Formation Of Corrosion Products Research Articles

Corrosion behavior of lithium silicate-based Zn-5.5Al-4.5Mg-0.3Ce coating

The effects of Ce on the microstructure of Zn-5.5Al-4.5Mg alloy and the corrosion mechanism of Zn-5.5Al-4.5Mg-0.3Ce alloy and its coatings were investigated in this research. The results show that the structure of Zn-5.5Al-4.5Mg-0.3Ce alloy is composed of hcp-Zn, fcc-Al and ternary eutectic structure (Zn/Al/MgZn2/Mg2Zn11), without dendritic tissue and MgZn2. The salt spray corrosion performance of Zn-5.5Al-4.5Mg-0.5CMC-2C coating was significantly better than that of Zn-5.5Al-4.5Mg-0.3Ce coating and Zn-5.5Al-4.5Mg-0.5CMC coating. The corrosion current density of Zn-5.5Al-4.5Mg-0.5CMC-2C coating was 3.217 μA cm−2, which was significantly lower than 3.96 μA cm−2 of Zn-5.5Al-4.5Mg-0.3Ce alloy and 5.879 μA cm−2 of Zn-5.5Al-4.5Mg alloy. Sodium carboxymethylcellulose and graphene improve the film formation and electrical conductivity of water-based lithium silicate resin. During corrosion of Zn-5.5Al-4.5Mg-0.3 Ce series alloy and coatings, Zn and Mg2Zn11 dissolved preferentially to form corrosion products of Zn5(OH)6(CO3)2 and Zn5(OH)8Cl2·H2O. Mg6Al2(OH)16CO3·4H2O and Zn4Al2(OH)12CO3·3H2O bimetallic hydroxide colloidal membranes were formed, which attached and wrapped on the surface of early corrosion products. Mg6Al2(OH)16CO3·4H2O and Zn4Al2(OH)12CO3·3H2O, preventing the dissolution of soluble corrosion products, improved the corrosion resistance of Zn-5.5Al-4.5Mg-0.3Ce alloy.

Enhancement of corrosion resistance by lamination of Mg film on Zn-55Al-1.6Si-coated steel by physical vapor deposition

Pure Mg films are laminated on hot-dip Zn-55Al-1.6Si-coated steel by physical vapor deposition. Ar gas pressure and deposition time are varied to control the crystal alignment and thickness of the Mg films. Potentiodynamic polarization tests, neutral salt spray tests (SST) and cyclic corrosion tests (CCT) are employed to evaluate the corrosion resistances of Mg laminated Zn-55Al-1.6Si-coated steel. The results show that lamination of Mg films enhance the corrosion resistance, especially against the cross-section by a factor of 2 to 10. The corrosion resistance is significantly affected by Mg film thickness. Polarization test results indicate that corrosion resistance of Mg films is slightly improved by crystal alignment against (101) plane. The effects of Mg films on corrosion process of Zn-55Al-1.6Si-coated steel are investigated via analysis of corrosion products formed during the SST and CCT. Lamination of Mg films seem to hinder the corrosion process by cathodic protection and formation of protective Mg corrosion products at defective areas through Mg ion migration while the underlying Zn-55Al-1.6Si coating protects steel substrate surface. The improvement in corrosion resistance under different corrosion environments of SST and CCT is explained by the transformation of Mg corrosion products from magnesium hydroxide to a more protective hydromagnesite.

Corrosion of 304 stainless steel in carbonates melt– a state of enhanced dissolution of corrosion products

An in-situ corrosion pretreatment is carried out to create a reference to understand the severe corrosion of 304 stainless steel in carbonates melt kept under CO2-2%O2 environment at 650 °C. N2-20%O2 gas is passed into the melt for the initial 24 h to form corrosion products including LiFeO2 layers, and then corrosion is observed in the melt with CO2-2%O2 environment. EIS spectra, mass loss and corrosion microstructure analyses reveal that corrosion products dissolve significantly into the melt when corrosion is initiated in the melt under CO2-2%O2. The pretreatment suppresses the dissolution and reveals that corrosion resistance can be undermined by ∼3 orders of magnitude by the dissolution.

