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This study discusses the comparative weatherability of three microstructures of ferrite, ferrite + bainite, and bainite, obtained by controlling the rolling and cooling processes of a well-established weathering steel by adding 3 wt% Ni in tropical marine atmospheric environments. In field exposure in the Trat (Thailand) and accelerated laboratory testing, the bainitic steel showed superior weathering resistance compared to the other microstructures. The corrosion rates after 12-month of exposure were 0.012 mm/y for ferritic steel, 0.010 mm/y for ferritic + bainite steel, and 0.009 mm/y for bainitic steel. An analysis of the electrochemical properties of the rust layer, its phase composition, and morphological observations revealed that the influence of microstructure on the atmospheric corrosion rate of weathering steels primarily depend on the density of the rust and the adhesion between the rust layer and the substrate. [ABSTRACT FROM AUTHOR]

This study investigated the corrosion resistance of weathering steels (WS) with five different Mo/Ni ratios (blank, 1:2, 1:1, 3:1, and 5:1) in a simulated tropical marine atmosphere. The results revealed that the synergistic effect between Mo and Ni with a 3:1 ratio led to the optimal atmospheric corrosion resistance of WS. Reducing Ni promotes the growth of nano-structured Fe(O,OH)6, while increasing Mo disrupts the structure of Fe(O,OH)6, creating additional nucleation sites for corrosion products. The evolution of the rust layer is governed by the growth of Fe(O,OH)6 at low Mo/Ni ratios, while the destruction of Fe(O,OH)6 by Mo becomes dominant as the Mo/Ni ratio increases. The optimized ratios (3:1) contain more α-FeOOH in the rust layer, compact the rust layer, reduce defects, and inhibit the charge transfer processes, thereby attaining better atmospheric corrosion resistance of WS. [ABSTRACT FROM AUTHOR]

The corrosion behavior of ship steel with different microstructure in a simulated tropical marine atmosphere was systematically investigated. Results show that homogeneous martensite structure obtained by water-quenching process was much more resistant to corrosion than that of ferrite-pearlite. Furthermore, the precipitation of carbide resulting from tempering process promoted the formation of corrosive microcells within the matrix and further intensified the corrosion. Rapid cooling rate led to the enrichment of chromium, which formed protective corrosion products such as FeCr2O4 and Cr(OH)3. This also facilitated the conversion of α-FeOOH and significantly enhanced the corrosion resistance.

The corrosion behaviors of self-designed Ni-containing low-alloy steels with Mo addition in tropical marine atmosphere was investigated by surface analysis and electrochemical methods. The results showed that with the increase of Ni content, the weight gain rates of low-alloy steels gradually decreased and the composition phases of inner product films changed from γ-FeOOH to α-FeOOH. The synergistic effect of Ni and Mo on the corrosion resistance of low-alloy steel was related to the Ni content in the steel matrix. The corrosion resistance can be significantly improved by adding Mo when the Ni content was in the range of 1.72 to 2.87 wt.%. The enhanced synergistic effect of Ni and Mo could further promote the formation of more protective corrosion products, such as α-FeOOH, NiFe2O4 and Mo6+. [ABSTRACT FROM AUTHOR]

In this work, the effects of the tropical marine atmospheric environment on the corrosion behaviour of the 7B04-T74 aluminium alloy were systematically investigated by using accelerated testing, together with corrosion kinetic analysis, microstructure observation, product composition analysis, and potentiodynamic polarization curve tests. The weight loss method was used for the corrosion kinetics analysis. The surface morphology and corrosion products transformation law were investigated by OM, SEM, EDS, and XPS. The electrochemical characteristics were studied using potentiodynamic polarization curves. The research indicated that the 7B04-T74 aluminium alloy has eminent corrosion resistance in the tropical marine atmospheric environment. Localized pitting corrosion occurred rapidly in the tropical marine atmosphere. In the later stage of corrosion, the corrosion of aluminium alloy did not become serious. Specifically, no obvious intergranular corrosion was found, which is related to the thermal treatment method. Corrosion products included Al(OH)3, Al2O3, and AlCl3, of which Al(OH)3 is the most notable. [ABSTRACT FROM AUTHOR]

