ABSTRACTA dual-circulation testing system was constructed to study the effect of heat flux density (q, −22.28 ∼ +22.54 kW m-2) on the corrosion of an Al–Cu–Mg–Mn alloy in 0.5 M H2SO4 at a constant metal surface temperature (50°C) under different controlling heat transfer states. Their heat transfer parameters were determined quantitatively by COMSOL simulation to keep them except q in constant, highlighting the influence of q on metal corrosion. Compared with the q = 0 condition, the positive heat flux (q > 0) improved the surface layer performance of the Al alloy to inhibit its anodic corrosion process, decreasing its corrosion current density (icorr) by 56% with increasing q to 22.54 kW m-2. While q < 0, the opposite effect occurred, increasing its icorr by 52% at q = −22.28 kW m-2. Heat flux also changed the corrosion reaction’s apparent effective activation energy (Ea) and pre-factor, and Ea played a dominating role in changing icorr.

ABSTRACTA CoCrFeNiMo high-entropy alloy (HEA) coating was prepared on Ti6Al4V alloy by plasma spraying technique. The microstructure and phases of the obtained coating were analysed using an ultra-depth-of-field microscope and X-ray diffraction, respectively. The electrochemical corrosion properties of CoCrFeNiMo HEA coating in 3.5% NaCl, 0.1 M H2SO4 and 0.1 M NaOH solutions were comparatively evaluated using an electrochemical workstation, and the corrosion mechanisms were also discussed by the corrosion models. The results show that the CoCrFeNiMo HEA coating is mainly a solid solution structure of face-centered cubic, which forms mechanical bonding at the coating interface. The charge transfer resistance Rct of 597 × 1016 μA·cm–2 in 0.1 M H2SO4 solution is the highest and the corrosion current density icorr of 4.203 × 10–7 μA·cm–2 also presents the lowest among the three kinds of corrosive solutions, which shows the highest electrochemical corrosion resistance. As a result, the sequence of corrosion resistance is in 0.1 M H2SO4 solution > in 3.5% NaCl solution > in 0.1 M NaOH solution, in which the corrosion resistance is further improved by the passive film on the CoCrFeNiMo HEA coating.

ABSTRACTThe corrosion behavior and mechanism of CoCrFeNi HEA under AC application were systematically studied in 3.5 wt.% NaCl solution containing different concentrations of NaHSO3. The results showed that the existence of HSO3− increased the defect density within the passivation film on the surface of the HEA. With the increase of HSO3− concentration, the passivation zone became narrow, Ep shifted negatively, and ip increased, which indicated that the corrosion resistance of the HEA was weakened. When the HSO3− concentration increased to 0.1 M, a clear transition zone appeared, indicating that the increase in concentration reduced the passivity of the HEA, and its corrosion behavior changed from the typical passivation characteristic to the activation state with increasing iAC. The imposed AC promoted the nucleation of metastable pitting, which was indicated by a remarkable increase in the average pitting number density (Navg) from 5 to 17 mm−2 with the enhancement of iAC.

ABSTRACTMetal corrosion has been one of the most urgent problems bringing about great economic loss and life threats worldwide. Corrosion inhibitors can prevent metal from corrosion when directly added to corrosive media, which is a low-cost and efficient strategy towards corrosion protection. However, the corrosion protection ability cannot be durable without continuous additions of corrosion inhibitors. Micro/nano-containers encapsulated with corrosion inhibitors offer promising opportunities for delaying the corrosion process and prolonging the service period, regarded as smart corrosion inhibitors that can respond to stimuli spontaneously and release on demand. The review focuses on the diverse ambient stimuli and actively responsive mechanisms of smart corrosion inhibitors. Furthermore, the present drawbacks and underexplored possibilities of current research studies are pointed out, and future perspectives are prospected for smart corrosion inhibitor applications.

ABSTRACTAlloy 625 is a Ni-based alloy used in aerospace, energy, chemical, oil and gas industries, mainly as cladding material due to its corrosion resistance, high strength and creep resistance at high temperature. In this study, microstructural evaluation and susceptibility to intergranular corrosion of base metal and friction stir welded Alloy 625 were investigated. Friction stir welded joints exhibited a lower corrosion rate and degree of sensitisation compared to the base metal. It is mainly due to grain refinement and lower cooling rate of the friction stir welding process. The stir zone of the present weld has a finer grain structure and lower density of twin boundaries than the base metal and it led to the improvement of the mechanical and corrosion properties.

