•Residual cementite enhances micro-galvanic corrosion for 16Mn and 16MnCu steels.•Nano-sized Cu particle is deposited on the steel surface during corrosion process.•Galvanic corrosion is restrained by the deposited Cu.The effect of Cu on the corrosion behavior of low alloy steel under the simulated bottom plate environment of cargo oil tank was investigated by using gravimetric test, SEM, EDS, XPS, XRD and electrochemical measurements. The results indicate that the corrosion resistance of 16MnCu steel was higher than that of 16Mn steel, which was related to the deposition of nano-sized Cu particles on the surface. The deposited Cu might suppress both the anodic dissolution of the ferrite and hydrogen evolution reaction on the cementite, leading to the reduction of the galvanic effect caused by the residual cementite.

There have been extensive experimental observations of the anisotropic corrosion behavior of metals and alloys, and their mechanisms were assumed to be correlated with the so-called surface energy or the work function. However, to date, a specified mechanism or theory to interpret anisotropic corrosion behavior remains unclear. Here, we determine the anisotropic anodic dissolution of metals and alloys in corrosive environments by developing a formula to specify the relationship between the electrode potential (U) and the current density (I) by considering the basic parameters of our defined surface energy density (Esurf/ρ) and the work function (Φ). Therefore, we build an ab initio model to evaluate the anisotropic anodic dissolution behavior of metals and alloys using the inputs obtained within density functional theory. This theory is further validated in the case of variations in the crystallographic planes of Mg. Moreover, some selected alloying additions such as Ga, Cd, Hg, In, As, and Cr are theoretically elucidated to effectively reduce the anodic dissolution rates of the Mg matrix to some extent, in close agreement with available experimental observations. This model is capable of predicting the anisotropic anodic dissolution behavior, providing a promising perspective for designing better corrosion-resistant alloys.Download high-res image (334KB)Download full-size image

The effect of Cl− ions on corrosion evolution of NiCu low alloy steel during immersion tests (up to 70 days) in deaerated 0.05 mol/L bicarbonate solutions was investigated by in-situ electrochemical measurements combined with X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) characterisations. The results showed that due to the acceleration of substrate dissolution in the presence of Cl−, corrosion of NiCu low alloy steel underwent only two stages, i.e., a quick oxidation process followed by a final metastable passive state, without the initial slow anodic dissolution as observed in the Cl−-free bicarbonate solution. The main components of the formed rust layer in the Cl−-free bicarbonate solution were α-FeOOH and Fe3O4, while apart from α-FeOOH, Fe6(OH)12CO3 was found evident instead of Fe3O4 in the Cl−-containing solution. Metastable pits were only found in the Cl−-containing solution where Cl− accumulated after the immersion test, confirming the attack of Cl− on the substrate after penetrating the formed corrosion product layer.

Corrosion evolution during immersion tests (up to 43 days) of NiCu steel in deaerated 0.1 mol/L bicarbonate solutions was investigated by electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results show that NiCu steel transformed from the anodic dissolution in the early stage of immersion to a metastable passive state in the final stage as the open-circuit potential value shifted positively, which was aroused by the precipitation of corrosion products. This process was mainly promoted by the trace amount of oxygen. Simultaneously, dominant cathodic reaction transformed from the hydrogen evolution in early stage to reduction processes of corrosion products in later stages. Possible corrosion processes were discussed with the assistance of a corresponding Pourbaix diagram.

Electrochemical impedance spectroscopy (EIS) and film thickness measurement have been employed to study the atmospheric corrosion of a weathering steel covered with a thin electrolyte layer in a simulated coastal–industrial atmosphere. The results indicate that the corrosion rate is a function of the covered electrolyte thickness and the wet/dry cycle. Within each wet/dry cycle, the increased corrosion rate is related to the increased Cl− and SO42− concentration and an enhancement of oxygen diffusion rate with the evaporation of the electrolyte. In addition, the corrosion rate increases during the initial corrosion stage and then decreases as the wet/dry cycle proceeds. Moreover, one mathematical approach based on the numerical integration method to obtain corrosion mass loss of steel from the measurements of EIS has been developed, and this would be useful for the development of indoor simulated atmospheric corrosion tests.