In this work, the effects of the tropical marine atmospheric environment on the corrosion behaviour of the 7B04-T74 aluminium alloy were systematically investigated by using accelerated testing, together with corrosion kinetic analysis, microstructure observation, product composition analysis, and potentiodynamic polarization curve tests. The weight loss method was used for the corrosion kinetics analysis. The surface morphology and corrosion products transformation law were investigated by OM, SEM, EDS, and XPS. The electrochemical characteristics were studied using potentiodynamic polarization curves. The research indicated that the 7B04-T74 aluminium alloy has eminent corrosion resistance in the tropical marine atmospheric environment. Localized pitting corrosion occurred rapidly in the tropical marine atmosphere. In the later stage of corrosion, the corrosion of aluminium alloy did not become serious. Specifically, no obvious intergranular corrosion was found, which is related to the thermal treatment method. Corrosion products included Al(OH)3, Al2O3, and AlCl3, of which Al(OH)3 is the most notable.

In this study, three kinds of welding materials (WMs) with different Ni contents were designed for low alloy steel welded joints made by CO2 gas‐shielded welding technique. The corrosion resistance of welded joint was studied through the atmosphere exposure test in Trat, Thailand. Results show that there was a hardening in the heat‐affected zone after welding, and the average hardness slightly increased with the increase of Ni in WM. The electrochemical activity at the fusion line zone enhanced with the addition of Ni in WM, indicating a more obvious local galvanic corrosion effect there. Ni promoted the generation of NiFe2O4 and the transformation from γ‐FeOOH to α‐FeOOH, which increased the impedance modulus of the rust film on WM. More important, the corrosion performance assessment illustrated that the addition of Ni in WM hardly had an effect on the corrosion step values after half a year test, which confirmed that there is no need to add excessive content of Ni. [ABSTRACT FROM AUTHOR]

This study compares the weathering performance of steel with three microstructures: ferrite, ferrite + bainite, and bainite. The rolling and cooling processes of a novel and cost-effective Ni–Mo weathering steel (WS) were manipulated to achieve these microstructures. Validation of these microstructures was performed through field exposure tests in Trat, Thailand, representing a typical tropical marine atmospheric environment. The bainitic steel exhibited better weathering resistance. After 12-month of exposure, the corrosion rates for ferritic + bainite and bainitic steels were 105.6 % and 97.9 % respectively, compared to ferritic steel. The variation in corrosion rates confirm that modifying the microstructure, with emphases on bainite, improves the WSs' resistance to atmospheric corrosion. Changes in corrosion rate mainly depend on the rust layer's adherence. Interestingly, α-FeOOH predominantly characterizes the rust layer on the skyward side of the samples following 12-month of exposure, while the earthward side is mainly composed of Fe 3 O 4 /γ-Fe 2 O 3. • Field tests in Thailand compared microstructure's impact on Mo–Ni WS corrosion. • Bainitic micro-structured weathering steel exhibited better corrosion resistance. • Skyward: α-FeOOH dominates the rust layer; Earthward: Fe 3 O 4 /γ-Fe 2 O 3 dominates. [ABSTRACT FROM AUTHOR]

The corrosion behaviors of carbon steel and Ni-advanced weathering steel exposed to tropical marine atmosphere were investigated by using electrochemical impedance spectroscopy, x-ray diffraction, transmission electron microscope and x-ray photoelectron spectroscopy measurements. The results showed that Ni had almost no effect on corrosion kinetics and electrochemical performance of Ni-advanced weathering steel in mild atmosphere. Conversely, Ni-advanced weathering steel in marine atmosphere showed a significant superiority in improving corrosion resistance compared with carbon steel. Moreover, the mass formation of NiFe2O4 in the inner rust layer promoted the stability of the corrosion resistance improvement rate of Ni-advanced weathering steel with the increase in exposure time. Therefore, Ni-advanced weathering steel seemed to be more suitable for the harsh marine atmospheric environment. [ABSTRACT FROM AUTHOR]

