Effectiveness of pretreatment method to hinder rebar corrosion in concrete

Abstract

This paper aims to evaluate the ability of phosphate pretreatments applied on steel rebars to hinder the corrosion reinforcements using synthetic pore electrolyte and mortar contaminated by chloride ions. The electrochemical behaviour of the pretreated substrate was assessed by corrosion potential, polarisation resistance and electrochemical impedance spectroscopy measurements. The results have demonstrated that the treatment of the rebar by immersion in the Na₃PO₄ (0·5M) solution favours the formation of a passive layer on the steel rebar surface, which increases the resistance to corrosion initiation up to 0·3M Cl^– instead of 0·1M Cl^– without treatment. The pretreatment also provides enhancement of corrosion protection of the steel rebar in mortar. The evolution of the impedance spectra in function of chloride concentration is in a fairly good agreement with the results obtained from R_P measurements.     

Effect of Cr3+- based conversion films on the corrosion behaviour of electrodeposited composite ternary zinc alloys with embedded carbon nanotubes

ABSTRACT

Composite ternary zinc-based alloys Zn-Ni-P and Zn-Fe-P containing embedded multi-walled carbon nanotubes (CNT) were obtained electrochemically. They were treated additionally in an environmentally friendly chemical conversion solution for passivation based on Cr³⁺-compound. The surface morphology, chemical and phase composition of the coatings after their treatment in the passivating solution were determined by SEM, EDS and XRD analysis respectively. The corrosion behaviour of composite alloy deposits both with or without additional conversion film (CF) was investigated in a model corrosion medium of 5% NaCl using selected methods such as potentodynamic polarisation (PD) curves, electrochemical impedance spectroscopy (EIS) and polarisation resistance (R_p) measurements. The changes in the surface morphology and phase composition of the corrosion tested samples were determined by SEM and XRD analysis. The corrosion studies indicated that the presence of a CF leads to a delay in the anodic dissolution process during the polarisation.     

Simulation eines Korrosionselementes bestehend aus einer beschichteten Stahlelektrode mit einer Verletzung

Simulation of a corrosion element consisting of a coated steel electrode with a damage Cathodic blisters are generated on samples with an anodic electrocoating bearing no conversion layer to simulate the corrosion element coated steel electrode/bar steel. Therefore potentiostatic polarisation experiments were carried out in 3 wt.% sodium chloride solution. The coating pigmented with titanium dioxide consists of oil-free polyester, melamin resin and polyalcohol. In our investigations we varied the polarisation potential, the time of corrosion loading and in a restricted manner the coating thickness of the coating. The polarisation potential ranged between the limiting values of the free corrosion potential of a coated electrode and that of a bar steel electrode. The galvanic current and the delaminated area consisting of the number of blisters as well as the growth-rate of blisters are measured. There exists a constant current density at the delaminated areas, because for the galvanic current and the delamination has been found the same time behaviour. The current density depends on the polarisation potentials in a non linear way. This shows that the ohmic resistance of the coating does not determine the rate of the electrochemical reactions at the phase boundary exclusively. Blister generation and blister growth-rate increase linearly with time of corrosion loading, but only the blister growth-rate is influenced by the polarisation potential.     

Corrosion behaviour in alkaline medium of zinc phosphate coated steel obtained by cathodic electrochemical treatment

The present work evaluated the ability of zinc phosphate coating, obtained by cathodic electrochemical treatment, to protect mild steel rebar against the localized attack generated by chloride ions in alkaline medium. The corrosion behaviour of coated steel was assessed by open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. The chemical composition and the morphology of the coated surfaces were evaluated by X-ray diffraction and scanning electron microscopy. Cathodically phosphated mild steel rebar have been studied in alkaline solution with and without chloride simulating the concrete pore solution. For these conditions, the results showed that the slow dissolution of the coating generates the formation of calcium hydroxyzincate (Ca(Zn(OH)₃)₂·2H₂O). After a long immersion time in alkaline solution with and without Cl⁻, the coating is dense and provides an effective corrosion resistance compared to mild steel rebar.     

Microstructural characterization and surface properties of zinc phosphated medium carbon low alloy steel

The present study concerns an in-depth investigation of the influence of process parameters on zinc phosphating of medium carbon steel to have an understanding of nucleation and growth mechanism and the microstructures developed thereafter. Furthermore, the wear and corrosion behavior of phosphated steel were evaluated in details. Phosphating was carried out in zinc phosphate bath (6–8% ZnPO₄) using 0.43 wt.% Fe as catalyst at 90 °C with variable time ranging from 60 s to 1800 s followed by water rinsing and oil sealing. The microstructural evolution with time was recorded to understand the nucleation and growth phenomena. The morphology of coating was both plate and needle shaped. The microhardness of the coated layer is significantly lower than that of as-received steel. Fretting wear behavior against hardened steel ball showed that phosphating reduced the wear resistance which was possibly attributed to the presence of phosphate needles causing an increased coefficient of friction and the reduced hardness of the phosphate layer. Corrosion potential of the phosphated steel shifted towards noble direction as compared to the as-received one. The study of the corrosion rate also shows that the rate is considerably reduced by phosphating. Pitting corrosion resistance was also found to be increased (in terms of increased critical potential for pit formation) due to phosphating as compared to as-received steel.     

