THE EFFECTS OF CATHODIC POTENTIAL AND CALCAREOUS DEPOSITS ON CORROSION FATIGUE CRACK GROWTH RATE IN SEA WATER FOR TWO OFFSHORE STRUCTURAL STEELS

The effects of cathodic protection potential, corrosion products and stress ratio on corrosion fatigue crack growth rate have been studied on offshore structural steels. These materials were cathodically polarised in seawater and 3% sodium chloride solution at three potentials of -0.8, -1.0 and -1.1 V(SCE). The corrosion fatigue crack growth rate in seawater was greater than that in air and increased with more negative potentials. The maximum acceleration of crack growth rate in seawater was observed at the crack growth plateau which was independent of ΔK. Calcareous deposits precipitated within the cracks resulted in an increase of crack opening level and contributed to a reduction of the corrosion fatigue crack growth rate. Such a corrosion-product-wedging effect could be evaluated by using an effective stress intensity range, ΔK_eff. The estimation of corrosion fatigue crack growth rate in terms of ΔK_eff clarified the effect of hydrogen embrittlement under a cathodic potential. Thus the processes of cracking in seawater at cathodic potentials resulted from mechanical fatigue and hydrogen embrittlement with calcareous deposits reducing the crack growth rate. All these three mechanisms were mutually competitive.     

Mechanisms for corrosion fatigue crack propagation

ABSTRACT The corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high‐strength low‐alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open‐circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non‐single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (K_max) exceeded a certain well‐defined critical value characteristic for a given material‐solution combination. When K_max was lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen‐induced cracking (HIC). Therefore, critical values of K_max, as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated as K_HIC and ΔK_HIC. HIC was the main mechanism of corrosion FCG at K_max > K_HIC (ΔK > ΔK_HIC). For most of the material‐solution combinations investigated, stress‐assisted dissolution played a dominant role in the corrosion fatigue crack propagation at K_max < K_HIC (ΔK < ΔK_HIC).     

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, i _crit, and passive current density, i _p, was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, t _f, with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, K _ISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight loss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from this at higher BTA concentrations. The paper discusses the possible reason for this deviation.     

CORROSION FATIGUE FRACTURE BEHAVIOUR OF A SiC WHISKERALUMINIUM MATRIX COMPOSITE UNDER COMBINED TENSIONTORSION LOADING

We describe an investigation into the fatigue fracture behaviour under combined tension–torsion loading of a SiC whisker-reinforced A6061 aluminium alloy fabricated by a squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conducted on both the composite and its unreinforced matrix material, A6061-T6, under load-controlled conditions with a constant value of the combined stress ratio, α = τ_max /σ_max in laboratory air or in a 3.5% NaCl solution at the free corrosion potential. The corrosion fatigue strength of both the matrix and composite was less in the solution than in air. The dominating mechanical factor that determined the fatigue strength in air was either the maximum principal stress or the von Mises-type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the maximum principal stress, irrespective of the combined stress ratio. In the case of the matrix material, crack initiation occurred by a brittle facet normal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but at a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resulting in final failure. Pitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiation at the bottom of a pit was close to the terminal stage, i.e. about 70% or more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assuming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension–torsion loading were examined in detail.     

Propagation of steel corrosion in concrete: Experimental and numerical investigations

This paper focuses on experimental and numerical investigations of the propagation phase of reinforcement corrosion to determine anodic and cathodic Tafel constants and exchange current densities, from corrosion current density and corrosion potential measurements. The experimental program included studies on RC specimens with various binder compositions, concrete cover thicknesses, and concrete cover crack widths. Modelling and fitting of experimental data using an electrochemical model allowed for the determination of parameters, which are key parameters for electrochemical modelling tools. The numerical model was, furthermore, used to identify electrochemical parameters, which are independent of concrete cover thickness and crack width and at the same time allow for determination of the corrosion current density and corrosion potential of concrete structures within an acceptable error.Very good comparisons between the experimentally measured and numerically simulated corrosion current densities and corrosion potentials were found for the various RC specimens. Anodic and cathodic Tafel constant between 0.01 and 0.369 V/dec and 0.01 and 0.233 V/dec, respectively, were found in the present study through numerical simulations of the experimental data. Anodic and cathodic exchange current densities ranged from 1.0E–12 to 1.0E–09 A/mm² and 1.0E–12 to 1.1E–09 A/mm², respectively.     

