Effects of pH-value and Chloride Ion Concentration on Passivation Behavior of Steel Rebar in Different Surface Conditions
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Electrochemical experiments were used to study the passivation behavior of the steel rebars with polished surface,oxide skin and rusty skin in simulated concrete pore solution.The results indicated that after the corrosion potential became stable,the steel rebar with nude surface had the highest potential while the steel rebar with rusty skin had the lowest potential.With the decrease of solutions' pH value,the passivation of the three kinds of rebar decreased.The passivation of nude surface steel rebar and oxide skin rebar was apparently better than that of rusty skin rebar.The threshold pH values for polished steel rebar,oxide skin rebar and rusty skin rebar were 11.2,11.0and 12.4,respectively.With the increase of chloride ion concentration,the passivation property of the three kinds of rebar decreased.The passivation of rusty rebar was apparently worse than that of nude surface rebar and oxide skin rebar.The threshold of chloride ion concentration for polished steel rebar,oxide skin rebar and rusty skin rebar were0.03mol/L,0.03mol/L and 0.01mol/L,respectively.Keywords:
Rebar
Passivation
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The corrosion behavior of rebars with different surface states was studied by electrochemical technique and accelerated atmospheric corrosion tests.The reinforcing steel with different surface states had different corrosion characteristics.In the accelerated laboratory tests,the steel with oxide skin had larger corrosion weight loss than the steel with nude surface.In the simulated concrete solutions with no salt,both steels were protected.When the concentration of NaCl increased,the corrosion behavior of the steel varied considerably,and the corrosion rate increased,however,the steel with oxide skin showed better corrosion resistance than the steel with nude surface.
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Alkaline battery
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The corrosion evolution of scaled rebar cooled by a chemical reagent FM in concrete immersed in 3·5%NaCl aqueous solution was studied by electrochemical impedance spectra and compared with bare rebar. The results show that the corrosion evolution process of FM cooled rebar consists of three stages during the immersion: the passive stage, the accelerated corrosion stage dominated through the charge transfer step and the steady corrosion stage dominated through the mass transfer step. The corrosion rate of FM cooled rebar reaches a steady rate (<0·7 μA cm−2) after 80 days. However, the corrosion rate of bare rebar always increases at all immersion times. After immersion for ∼1 year, the corrosion rate of bare rebar is nine times larger of that of FM cooled rebar. Therefore, the corrosion resistance of the rebar has been improved significantly by applying the FM cooling process.
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Immersion
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Stainless steel-clad rebar provides an opportunity to significantly increase the chloride threshold concentration associated with active corrosion initiation compared to plain carbon steel. However, threshold chloride concentrations for 316L stainless steel-clad rebar are unknown. Moreover, the impact of possible galvanic corrosion between the clad layer and any exposed carbon steel core has not been investigated. The chloride threshold concentrations for corrosion initiation on clad 316L stainless steel (with a thickness of about 1 mm over a carbon steel core), solid 316LN stainless steel, and plain carbon steel were examined in saturated calcium hydroxide plus various concentrations of sodium chloride. The electrochemical properties of intact 316L stainless steel-clad rebar were found to be similar to those of solid 316LN stainless steel according to several electrochemical criteria. The chloride threshold concentrations for corrosion initiation were increased to chloride/hydroxide molar ratios as high as 17 to 24 even at high anodic potentials for intact 316L-clad and solid 316LN stainless steel, respectively. Thus, active corrosion of intact 316L-clad rebar could be delayed for many years due to the high chloride/hydroxide molar ratios required at the stainless steel/concrete interface and the slow transport rate of chloride in concrete. In contrast, the threshold chloride concentration for corrosion initiation on carbon steel was low (chloride/hydroxide molar ratio < 1.5) at all potentials. Cladding with a physical breech exhibited chloride/hydroxide thresholds dominated by the exposed plain carbon steel. Galvanic coupling between exposed plain carbon steel and the stainless steel-cladding accelerated corrosion of the plain carbon steel only at and above the chloride/hydroxide ratio necessary for corrosion initiation on carbon steel.
