With the increasing utilization of electrochemical impedance spectroscopy (EIS) in the field of corrosion research, the EIS analysis gradually become a great and important means for corrosion scientists. Especially in recent years, the corrosion processes involved in EIS studies become more and more complicated, not only for the diversity of corrosion environments and metallic materials, but also superficial- and/or interfacial-membranes on metal surface, thus such complex systems may bring too much difficulties for dealing with the relevant items, therewith the requirements for the normal and simple EIS analysis may not be satisfied any more. Comparing with electrochemical kinetics, the EIS analysis with the simulated equivalent circuit (SEQC) is fast propagating recently, because it is more simple, intuitive and accepted for the corrosion scientists, especially the ones of non-professional electrochemists. However, there exists inherent deficiency, shortages in preciseness and normalization for that technique, which may result in lower credibility of the relevant model of SEQCs. Therefore, the practical and reliable means in analysis of EIS should be created for the corrosion scientists with inadequate experience in electrochemistry, hence, the issues related with EIS analysis of corrosion processes were examined by the authors based on their experience in the field: i.e. the review of the present situation and the advantages and disadvantages of the application of the EQCS in EIS analysis of corrosion processes, and the discussion on the necessity and feasibility to enhance the credibility of SEQC in corrosion EIS analysis so that to establish tentatively routs for precise and normalized analysis of corrosion EIS by means of SEQCs.
The addition of inhibitors is one of the main methods to control the corrosion of metals and alloys. At present, studies and applications about inhibitors are mainly focused on the inhibition for uniform corrosion, but less on the inhibition for localized corrosion, particularly for pitting corrosion. In this paper, the inhibition effect and mechanism of several inhibitive species for pitting corrosion are summarized and compared, involving mainly inorganic anions, organic molecules/ions and rare earth elements. Relatively speaking, there are many studies about the inhibition of inorganic anions in recent years, but the published reports about the inhibition of rare earth elements are relatively few. The objective of this paper is to provide a reference for the development of novel and composite inhibitors in order to control uniform corrosion and pitting corrosion simultaneously. Finally, the further studies and applications are predicted.
This paper focuses on the nowadays research status of corrosion and biofouling phenomena occurred on marine propeller as well as the relevant protection methods, namely external protection technologies for the propellers in service and new corrosion-resistant materials for the propeller fabrication. The protection technologies of marine propeller include mainly cathodic protection technologies, anticorrosion coatings, and electrolytic antifouling technologies, which are suitable for the in-service propellers. Except for common copper alloys, new materials applied to making propeller developed rapidly, such as stainless steels and composite materials which have attached much attention. Several protection methods and new materials for marine propellers are introduced in the paper. Finally, the trend of research and development of materials and protection technologies for marine propeller in the future was also pointed out.
In the oil and gas industry, the effect of stress on corrosion of steels for pipes can not be ignored due to the environment of complex corrosive media and the applied multiple stress loads on pipes during service. The synergistic effect of stress and corrosive medium can not only cause stress corrosion cracking of oil and gas pipes, but also influence corrosion behavior of steels for oil and gas pipes by changing their electrochemical process. Therefore, the research on the stress induced corrosion electrochemical behavior of steels for oil and gas pipes was commented in this paper. The effect of stress on corrosion thermodynamics, corrosion kinetics and corrosion product film of steels for oil and gas pipes was analyzed. At last, the research trend of the stress influenced corrosion electrochemical behavior of steels for oil and gas pipes was discussed.
Tinned steel sheet is one of the most frequently used metal packaging materials for food industry, due to its high strength, good barrier property, and excellent processing performance. However, one shortcoming related with the application of tinned steel sheet is the trouble of corrosion, peculiarly when the stored food is highly corrosive. Therefore, the corrosion behavior of tinned steel in food environment as well as the relevant corrosion mechanism of the bare- and lacquered-tinned steel are introduced and discussed in the paper. It seems that the corrosion process was closely linked to pH value, dissolved oxygen and some corrosive ions in the food-containing electrolyte. It is noted that new surface treatment methods and new organic coatings for the tinned steel sheet are needed to develop. Finally, the future work on this topic is also pointed out.