The characterization and corrosion resistance of cerium chemical conversion coatings for 304 stainless steel

A golden yellow-colored cerium conversion coating was obtained on 304 stainless steel surface by immersing the steel into a solution containing cerium (III), KMnO₄ and sulfuric acid. The corrosion resistance of the coatings was evaluated by electrochemical methods, potentiodynamic polarization experiments and electrochemical impedance spectrum. The experimental results indicated that the corrosion resistance for the conversion coated 304SS in 3.5% NaCl solution increased markedly. The corrosion potential of the treated steel increased to a more noble level, the pitting corrosion potential increased also, the passive potential range was enlarged markedly and the passive current density decreased about one order compared to that of the untreated steel. The cathodic and anodic reaction were both inhibited to some extent. The chemical state of the elements in the coatings was investigated by XPS. The cerium element was in the form of tetravalent state. And AES depth profile analysis suggested that the thickness of the conversion coatings was less than 66 nm. The mechanisms of coatings formation and corrosion resistance are discussed.     

Corrosion of reinforcing steel in simulated concrete pore solutions: Effect of carbonation and chloride content

The corrosion susceptibility of as-received reinforcing steel bars (rebars) in solutions simulating the pore liquid of alkaline and carbonated concrete has been studied by means of potentiodynamic polarisation tests and polarisation resistance measurements. The effect of different degrees of carbonation and the presence of several chloride contents in the simulated pore solutions was investigated. Results show the beneficial effect of high alkalinity on the localised corrosion of steel caused by chloride ions. From the results of the potentiodynamic tests a critical chloride concentration above which pitting could take place was evaluated for each solution. The chloride threshold values here found are of the same order than those previously reported in the literature for film-free steel. The results obtained in solutions simulating carbonated concrete showed that under weak carbonation conditions carbon steel does not passivate while in the presence of high levels of carbonate and bicarbonate the resistance to localised corrosion is improved.     

Corrosion behaviour of an industrial shot-peened and coated automotive spring steel AISI 9254

A high-strength low-alloy steel, AISI 9254 (54SiCr6), is widely used for suspension spring production in the automotive industry. In this work, industrially manufactured zinc phosphate coated helical springs are subjected to detailed microstructural and surface analysis for better understanding of corrosion evolution. The material’s free corrosion potential and anodic/cathodic behaviour were investigated in NaCl solutions and corrosion propagation mechanisms were studied using potentiostatic polarisation on cross-sectional and external surfaces. The bulk material is fully martensite with uniformly distributed MnS inclusions, while the spring surface has a 2–3?μm mechanically deformed region introduced by shot-peening and a thin zinc phosphate coating. The corrosion open circuit potential of bulk material and shot-peened spring surface was about –0.7V_SCE without significant difference, while phosphated surface is more noble (more positive potential). MnS inclusions, stimulating the anodic attack in the steel, influence corrosion propagation and pit morphology to a large extent that can have an impact on the spring performance.     

氟硅酸钾封闭对钢铁表面氟铁酸钾转化膜耐蚀性能的影响

采用扫描电镜(SEM)、能谱分析(EDS)、电化学测试和中性盐雾试验对经过氟硅酸钾溶液封闭处理后的氟铁酸钾转化膜的表面形貌、元素组成和耐蚀性进行了分析测试。结果表明,氟硅酸钾水解产生的SiO2胶体对钢铁表面氟铁酸钾转化膜的封闭作用使得钢铁样品的腐蚀电流密度降低,防腐性能增强。     

Effect of porosity sealing treatments on the corrosion resistance of high-velocity oxy-fuel (HVOF)-sprayed Fe-based amorphous metallic coatings

