Behaviour of stainless steel in simulated concrete pore solution

Abstract

The localised corrosion behaviour of austenitic, martensitic, and duplex stainless steels has been studied in several solutions simulating the liquid present in the pores of both alkaline and carbonated concrete in the presence of chloride ions. The work aimed to evaluate the suitability of these types of stainless steel as rebars in reinforced concrete structures exposed to very aggressive environments. Electrochemical tests have been performed in solutions with pH values in the range 7·6-13·9, chloride concentration in the range 0-10%, and temperatures of 20 and 40°C. The adverse effect of a decrese in pH on the critical chloride content is discussed, as a function of stainless steel composition and temperature, and the inhibitive effect of high carbonate/bicarbonate concentrations is also shown.     

A study of organic substances as inhibitors for chloride-induced corrosion in concrete

Chloride-induced corrosion of carbon steel reinforcement is the most important cause of premature failure on reinforced concrete structures. Among available methods, corrosion inhibitors offer a simple and cost effective prevention technique, primarily to prevent and stop chloride-induced corrosion. Nevertheless, performance of commercial inhibitors is only partially satisfactory. This paper deals with a basic study on the inhibitive action of organic substances toward chloride-induced corrosion on carbon steel rebar in alkaline environment. The effect of aminic and carboxylic groups was investigated through electrochemical potentiodynamic polarisation tests in simulating concrete pore solution in the presence of chlorides, to ascertain inhibitor effectiveness to increase pitting potential. Results are discussed taking into account the most likely mechanism of inhibition, in relation to the functional group of tested organic compounds.     

Corrosion behaviour of pure Mg,AS31 and AZ91 in buffered and unbuffered sulphate and chloride solutions

Abstract

The low corrosion resistance of magnesium limited its application in industrial affairs. The main corrosive anions for magnesium are sulphate and chloride. This paper deals with their effect at low concentrations on the corrosion behaviour of Mg and Mg based alloy (AS31 and AZ91) in presence/absence of buffer solutions (pH 8). The electrochemical measurements used are open circuit potential and potentiodynamic polarisation. The results exposed that in the absence of a protective layer, general corrosion is observed to occur at greater rate than when sulphate is present; however, the general corrosion is transformed to localised corrosion faster in presence of chloride. In the presence of a protective layer, the localised corrosion is perceived in the case of chloride more than sulphate. The presence of aluminium in the alloys has two contradictory actions. One increases the passivity of the formed layer and the other increases the localised corrosion. The percentage of aluminium in the alloy controls these actions.     

Detection and identification of concrete cracking during corrosion of reinforced concrete by acoustic emission coupled to the electrochemical techniques

The tenacious oxide passive film, which is formed on the surface of embedded reinforcing steel under high alkaline condition of concrete, protects the steel against corrosion. However, the condition of passivity may be destroyed, due to processes such as leaking out of fluids from concrete, atmospheric carbonation or through the uptake of chloride ions. Passive steel reinforcing corrosion induced by chloride is a well-known problem, especially where chloride-containing admixtures or chloride contaminated aggregate are incorporated into the concrete. The objective of this work is on one hand to study the effect of chloride ions on passivity breakdown of steel, respectively, in simulated concrete pore solution (SCP) and in concrete reinforcement, and on the other hand to reproduce the carbonation phenomena by applying to the concrete samples a heating–cooling cycles. In this context, the acoustic emission coupled to the electrochemical techniques (potentiodynamic and electrochemical impedance spectroscopy (EIS)) are used.

The results show clearly that [Cl⁻]/[OH⁻] ratio of 0.6 is the critical threshold where the depassivation set-up can be initiated. In addition, the carbonation process is very aggressive with chloride ions and shows a perfect correlation with acoustic emission evolution.

A physical model of the reinforcement/electrolyte interface is proposed to describe the behavior of the reinforcement against corrosion in chloride solution.     