Effect of Concentrations of Fe2+ and Fe3+ on the Corrosion Behavior of Carbon Steel in Cl− and SO42− Aqueous Environments

The concentration of metal ions in aqueous environments significantly affects the formation of corrosion products and further metal corrosion. In this paper, the electrochemical behavior of carbon steel in the presence of Fe2+ or Fe3+ concentration in Cl− and SO42− aqueous environments have been investigated using conventional electrochemical methods such as linear polarization and alternating current impedance spectroscopy. The morphology and composition of carbon steel corrosion products were studied using scanning electron microscopy and laser Raman spectroscopy. The effects of corrosion products and iron ions concentration on the corrosion of carbon steel were discussed. Corrosion products of carbon steel in aqueous environments were γ-FeOOH, γ-Fe2O3 and a small amount of α-Fe2O3. The addition of Fe2+ affected the cathode reaction of the electrode reaction, and promotes the formation of γ-FeOOH and Fe3O4. And with the increase of Fe2+ concentration in the solution, the anode process of electrode reaction was suppressed. The addition of Fe3+ promoted the formation of γ-Fe2O3 and Fe3O4. Fe3+ affected the ionization of water, causing the pH of the solution to drop. Fe3+ undergoes a redox reaction with the matrix. Addition of Fe3+ ions promoted the formation of FeOOH, an intermediate product, which reacts with the anode product Fe2+. These factors all accelerated the corrosion process.

Corrosion fatigue behavior of additively manufactured biodegradable porous zinc

Additively manufactured (AM) biodegradable porous zinc exhibits great potential as a promising bone-substituting biomaterial. However, there is no information whatsoever available regarding its corrosion fatigue behavior. In this study, we used direct metal printing to fabricate topologically ordered biodegradable porous zinc based on a diamond unit cell. We compared the compression-compression fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous zinc was high in air (i.e., 70% of its yield strength) and even higher in r-SBF (i.e., 80% of its yield strength). The high value of the relative fatigue strength in air could be attributed to the good ductility of pure zinc itself. The formation of corrosion products around the strut junctions might explain the higher fatigue strength of AM zinc in r-SBF. Furthermore, we compared the fatigue behavior of a uniform design of the AM porous zinc with a functionally graded design. The functionally graded structure exhibited higher relative fatigue strengths than the uniform structure. The inspection of the fatigue crack distribution revealed that the functionally graded design controlled the sequence of crack initiation, which occurred early in the thicker struts and moved towards the thinner struts over time. The theoretical fatigue life models suggest that optimizing the functionally graded structure could be used as an effective means to improve the fatigue life of AM porous zinc. In conclusion, the favorable fatigue behavior of AM porous zinc further highlights its potential as a promising bone-substituting biomaterial. Statement of SignificanceAdditively manufactured (AM) biodegradable porous zinc exhibits great potential for the treatment of large bony defects. However, there is no information available regarding its corrosion fatigue behavior. Here, we compared the fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The fatigue strength of AM porous Zn was even higher in r-SBF than in air, which were attributed to the formation of corrosion products. Furthermore, we found that the functionally graded structure controlled the sequence of crack initiation in differently sized struts and exhibited higher relative fatigue strengths than the uniform structure, suggesting that optimizing the functionally graded structure could be an effective means to improve the fatigue life of AM porous Zn.