ABSTRACTTungsten has unique physical and chemical properties that make it ideal for high-temperature applications. At room temperature, it is being considered for medical applications due to its protection by an oxide/hydroxide film. However, breakdown of the oxide film and tungsten dissolution can have adverse effects on human health. This study investigates the corrosion of tungsten with and without dispersed oxides of rare-earth elements (ThO2, CeO2, and La2O3) in a 3 wt-% NaCl solution using electrochemical techniques. The results suggest that tungsten dissolution occurs after the formation of an oxide film, likely WO3, on the surface of tungsten and dispersed oxide tungsten. La2O3 and CeO2 may decrease the corrosion rate of tungsten, while WThO2/tungsten has similar corrosion rates to tungsten. The study concludes that CeO2 or La2O3 could replace ThO2 in tungsten due to the radioactive nature of Th.

ABSTRACTThe susceptibility to hydrogen embrittlement of supermartensitic stainless steel (S13Cr), martensitic-ferritic stainless steel (17Cr), carbon steel (P110) and austenitic-ferritic stainless steels (2205 and 2507) was evaluated by slow strain rate tests using a modified NACE TM-0177 solution, with 10–3M NaS2O3 substituting the saturated H2S standard solution. Tests were conducted at 25°C, pH 2.7 and under galvanostatic polarisation with a cathodic charging of 10 mA/cm2. S13Cr, 17Cr, P110 and 2507 steels were susceptible to hydrogen embrittlement inthiosulphate, with S13Cr and 17Cr being the most sensitive to this mechanism, while 2205 showed no sign of ductility loss. This harmful effect was attributed to the hydrogen sulphide generated from thiosulphate.

ABSTRACTCorrosion under disbonded coatings (CUDC) remains a major unsolved issue and a prime risk to underground steel structures such as buried gas pipelines. CUDC could not be effectively mitigated by conventional cathodic protection (CP) or be detected through usual inspection and survey technologies due to CP shielding. This article provides a brief review of recent progresses made through laboratory and field probing of CUDC and its influence factors. Discussions on CUDC processes and mechanisms are made based on results from an extensive laboratory and field-testing program using electrode array-based corrosion probes. It is shown that CUDC is affected not only by well-known factors such as the CP level, but also by coating disbondment geometry and soil conditions especially the seasonal dry-wet changes in soil saturation status. These findings have led to improved understanding of CUDC, and could lead to the development of better CUDC mitigation and control methods.

ABSTRACTCorrosion under insulation is a major economic concern in the safe operations of water, oil and gas steel pipes. Herein, the external corrosion of X80 pipeline steel was examined beneath a disbonded insulation in groundwater containing sulphate-reducing bacteria (SRB). The results show that the steel under the insulation exhibited different electrochemical behaviour from the uninsulated sample in the bulk solution. A galvanic effect was detected between the steel under insulation and the uninsulated sample. Spectroscopic analysis reveals the rust layer of the insulated steel immersed in the SRB-inoculated groundwater was enriched by FeS from microbial metabolism. The steel under insulation recorded severe localised perforation, while corrosion severity was reduced on the uninsulated sample. Diffusion of ions, film development and insulation degradation played important roles in steel corrosion.

ABSTRACTIn the present study, laser surface melting (LSM) of 304L stainless steel (SS) was performed using 250 W pulse Nd: YAG laser which resulted in a 250 µm thick melted layer with refined microstructure on the surface. Potentiostatic polarisations at various potentials in the transpassive regime in 6 M HNO3 solution at 95°C were used to quantify the IGC rate. The transpassive dissolution rate was significantly reduced after LSM. The pitting corrosion susceptibility was assessed by potentiodynamic polarisation in 3.5 wt-% NaCl solution. LSM resulted in an increase in pitting potential. Following electrochemical tests, sample surfaces were examined using optical and scanning electron microscopes besides a 3-D optical profilometer. The depths of IGC attack and pit were significantly reduced after LSM. The improvement in resistance to pitting corrosion and transpassive dissolution was attributed to the elimination of inclusions and impurity segregation at the grain boundaries brought about by LSM.