Investigations of an ethanolamine‐based corrosion inhibitor system for surface treatment of reinforced concrete

Laboratory investigations were performed to assess the efficacy of a proprietary ethanolamine‐based corrosion inhibitor system when applied to the surface of reinforced concrete specimens that were chloride‐contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately eighteen months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. Analysis of aqueous extracts from treated concrete revealed that the ethanolamine component of the inhibitor system penetrated to depths of more than 15 mm within the concrete. It was found that, for inhibitor‐treated specimens, there was some reduction in the corrosion rate of pre‐corroding steel at low cover depths in non‐carbonated concrete with modest levels of chloride contamination. At higher levels of chloride contamination and in carbonated specimens, however, the ethanolamine‐based inhibitor was apparently ineffective under the conditions investigated.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

不锈钢在模拟混凝土孔隙液中的腐蚀行为研究

利用动电位扫描和交流阻抗法研究了普通碳钢与不锈钢在模拟混凝土孔隙溶液中的腐蚀行为及其差别,以及Cl-浓度、溶液pH值对腐蚀行为的影响,从而探讨了不锈钢取代碳钢作为混凝土钢筋骨架的可能性.结果表明：即便混凝土碳化导致孔隙液pH值下降后,氯离子对不锈钢的影响仍然较小,钝化膜致密完好,而普通碳钢的耐蚀性明显变差,钝化膜极易被氯离子侵蚀而破坏.     

Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 2: sodium monofluorophosphate

Sodium monofluorophosphate (MFP) has been applied in the form of concentrated aqueous solutions to the surfaces of concrete structures with the aim of inhibiting corrosion of embedded reinforcing steel which has become depassivated as a consequence of carbonation and/or chloride contamination. To evaluate the effectiveness of such treatments, a series of laboratory investigations was undertaken with reinforced concrete specimens that were chloride-contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately 18 months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. It has been found that there were no marked reductions in the corrosion rates of the steel under the conditions investigated. Analysis of aqueous extracts from the treated concrete specimens by means of ion chromatography revealed that negligible penetration of soluble MFP ions had occurred into any of the specimens. Hydrolysis products of MFP (phosphate and fluoride) were present at significant depths in aqueous extracts of the carbonated concrete specimens but only fluoride was detectable in similarly obtained aqueous extracts of non-carbonated specimens.     

Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 1: calcium nitrite

Laboratory investigations were performed to assess the efficacy of calcium nitrite as an inhibitor when used in surface treatments applied to reinforced concrete specimens that were chloride contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately 18 months thereafter. On completion of the exposure tests, measurements of corrosion weight losses and their distribution on the steel surfaces were also made. In non-carbonated specimens with high levels of internal chloride and carbonated specimens with even low levels of internal chloride, the surface-applied inhibitor treatment appeared to be ineffective under the conditions of the experiments and enhancement of local corrosion rates was observed in some specimens.     

Effect of chloride ions on corrosion behaviour of austenitic nickel and nickel free stainless steels in phosphoric acid solutions

The effect of chloride ions' presence (0·005–1·0M NaCl) in phosphoric acid solutions (5, 40 and 75%) on the corrosion behaviour of three austenitic stainless steels (an experimental steel Fe–18Cr–12Mn–0·6N and two trade grades, Fe–18Cr–9Ni and Fe–14Cr–15Mn–0·2N) has been studied by potentiodynamic polarisation measurements. The surface examinations of the samples tested involved X-ray photoelectron spectroscopy as well as optical and scanning electron microscopy. It was established that chlorides added to phosphoric acid solutions deteriorate the general corrosion resistance, and under anodic polarisation, they provoke pitting corrosion. The composition of the stainless steels significantly influences its corrosion behaviour in the phosphoric acid solutions containing chloride ions. The replacement of nickel with manganese and nitrogen on top of lower chromium content has a strong negative effect on the corrosion resistance.     

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.     