Calibration Curves for rapid monitoring of corrosion rates

The linear polarization method of corrosion rate determination has been investigated on the basis of the equation of Stern and Geary. It has been demonstrated that use of an average value for the parameter B does not lead to accurate results. A procedure has been suggested for rapid monitoring and more accurate measurements of corrosion rates, and this is based on the calculation of B from independent measurements of the electrode-kinetic parameters of the corrosion system under investigation. Values of B have been calculated for a wide range of Tafel slopes for the case when both cathodic and anodic partial reactions are controlled by charge transfer. Using the correct value of B, potential/corrosion current density curves are constructed for various polarizing current densities. It has been shown that these calibration curves are very helpful in the rapid monitoring of corrosion rate using the linear polarization method at constant current density with a three-electrode arrangement.     

FATIGUE LIFE BEHAVIOR OF MONOCRYSTALLINE COPPER IN 0.1 M PERCHLORIC ACID

We have studied the fatigue lives of single crystals of copper in 0.1 M HClO₄under different polarization potentials. Perchloric acid was chosen for the aqueous environment because it allows us to control the corrosion reactions rigorously. Persistent slip band (PSB) behaviour and crack nucleation were studied during life, and fracture surfaces after failure. Different behavior was observed depending on strain amplitude. At 2 × 10^-3 plastic shear strain amplitude, anodic potential was observed to decrease life, whereas cathodic potenlial was found to be less damaging than laboratory air. Crack nucleation and propagation occurred along the primary slip plane for both conditions. The reduction of fatigue life under anodic potential is explained by enhanced localized strain at the PSB's and preferential dissolution within them. However, for a strain amplitude of 4 × 10^-3, cracks nucleated and propagated along the secondary slip system. We observed crack nucleation to be associated with deformation-induced stress concentrations, and the aqueous solution showed no aggressive effect under either anodic or cathodic potential.     

Corrosion Fracture of Structural Metallic Materials: Effect of Electrochemical Conditions in Crack

Abstract: The work is a compressed review based on the summarised results and the original approach for study of corrosion crack growth, taking into account local electrochemical conditions in the crack tip, which was developed at the Karpenko Physico‐Mechanical Institute of NASU. The model scheme of the pre‐fracture zone in the corrosion crack tip, which can be defined by the local values of pH of solution, electrode potential of metal E and stress intensity factor K_I is proposed. For its realisation, the special method and testing equipment for corrosion crack growth study and local electrochemical measurements in the crack were developed. The variation of the electrochemical conditions in corrosion cracks was studied, and it has been found that some stabilised levels of the pH and E values can be achieved in the tip of a non‐propagating and a propagating crack under static and cyclic loading during of exposure time. On this ground, the method for forecasting of the threshold stress intensity factor K_ISCC under stress corrosion cracking was proposed using these characteristic values of pH and E. This method was also adopted for the determination of the threshold stress intensity factor K_th under corrosion fatigue. The special method for determining corrosion fatigue crack growth rate diagrams based on consideration of extreme electrochemical conditions in the crack tip was developed. It has been proven that such diagrams reflect the extreme influence of the environmental factor on corrosion fracture of material, and they may be recommended as the base for the remaining lifetime calculation of the structural elements exploited under environmental conditions.     