High-velocity oxy-fuel-sprayed FeCrMoMnWBCSi amorphous metallic coatings were sealed with sodium orthosilicate (Na₃SiO₄), aluminium phosphate (AlPO₄), and cerium salt sealants. The microstructure of the sealed coatings was characterised by scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction. Corrosion behaviour was examined using electrochemical methods of potentiodynamic polarisation, cyclic polarisation, electrochemical impedance spectroscopy, and Mott-Schottky tests. The results indicated that the uniform corrosion resistance of the three sealed coatings was enhanced greatly, and the passive current densities were decreased by one order of magnitude after the sealing treatments. The AlPO₄ sealant can penetrate the coatings by no less than 50 μm and enhance their hardness, which exhibited a more uniform corrosion resistance, fairly good pitting corrosion resistance, and can be applied in long-term corrosive and/or abrasive environments. The cerium salt-sealed coating showed better pitting corrosion resistance but inferior corrosion resistance in the local regions of micro-cracks, which was practically used for temporary corrosion protection. The Na₃SiO₄-sealed coating showed better uniform corrosion resistance and inferior pitting corrosion resistance, which can be applied in short-term corrosion environments. The stability of the passive film affected the corrosion behaviour of the sealed coatings. The AlPO₄-sealed coating performed better as a protective passive film during the long-term immersion test for a lower defect concentration and a more protective passive film.     

Electrochemical characterisation of the porosity and corrosion resistance of electrochemically deposited metal coatings

Electrochemical techniques for the assessment of porosity in electrodeposited metal coatings are reviewed. The determination of porosity and corrosion, resistance is illustrated by electrochemical data from three coating/substrate systems namely: electroless nickel on aluminium and steel and immersed gold coatings on an electroless copper-plated ABS polymer. Nickel coatings were up to 24 μm thick while gold deposits had thickness between 75 and 190 nm. Tafel extrapolation and linear polarisation resistance methods were used to determine the corrosion rate of the coated substrates. The aluminium samples were tested in 5% w/v (0.85 mol dm^− 3) NaCl, while coated steel and ABS samples were immersed in 0.125 mol dm^− 3 H₂SO₄ and 0.1 mol dm^− 3 NaBH₄, respectively, at 295 K. Current vs. time curves and anodic polarisation behaviour have also been considered.     

Electrochemical behaviour of conversion coatings on ferritic stainless steel for solar absorbers

Abstract

The electrochemical behaviour, in neutral solutions, of selective solar absorber coatings prepared from ferritic stainless steel using an original conversion treatment is studied by a polarisation technique. After preparation, these coatings present good resistance to corrosion in a humid environment. After thermal treatment in air at high temperatures, a corrosion peak may appear on the polarisation curve; this phenomenon cannot occur for flat collectors where the working temperature is lower than 250°C. The corrosion peak which appears after heating at high temperatures is observed only during the phase of structural reorganisation of the coating; it disappears after longer treatment times.     

Improving corrosion resistance of aluminium metal matrix composites using cerium sealed electroless Ni–P coatings

Abstract

A conversion coating treatment using cerium salts was developed for the surface sealing of electroless nickel–phosphorus (Ni–P) coatings on carbon fibre reinforced aluminium (Cf/Al) composites. The corrosion resistances of uncoated and coated materials (i.e. the Ni–P coating, the Ce conversion coating and Ce sealed Ni–P coatings) were evaluated in 3·5 wt-%NaCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy. Ce sealed Ni–P coating showed the highest corrosion resistance and clearly improved the overall corrosion resistance of Cf/Al composites. Thus, the Ce sealed Ni–P coating had no obvious microcracks that were generally evident in the more conventional Ce conversion coatings. This is presumed to occur because the electroless nickel surface is relatively homogeneous, compared with the Cf/Al composite surface on which different local coating thicknesses would encourage increased microcrack formation. X-ray photoelectron spectroscopy analysis showed that the Ce conversion coating mainly contained both Ce³⁺ and Ce⁴⁺ species; however, Ce⁴⁺ species were the dominant oxidation state on Ce sealed Ni–P coatings.     

Corrosion protection of steel reinforcement by a pretreatment in phosphate solutions: assessment of passivity by electrochemical techniques

Abstract

The aim of this work is to study the protection of steel reinforcement against corrosion by pretreatment in phosphate (Na₃PO₄) solution. The work has been carried out using electrochemical techniques, i.e. corrosion potential E _corr, polarisation resistance R _P and electrochemical impedance spectroscopy (EIS) measurements. The results have been validated by a gravimetric method. It has been stressed that R _P measurements, determined by DC techniques, include the charge transfer resistance R _t plus the resistance associated with the redox process R _ox, both determined by EIS. Also the results have demonstrated that the treatment of the rebar by immersion in the Na₃PO₄ (0·5M) solution favours the formation of a passive layer on the steel rebar surface, which is able to resist longer the action of chlorides to initiate corrosion. However, the resistance of the passive layer against chloride depends on the duration of the treatment by immersion of the rebar within the phosphate solution.     