Two welded joints applied for structural steel were exposed in the tropical marine atmospheric environment enduring 12 months in Thailand Trat to evaluate the corrosion resistance. The investigation results disclosed that the difference in microstructure resulted in uneven distribution of hardness and Volta potential in joints. For welds with a high content of Cr and Mn (W2), the hardness and Volta potential values of weld zone (WZ) were higher than those of another weld joint (W1), which was attributed to the addition of Mn and Cr in W2 fill metal. Meanwhile, the values of Ecorr of W2 fill metal were − 0.329 and − 0.601 V for rust and de-rusted conditions, respectively, which were more positive than that of other zones. Electrochemical impedance spectra (EIS) data presented that the corrosion resistance of heat-affected zone (HAZ) in both weld joints were more miserable than other zones in the same joint. In contrast, the inconspicuous corrosion steps between WZ and HAZ indicated a reasonable alloy composition design for both weld joints. The differences in corrosion resistance of subzones indicated that the local galvanic effect is majorly affected by material composition rather than microstructure. [ABSTRACT FROM AUTHOR]

The product film formed on Ni-advanced weathering steel in a tropical marine environment was investigated in detail through outdoor exposure by using diverse surface analysis techniques combined with electrochemical impedance spectroscopy and scanning kelvin probe measurements. The results showed that the product film was mainly composed of nanophasic goethite in the inner layer and maghemite, akaganeite, and hematite in the outer layer. Moreover, the resistance to atmospheric corrosion gradually increased from the outermost product film to the innermost film. Ni was significantly enriched in the inner layer in the form of the spinel phase NiFe 2 O 4 , which transformed lepidocrocite to fine-grained goethite, withstood the invasion of chloridion, and improved the corrosion potential of the product film in a tropical marine atmosphere. [ABSTRACT FROM AUTHOR]

The effect of using 1 wt% Cr instead of 1 wt% Ni on the corrosion behavior of 3 wt% Ni steel in the simulated tropical marine atmospheric environment is studied. The results show that replacing part of Ni in 3 wt% Ni steel with Cr can effectively promote the conversion of γ-FeOOH to α-FeOOH during the early stages of corrosion, which facilitates the densification of the rust layer to prevent the intrusion of corrosive media. As corrosion test periods and Cl− concentration increased, the hydrolysis effect of Cr causes local acidification leading to corrosion pits develop deeper and severe local corrosion, while destroying the protective nature of the rust layer and exhibiting strong electrochemical reaction activity. • The effects of using Cr instead of part of Ni on the corrosion behavior of Ni-steel is studied. • More α-FeOOH are formed during the early stages of corrosion. • The hydrolysis effect of Cr gradually increases electrochemical activity of Ni–steel. [ABSTRACT FROM AUTHOR]

The effect of Cr content on the corrosion behavior of Ni-Mo steel in the tropical marine atmospheric environment is systematically investigated. The increase of Cr content accelerates the aggregation and the enrichment of Ni element, and thus promotes the formation of an inner electronegative film which attracts positively charged ions including Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels negatively charged Cl- ions. Cr promotes the formation of more nano-FeOOH and the conversion of γ-FeOOH into α-FeOOH. A small amount of chromium promotes the formation of micro-pits at the initial stage, and with time, those micro-pits gradually accumulate into pieces. • Adding a small amount of Cr into Ni-Mo steel reduces the corrosion rate. • An electronegative inner film composed of Cr and Ni elements attracts Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels Cl- ions. • Cr 2 O 3 acts as the nucleation site of FeOOH, generating more nano FeOOH, and doping Cr promotes the transformation of γ-FeOOH to α-FeOOH. [ABSTRACT FROM AUTHOR]

