The Corrosion Behavior of High-Alloy CrMnNi Steels—A Research Work on Electrochemical Degradation in Salt- and Acid-Containing Environments
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The electrochemical corrosion behavior of high-alloy cast steels as well as of steel/ceramic composites was analyzed by conventional polarization, electrochemical impedance and long term-outdoor exposure tests in sulfuric acid and chloride containing solutions. Additionally, for the determination of corrosion initiation processes and its subsequent implications, potentiodynamic polarization and electrochemical noise were combined with the acoustic emission technique. The results of these studies show that several corrosion mechanisms occur simultaneously when the surface is potentiodynamically stressed, and that pitting corrosion starts with a specific time-shift to its initiation at the surface.
KeywordsCyclic polarization Impedance spectroscopy Outdoor exposure Acoustic emission Electrochemical noise
A wide range of techniques and analysis tools have been developed for studying the electrochemical corrosion behavior of high-alloy steels, with such techniques providing detailed information about the specific characteristics of steels in defined environments. The subsequent chapter explains how the electrochemical corrosion of high-alloy TRIP/TWIP steels with zirconium dioxide particle reinforcement was analyzed by a number of methods, such as conventional cyclic polarization, electrochemical impedance, indoor and outdoor exposure tests and special analytical tools, such as the combination of linear polarization and electrochemical noise measurement, as well as the acoustic emission method. For the presented results, the corrosion behavior of the analyzed steel and composite materials in salt- and/or acid-containing media was dictated primarily by the main alloying elements chromium, manganese and nickel as well as by the zirconia particle reinforcement. Due to the pronounced passivity, pitting and uniform corrosion were observed, which are the predominant corrosion mechanisms and the focus for the subsequent interpretation of data in the relevant media.
17.2 The Effect of Transformation-Induced Plasticity (the TRIP Effect) on the Electrochemical Degradation of a High-Alloy CrMnNi Steel
Due to the large number of their excellent mechanical properties, their high corrosion resistance and acceptable cost-benefit ratios, high-alloy stainless steels are commonly used in a wide range of applications. As a certain type of this material class, high-alloy TRIP steels offer further enhancement of the mechanical characteristics due to the transformation induced plasticity effect, which arises from a microstructural phase transformation of the metastable austenite phase into a martensite phase during an energy-absorbing process [1, 2, 3]. This characteristic combined with good corrosion durability facilitates their integration in the technical configuration for safety-relevant components by reducing costs for subsequent corrosion-protection measures.
With the focus on the material’s corrosion behavior, a particular advantage of high-alloy steels is their good corrosion resistance and durability in most aggressive environments. This corrosion resistance is predominantly due to the formation of a chromium-enriched passive oxide layer that restrains the metal-dissolution process during corrosion attack to a technically acceptable level. A further improvement is achieved by the addition of nickel and manganese. Nickel tends to agglomerate as an interlayer at the metal/oxide interface in its metallic state and can also affect the dissolution rate of the entire steel component, whereas manganese increases the solubility of nitrogen and molybdenum, elements that significantly enhance the pitting-corrosion durability of steels .
Chemical composition of high-alloy CastX6CrMnNi16-7-3 TRIP steel in wt%
The anodic polarization behavior clearly exhibited a strong dependency on the state of deformation. The initial state revealed the expected passivation and the initiation of metastable pitting corrosion when the anodic potential increased. For ε = 9%, the corrosion potential ECorr,1 increased significantly, but the range of passivity was reduced. It was assumed that this effect could be attributed to the homogeneously formed and distributed micro-anodes inside the large austenite grains (Fig. 17.1b). For the highest analyzed degree of deformation of ε = 18%, the corrosion potential dropped again to the level of the initial state, and no significant passivation or pitting behavior was detected. In comparison to the initial state and ε = 9%, the highest degree of deformation also resulted in the highest degree of corrosion sensitivity during anodic polarization. In comparison, the reverse scan did not indicate any clear difference in polarization behavior, with all deformation states exhibiting similar curve trends and secondary corrosion potentials ECorr,2. This characteristic clearly shows that after its initiation, the pitting corrosion attack—which was found for each state after the cyclic polarization (Fig. 17.3)—dictates the ongoing corrosion behavior of the whole steel sample and is independent of the degree of deformation.