ABSTRACTThe random forest (RF) algorithm was used to develop two models for predicting the first-year corrosion losses (C1) of carbon steel in open air in various regions of the world. The first RF model built using combined databases of international programmes ISO CORRAG, MICAT and ECE/UN and tests conducted in Russia is intended for estimation of C1 in various types of atmospheres in various regions of the world. The second RF model enables the prediction of C1 in continental areas of the world. The accuracy of C1 predictions by the two RF and two dose–response functions, i.e. the function presented in ISO 9223 standard and the new version for a non-marine atmosphere, was compared. The reliability of the two RF models was shown to be significantly higher than that of the dose–response functions with exception of the predictions for corrosion losses of carbon steel in regions of Russia with a cold climate.

ABSTRACTThe corrosion behaviour of equiatomic CoCrFeNi and CoCrFeNiMn high entropy alloys manufactured by the vacuum arc melting were studied in 3.5 wt-% NaCl solution. And the reasons for the differences in corrosion resistance were investigated in terms of microstructure, electrochemical behaviour and oxide film. The results show that the two alloys exhibit different corrosion characteristics, with CoCrFeNi alloy displaying typical passivation characteristics and CoCrFeNiMn alloy exhibiting activation behaviour. The CoCrFeNi HEA with compact and protective passive film has lower corrosion current density, larger film resistance and polarisation resistance, meaning a lower corrosion rate and superior corrosion resistance compared with those of CoCrFeNiMn alloy. Moreover, CoCrFeNiMn alloy forms a discontinuous and loose corrosion product film with higher Mn content and lower contents of Cr, Fe and Co compared to the matrix, which decreases its protection performance. The addition of Mn changes the electrochemical behaviour, and higher Mn oxides/hydroxides within oxide film are unfavourable to forming a stable passive film.

ABSTRACTThe corrosion behaviours of Ni-Mo-Cr alloy and 316 stainless steel samples after immersing in NaCl-KCl-AlCl3 salt at 400.0°C for 168 h were investigated. The surface of Ni-Mo-Cr alloy basically kept its pre-corrosion morphology, with only a few corrosion pits. While NaCl-KCl-AlCl3 salt had stronger corrosivity to 316SS. After corrosion, the grain boundary of the surface of 316SS alloy was clearly visible, and the Fe and Cr atoms on the surface of 316SS were selectively corroded, leaving a Ni- rich layer of about 6.5 µm on the outmost surface of 316SS. At the temperature of 400.0°C, the corrosion resistance of Ni-Mo-Cr alloy to molten NaCl-KCl-AlCl3 salt was better than that of 316SS.

ABSTRACTIn this study, the mechanical and biocorrosion behavior of as-cast Zn-xCu-0.1Sr (x = 1, 2, and 3wt%) alloys for potential application was studied. The microstructure, mechanical properties, and in vitro degradation of Zn-xCu-0.1Sr alloys were investigated. The results show that as Cu content increased from 1.0 to 3.0wt%, the number of CuZn5 phases increased and the average grain size of CuZn5 phases increased from 6.96 μm to 10.92 μm. The 2wt% addition of Cu resulted in the best mechanical properties with 129.4 MPa in ultimate tensile strength and 108.4 MPa in yield strength. Furthermore, the higher corrosion rate was experienced in the alloys with higher Cu content after immersion. Zn-xCu-0.1Sr alloys all presented appropriate in vitro degradation rates in a basically uniform degradation mode. The strength, ductility and biocorrosion property exhibited by the Zn-2Cu-0.1Sr alloy compositions make it a promising candidate for cardiovascular stent applications.

ABSTRACTIn this study, the corrosion behavior of turmeric-coated AZ51 alloy is studied in a simulated bodily fluid (SBF) and then correlated using different characterization techniques. Optical microstructures of all the samples reveal the presence of grains of the α-Mg alloy and precipitates of β-Mg17Al12. Two well-defined peaks were obtained from the XRD patterns -Mg and Mg17Al12, correlating with the optical microscopy results. The potentiodynamic polarisation investigation illustrates the corrosion behavior of uncoated (UC) and turmeric-coated magnesium samples dipped for 24 (DC-24), 48 (DC-48), and 96 hours (DC-96). Results confirmed that the DC-48 sample shows the maximum corrosion resistance among all the samples, and the EIS study correlates well with the study. The mineralization study confirms that the DC-48 sample has the maximum corrosion resistance due to the uniformity and compact nature of the coating.