Corrosion Resistance of Micro-Textured Surface Modified Alumina-Titania Coating

Effect of micro-texture on the corrosion resistance of alumina-titania coated mild steel was investigated. The micro-texture was fabricated on the coating surface via laser surface texturing technique. Tafel extrapolation and immersion test was conducted to measure the corrosion resistance and corrosion mechanism of the coating in 3.5% NaCl solution. The results indicated that the micro-texture contributes to a significant improvement of corrosion resistance due to the formation small volume of air trapped in the micro-grooves, which resist the penetration of corrosive ions and reduce the area of solid – liquid interface. The WCA indicated that the textured surface had low wettability. The SEM analysis showed the occurrence of uniform corrosion. The analysis of EDS revealed that there was formation of corrosion product at the coating-substrate interface. In short, the resistance towards corrosion was increased up to 73% indicating that the resistivity of the coating against corrosion was improved by engraving the micro-texture on its surface.

Corrosion behaviors of Cr2AlC/α-Al2O3 composites in 3.5 wt. % NaCl aqueous solution

Cr2AlC and its composites containing α-Al2O3 (6.1 and 15.2 wt %) were prepared by hot pressing and their corrosion behaviors in air-saturated 3.5 wt % NaCl aqueous solution were investigated by electrochemical testing methods. It was revealed that the secondary phase of Al2O3 particles mainly distributed along grain boundaries of Cr2AlC matrix. The potentiodynamic polarization measurements showed that the corrosion current densities of these Cr2AlC composites were lower than that of the pure Cr2AlC. The Aluminum in Cr2AlC was prone to be attacked more easily. When immersed at open circuit potential (OCP), Al readily slipped out from Cr2AlC matrix into NaCl solution in the form of dissoluble species. But in the case of polarization, regardless of potentiostatic polarization or potentiodynamic polarization, more de-intercalated Al, reacted with the electrolyte to form corrosion products of Al2O3 and/or AlOOH and deposited on the sample surface. For Cr2AlC/α-Al2O3 composites, the presence of Al2O3 weakened the corrosion along grain boundaries by partly blocking the permeation of electrolyte and inhibiting the anodic dissolution process.

Hydrogen isotope permeability of reduced activation ferritic/martensitic steel CLF-1 corroded by Li4SiO4

In a solid tritium breeding blanket, Li4SiO4 is in contact with reduced activation ferritic/martensitic (RAFM) steel at high temperatures for years and forms a corrosion layer. The effects of the corrosion layer on the hydrogen isotope permeability of a RAFM steel CLF-1 were thus investigated. X-ray diffraction, secondary ion mass spectroscopy and spherical aberration corrected transmission electron microscopy demonstrated the formation of a three-layered corrosion product, comprising a middle layer of LiFeO2 and an inner layer of LiCrO2. Li4SiO4 powder with the (104) crystal plane orientation was adsorbed on the outermost surface of the CLF-1, forming a nanoscale layer with a thickness of ∼150 nm. Deuterium permeation tests show that the deuterium permeability of the corroded samples was two orders of magnitude higher than that of the blank substrate that was pre-heated for 30 days, but one order of magnitude lower than that of the blank substrate that was not preheated. The CLF-1 corroded for 10 days exhibited the highest deuterium permeation flux. There was a decrease in the deuterium permeability of the steel that corroded for 15 and 30 days; thus, there was a decrease in the permeability with an increase in the corrosion period. By the weighting method and TEM analysis, a cracking and reoxidation process was proposed to explain the decreasing deuterium permeability of CLF-1 during corrosion by Li4SiO4. Our study provides a promising approach for preventing tritium permeation.

High‐temperature corrosion behaviors of typical nickel alloy coatings in a simulated boiler coal ash/gas environment in the Zhundong region

AbstractThe high‐temperature corrosion behaviors of five nickel alloy coatings used in coal‐fired boilers in the Zhundong region (Xinjiang province) were investigated in simulated coal ash and coal‐combusted flue gas environment at 650°C for 250 and 500 hr. The samples were analyzed by weight gain experiment, X‐ray diffraction test, and scanning electron microscopy technique with energy‐dispersive spectroscopy. The results indicated that the corrosion level is in the order of NiCrMo13 ≈ Hastelloy C‐276 (276) > NiCrBSi > Inconel 718 (718) > 45CT. The compositions of the corrosion scale in five nickel alloy coatings mainly consist of NiO, Ni3S2, and Cr2O3. The enrichment of Cr in the corrosion scale in 45CT, 718, and NiCrBSi coatings inhibits the formation of oxide and sulfide on the coating surface. The presence of W and Mo in nickel alloy coatings accelerates the formation of corrosion products, thus weakening the corrosion resistance of NiCrMo13 and 276 in simulated coal ash and coal‐combusted flue gas environment.