•A four layered phosphate coating was electro-deposited on AZ31 alloy in deaerated 0.1 M K2HPO4 solution with pH 9.5.•The formation process of phosphate conversion coating contains five characterization stages.•Amorphous Mg(OH)2, MgHPO4 and crystallized KMgPO4•6H2O are in sequence deposited to form the conversion coating.•The innermost layer of the coating consists of Mg(OH)2, and the inner layer consists of Mg(OH)2 and MgHPO4.•The middle layer consists of Mg(OH)2, MgHPO4 and KMgPO4•6H2O, and the topmost layer consists of MgHPO4 and KMgPO4•6H2O.The coating process of a four layered phosphate mineral conversion coating on AZ31 Mg alloy deposited at −0.8 V in 0.1 M K2HPO4 solution with pH 9.5 was studied by in situ potentiostatic polarization, together with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetry (TG), differential scanning calorimetry (DSC), differential thermal gravimetry (DTG) and electron probe micro-analyzer (EPMA). Our results show that the coating process can be divided into five stages. The initial two stages are caused by the rapid decrease of charging current of the electrical double layer capacitance and the slow increase of faraday current density on AZ31 Mg alloy, and Mg(OH)2 was detected in the corrosion product in these two stages. The third process corresponds to the deposition of amorphous MgHPO4 film, followed by the fourth process corresponding to the decrease of the corrosion resistance of MgHPO4. In the last stage, crystallized KMgPO4 · 6H2O (struvite-K) forms. The SEM, XPS, TG, DSC, DTG and EPMA indicate that the deposited coating consists of four layers. The innermost layer is composed of a corosion product, Mg(OH)2, followed by the inner layer composed of Mg(OH)2 and MgHPO4, subsequently the middle layer consisting of Mg(OH)2, MgHPO4 and struvite-K, and finally the topmost layer consisting of MgHPO4 and struvite-K.

The corrosion behaviors of the isolated short and vertical long scale Q235B steel in a simulated tidal zone were studied by electrochemical impedance spectroscopy (EIS) monitoring and corrosion weight loss calculation in an experimental indoor simulating trough. The results show that the corrosion rate of the isolated short scale Q235B steel in the tidal zone acquired by the EIS agrees with the corrosion weight loss result. The corrosion rates of the short scale steel are in the order of middle tidal zone > the central zone between the middle tidal zone and low tidal zone > high tidal zone > low tidal zone. The fastest corrosion rate in the middle tidal zone is attributed to the longest wet time in a tidal cycle. According to the comparison of corrosion weight loss between the vertical long scale and isolated short scale specimens, the corrosion rate of vertical long scale specimens of Q235B steel is lower than that of the isolated short scale specimens in the tidal zone, but the result is contrary in the immersion zone.

•A double layered fluoride conversion film was electro-deposited on AZ31 Mg alloy in deaerated 0.1 M KF solution with pH 7.5.•The coating process of fluoride conversion film contains five characterization stages.•Amorphous Mg(OH)2, MgF2 and crystallized KMgF3 are in sequence deposited to form the conversion film.•The inner layer of the film consists of Mg(OH)2 and MgF2, and the outer layer consists of Mg(OH)2, MgF2 and KMgF3.The coating process of fluoride conversion film deposited at −1.4 V on AZ31 Mg alloy in 0.1 M KF solution of pH 7.5 at room temperature was studied by in situ potentiostatic polarization, together with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Our results show that the coating process can be distinguished into five stages. The initial two stages are caused by the rapid discharge of the electrical double layer capacitance and the slow increase of the faradaic current density on AZ31 Mg alloy and Mg(OH)2 was detected in the corrosion product at these stages. The third process corresponds to the deposition of amorphous MgF2 film followed by the failure of the protection of MgF2 caused by Mg(OH)2 at the fourth process. Finally, the deposition of crystallized KMgF3 occurs. The SEM and XPS depth profile indicate that the deposited coating consists of two layers. The inner layer is composed of amorphous Mg(OH)2 and MgF2 with a thickness of about 300 nm, and the outer one is comprised of Mg(OH)2, MgF2 and KMgF3 with thickness of about 260 nm.