The integral effects of Ca and Sb on the corrosion properties of E690 steel were studied in a simulated tropical marine environment through immersion and dry/wet cyclic tests, focusing on localized and long-term corrosion properties. Results showed that Ca improved the corrosion resistance of steel by modifying the MnS inclusion into MnS-CaAl 2 O 4 inclusion, and Sb enhanced the consumption of H+ to decrease the electrochemical activity on steel surface. The protectiveness of the rust layer was increased by the enrichment of Ca and Sb, and the alkalization caused by Sb promoted appositional enrichment of Sb and Ca in the rust layer. • Ca inhibited the dissolution process of inclusions and reduced the susceptibility of pitting corrosion. • Sb can consume H+ to promote the formation of Ca(OH) 2 in the rust layer. • The enrichment of Ca and Sb in the rust layer promoted the α/γ, and enhanced the protective properties of the rust layer. [ABSTRACT FROM AUTHOR]

The galvanic corrosion behaviour of aluminium 3004 - ∝ brass with different area ratios was studied in the tropical marine atmosphere at Tuticorin harbour over a period of 426 days. The area ratios, viz. A: A, studied were 0.125, 0.25, 0.5, 1, 2, 4 and 8. The galvanic corrosion behaviour of the metals was studied in terms of the relative increase in the corrosion rate of aluminium due to galvanic coupling with ∝ brass, the relative decrease in the corrosion rate of ∝ brass due to galvanic coupling with aluminium, and the susceptibility of aluminium to pitting owing to galvanic coupling with ∝ brass. The galvanic potential and galvanic current of the system were monitored. Pits of different dimensions ranging from mild etchings to perforations were experienced on the borders and the surfaces of the interface of aluminium in contact with ∝ brass. The corrosion products resulting from galvanic corrosion were analysed using XRD and the pitting on aluminium as a result of galvanic corrosion was highlighted in terms of pit depth, size and density of pit, using a high resolution microscope. The most favourable area ratio of aluminium - ∝ brass in marine atmosphere in terms of gravimetric corrosion rate is 8:1 and the most unfavourable area ratio of aluminium - ∝ brass is 1:4. [ABSTRACT FROM AUTHOR]

The galvanic corrosion behaviour of aluminium 3004 and copper with different area ratios were studied in the tropical marine atmosphere at Tuticorin harbour over a period of 426 days. The area ratios of A: A, studied were 1:1, 1:2, 1:4, 1:8, 2:1, 4:1 & 8:1. The galvanic corrosion behaviour of metals was studied in terms of relative increase in the corrosion rate of aluminium due to galvanic coupling with copper, relative decrease in the corrosion rate of copper due to galvanic coupling with aluminium, and the susceptibility of aluminium to pitting owing to galvanic coupling with copper. The galvanic potential and galvanic current of the system were monitored. Pits of different dimensions ranging from mild etchings to perforations were experienced on the borders and the surfaces of the interface of aluminium in contact with copper. The weathering parameters and the environmental pollutants which have a major role in influencing the galvanic corrosion of metals were also monitored. The corrosion products resulting from galvanic corrosion were analysed using XRD and the pitting on aluminium resulting from galvanic corrosion has been highlighted in terms of pit depth, size and density of pit, using a high resolution microscope. [ABSTRACT FROM AUTHOR]

Through exposing the 3%Ni (wt. %) steel welded joints in a tropical marine environment, the effects of welding methods and the corresponding parameters on the corrosion resistance of fusion zone (FZ) were investigated in detail. Results indicated that the microstructures of FZ were mainly composed of more finer primary austenite grains for welded joint made by CO 2 gas metal arc welding (GMAW), compared with shielded metal arc welding (SMAW) and automatic submerged arc welding (SAW), which resulted in more uniform Volta potential distribution on FZ (0.02–0.21 V for GMAW, 0.37–0.64 V for SMAW, −1.21–1.2 V for SAW). The uniform corrosion morphology facilitated by the fine primary austenite grains on GMAW FZ improved the combination of the rust layer on matrix, presenting highest corrosion potentials after exposure tests (E corr = −577 mV for one year and −408 mV for two years). The rust layer formed on GMAW FZ contained more amorphous substance and presented better corrosion protectiveness (α/γ = 1.36) compared with other two welding processes (0.86 for SMAW and 0.75 for SAW), which was attributed to the electrode welding materials with fine diameter (1.2 mm) and the moderate welding heat input (14.5–15 kJ/cm) for GMAW. • The Volta potential distributions of 3%Ni steel welded joints fabricated by three welding methods were characterized. • The effects of welding methods on the microstructure and the properties of the rust layer on FZ were discussed. • Based on the exposure test results in Thailand, a reasonable welding process was proposed. [ABSTRACT FROM AUTHOR]