Influence of corrosion products on the inhibition effect of pyrimidine derivative for the corrosion of carbon steel under supercritical CO2 conditions

The effect of corrosion products on the inhibition effect of pyrimidine derivative inhibitor (DABTP) for the corrosion of N80 carbon steel in supercritical CO2 oilfield produced water was studied by electrochemical measurements, surface characterization, and theoretical calculations. It is found that DABTP exhibits high inhibition performance for bare carbon steel, while the formation of corrosion products (Fe3C, FeCO3) after pre-corrosion reduces the inhibition effect of DABTP. Theoretical calculations indicate that DABTP preferentially adsorb on FeCO3 film instead of Fe surface, which results in the reduction of inhibition effect of DABTP after FeCO3 film formed on the carbon steel surface.

Corrosion behavior of double-glow plasma copperizing coating on Q235 steel

In this paper, the copperized layerwas fabricated on the surface of Q235 steel via double glow plasma surface alloying technology to improve the marine fouling organisms attached on the surface of marine structures. The microstructure and phases composition of the coating were analyzed by SEM and XRD. The corrosion characteristics of substrate and coating were investigated in 3.5wt.% NaCl solution.The results indicated that the protective coating had a novel structure of an outermost deposition layer (85?m) and inner diffusion layer (51 ?m), which exhibited great adhesion stress because of the metallurgical bonding effects. The corrosion characteristics of substrate and coating were analyzed in detail. The corrosion products of Q235 steel were mainly ?-FeOOH, ?-FeOOH, and ?-FeOOH in the early stage of corrosion process. Then the whisker-like structure of ?-FeOOH grew on the surface of ?-FeOOH and the ?-FeOOH transformed into ?-FeOOH to achieve thermodynamical equilibrium. The dissolved copper ions in solution resulted in a unique difference in the formation of corrosion products, which inhibited the transition of iron hydroxide and promoted the formation of amorphous phases. Based on electrochemical measurement, the coating hadlower corrosion current density and higher charge transfer resistance than substrate. The corrosion resistance of copperized layer wasbetter than that of Q235 steel.

Evaluation of the behavior of D-Gun sprayed coatings on T-11 boiler steel at 900 °C temperature

D-Gun thermal spray technique was used to deposit Cr3C2-NiCr and Cr2O3 coatings on T-11 boiler steel in the present study. Hot corrosion behavior of coated and uncoated samples of T-11 boiler was analyzed for 50 cycles at 900 °C in the simulated Na2SO4-60%V2O5 condition. Kinetics of corrosion was measured by using thermogravimetric studies. Surface and cross sectional analysis has been reported by using XRD and SEM/EDS to know the formation of corrosion products. Uncoated sample has shown spalling and degradation of oxide scale. Cr2O3 coated sample has shown detachment of coating from the substrate after 1st cycle. Cr3C2-NiCr coating has shown the protective behavior as well as corrosion resistance in the reported environment by virtue of protective oxides.