Corrosion properties of HVOF-coated steel in simulated concrete pore electrolyte and concentrated chloride environments

The corrosion susceptibility of steel and HVOF-coated steel in solutions simulating the alkaline concrete pore environment and with the addition of chloride was investigated using potentiodynamic polarization and potential step techniques. The surface characterization was performed using SEM and the surface elemental analysis was determined by EDS. The concentration of chloride was 2.8 M to simulate the concentration of chloride spread in many local regions of Saudi Arabia and called Sabkha. It was found that, in the case of the simulated concrete pore electrolytes, the HVOF coating resulted in an anodic shift of the corrosion potential with marginal effect on the corrosion current. However, upon addition of 2.8 M chloride solution, the corrosion rate of the HVOF-coated steel was found to increase by a factor of two. SEM showed a network of pores within the coating which provides a path for the electrolyte. This would result in preferential corrosion around splat boundaries and confirmed by EDS which showed that the corroded splats have higher oxide contents. Potential step experiments at 400 mV above open circuit potential showed a suppressed current of the HVOF-coated steel compared to the steel substrate alone. The corrosion potential versus time experiments resulted in a more anodic Ecorr which decreased with time and became equal to the Ecorr of the bare steel after 34 h. After that, the corrosion potential of the HVOF-coated steel decreased due to the increase in galvanic coupling between the steel and the HVOF coating.     

Feasibility of utilizing 2304 duplex stainless steel rebar in seawater concrete: Passivation and corrosion behavior of steel rebar in simulated concrete environments

The feasibility of using 2304 duplex stainless steel rebar in seawater concrete was determined by studying the passivation and corrosion behavior of steel in solutions simulated curing and service stage of concrete, respectively. The results demonstrate that 2304 duplex stainless steel rebar could be used with seawater concrete because of a stable passive film formed on the steel surface during the curing stage of concrete even in the presence of 2 M chloride ions. However, due to the synergistic effect of concrete carbonation, the rebar suffered a corrosive attack by chloride due to the lack of OH^? inhibition.     

碳化对模拟混凝土孔溶液中HRB335钢腐蚀行为的影响

应用电化学阻抗谱、循环伏安与动电位极化等方法研究了碳化后模拟混凝土孔溶液pH值的变化对钢筋腐蚀电化学行为的影响.结果表明,随着pH值的下降钢筋表面钝化膜的稳定性与耐蚀性不同程度地降低.当模拟液pH值为12.5与11.5时,钝化膜的稳定性处于因pH值降低导致的钝化膜溶解与表面沉积物CaCO3或含钙氧化物CaFe2O4等耐...     

Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Effects of pH solution and chloride (Cl⁻) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E_pit and corrosion E_cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).     

Zum Einfluss der kathodischen Reaktion auf die chloridinduzierte Bewehrungskorrosion

Investigations on cathodic control of chloride‐induced reinforcement corrosion Regarding the mechanisms of reinforcement corrosion there are still contradictions with respect to the controlling/rate‐determining factors of the corrosion process. It is often discussed, that the electrolytic resistance of the concrete is the controlling factor and that the corrosion rates can subsequently be calculated from concrete resistivity. However, extensive research carried out by the authors has clearly demonstrated, that instead of concrete resistivity the resistance to cathodic polarisation normally is the controlling factor in the case of chloride‐induced macrocell‐corrosion. Generally, cathodic control can be related to restricted oxygen diffusion or activation control. In the present paper, these relationships are discussed in detail by results of numerous tests on the cathodic polarisation behaviour of passive reinforcement. For simple defined geometrical conditions simulating practical cases it is shown by a numerical analysis that the resistance to activation is normally the controlling factor for the corrosion rate and that oxygen diffusion has only to be taken into account, when the concrete is permanently water saturated or extremely dense. To verify, whether it is correct to estimate corrosion rates from resistivity data, tests should be carried out to check the influencing parameters on concrete resistivity and cathodic activation of passive steel surface areas.     

Organic substances as inhibitors for chloride‐induced corrosion in reinforced concrete

Corrosion inhibitors are used to prevent chloride‐induced corrosion in reinforced concrete structures. Since performance of commercial organic inhibitors is only partially satisfactory, a 7‐year long research has been carried out in order to set‐up a new organic inhibitive mixture, able to prevent chlorides‐induced corrosion. A first screening, by means of potentiodynamic polarisation test in alkaline synthetic pore solution, was performed on 80 organic compounds, mainly primary and tertiary amines, aminoalcohols, carboxylates compounds and aminoacids, in order to select the best inhibiting substances. The nine best inhibitive organic substances were selected for long‐term tests: 2 amines (dimethylethanolamine and triethylentetramine), 4 aminoacids (aspartate, asparagine, glutamate and glutamine) and 3 carboxylates compounds (tartrate, benzoate and EDTA). Potentiostatic polarisation and free corrosion tests in synthetic pore solution were performed, as well as tests in concrete exposed to accelerated chlorides penetration. Five years of tests allow estimating the efficiency of the substances in preventing chlorides‐induced corrosion, in term of influence on chlorides penetration and on critical chlorides threshold.     