ABSTRACTThis research mainly focuses on selecting phase change materials (PCM) and container materials for next-generation concentrated solar power plants (CSP). The present study investigates the hot corrosion behaviour of Inconel 617 and 625 in Na2SO4 + 45 wt-% diatomite PCM environment at 800°C for 40, 80 and 120 h. Oxide scales developed on the alloy surfaces were examined by Scanning electron microscope (SEM) and X-ray diffraction (XRD) to study morphology and oxide phases. The result indicates that Inconel 617 and 625 weight changes were 1.239 and 0.925 mg cm−2 at 800°C for a maximum duration of 120 h. The dense oxide of Cr2O3 is formed on alloys 617 and 625, which gives better oxidation resistance in a PCM environment. Moreover, the Inconel 625 shows lesser weight gain compared to Inconel 617 due to the presence of higher wt-% of Cr in the alloy and Nb formation oxides reducing corrosion species diffusion rate.

ABSTRACTHigh-strength Mg-Zn-Y alloy with a long-period stacking ordered (LPSO) phase was chosen as the base alloy, and different Ca were added to it. The phase component and microstructure of the alloys were examined using diffraction of X-rays (XRD) and scanning electron microscopy (SEM). A new phase, Ca2Mg6Zn3, was found to be formed by the addition of Ca. The hydrogen evolution corrosion method and an electrochemical test showed that the amount of hydrogen evolution without Ca addition was twice that with Ca addition. The corrosion resistance of the magnesium alloy improved when the amount of Ca added was. In order to understand the dynamic corrosion process, we soaked Mg97Zn1Y2-X wt-% Ca (X = 0, 0.1, 0.3, 0.6, 1) alloy in 3.5 wt-% NaCl solution for 2, 6, 12 h. It was observed that with an increase in the Ca content, the degree of corrosion of the alloy gradually decreased, and the amount of granular corrosion products covering the alloy’s surface also decreased.

ABSTRACTAvailable chlorine is known to play a significant role in metal corrosion due to its oxidising properties. In this paper, the effect of available chlorine concentration on corrosion behaviour of the low alloy marine steel was investigated by electrochemical and immersion tests. Experimental results show that the corrosion rate of low alloy marine steel is accelerated with the increase of available chlorine concentration in seawater. The localised electrochemical dissolution of the steel is more active with the concentration of available chlorine increasing. Energy dispersive spectroscopy (EDS) was performed to understand the composition of the corrosion products that the large enrichment of Cl element occurred for the high available chlorine concentration (100 ppm) after 720 h exposure to seawater. The influence of available chlorine concentration on mechanical property of low alloy marine steel indicatesthe less impact on the tensile and yield strength of the low alloy marine steel.

ABSTRACTCurrent successes recorded in the use of plant leaves as corrosion inhibitors have been globally admitted as one of the best options. Little attention is paid to the future implication or risks concerning food insecurity, economic challenges and competition in various sectors that employ plants as their raw materials. This study reviewed the potential of plant wastes (such as peels, rinds and other components) as alternative sources of sustainable corrosion inhibitors. The review reveals that through the considerations of experimental and quantum chemical data as well as their chemical constituents, most plant wastes have excellent corrosion inhibition capacity. Those tested and confirmed as good corrosion inhibitors have also been reviewed. Computational chemistry has been implemented to demonstrate how some challenges confronting the understanding of their inhibition mechanism can be resolved.

ABSTRACTThe selection methodology for thickness loss measurement is very important to determine the extent of corrosion damage, as well as in formulation of corrosion prediction models and inspection/maintenance plans for offshore structures. This paper introduces a more accurate corrosion measurement technique, based on the pre-exposure dimensional metrology and post-exposure optical microscopy/image analysis on the cross-sections of steel samples. During this corrosion test, the surface grinded and uncoated steel samples were submerged vertically in an artificial seawater solution, for a duration of up to a maximum of 365 days. The corrosion damage experienced on the steel samples means that the dimensional metrology can be more accurate, and useful approach to measure both uniform and localised corrosion losses simultaneously than the conventional average mass loss method.