THE EFFECT OF CATHODIC POLARIZATION ON CORROSION FATIGUE OF A HIGH STRENGTH STEEL IN SALT WATER

Abstract— Corrosion fatigue crack growth rates in high strength steel are often increased when a large cathodic polarization is applied. The corrosion fatigue mechanism in this case is generally considered to be due to hydrogen embrittlement. In the present study the crack growth process was carefully monitored by taking replicas from initially smooth specimens of a high strength steel under fully reversed push-pull loading while: (1) exposed to laboratory air, (2) immersed in a 0.6 M sodium chloride (NaCl) solution at open circuit potential (OCP) and (3) with an applied cathodic potential of —1250 mV (SCE). It is shown that the effect of cathodic polarization is dependent on the applied stress level and the nature of the cracking process, which in turn, is related to the sue of the crack. For stress levels at or below the in-air fatigue limit, failure did not occur for cathodically polarised specimens despite the number of loading cycles being 10 times that of the lifetime of identical tests in solution at the open circuit potential. At stress levels above the in-air fatigue limit the reduction in fatigue endurance caused by the presence of the corrosive environment can be partially recovered through cathodic polarization. The role of non-metallic inclusions in the cracking process under various exposure conditions is discussed, and a cracking mechanism is proposed.     

A FATIGUE-CRACK-GROWTH-BASED ANALYSIS OF TWO-STEP CORROSION FATIGUE TESTS

Abstract A method for the analysis of two-step fatigue level sequences is proposed and compared with experimental results. Two-step loading tests of the aluminum alloy 2017-T4 in 3% sodium chloride solution have been carried out in conjunction with a replica technique used to monitor the growth of fatigue cracks. Fatigue cracks were nucleated at corrosion pits 10–15 μm in size, and crack growth rather than crack initiation was found to take up the major portion of the fatigue lifetime in these tests. The results could therefore be analyzed on the basis of the following constitutive relation for fatigue crack growth. da/dN=A(ΔK_eff-ΔK_effth)² This analysis was simplified since the influence of transients in the crack growth rate induced upon change in load level was found to be minimal. The approach provides a rational basis for dealing with load-sequence effects.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

PASSIVATION EFFECTS ON CRACK CLOSURE IN THE FATIGUE CRACK PROPAGATION OF A PEAK-AGED Al-Li-Zr ALLOY IN NaCl AND Na2SO4 SOLUTIONS

Abstract Fatigue cracking of a peak-aged Al-Li-Zr alloy was investigated by measuring crack closure as a function of applied anodic potential in 0.6 M NaCl and 0.5 M Na₂SO₄ solutions with an unloading elastic compliance technique, and by comparison with crack closure in dry air. The present work involves complementary anodic behaviour of the Al-Li-Zr alloy in both solutions by potentiodynamic polarization and potentiostatic current transient experiments. From the repassivation rates in the passivation potential range in both solutions, it is indicated that a more stable passive film is formed at lower applied anodic potential than at higher applied anodic potential. The intrinsic fatigue crack propagation (FCP) rates under unstable passivation potential in both solutions were significantly larger than those obtained in dry air. Under stable passivation potential in both solutions, however, the intrinsic FCP rates in the low ΔK_eff range were slightly lower than those obtained in dry air. The crack closure in the low ΔK_eff range increased under stable passivation potential, in dry air and under unstable passivation potential. The high crack closures appearing in the low ΔK_eff range were characterized by a tortuous fracture surface in dry air, and the occurrence of various crack paths such as rolling plane delamination under unstable passivation potential. The difference between environmental crack closures under stable and unstable passivation conditions is discussed in terms of environment-assisted crack-tip damage processes.     