S32750 super duplex stainless steel (SDSS) was aged in the temperature range 650 ℃ to 1000 ℃, and then was characterized by means of optical microscope (OM), scanning electronic microscope (SEM) and X-ray diffractometer (XRD). The corrosion resistance of the aged steel in 5%(volume fraction) HF solution was assessed by measurements of potentiodynamic polarization, electrochemical impedance spectra (EIS) and Mott-Schokkty plots. Results show that the σ phase precipitated in the steel in the temperature range from 750 ℃ to 950 ℃, but not at 650 ℃ and 1000 ℃ respectively. Among others, the amount of σ phase is the highest and correspondingly, the corrosion resistance is also the worst for the steel aged at 850 ℃. This is mainly because of that the precipitates of σ phase lead to the segregation of Cr and Mo within the steel, thus the non-uniform distribution of the alloying elements in the steel may induce galvanic corrosion within the steel , which accelerates the dissolution of SDSS. At the same time, the σ phase precipitation may also induce the increase of carrier densities of the formed passive film of the steel in HF solution, which enhanced the adsorption of F-, and in turn the dissolution rate of passive film and finally reduced the stability of the passive film, thus, the corrosion rate of SDSS in HF is accelerated.
The effect of Cl- concentration on stress corrosion cracking (SCC) in alkaline solutions of NaCl/Na2S was assessed by means of slow strain rate test (SSRT) and U-type bending immersion test, as well as measurements of potentiodynamic polarization, electrochemical impedance spectra (EIS). 316L stainless steel exhibits SCC susceptibility to a certain extent in alkaline NaCl/Na2S solution, while Cl- and S2- have a competitive effect on the corrosion process of 316L stainless steel, resulting in the extreme value of electrochemical impedance. With the increasing of Cl- concentration, the effect of hydrogen-induced plasticity (HIP) can increase the elongation to a certain extent and decrease the plastic loss of 316L stainless steel, but it has little effect on the tensile strength of 316L stainless steel.
The microbe induced corrosion of Cu in circulation cooling water system was investigated in the presence of sulfate reducing bacteria by means of electrochemical measurement and surface analysis techniques. Meanwhile, the inhibition effect of cysteine on Cu was also assessed. The results show that, polarization resistance of Cu increased first and then decreased in the water with sulfate reducing bacteria, and the polarization resistance of Cu is 25.82 kΩcm2 lower than that in the sterile water. The corrosion current density of Cu reduced first and then increased, while the electrochemical impedance spectroscopy presented the same tendency. In the sulfate reducing bacteria containing water, the addition of different concentrations of L-cysteine can lead the decline of the corrosion current density Icorr and the expansion of the radius of electrochemical impedance of Cu, particularly with the concentration of 10-3 mol/L, L-cysteine exhibits the best corrosion inhibition effect for Cu in the water with sulfate reducing bacteria.
The evolution of cavitation-corrosion of pure titanium TA2 in LiBr solution was investigated by roughness profiler, scanning electronic microscope (SEM) and three-dimensional video microscope, while the combined action of corrosion and cavitation was examined by electrochemical measurement technologies. Results show that there was uneven plastic deformation firstly under the action of cavitation, and as the cavitation test continued, the degree of plastic deformation and the dislocation density at grain boundaries increased, causing the concentration of stress and slight falling off of materials of grain boundaries. Cracks were generated locally, while the surface roughness value (Rq) increased linearly with time. Then cracks propagated along grain boundaries or through grains, then the falling of material increased. At the same time, the passive film on the surface was easily attacked and therewith the fresh titanium substrate was exposed, which promoted the corrosive effect of lithium bromide. The synergistic effect of mechanics and corrosion may accelerate the deepening and expending of corrosion pits, eventually the materials on the periphery of corrosion pits fell off, and thereby the degree of surface concave and convex became smaller, the growth rate of Rq decreased, at the end, the value of Rq tended to stabilize.