To optimize Ni, Cu, Mo element content of low Cr steel rebars in tropical marine atmosphere environment, five newly steel rebars were exposed in Dalat to carry out the outdoor exposure tests including weight loss method, surface characterization method and electrochemical test. The results show that when 0.15 wt% Mo is added in the steel rebar, the corrosion resistance is 31.8% higher than that of the steel rebar with 0.3 wt% Cu. The corrosion resistance of 0.3 wt% Cr added steel rebar is 63.4% higher than that of 0.2 wt% Ni added steel rebar. With the addition of 0.3 wt% Cu and 0.3 wt% Cr, the corrosion resistance is 46.3% higher than that of the steel rebar with 0.2 wt% Ni and 0.15 wt% Mo. Mo and Cr are the main elements to improve the protection of steel rebar rust layer. The corrosion products of the five steel rebars are composed of α-FeOOH, γ-FeOOH and Fe 2 O 3 or Fe 3 O 4. The higher the content of Cr is, the higher the ratio of α - FeOOH to γ-FeOOH (α/γ) in the rust layer becomes. The protective layer composed of CaMoO 4 and Fe 2 (MoO 4) 3, formed by molybdate in alkaline environment hinders the further corrosion of steel rebars. Cr(OH) 3 produced by hydrolysis of Cr in low alloy steel inhibits the growth of FeOOH in rust, resulting in the decrease of FeOOH particle size and the rust layer more compact. On the whole, the newly designed low alloy steel rebar with a little of 1.6Cr-0.15Mo elements can be well used in tropical marine atmospheric environment. [ABSTRACT FROM AUTHOR]

• Newly designed low-cost steel bar is well used in tropical marine atmospheric environment. • Corrosion product films of newly designed low alloy steel bar and carbon steel bar is studied by outdoor exposure and immersion test. • Fixture for simulating concrete steel rebar is designed. • Improvement mechanism of Cr and Mo on the compactness of corrosion product films of steel rebar is proposed. Carbon steel rebar seriously corrodes in severe tropical marine atmospheric environment, threatening the safety of building structures. Hence, it is of great significance to design and manufacture low-alloy steel rebar which can be used in tropical marine atmospheric environment. In the paper, both of the self-designed low alloy (LA-1) steel rebar and carbon steel (HRB400E) rebar has been exposed in Sriracha for two years. The corrosion rate of LA-1 steel rebar is only half of that of carbon steel rebar after two years. However, it is difficult to study the root cause of low alloy steel rebars' enhanced corrosion resistance by alloy elements due to the presence of a large amount of CaCO 3 in the rust layer. Therefore, the mechanism of improving the protective effect of alloy elements on rust layer is further studied by dry/wet cycling corrosion tests to simulate a tropical marine atmosphere environment. The formation of dense product film on LA-1 steel surface is mainly because of the addition of Cr and Mo elements, leading to the corrosion rate of LA-1 steel rebar far less than that of HRB400E steel rebar. The protective film formed by molybdate in alkaline environment hinders the further corrosion of steel rebars. Cr(OH) 3 formed by hydrolysis of Cr in low alloy steel rebars inhibits the growth of FeOOH in rust, resulting in the generation of smaller size of FeOOH particles. The experiment proves that the newly designed LA-1 steel rebar can be well applied in tropical marine atmospheric environment. [ABSTRACT FROM AUTHOR]