THE EFFECTS OF A MARINE ENVIRONMENT ON THE CORROSION FATIGUE CRACK PROPAGATION RATE OF PURE TITANIUM AND ITS WELD METAL

The effects of stress ratio and microstructure on fatigue crack growth rate in air and natural seawater were investigated for pure titanium and its weld metal. The corrosion fatigue characterization of pure titanium was also studied under a cathodic potential in natural seawater. In air, the fatigue crack growth rates of pure titanium and its weld metal increased with increasing stress ratio. In natural seawater, the effect of stress ratio was similar to that in air. However, the crack growth rates were greater for pure titanium than for the weld metal. These results indicate that the corrosion action is sensitive to the microstructure in front of the crack tip. When the crack growth rate for the weld metal was plotted using the effective stress intensity factor range, the crack growth rate in natural seawater was coincident with that in air, regardless of stress ratio. For the base metal, there is a significant difference in the crack growth rate between natural seawater and air.     

The effect of acetic acid on the CO2 corrosion of grade X70 steel

The effect of acetic acid (HAc) on the CO₂ corrosion of grade X70 steel was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), polarization tests and electrochemical impedance spectroscopy (EIS). In the absence of acetic acid, a fairly dense layer of iron carbonate (FeCO₃/siderite) was formed. At 500 ppm HAc, FeCO₃ layer became more porous. In addition, anodic/cathodic polarization curves were activated with the more pronounced effect on the cathodic side. By adding 1000 ppm HAc, similar polarization behavior was obtained and FeCO₃ layer became yet more porous than previous conditions. At 2000 ppm HAc, FeCO₃ layer disappeared completely, while polarization behavior changed and the limiting diffusive current density was observed in the cathodic side. There were two major increases in the corrosion rate at 500 and 2000 ppm HAc. The EIS results reflected similar behavior for the specimens exposed to the solutions with 0–1000 ppm HAc. Under these conditions, a charge transfer controlled behavior due to the FeCO₃ layer was observed which was accelerated by increasing HAc concentration. At 2000 ppm HAc, the corrosion behavior changed considerably and the formation/adsorption of corrosion product followed by the dissolution process was observed.     

Effects of pH on the electrochemical behaviour of titanium alloys for implant applications

The electrochemical behaviour of two commercial titanium alloys Ti-6Al-4 V (ASTM F136) and Ti-13Nb-13Zr (ASTM F1713) was investigated in Ringer physiological solution at two pH values (5.5 and 7.0). The corrosion properties were examined by using electrochemical techniques: Potentiodynamic anodic polarization, cyclic polarization and electrochemical impedance spectroscopy (EIS). The electrochemical corrosion properties of both alloys at different conditions were measured in terms of corrosion potential (E _corr), corrosion current density (i _corr) and passivation current density (i _pass). Equivalent electrical circuits were used to modulate EIS data, in order to characterize alloys surface and better understanding the pH effect on the interface alloy/solution.     

CRACK INITIATION AND PROPAGATION BEHAVIOUR OF A HEAT-TREATED CARBON STEEL IN CORROSION FATIGUE

Abstract— When estimating fatigue damage quantitatively it is important to clarify its physical basis. In this study, rotating bending fatigue tests of a heat-treated 0.45% carbon steel were carried out in 3% NaCl solution, in order to clarify the physical basis of corrosion fatigue damage from successive observations of plastic replicas. The results show that corrosion pits are generated during the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is observed only in the case when the stress range is relatively large, and in the range of stress for which slip bands are produced in air. After initiation of a crack, the crack propagates by frequent interactions and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air.     

09Cr2AlMoRE钢和N80油套管钢的CO2腐蚀电化学行为

利用交流阻抗技术对比研究了N80油套管钢和09Cr2AlMoRE钢（称新型合金）在饱和CO2的地层水中的腐蚀电化学行为,同时利用SEM观察了两种材料在1MPa,120℃腐蚀7d（天）所成膜的形貌。结果表明,N80钢的阳极电化学阻抗谱出现了高频的容抗弧、中低频的感抗弧和低频的容抗弧;而新型合金的阳极电化学阻抗谱只出现了高、低频容抗弧,感抗弧已很少了。对于阴极电化学阻抗谱,N80钢的阻抗谱只由一容抗弧组成,而新型合金则由高频容抗弧、中频Warburg阻抗和低频容抗弧共同组成。对两种材料阴、阳极过程中极化电阻的计算表明,新型合金阴、阳极过程中的极化电阻大于N80钢相应过程的极化电阻,表明新型合金能有效地抑制了CO2腐蚀。     

