Environmental factors affecting the corrosion behaviour of reinforcing steel: I. The early stage of passive film formation in Ca(OH)2 solutions

Oxide film thickening on reinforcement steel at early stage of formation is followed in naturally aerated Ca(OH)₂ solutions, recalling the natural behaviour in concrete, by measuring the open-circuit potential, E, with time up to 4 h. The final potentials, E_fin, are reached from negative values indicating oxide film growth. E varies with the Ca(OH)₂ concentration according to a straight line relationship. Oxide film thickening, at early stage of immersion, follows a direct logarithmic growth law as evident from the linear relationship between E and log t. The rate of oxide film thickening deceases by increasing the concentration and pH of the solution and by raising the temperature. The free activation energy of oxide film thickening is determined and found to be 29.28 kJ/mole, indicating that the process of oxide film growth is under diffusion control.     

Environmental factors affecting the corrosion behaviour of reinforcing steel. V. Role of chloride and sulphate ions in the corrosion of reinforcing steel in saturated Ca(OH)2 solutions

The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)₂ solutions in absence and presence of different concentrations of NaCl, NH₄Cl, Na₂SO₄ and (NH₄)₂SO₄ is followed by measuring of the open circuit potential complemented with SEM and EDS investigation. These salts cause breakdown of passivity and initiation of pitting corrosion. The rates of oxide film thickening by OH⁻ ions and oxide film destruction by the aggressive ions follow a direct logarithm law and depend on the concentration and type of aggressive salts anions and cations. The values of the activation energies for oxide film thickening are calculated and discussed.     

Environmental factors affecting the corrosion behavior of reinforcing steel II. Role of some anions in the initiation and inhibition of pitting corrosion of steel in Ca(OH)2 solutions

Potential-time curves are constructed for the steel electrode in naturally aerated Ca(OH)₂ solutions simulating the corrosion behavior in concrete. Cl⁻ and SO₄²⁻ ions cause the destruction of passivity and initiation of pitting corrosion. The rate of oxide film growth by Ca(OH)₂ and oxide film destruction by Cl⁻ and SO₄²⁻ ions follows a direct logarithmic law as evident from the linear relationships between the open-circuit potential and the logarithm of immersion time. Chromate, phosphate, nitrite, tungstate and molybdate ions inhibit the pitting corrosion of steel. The rate of oxide film healing and thickening increases with their concentrations. In presence of constant inhibitor concentration, the efficiency of pitting inhibition increases in the order: (weak) CrO₄²⁻ < HPO₄²⁻ < NO₂⁻ < WO₄²⁻ < MoO₄²⁻ (strong).     

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.     

Electrochemical investigation of chloride-induced depassivation of black steel rebar under simulated service conditions

Electrochemical measurements of chloride thresholds are reported in simulated concrete pore solution for as-received and surface-modified rebar in an experimental apparatus designed to simulate service conditions. Surface modification led to higher chloride thresholds and reduced variability. The variability in thresholds for as-received rebar was represented by a log-normal distribution; therefore, simple averages of chloride thresholds, without reference to underlying distribution, might not provide reliable indicators of depassivation. The relative constancy of electrochemical measurements below thresholds, and the dependence of the thresholds on surface conditions, suggests that rebar depassivation is likely caused by local critical chemical conditions at the steel surface.     

Inhibitory effects of 5-aminouracil on cathodic reactions of steels in saturated Ca(OH)2 solutions

The inhibitory effects of 5-aminouracil on cathodic reactions of steels were investigated using saturated Ca(OH)₂ solution to simulate the environment in pores in concrete. Polarization measurements showed that 5-aminouracil effectively inhibited these cathodic reactions, indicating the potential of 5-aminouracil as an inhibitor for macro-cell corrosion in concrete/soil systems, which are caused by cathodic reactions in concrete. Cathodic reactions were inhibited in accordance with the Langmuir adsorption isotherm. FT-IR and XPS measurements suggested that 5-aminouracil was adsorbed onto cathodic portions of the steel through the formation of a coordination bond with the amino group, mainly in the form of dimer.     