• Corrosion behavior of the weathering steels (WSs) with different Cr content in a tropic marine atmosphere environment was studied. • Both phase composition and valence states of Cr in the rusts were investigated. • Based on the statistics of Cr(OH) 3 and Cr 2 O 3 in the rusts, the facilitation of Cr on the formation of α-FeOOH with great corrosion resistance was proposed. The atmospheric corrosion behavior of weathering steels (WSs) with dissimilar Cr contents was studied through dry/wet cycling corrosion tests combined with electrochemical measurements and characterizations of morphology and composition of the rusts. The results presented that the corrosion rates of WSs decreased as the Cr contents in WSs increased, which manifested as an increase in the impedance for rust layers after various corrosion periods. The enriched area of Cr in the inner rust layer was more evident in blocking the invasion of Cl-, which is related to the positive ion selectivity of the rust. High content of Cr in steel promotes the generation of Cr(OH) 3 and Cr 2 O 3 , which acted as nucleation sites for the formation of Fe 3 O 4 and α-FeOOH, presenting high value of α*/γ (the sum of α-FeOOH and Fe 3 O 4 / γ-Fe 2 O 3 to γ-FeOOH). [ABSTRACT FROM AUTHOR]

• The effect mechanism of Cl− on corrosion resistance was investigated. • The effect of Cl− on corrosion resistance was related to the Cl− concentration. • The corrosion resistance reduced with the inhibition of Cl− on NiFe 2 O 4 enhanced. • The transformation from γ-FeOOH to α-FeOOH was limited in high-Cl− environment. The effect of chloride ion on corrosion resistance of Ni-advanced weathering steel in simulated tropical marine atmosphere was investigated by surface analysis and electrochemical methods. The results showed that the effect of chloride ion on corrosion resistance of steel was primarily related to chloride ion concentration. As the chloride ion concentration increased, the inhibition of chloride ion on the formation of NiFe 2 O 4 was gradually enhanced and the corrosion resistance of steel was reduced. Meanwhile, the main composition phase of inner product film changed from α-FeOOH to γ-FeOOH, whereas that of outer film changed from γ-FeOOH to β-FeOOH. Moreover, Ni was mainly concentrated in the inner film instead of being distributed in the whole product film. [ABSTRACT FROM AUTHOR]

A fast mobility particle sizer operating at a one-second time resolution was used to measure aerosol particle number size distribution (5.6–560 nm) in marine conditions over the South China Sea (SCS) from 29 March to 2 May 2017 and in the tropic zone of the Northwest Pacific Ocean (NWPO) from 10 to 29 October 2018. The clean background number concentration of nucleation mode atmospheric particles (<30 nm) was approximately 0.6 × 103 cm−3 in these areas. Two nighttime and five daytime strong new particle formation (NPF) events were observed to occur extending over a spatial scale from 2 to 140 km in the SCS, with a net increase of nucleation mode particles of 4.5 × 104 cm−3 ± 3.4 × 104 cm−3 during five of the seven events. Nighttime NPF events were unlikely associated with sulfuric acid vapor because of lack of photochemical reactions. Daytime NPF events share several common features with nighttime NPF events, e.g., dramatic spatiotemporal variations in the number concentration of the nucleation mode particles. Without aerosol precursor measurements we cannot address the vapors driving the formation process. However, our results show no banana-shaped growth of the particles. The growth into larger particle sizes seems to be restricted by the availability of condensable components in the gas phase. The nucleation mode was observed and sometimes even dominated the number concentration over other particle modes in the marine atmosphere over the tropic zone of the NWPO. In addition, more data obtained during the two campaigns and other campaigns were also applied to strengthen the analysis in terms of origins, formation and absent growth of nucleation mode particles in the marine atmospheres over the two tropic zones. Unlabelled Image • Strong and weak local turbulence-driven NPF events were observed over SCS. • A clear nucleation particle mode was observed over the tropical zone of NWPO. • No growth of the nucleation mode was detected due to limited key precursors. [ABSTRACT FROM AUTHOR]