Influence of steel-concrete interface condition on galvanic corrosion currents in carbonated concrete

This paper presents specific experiments which were developed in order to assess galvanic currents in macrocell corrosion specimens involving active steel in carbonated concrete and passive steel in sound concrete. The influence of the steel-concrete interface condition on the galvanic current was also experimentally investigated. To focus on macrocell corrosion rate assessment, the initiation time of the corrosion process (concrete carbonation) was accelerated. FEM simulations were carried out in order to enhance the physical comprehension of these corrosion experiments. It was found that, in realistic condition, the electrical coupling of active and passive steel areas leads to high galvanic currents and consequently high corrosion levels according to RILEM recommendation. Moreover, steel-concrete interfacial defaults significantly increase the macrocell driving potential and, therefore, the galvanic corrosion current.     

Effect of nitrite in corrosion of reinforcing steel in neutral and acid solutions simulating the electrolytic environments of micropores of concrete in the propagation period

Inhibitors in concrete are usually used to be effective in alkaline or neutral solutions, however, when corroding by chlorides, there is a local acidification and therefore it is necessary to study the effect of the inhibitor in acid pH values. Measurements of the corrosion rate of corrugated steel bars have been carried out in solutions simulating electrolytic chloride environments in the micropores of concrete in the propagation period. It has been studied the effect of sodium nitrite as a corrosion inhibitor when added to the mentioned solutions. The solutions considered consisted of sodium chloride for neutral condition and ferrous chloride for acid condition. This is the soluble compound produced during the corrosion of steel as a result of chloride attack. Comparison of the results of polarization resistance with gravimetrically determined weight losses are presented. Also, results of electrochemical impedance spectroscopy are presented here. It has not been observed a significative improvement in using nitrite as inhibiting agent in these systems. The corrosion seems to be related to the [Cl⁻]/[OH⁻] ratio in three different regions of pH identified from acid to alkaline pH values.     

Untersuchungen zum kritischen korrosionsauslösenden Chloridgehalt von Stahlfasern in künstlicher Betonporenlösung

Investigations into the critical corrosion‐inducing chloride content of steel fibres in artificial concrete pore solution It is well known, that reinforcement steel in concrete is normally protected against corrosion due to the high pH‐value of the pore solution of the concrete. This alkalinity leads to a passive layer on the steel surface, which prevents further corrosion. The passive layer can be destroyed by chloride ions diffusing into the concrete. The concentration of chloride in the concrete which leads to a destruction of the passive layer and therefore to corrosion of the steel is defined as the critical chloride content. Investigations in artificial concrete pore solutions show that the critical chloride content of black steel is strongly dependent on the pH‐value of the solution: the higher the concentration of the OH⁻‐ions the higher the critical chloride content. For steel fibres earlier investigations have shown, that steel fibres do not corrode in concrete even at high chloride contents. Therefore it could be assumed, that the critical corrosion‐inducing chloride content of steel fibres in concrete is distinctly higher than of conventional reinforcing steel. To verify this assumption the corrosion‐inducing chloride content of steel fibres is investigated in artificial chloride‐containing concrete pore solutions at different pH‐values. 5 different types of steel fibres, 1 lashing wire and as reference 1 reinforcing steel are investigated at 3 different pH‐value ranges. The concentration of chloride within the pore solution is gradually increased in time steps of 12 h. The beginning of corrosion is determined by current as well as potential measurements. Furthermore additional investigations are carried out with intermediate products of the fibre production (steel wires with different diameters) to investigate if the critical chloride content of the wires is increasing gradually with decreasing diameter. The investigations show, that steel fibres in artificial chloride‐containing pore solutions indicate an distinctly increased resistance against chloride‐inducing corrosion compared with conventional reinforcing steel for high pH‐values. With decreasing diameter of wires the critical chloride content increases gradually.