Effect of Sodium Phosphate and Calcium Nitrate Sealing Treatment on Microstructure and Corrosion Resistance of Wire Arc Sprayed Aluminum Coatings

Aluminum coating was deposited by arc thermal spraying process onto the steel substrate for the corrosion protection in aggressive environment. However, the arc thermal sprayed coating possesses defects in the coating. Thus, it is important to reduce the defects and enhance the corrosion resistance properties of the deposited coating using post-treatment. In the present study, we have used different concentrations of sodium phosphate mono basic (NaH2PO4) with 0.1 molar (M) calcium nitrate [Ca(NO3)2] as post-treatment solution to fill out the defects of the Al coating. It was observed by scanning electron microscopy (SEM) that 1 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample exhibited 71% reduction in defects compared to as coated samples. X-ray diffraction (XRD) was performed to determine the phases formed on the coating surface after treatments. XRD confirms the formation of sodium aluminum hydrogen phosphate (Na3Al(OH)(HPO4)(PO4)) and brushite (Ca(HPO4)(H2O)2) as composite oxides on the Al coating. Electrochemical results show that 0.5 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample has exhibited the highest charge transfer resistance and the lowest corrosion current density after 89 days of exposure in 3.5 wt.% NaCl solution. The enhancement in corrosion resistance of 0.5 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample attributed to the formation of adherent, sparingly soluble, and stable corrosion products. The volume fraction result of the corrosion products formed on 0.5 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample after 89 days of exposure in 3.5 wt.% NaCl using XRD confirms the highest amount of Bayerite (α-Al(OH)3) deposition, thus, the corrosion rate of this sample was the lowest.

Evaluating Deposit Formation of Iron-Corrosion Products in High- and Low-Pressure Evaporative Circuits When Starting a Combined-Cycle Plant

One of the main causes of equipment damage at TPPs is corrosion of structural materials and deposit formation on the heat-transfer surfaces of the steam-water tract. This process occurs during stationary operation of the equipment and during start-ups and shutdowns of the power unit. In this paper, we assess the deposit formation of corrosion products in the evaporative circuits during the start-up of a combined cycle plant with the subsequent prediction of the number of starts after which it is necessary to carry out a chemical washing of the boiler-utilizer. The changes in the product concentration of iron corrosion in water and steam when starting a 450-MW power unit from a cold state are shown. The probability for the deposit formation of iron-corrosion products is estimated based on the experimental data obtained at start-up, with the subsequent calculation of the material balance for iron-corrosion products for low- and high-pressure circuits. A comparative analysis of the probability for deposit formation of iron-corrosion products in the high- and low-pressure circuits during the start-up period and at nominal operating parameters is performed. As a result of studies, it was found that the iron concentration in the feed and boiler water and saturated steam of the high- and low-pressure circuit significantly exceeds the normalized values by the time when the boiler-utilizer reaches its operating parameters. The specific amount of iron-corrosion products that can be deposited on heat-transfer surfaces in one start for the low- and high-pressure circulation circuit is determined. To reduce the probability for deposits of corrosion products during start-up, it is proposed to use feed water with a lower iron concentration or to increase the purge of the boiler-utilizer. Recommendations on the shutdown of the boiler-utilizer for washing when the maximal specific deposit amount of 200 g/m2 is reached taking into account the number of permissible starts for the low-pressure circuit are given.

Laboratory accelerated and field exposure testing of MgRP and MgORP on AA2024-T351: Chemical and electrochemical protection effects