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Carbon steel
Cladding (metalworking)
Galvanic cell
Galvanic corrosion
Sodium hydroxide
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The chloride-induced corrosion of reinforcing steel bars (rebar) in concrete seriously limits durability of reinforcing concrete structures. This investigation examines key issues in pitting corrosion and chloride corrosion threshold of rebar in alkaline solutions. The effect of rebar surface condition on the threshold chloride concentration values was evaluated using the open-circuit immersion method. Results indicate that sandblasted rebar had higher threshold chloride concentration values than as-received or prerusted rebar. The threshold chloride-to-hydroxide ratio increased with pH, suggesting that the intrinsic inhibiting effect of hydroxide ions was stronger at higher hydroxide levels. Ranges of total chloride threshold in concrete were estimated based on the threshold chloride concentration from liquid solution measurements and on available chloride binding data, and were found to be consistent with values observed in practice. The effect of several important factors (chloride concentration, pH, rebar surface finish, polarization scan rate, passive film maturity, specimen size) on pitting and repassivation potentials in alkaline solutions (pH 11.6 to 13.6) was investigated with the cyclic polarization technique. It was found that pitting was not a unique function of the test condition and that its variability tended to increase with decreasing chloride-to-hydroxide ratio. The average value of pitting in replicate tests decreased with solution chloride concentration, specimen size, and steel surface roughness but increased with solution hydroxide concentration and passive film maturity. In contrast, repassivation was nearly insensitive to those parameters. The functional form of the distribution of pitting for a given solution and steel surface condition was not clearly apparent. The dependence of pitting on specimen size was explained using stochastic pitting initiation theory. The passivation behavior of rebar steel in alkaline solutions (pH 8.2 to 13.6) was also explored. Recommendations included: incorporating a total chloride threshold in concrete variability term in future service life prediction procedures; using a representatively large exposed steel surface area for future testing to determine the total chloride threshold in concrete; performing exploratory cost/benefit analyses to assess possible use of sandblasted rebar surfaces and performing corresponding tests in concrete; careful assessment of effect of lowering pore water pH in new concrete formulations; and taking into consideration potential dependence of the total chloride threshold in concrete in future durability models.
Rebar
Pitting Corrosion
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The corrosion behavior of rebar HRB400 in 0.5 wt.%, 1.5 wt.% and 3.5 wt.% NaCl solutions was investigated by corrosion rate calculation and corrosion morphology observation. The results showed that, with the increase of corrosion time from 240h to 720h, the corrosion rate of rebar HRB400 in each NaCl solution increases. With the increase of NaCl solution concentration, the corrosion rate of rebar HRB400 increases. The corrosion morphologies show strong consistency with the test results of corrosion rate.
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Rebar
Cage
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To improve the corrosion resistance of rebar,a new type of corrosion inhibition quenched agent was prepared.The corrosion resistances of quenching rebar were researched by atmospheric exposure test(AE)and wet/dry cyclic accelerated corrosion test(CCT).The inhibition mechanism was also investigated by electrochemical test,FTIR,SEM and XRD.The results showed that the dense passive film(thickness about 10μm)was appeared on the surface of the rebar;the composition of the AE products consisted ofα-Fe2O3,γ-Fe2O3,α-FeOOH and iron silicate,and the composition was unchanged but the relative content of each composition was changed;the activation energy of the corrosion reaction increased from 73.8kJ/mol to 111.3kJ/mol after quenched with the obtained agent.The corrosion resistance of rebar was significantly enhanced.
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The corrosion performance of stainless steel clad reinforcing bar (SCR) was investigated. Corrosion potential of SCR in various alkaline media as a function of time was measured periodically and corrosion rates were estimated using Electrochemical Impedance Spectroscopy (EIS). At room temperature, SCR without cladding breaks was free of corrosion for up to one year in all the testing conditions: saturated Ca(OH)2 solutions (SCS, pH ~12.6), simulated pore solution (SPS, pH ~13.6), simulated carbonated concrete pore solution (CPS, pH <10) (each with 15 wt% chloride by the end of test), and concrete with chloride up to 8% chloride by weight of cement. SCR without cladding breaks also remained passive at 40 o C in SCS with 15% chloride and in concrete with 8% chloride. SCR with a 1 mm hole corroded actively in SCS with 15% chloride. The results suggest that carbon steel exposed by the small hole was corroding actively at a high local rate. A model for SCR with a single cladding break at one cut-end in concrete was modeled using a finite differences appr oach. Calculations indicated that the resistivity of concrete and size of cladding breaks were critical parameters in establishing the rate of corrosion, and that corrosion of SCR with sub-millime ter breaks in high qualit y concrete would cause concrete cracking only after long service times.
Rebar
Cladding (metalworking)
Carbon steel
Steel bar
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