Effect of hydrolysis of AlN particulates on corrosion behavior of Al/AlNp composite in neutral chloride solution

The effects of hydrolysis of the AlN particulates on corrosion behavior of Al/AlN_p composite were examined by analyzing potentiostatic anodic current transient, cyclic voltammetry curves and potentiodynamic cathodic polarization curves. It is found that the degree of hydrolysis of AlN particulates in the composite increase with the pre-hot water treatment time, which increase the occurrence of metastable pits in the composite. In addition, hydrolysis of AlN particulates also affects the electrochemical corrosion behaviors of the composite, and makes Al/AlN_p composite show intergranular corrosion behavior to some extent.     

A Fracture Probability Competition Mechanism of Stress Corrosion Cracking

The stress corrosion cracking (SCC) of austenitic stainless steel was studied via polarization, slow strain rate and scanning electron microscope (SEM) techniques. Many SCC mechanisms have been proposed in which hydrogen embrittlement and passive film rupture-repassivation theories are generally accepted, but they can hardly explain the SCC mechanism of austenitic stainless steel in acidic chloride solution adequately, because the steel is in active dissolution state and cathodic polarization can prevent it from occurring. Our experiment shows that the anodic current increases the creep rate and decreases the plastic strength of the material on single smooth specimen as well as at the SCC crack tip. The fractured surface was characterized as brittle cleavage, while the surface crack of smooth specimen was almost vertical to the tensile strength, which can confirm that the cracks were caused by tensile stresses. A fracture probability competition mechanism of SCC was proposed on the basis of the experimental results combined with the viewpoint of ductile-brittle fracture competition. When the anodic dissolution current is increased to a certain degree, the probability of fracture by tensile stress will exceed that by shear stress, and the brittle fracture will occur. The proposed SCC mechanism can riot only explain the-propagation of SCC cracks but can explain the crack initiation as well. The strain on the surface distributes unevenly when a smooth specimen is deformed, so does the anodic current distribution. The crack will initiate at a point where the anodic current density is large enough to cause the material at a specific point to fracture in brittle manner.     

Effects of phosphate on the chloride-induced corrosion behavior of reinforcing steel in mortars

The influence of phosphate as a corrosion inhibitor on the corrosion behavior of as-received and pre-rusted reinforcing steels in mortar specimens was investigated after 360 days exposure in 3.5% NaCl solution. This involved the use of electrochemical techniques for studying the steel surface reactions and microscopic observations of the steel–mortar interface. The electrochemical methods, including electrochemical impedance spectroscopy (EIS) and measurements of corrosion potential (E_corr) and linear polarization resistance (LPR), were employed to evaluate the corrosion tendency and general corrosion rate of steel. In addition, the pitting corrosion resistance of steel was also determined by cyclic polarization (CP) measurements. The results indicate that different from nitrite, which is generally accepted as an anodic inhibitor, phosphate may be a cathodic inhibitor according to its reduced corrosion rate and more negative E_corr at the same dosage as nitrite in mortar specimens. The study also reveals that the inhibiting efficiency of phosphate against general corrosion of both as-received and pre-rusted specimens is lower than 10%, which is inferior to nitrite in some respects. However, as indicated by cyclic polarization measurements, the presence of phosphate provides slightly higher pitting corrosion resistance in comparison to nitrite. Furthermore, it suggests that the corrosion inhibition mechanism of phosphate in mortars mainly depends on a dual effect occurring at the steel–mortar interface. Furthermore, it is confirmed that phosphate has little effect on the long-term mechanical properties of mortars.