The trivalent chromium passivation film on the surface of Zn-plated Q235 steel was fabricated by immersion method. The microstructure and corrosion performance of the passivation film were studied by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), as well as polarization curve measurement, electrochemical impedance spectroscopy (EIS) and neutral salt spray (NSS) test, respectively. The results show that the Zn-coating is compact without obvious cracks but with excellent anticorrosion performance. The trivalent chromium passivation film on Zn-coating has smooth surface with a large number of cracks. The major constituents of the trivalent chromium passivation film were Zn, Cr and O, while a small amount of P and N was also detected, which probably in the form of compounds such as Cr(OH)3, Zn(OH)2, Cr2O3, and ZnO. The forming process of passivation film possibly consists of three steps such as zinc dissolution, passivation film forming and dissolution, and finally drying to become passivation film. It is noted through NSS test that the trivalent chromium passivation film can enhance the corrosion resistance of Zn-coating by c.a. 7 times．
Mixed solutions of perfluorooctanoic acid (PFOA) with silane coupling agents KH550 and KH560 respectively were used to pre-treat the surface of carbon steel Q235 and then the effect of the two surface treatment agents on the properties of the applied epoxy coatings was studied by means of FT-IR, SEM with EDS, as well as polarization curve measurement, contact angle measurement and dropping corrosion test with CuSO4 solution. Results show that the performance of the steel treated with PFOA/KH550 and PFOA/KH560 has been improved. Firstly, contact angle increased by 17.2% and 11.7% respectively. Then the time for corrosion occurrence during dropping corrosion test was increased by 64.7% and 38.8% respectively. Finally, corrosion current was reduced by 93.5% and 78.7% respectively. The performance of the epoxy coating applied on the rust steel surface treated with PFOA/KH550 was significantly improved, i.e. the adhesion and impact strength were increased by 14.3% and 11% respect tively, and corrosion current decreased by 96.2% compared with those of the coating applied on the as received rust steel surface. Besides, there was no significant change for the above-mentioned coating after 62 d soaking in NaCI solution.
A kind of circular loop microelectrode was designed and embedded in the coating body during coating fabrication. Then a novel electrochemical impedance spectroscopy (EIS) measurement system was developed based on the embedded-microelectrode (EM), which was suitable for in situ evaluation of the coating in alternating hydrostatic pressured (AHP) sea-water with the purpose of simulation of deep sea environments. The results showed that EM could avoid difficulties related with the installation of an external electrode and could overcome drawbacks that the detected signals of the corrosion of metal beneath thick coatings was too weak for the field measurement by an external electrode. Therefore, the EM can be utilized in high hydrostatic pressured or AHP sea-water environment. The EIS results of the EM system were consistent well with those of the traditional three-electrode measuring device. Consequently, the validity and reliability of EM were reasonably confirmed.
The Cd-plated AISI 4130 steel is being widely used for aircraft landing gear and potassium formate is the main composition of the common deicing fluid. Therefore, it is worthy to investigate the corrosion electrochemical behavior of Cd plated AISI 4130 steel beneath thin electrolyte layer of potassium formate solution at low temperature. Results showed that the corrosion process of the Cd-plated AISI 4130 steel in potassium formate solution is mainly controlled by the diffusion of dissolved oxygen, and influenced by the deposition of corrosion product beneath the thin electrolyte layer. When the thickness of the electrolyte layer is higher (240 μm or more), there is little influence of the thickness of the electrolyte layer on the cathodic limited diffusion current density and solution resistance. When the thickness of the electrolyte layer is thinner, the cathodic limited diffusion current density and solution resistance are increased with the decrease of the thickness of the electrolyte layer. For the same thickness of the electrolyte layer, the corrosion potential of the Cd-plated AISI 4130 steel decreases with the increase of solution concentration. For the dilute solutions, the corrosion rate of the Cd-plated AISI 4130 steel is lower, and the corrosion potential and corrosion rate are less affected by the thickness of the electrolyte layer. When the concentration of potassium formate solution is higher, the corrosion potential of the Cd-plated AISI 4130 steel is positively shifted with the decrease of the thickness of the electrolyte layer. The corrosion rate of the Cd-plated AISI 4130 steel beneath thin electrolyte layer is obviously higher than that of full immersion test, and the corrosion rate for the former case increased first and then decreased with the decreasing thickness of the electrolyte layer, and reached the maximum value when the thickness of the electrolyte layer is about 150 μm.