Two Mg-based coatings consisting of Mg-rich primer (MgRP) and MgO-rich primer (MgORP) with and without topcoat were exposed to laboratory accelerated lifecycle testing (LALT) and outdoor field environments. The coatings were tested in parallel exposure studies utilizing ASTM B117, modified ASTM B117 (cyclic wet-dry, dark-UV exposure in ASTM synthetic ocean water), rural inland (Charlottesville, VA), and coastal marine (Kennedy Space Center) field exposure. Several characterization and electrochemical diagnostic techniques were used to continuously assess coating performance as a function of time in various exposures, enabling comparison of LALT to field exposures. The coating systems were evaluated with respect to the intact coating and to a machined scribe defect. The fate of Mg2+ upon dissolution of Mg and MgO with respect to pigment depletion, corrosion product formation, and repartitioning from the coating into the scribe was monitored with XRD and EDS. The capacity for MgRP and MgORP to provide barrier, cathodic, and chemical corrosion protection to AA2024-T351 substrate was evaluated. The ultimate ability of these coatings to protect the AA2024-T351 substrate both in the scribe and under the coating was gauged with optical profilometry assessment of corrosion damage. The Mg and MgO-based coating systems exhibited several desirable aspects of a corrosion preventative coating. The corrosion damage quantified both under the intact coating and within the scribe was much less than the corrosion damage measured on uncoated controls. Within a given exposure, the reduction of corrosion damage in the scribe correlated with the amount of Mg2+ which was released from the coating and repartitioned into the scribe. The open circuit potential of the coated substrate was depressed to more negative potentials than the bare substrate in 5 wt% NaCl, with non-topcoated samples establishing more negative potentials. The barrier properties remained high throughout the exposures for topcoated samples, which also better preserved the pigment from depletion. Comments are made regarding differences between the Mg and MgO-based coatings and the various laboratory and field exposures.

Microbiologically influenced corrosion of marine steels within the interaction between steel and biofilms: a brief view.

Marine is the harshest corrosive environment where almost all marine underwater equipment and facilities undergo corrosion caused by marine microorganisms. With the development of marine resources globally, the marine engineering and relevant infrastructures have increased exponentially. Microbiologically influenced corrosion (MIC) leads to severe safety accidents and great economic losses. The specific aggregation of corrosive microbial communities and their interactions with materials conform to a typical ecological adaptation mechanism. On the one hand, corrosive biofilms in the marine environment selectively colonize on a specific steel substrate by utilizing their complex community composition and various extracellular polymeric substances; on the other hand, the elemental composition and surface microstructure of different engineering steels affect the microbial community and corrosive process. MIC in the marine environment is a dynamic process evolving with the formation of corrosive biofilms and corrosion products. In this mini-review, the interactions between corrosive biofilm and steel substrates are explored and discussed, especially those conducted in situ in the marine environment. Herein, the important role of iron in the dynamic process of marine corrosion is highlighted.

Масс-спектрометрическая визуализация для исследования поверхности стали

Визуализация масс-спектрометрическими методами исследования поверхности широко используется для построения карт распределения различных соединений. В настоящей работе данныйинструмент впервые применён для исследования морфологии и состояния различных промышленных сталей. Ранее метод применялся только для визуализации с тонких поверхностей. Авторами показана принципиальная возможность оценки состояния поверхности стали и скорости протекания на ней коррозионных процессов. Показано, что при образовании продуктов коррозии последние легко и на низком уровне обнаруживаются в виде кластерных ионов, что позволяет легко определять начинающуюся коррозию. Визуализация наглядно показывает степень локализации и распространения коррозии, которая на сталях проходит сразу почти по всей поверхности. Также на поверхности стали обнаружены следы адсорбции хлора, который является потенциальным активатором коррозии и показана его локализация. Также возможно обнаружить продукты коррозии и адсорбированные на поверхности соединения, построить диаграмму распределения по широкой области поверхности любого обнаруженного соединения, провести локализацию исследуемой области для изучения её в сочетании с классическими методами исследования поверхности, проводить общий контроль состояния поверхности сплава во времени при его эксплуатации. Метод является уникальным, не имеет аналогов в мире и может быть распространён на любые другие поверхности и материалы