17.3 Influence of Particle Reinforcement on the Corrosion Behavior of a High-Alloy Steel in Sodium Chloride Solution
Chemical composition of the high-alloy X5CrMnNi14-6-6 steel in wt%
Chemical composition of the Mg-PSZ powder in wt%
The polarization curves of the material without (0 vol%) and with 5 vol% particle content showed slight passivation, which was more pronounced for the unreinforced material than for the composite. In comparison, the composite material with 10 vol% Mg-PSZ exhibited the lowest corrosion potential and no significant passivation behavior, which indicated a higher degree of corrosion sensitivity.
The comparison clearly shows that significant increases in the contents of iron, chromium, nickel and manganese are detectable in the solution when the Mg-PSZ content is increased up to 10 vol%. The electrolyte analysis confirms that the passivation behavior of the steel matrix is drastically reduced when the particle content increases and the entire composite is more susceptible to an aggressive medium.
In a further study, the corrosion behavior of the unreinforced material (0 vol%) and the composite with 10 vol% Mg-PSZ particle content was investigated by a one-year outdoor exposure test and compared to the results obtained after potentiodynamic polarization in a 5 wt% sodium chloride solution . For the outdoor exposure test, the specimens were stored in an outdoor exposure test rig in Dresden, Germany and sprayed weekly with a 3 wt% sodium chloride solution.
After polarization, the material without particle reinforcement was attacked by pitting corrosion (I in Fig. 17.8a), whereas the particle-reinforced composite exhibited a uniform corrosion attack and an attack at the particle/matrix interface (II in Fig. 17.8b), which was accompanied by intensified particle detachment (III in Fig. 17.8b). Similar behavior was observed after one year of outdoor exposure testing. Pits were identified on the surface of the unreinforced material (Fig. 17.8c), while a large number of small cavities were found on the composite material that had resulted from the detachment of Mg-PSZ particles (Fig. 17.8d). In addition to these localized corrosion attacks, the intensified formation of red rust was observed. This behavior was more pronounced for the composite material and also confirmed the higher corrosion sensitivity and lower passivation ability that was identified during the polarization test. The results of the study show that under similar conditions, the results of the short-time polarization test can be used to approximate the corrosion behavior associated with long-term outdoor exposure tests.
17.4 Electrochemical Corrosion of the Particle-Reinforced High-Alloy Steel at Different Temperatures
Impedance curve fitting results before polarisation in 5 wt% NaCl solution on 10 vol% Mg-PSZ particle-reinforced steel
Re (Ω cm2)
Q (µΩ−1 cm−2 sα)
Rt,bp (kΩ cm2)
Impedance curve fitting results after polarisation in 5 wt% NaCl solution on 10 vol% Mg-PSZ particle-reinforced steel. T in °C, Re, Rt,ap in Ω cm2 and Rf in kΩ cm2, Q1, Q2 in µΩ−1 cm−2 sα
The correlation revealed a slight increase in current density up to a temperature of 34 °C, including a peak between 32 and 33 °C. This characteristic was related to particle detachment. When the temperature increased further, the current density increased significantly and exhibited fluctuations up to a temperature of 38 °C—which was related to the intensified particle detachment that was evident. For higher temperatures, the current density increased drastically and indicated pitting corrosion attack. Consequently, the critical temperature for intensified particle detachment and subsequent pitting corrosion attack on the composite was approximately 34 °C.
