore reflecting corrosion behaviour of samples were evaluated. From the results, it was found that samples plated in bath containing ZrO₂ exhibited improved protective properties as a result of the structural characteristics of the coatings obtained; namely, the size and shape of pores.     

Corrosion and recession behavior of zircon in water vapor environment at high temperature

A corrosion test in static state water vapor environment and a recession test in high velocity steam jet environment for zircon bulk were performed at 1300 °C. The trace of the water vapor corrosion could be recognized on the grain surfaces and at the grain boundaries for the sample after the static state corrosion test. Sand ripple like morphology was generated on the grain surfaces and etch pits with less than 0.1 μm size were formed at the grain boundaries. A porous structure was formed on the bulk surface of the sample after the steam jet test. A glassy phase enriched with silica could be recognized on the surface of the sample after the test. Cracks were induced on the bulk surface during the test. The zircon phase at the bulk surface decomposed into monoclinic zirconia phase and the fraction of silica component at the bulk surface decreased by the steam jet test. The monoclinic zirconia phase was observed to re-generate and grow on the bulk surface.