The evolution of atmospheric corrosion of MnCuP weathering steel in a simulated coastal-industrial atmosphere was investigated by weight gain, SEM, XRD, and electrochemical measurements. The results indicate that the corrosion kinetics divides into two stages with a higher corrosion rate in the first stage and a lower rate in the second stage. The increased amount of α-FeOOH indicates an improved resistance of the rust. The rust enhances initially and then stabilizes the cathodic process, but the anodic process tends to be inhibited. The EIS results indicate that the protective ability of the rust layer can be evaluated by the charge transfer resistance.Highlights► The evolution of corrosion of MnCuP weathering steel in a simulated coastal-industrial atmosphere was studied. ► Instantaneous corrosion rate was used to characterise the corrosion kinetics instead of average corrosion rate. ► The formation of β-FeOOH in the presence of Cl− can be affected greatly by the co-existence of SO42-. ► The increased relative amount of α-FeOOH in the rust layer indicates its improved corrosion resistance. ► The protective ability of the rust layer can be evaluated by the charge transfer resistance.

The evolution of rust on MnCuP weathering steel submitted to a simulated coastal atmosphere was investigated by corrosion weight gain, scanning electron microscopy, X-ray diffraction, and electrochemical methods. The results indicate that the higher corrosion rate during the first stage than that during the second stage is related closely to the rust composition and electrochemical properties. The corrosion rate evolution is caused by the formation of a protective rust layer with a higher relative amount of α-FeOOH. The rust initially enhances and then stabilizes the cathodic process, but the anodic process tends to be inhibited by the protective rust layer.Highlights► The evolution of rust on MnCuP weathering steel submitted to a simulated coastal atmosphere has been investigated. ► The corrosion evolution of MnCuP weathering steel can be divided into two stages with distinct rust properties. ► A protective rust layer with higher amounts of α-FeOOH and lower Fe3O4 forms as the corrosion proceeds. ► The rust initially enhances and then stabilizes the cathodic process, but the anodic process tends to be inhibited.

The corrosion evolution of a Mo–Cu-bearing fire-resistant steel in a simulated industrial atmosphere was investigated by corrosion weight gain, XRD, EPMA, XPS, and polarization curves. The results indicate that the corrosion kinetics is closely related to the rust composition and electrochemical properties. As the corrosion proceeds, the relative content of γ-FeOOH and Fe3O4 decreases and α-FeOOH increases, and the rust layer becomes compact and adherent to steel substrate. Molybdenum and copper enrich in the inner rust layer, especially at the bottom of the corrosion nest, forming non-soluble molybdate and Cu(I)-bearing compounds responsible for enhanced corrosion resistance of the rust layer.Highlights► The rusting evolution of a Mo–Cu-bearing fire-resistant steel in a simulated industrial atmosphere was investigated. ► The rusting evolution of the steel is related to the rust composition, structure, and electrochemical characteristics. ► Increased content of α-FeOOH and decreased γ-FeOOH and Fe3O4 indicate the enhanced resistance of the rust. ► Mo and Cu are involved in the formation of molybdate and Cu(I)-bearing compounds in the rust.

In this work, atmospheric corrosion resistance of low cost MnCuP weathering steel in simulated coastal, industrial, and coastal–industrial atmospheric environments was investigated by wet/dry cyclic acceleration corrosion tests. The results indicate that MnCuP weathering steel exhibits high corrosion resistance in the three atmospheres. Besides, the alloying effect of Mn, Cu, and P elements on the anti-corrosion mechanism of MnCuP weathering steel was discussed by techniques of X-ray photoelectron spectroscopy, potential–pH diagram, and electron probe microanalysis.Highlights► Atmospheric corrosion resistance of a low cost MnCuP weathering steel was investigated by simulated wet/dry cyclic tests. ► The steel shows high corrosion resistance in simulated coastal, industrial, and coastal–industrial atmospheres. ► Mn and Cu are identified in bivalent and univalent respectively, leading to cation-selectivity of the rust layer. ► Phosphorus promotes the formation of non-soluble phosphates that may act as corrosion inhibitor in rust layer.

In order to clarify some features of atmospheric corrosion mechanisms, mild steel and two kinds of low alloy steels (1%Ni and 4%Ni-bearing steel) were investigated using 0.3%NaCl solution in wet/dry cyclic corrosion tests to simulate the coastal environment. The cross-section of rusted steels was analyzed by ESEM after 100 wet/dry cycles. The ion-selective permeability of rust layers on steels was investigated by measuring the distribution of Cl and Na element in the rust layer using EDXA. The results showed that the addition of Ni improved the ion-selective property: the rust layer of mild steel was anion-selective; the rust layer of Ni-bearing steel mainly was anion-selective, and its anion-selective property decreased with increasing content of Ni. The cation-selective property of the rust layer of Ni-bearing steel emerged when the content of Ni exceeded 4%.