The weld joint of S450EW steel was prepared by metal active gas arc (MAG) welding. The corrosion behavior of different zones of the welded joint was studied via electrochemical test and scanning electron microscope (SEM). The results showed that the microstructures were mainly composed of granular bainite of the base metal zone and weld zone, while the microstructure of the heat affected zone was composed of a mixture of ferrite and pearlite. Uniform corrosion was the typical corrosion pattern in the weld zone, and local corrosion occurred in the base metal zone and heat affected zone. Besides, severer local corrosion was observed in the heat affected zone. The better corrosion resistance of the weld zone may ascribed to the effect of high Ni content of the welding rod, while the high corrosion rate of the heat affected zone may be due to the dual phase microstructure of ferrite and pearlite.
Corrosion behavior of T91 steel by neutral salt spray with 0.1% (mass fraction) NaHSO3 was studied by means of mass loss measurement, macro-appearances observation and characterization of corrosion products with scanning electron microscope/energy dispersive spectrometer (SEM/EDS). Results showed the corrosion product on the steel surface increases gradually throughout the salt spray testing, and its coverage ratio, integrity and compactness increase accordingly, and finally the corrosion product completely covers the steel surface. The corrosion product composed mainly of FeOOH, Fe2O33H2O and Cr2O3 containing a little sulfur. The corrosion process of T91 steel by salt spray test can be expressed as follows: during the salt spray with NaHSO3 solution, localized corrosion sporadically generates on the steel surface at the initial stage, and then these localized corrosion develops into big corrosion spots, and finally those big corrosion spots spread and connected each other to generate corrosion on large area.
To improve the high-temperature oxidation resistance of austenitic stainless steels, the high-Al austenitic stainless alloy coatings were deposited on the surfaces of 316 austenitic stainless steel using the direct current magnetron sputtering. Cyclic oxidation tests were performed at 850 ℃ in air. The effect of the alloy coatings on the oxidation resistance of 316 austenitic stainless steel was studied by SEM,EDS and XRD. The oxidation resistance of 316 austenitic stainless steels with coatings was much better than that without coatings. The formation of a continuous and dense film on the surfaces of alloy coatings consisting of Al2O3, Fe(Cr, Al)2O4 and NiCrO3 is responsible for its excellent oxidation resistance. The poor oxidation resistance of 316 stainless steel can be ascribed to the formation of a rough and loose film on the surfaces consisting of Fe2O3, Cr2O3, FeCr2O4 and NiCr2O4.
Since the retrofitting of a thermal power unit for heat delivery, correspondingly, the supplied water of boiler significantly increased from ordinary 3%~5% to above 50%, thereby the conductivity of the water generally exceeded, resulting in serious acid corrosion troubles in low pressure cylinder of the steam turbine. It is known that humic acid is a typical organic substance related with corrosion troubles in the water system, thus the decomposition behavior of humic acid at 350 ℃ was studied in concentration range 0~20 mg/L, while decomposition products specially from the test by 7.5 mg/L of humic acid at different time intervals were extracted for composition determination with ion chromatography and TOC analyzer. Thereafter the effect of low molecular organic acids with impurities on the corrosion behavior of steel 1Cr13 was characterized by means of electrochemical methods as well as SEM, EDS and XRD. The results showed that high temperature decomposition products of humic acid contained low molecular organic acids mainly of acetic acid and formic acid,and impurity ions of SO42-, Cl-, NO3- and F-. Obviously, the ions of SO42-, NO3- and Cl- were aggressive to the steel 1Cr13, in the contrary, F- exhibited inhibition effect with the increasing concentration.
The corrosion of carbon steel in high temperature gas containing ammonium bisulfate (ABS) and ammonium sulfate (AS) was studied by means of weight loss method, SEM/EDS and XRD. Results show that in the range of 108~282 ℃, the ABS and AS in flue gas are both corrosive to carbon steel, while ABS causes more serious corrosion. As the concentration of ABS and AS in gas phase increases, the corrosion rate is accelerated; with the rising gas temperature, the corrosion rate is slowed down. The corrosion mechanism is speculated as follows: due to the acidity of ABS and AS solution, H+ acid corrosion is the main corrosion of carbon steel, but oxygen corrosion also takes place. The anode reaction generated Fe2+ may then induces the secondary reaction to form series of iron oxides, thereafter ammonium sulfate salt interacts with Fe2O3 and Fe3O4 to generate ammonium sulfate (NH4)Fe(SO4)2 and other double salts.