17.5 Potentiodynamic Polarization of CastX5CrMnNi16-7-9 in Sulfuric Acid Solution Combined with Acoustic Emission Analysis
Chemical composition of the high-alloy cast steel CastX5CrMnNi16-7-9 in wt%
Cluster C1 was characterized by a high spectral energy and low signal duration and corresponded to hydrogen bubble evolution. In cluster C2, the signals were typified by lower spectral energies and higher durations, and correlated to the hydrogen-assisted exfoliation process.
Moreover, when the potential was increased further up to E ~ 1.45 V versus Hg/Hg2SO4, the AE signals with a peak frequency of f > 100 kHz ended abruptly, which corresponded well with the disappearance of Type 1 oxygen bubbles in the gas stream. Up to the end of the polarization process, only Type 2 oxygen gas bubbles were identified in the gas stream (Fig. 17.23c).
Consequently, the rate at which oxygen bubbles was released at the pores had to decrease when the passive oxide dissolution increased. This behavior was in full accordance with the observed decline in the release of Type 1 oxygen bubbles and the identified potential limitation of AE events with frequencies of f > 100 kHz (Fig. 17.19d).
17.6 Analysis of Pit Initiation on CastX3CrMnNi16-7-9 by the Combination of Electrochemical Noise and Acoustic Emission Measurement
The electrochemical noise (EN) and acoustic emission (AE) methods are highly suitable monitoring and analysis tools for the investigation of corrosion initiation processes under steady state conditions [9, 10, 11, 12, 13, 14, 15, 16, 17]. The EN technique is based on the measurement of potential and/or current fluctuations, which facilitates the determination of the activity of a corrosion system. The AE technique—based on the measurement of elastic waves—is useful in identifying different corrosion mechanisms occurring at the same time on a surface . One outstanding characteristic is that both methods are non-destructive and highly sensitive, which facilitates the identification of corrosion initiation processes.
17.7 Analysis of Electrochemical Noise by Continuous Wavelet Transform
Chemical composition of the CastX15CrMnNiN19-3-4 in wt%
From the micrographs, it was obvious that the corrosion attack in the unmixed acid and the combined electrolyte was nearly identical. This behavior clearly indicated that the acid dominated the corrosion attack in the combined solution, and that the sodium chloride seemed to have little or no effect on the corrosion attack. When considering the CWT noise analysis, however, the influence of the salt became clear. The maxima in the low-frequency range (Fig. 17.32b)—indicating long-term corrosion processes—indicated a longer-lasting intergranular corrosion attack. Consequently, it was assumed that in the unmixed acid solution, the corrosion attack was characterized by spontaneous active dissolution of the grain boundaries, accompanied by spontaneous repassivation. In the combined electrolyte, the attack also occurred spontaneously, but the repassivation process was drastically hindered, which was predominantly attributed to the Cl− ions.
The study shows the advantage of utilizing electrochemical noise analysis, which facilitated the identification and isolation of short-time and long-term corrosion events simultaneously. In comparison, the effect was not identifiable by SEM observation after a two-hour exposure test.
In this chapter, several studies of the electrochemical corrosion behavior of high-alloy cast steels and steel/ceramic composite materials in sulfuric acid and chloride containing aqueous solutions are presented and discussed. For the analysis numerous polarization techniques, outdoor exposure tests and combined methods were carried out. The analyzed characteristics in dependence of applied potential, frequency and temperature give specific information about the influence of the TRIP effect to the corrosion characteristics, were used to design and describe the composite/electrolyte interphase and let extract specific parameters for the evaluation of the pitting corrosion sensitivity. Finally, the combination of polarization, electrochemical noise and acoustic emission was used to estimate and separate simultaneous occurring corrosion mechanisms, and more precisely, to identify the time-dependence of specific initiation phenomena and corresponding corrosions damage evolution processes.
The authors greatly acknowledge the financial support of this works by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) as part of the Collaboration Research Centre 799 “TRIP-Matrix Composites” project—Projektnummer 54473466—SFB 799, subproject B6.
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