Fluoride induced corrosion of steel rebars in contact with alkaline solutions, cement slurry and concrete mortars

Corrosion behaviour of mild steel rebars has been studied in contact with 0.01 N NaOH, saturated lime water, cement slurry and embedded mortars having different concentrations of fluoride ions. Weight-loss, electrochemical DC cyclic polarisation and polarisation resistance, surface topographic and X-ray diffraction (XRD) techniques have been used to investigate the effects of fluoride ion on the corrosion behaviour of the material. Under all the conditions of the exposure it is observed that a low content of fluoride (?25 ppm) in the corrodent has deleterious action on the performance of the steel, where as at its higher content (?100 ppm) the ion has rather an inhibiting effect on corrosion rate. XRD studies of the corrosion product accumulated on the steel surface in the presence of fluoride ion indicate the predominance of magnetite oxide phase. Accelerating effect of the ion is observed to be due to depolarisation of cathodic reaction of the corrosion process. Observations show that in accelerating range of fluoride (?25 ppm) it has almost double corrosive effect than noted for equal concentration of chloride ion.     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

Investigation of the corrosion behaviour of steel coated with amorphous silicon carbon alloys

Thin amorphous silicon (a-Si:H) and carbonated silicon (a-Si_1 − xC_x:H) layers were deposited on stainless steel substrates using plasma-enhanced chemical vapour deposition (PECVD) in a “low-power” regime. The carbon content of the carbonated silicon (a-Si_1 − xC_x:H) alloys was varied between 0.1 and 0.37 at.%. The performance of these interfaces as effective corrosion barriers in 3% sodium chloride aqueous solutions was evaluated. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS) were used next to X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to investigate the protection efficiency of the different barriers. The a-Si:H coated showed better corrosion resistance as compared to the carbonated silicon alloys. No degradation was observed after 14 days immersion of the steel substrate coated with a-Si:H in 3% sodium chloride aqueous solution, making this coating an attractive candidate as corrosion barrier.     

An investigation of the corrosion behaviour of a FeNiCoAlTa shape memory alloy in 3.5 wt-% NaCl solution

In this study, the corrosion behaviour of an FeNiCoAlTa (NCAT) shape memory alloy in 3.5% (w?w^?1) NaCl solution was evaluated. Linear polarisation resistance and potentiodynamic polarisation tests were conducted at 25°C. An open circuit potential (OCP) of ?381?mV (vs. saturated calomel reference electrode) and a corrosion rate (CR) of 0.0174?mm?y^?1 were obtained. The NCAT alloy did not show any passivation in the 3.5% (w?w^?1) NaCl solution. Its corrosion behaviour was very similar to that of G10180 (AISI 1018) carbon steel with the NCAT showing a lower CR and a less active OCP. Its corrosion in the solution was a combination of general dissolution and localised attack (pitting). Aging heat treatment caused β-Ni₃Al phase, and tantalum to precipitate on the grain boundaries, resulting in chemical species segregation between the grain boundaries and the grains. The chemical segregation caused intergranular corrosion of the alloy.     

Effect of surface mechanical attrition treatment (SMAT) on zinc phosphating of steel

The effects of surface mechanical attrition treatment (SMAT) of EN8 steel on the growth of phosphate coatings, morphological features and corrosion resistance of the resultant coatings have been studied. SMAT enabled the formation of a uniform surface profile although the average surface roughness is increased after treatment. SMAT increased the extent of metal dissolution and the rate of growth of phosphate coating. In spite of the similarity in phase composition, the phosphate crystallite size is relatively high for samples treated by SMAT. Compared to the untreated one, a cathodic shift in E_corr with a corresponding decrease in i_corr is observed for SMAT treated EN8 steel after phosphating. For all tested samples, zinc phosphate coatings deposited on EN8 steel after SMAT using 8 mm Ø balls for 30 min offers the highest corrosion resistance. The increase in surface roughness by SMAT is partly compensated by the expected improvement in corrosion resistance.     

热喷涂钢柱10年海水腐蚀行为

采用直流电弧喷涂和火焰喷涂技术,在碳钢管表面沉积了１２种类的Ｚｎ、Ａｌ和Ｚｎ－１３Ａ１涂层在日本的千仓海岸进行海水腐蚀实验,结果表明,在５年暴露期内,所有的涂层均没有发生明显的腐蚀;经７年暴露后,未经封闭和经封闭处理的Ｚｎ涂层在浸泡区出现严重的锈蚀,电弧喷涂并经封孔处理的Ａ１涂层,火焰喷涂并经封孔处理的Ｚｎ－Ａ１涂层和火焰涂并经重涂装的Ａ１涂层显示出优良的耐蚀性能,相比之下,未经封闭处理的火焰喷涂