Use of a 3D optical measurement technique for stochastic corrosion pattern analysis of reinforcing bars subjected to accelerated corrosion

The 3D corrosion patterns of 23 reinforcing bars subjected to accelerated corrosion are characterised using an optical surface measurement technique. A stochastic signal processing methodology is employed for corrosion pattern analysis of the measured data. The statistical analysis of corrosion pattern data shows that a lognormal distribution model can represent the non-uniform distribution of pitted sections along the corroded bars. It was observed that the frequency of corrosion is independent from the mass loss ratio and the length of the bars. Finally, a set of probabilistic distribution models for the geometrical properties of corroded bars is developed.     

Environmental factors affecting the corrosion behavior of reinforcing steel. IV. Variation in the pitting corrosion current in relation to the concentration of the aggressive and the inhibitive anions

The pitting corrosion current of reinforcing steel is measured under natural corrosion conditions in Ca(OH)₂ solutions in presence of Cl⁻ as aggressive ions and as inhibiting anions. The corrosion current starts to flow after an induction period which depends on solution composition (concentration, pH and presence or absence of the aggressive and the inhibiting anions). The limiting corrosion currents increase with increasing the Cl⁻ ion concentration and decrease with increasing the pH and inhibiting ions concentration. The inhibition efficiency of the studied inhibiting ions increases in the following order: , and depends on the way by which the inhibitor is added to the solution. Injection of the inhibiting anions in solution causes repassivation of the pre-formed pits through competition with Cl⁻ ions for adsorption sites on metal oxide surface. The adsorbability constant and the free energy of repassivation of the inhibiting anions are calculated.     

Effect of CO2 and R-ratio on near-neutral pH stress corrosion cracking initiation under a disbonded coating of pipeline steel

This study investigates the role of CO₂ and cyclic stress R-ratio (R = minimum stress/maximum stress) on near-neutral pH SCC initiation mechanism(s) under a disbonded coating of pipeline steel protected by cathodic protection (CP). It was found that depending on CO₂ concentration and level of CP, different localized environments with various pH could be formed under the disbonded coating. When cyclic loading was applied, different SCC initiation mechanisms were involved depending on the pH of the localized environments. Reducing the R-ratio had different effects on the initiation mechanisms.     

Chloride threshold for rebar corrosion in concrete with addition of silica fume

The effect of silica fume on the chloride threshold for the initiation of pitting corrosion of steel in concrete was investigated. Laboratory tests were carried out in concrete specimens made with ordinary Portland cement and with 10% of silica fume. Chloride contents up to 2% by mass of cement were added to the mixes, in order to investigate the corrosion rate of embedded bars made of both strengthened and mild steel. A lower chloride threshold was observed in the bars which were embedded in concrete with silica fume compared to those embedded in concrete made of Portland cement.     

Galvanic currents and corrosion rates of reinforcements measured in cells simulating different pitting areas caused by chloride attack in sodium hydroxide

Chlorides induce localized corrosion in the reinforcement. The pits formed a substantial reduction in the cross-sectional area. The smaller is the ratio between the size of the corrosion spot and the surrounding passive metal area acting as cathode, the higher is the corrosion rate or the progression in the pit depth. This, however, is not always the case. To understand well the macro or microcell behavior of the corrosion of reinforcements, this paper reports a study on the effect of decreasing sizes of anodic zones until relatively small, maintaining the size of the cathode constant. In addition, to enhance the pure anodic behavior of the corroding zone, an acidic solution was used as an anolyte. Measuring the galvanic current (I_g) and the corrosion rate (I_corr) by using zero resistance ammeter (ZRA) and polarization resistance (R_p) techniques, respectively, was crucial to differentiate the microcell contribution from that of the macrocell. The results indicate that while I_g increased as the anodic area decreased, I_corr presented a maximum for the area ratios, S_A/S_C = 1/3.     

The effect of silane-based hydrophobic admixture on corrosion of galvanized reinforcing steel in concrete

Sound or deliberately pre-cracked concrete specimens, with 0.5 or 1 mm wide crack, were manufactured with water to cement ratios (w/c) of 0.45 and 0.75, both in the presence and in the absence of a silane admixture. The specimens were exposed to wet-dry cycles in a 10% NaCl aqueous solution. The results, in terms of electrochemical measurements, and visual and metallographic observations carried out on the galvanized steel reinforcement removed from the specimens, showed that the hydrophobic concrete is able to protect galvanized steel reinforcement from corrosion even in the presence of cracks in the concrete cover, especially when a high w/c is used.     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