Role of copper in tarnishing process of silver alloys in sulphide media

Abstract Silver and silver alloys usually tarnish, which causes some changes in their aesthetic appearance and electrical properties, due to their exposure to sulphide environments (H2S), and this is a problem in the field of corrosion and conservation of cultural heritage metallic artefacts. In this study, the role of copper content in the tarnishing process of 0.925, 0.800 and 0.720 silver alloys in a 0.07 vol.% ammonium sulphide solution for different immersion periods was analyzed by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The polarisation curves showed that the copper content and sulphide increased the corrosion current density and delayed the passivation of silver alloys. The impedance spectra collected at the open circuit potential (OCP) showed a single capacitive, incomplete and depressed loop, indicating that the charge transfer resistance decreased as the copper content increased in the alloys. In contrast, the double-layer capacitance increased as the copper content increased. The SEM−EDS analysis confirmed that the copper-rich phase in the silver alloys was selectively dissolved due to the preference of S to react with Cu, resulting in a localised attack, thus delaying the formation of a passive film. A marked localised attack was observed in alloys with lower copper content. The mechanism for the tarnishing of silver alloys in sulphide media was dependent on the nature of the alloy and the greater affinity of copper for sulphur. The dissolution of Ag and Cu to form corrosion products was proposed as the rate determining step.

Corrosion of Carbon Steel in Artificial Geothermal Brine: Influence of Carbon Dioxide at 70 °C and 150 °C.

This study focuses on the corrosion mechanism of carbon steel exposed to an artificial geothermal brine influenced by carbon dioxide (CO2) gas. The tested brine simulates a geothermal source in Sibayak, Indonesia, containing 1500 mg/L of Cl−, 20 mg/L of SO42−, and 15 mg/L of HCO3− with pH 4. To reveal the temperature effect on the corrosion behavior of carbon steel, exposure and electrochemical tests were carried out at 70 °C and 150 °C. Surface analysis of corroded specimens showed localized corrosion at both temperatures, despite the formation of corrosion products on the surface. After 7 days at 150 °C, SEM images showed the formation of an adherent, dense, and crystalline FeCO3 layer. Whereas at 70 °C, the corrosion products consisted of chukanovite (Fe2(OH)2CO3) and siderite (FeCO3), which are less dense and less protective than that at 150 °C. Control experiments under Ar-environment were used to investigate the corrosive effect of CO2. Free corrosion potential (Ecorr) and electrochemical impedance spectroscopy (EIS) confirm that at both temperatures, the corrosive effect of CO2 was more significant compared to that measured in the Ar-containing solution. In terms of temperature effect, carbon steel remained active at 70 °C, while at 150 °C, it became passive due to the FeCO3 formation. These results suggest that carbon steel is more susceptible to corrosion at the near ground surface of a geothermal well, whereas at a deeper well with a higher temperature, there is a possible risk of scaling (FeCO3 layer). A longer exposure test at 150 °C with a stagnant solution for 28 days, however, showed the unstable FeCO3 layer and therefore a deeper localized corrosion compared to that of seven-day exposed specimens.

Evaluation of surface activity of hot-dip galvanized steel after alkaline cleaning

The surface activity of hot-dip galvanized steel was evaluated after alkaline cleaning by exposing the cleaned specimens in humid supercritical carbon dioxide, followed by extraction and quantification of the formed corrosion products. Different free alkalinities of the cleaning bath were studied to obtain information on the evolution of zinc surface activity at different levels of surface etching. Surface reactivity of uncleaned galvanized steel is restricted and local, with major contributions from grain boundaries and intermetallic particles. Formation of zinc corrosion products took place in uncleaned samples exclusively on and around these sites. Al2O3 removal by alkaline cleaning gradually increased the surface activity. The major increase in surface activity was achieved when increasing the free alkalinity from 0.5 to 4.0 mEq/L, which was shown by formation of zinc corrosion products within zinc grains. The surface activity was confirmed by applying a titanium hexafluoride pretreatment on the cleaned panels and measuring the lateral microscale uniformity of the formed layer. The uniformity increased when larger areas of galvanized steel became activated by alkaline cleaning. Reactivity of zinc is rather difficult to quantify when setting up an industrial cleaning sequence for galvanized steel. Controlled exposure of cleaned samples in humid supercritical carbon dioxide, followed by quantification of oxidized zinc, provides a straightforward method to evaluate zinc reactivity.