Comparison of the corrosion of bulk amorphous steel with conventional steel

The corrosion behaviour of one prominent bulk amorphous steel (BAS) alloy, (Fe_44.3Cr₅Co₅Mo_12.8Mn_11.2C_15.8B_5.9)_98.5Y_1.5, is compared to that of its crystalline counterpart and of the conventional steel X210Cr12. In acidic solutions, due to its single-phase nature, the BAS exhibits a much higher stability than the multiphase crystalline counterpart and the conventional steel. With increasing pH value, the overall elemental composition plays a more important role and the reactive elements Mo, Mn and Co, are assumed to be responsible for the observed lower stability of the BAS and its crystalline counterpart as compared to the conventional steel.     

Initiation and repassivation of pitting corrosion of carbon steel in carbonated concrete pore solution

The initiation, propagation, repassivation of metastable pits of Q345 carbon steel in Cl⁻ containing concrete pore solution were studied by electrochemical noise. The initiation rate of metastable pits increases exponentially up to a constant value with time. After stable pit occurs, the nucleation of metastable pits is mainly induced by the rust layer, and mainly takes place in the outskirt of the rust layer. Stable pits incline to expand horizontally to produce a shallow pit. In addition, high concentration of nitrite can inhibit the nucleation of metastable pits around the rust cover and accelerate the repassivation of stable pits.     

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.     

Studies of the corrosion of mild steel in alkali-activated slag cement mortars with sodium chloride admixtures by a galvanostatic pulse method

Corrosion of mild steel reinforcement in an alkali-activated slag (AAS) cement mortar containing NaCl admixtures has been investigated using an improved galvanostatic pulse technique. Corrosion rates were monitored at 90% relative humidity over a period of several months. On first exposure highest corrosion rates are seen with the lowest chloride levels, but over time different behaviours are seen for different chloride levels. The complex pattern of current and potential variation has been analysed using Tafel plots. Tentative conclusions are put forward to account for the behaviour seen, based on a model where oxidation of sulphide anions to sulphur prevents potential rising into the pitting regime and leads to inhibition of the oxygen cathode reaction.     

Influence of pH on the nitrite corrosion inhibition of reinforcing steel in simulated concrete pore solution

Measurements of corrosion rate of reinforcing steels have been carried out in solutions simulating electrolytic chloride environments in corrosion pits in the propagation period with sodium nitrite. A significant corrosion inhibition has been observed at relatively low chloride contents of 0.045 M due to the presence of nitrite in these systems, but its efficiency decreases when the pH is reduced. The corrosion intensity seems to be related to the [Cl⁻]/[OH⁻] ratio. Three different pH regions from acid to alkaline have been observed in terms of corrosion activity. An explanation on the behaviour of nitrite at different pH is given.     

Inhibition effect of chromate on the passivation and pitting corrosion of a duplex stainless steel in LiBr solutions using electrochemical techniques

The electrochemical behavior of duplex stainless steel (DSS) in LiBr media was investigated by anodic cyclic polarization curves and AC impedance measurements. The effect of bromide concentration and the presence of chromate in the solutions on the corrosion behavior of AISI 2205 was studied. Cyclic polarization curve analyses showed that there was different pitting susceptibility of passive films depending on the LiBr concentration. Pitting potential decreases with LiBr concentration in a semilogarithmic scale following two different slopes. Chromate presence displaces pitting potentials towards more positive values at low LiBr concentrations but it has no effect when LiBr concentration increases.The comparative analysis carried out in LiBr and LiBr chromate-containing solutions at two different concentrations, 0.016 M and 0.032 M, verifies the assumption that halogen ions facilitate inhibitor adsorption. The addition of halides strongly increased the inhibition efficiency of chromate. The passive film becomes more resistant when bromide concentration increases, although film thickness decreases.     

A numerical study of damage caused by combined pitting corrosion and axial stress in steel pipes

Localized pipe wall damage accounts for many failures. Numerical modelling of pipes under increasing axial load and constant internal pressure when there is corrosion pits on the exterior surface of the pipe is reported herein. It is shown that for the assumed ideal elastic–plastic material the shape and volume of the plastic field depend on pit depth and its geometry. Pipe wall fracture around a pit can be associated with a critical plastic section. The results reported herein should be relevant for estimating of the risk of perforation and of loss of contents for steel pipes under different loading.