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.
The potentiodynamic polarisation behaviour of a high-alloy Cr-Mn-Ni steel in its as-cast state was investigated in a 0.1 M H2SO4 solution by means of acoustic emission (AE) testing. Simultaneously, the evolution of hydrogen and oxygen gas bubbles was observed by long-distance microscopy. Three regions of acoustic activity were identified during the polarisation process from the cathodic range up to transpassivity. Bubble release at the pores and gas-assisted exfoliation of the passive oxide were identified as the AE sources. An evaluation of the gas stream revealed two types of oxygen bubbles that differed significantly in size. The effect was attributed to different points of origin at the electrode.
The corrosion behaviour of a high-alloy CrMnNi steel was investigated electrochemically in a 0.5 M sulphuric acid solution. The characteristic regions of the active, passive and transpassive state for the material were monitored by means of acoustic emissions. To this end, the steel was potentiostatically polarised and the acoustic emission signal was recorded on the specimen’s surface at a distance of 5 cm from the location of corrosion. Analysis of the acoustic emission signals revealed an increase in signal intensity due to hydrogen bubble formation when the material was cathodically polarised. Furthermore, for anodic polarisation, a continuous decline in the signal-time correlation occurred when the material was polarised in the active/passive state, whereas a sharp point of inflexion and drastic reduction in intensity was recognised when the material was polarised in the passive state. Moreover, a further increase in signal intensity was observed when the transpassive state was reached. The increase at transpassivity was related to the onset of oxygen bubble formation at the electrode.
In this study, the effect of non-metallic inclusions on the material behavior of 42CrMo4 was investigated. By adding aluminum oxide particles to steel powder, different inclusion contents were simulated. The mechanical and fracture toughness properties were measured under dynamic loading. The damage was examined using the acoustic emissions. With an increasing inclusion content, a decreasing strength, deformability and fracture toughness were observed. The distribution of the inclusions along the surfaces of the prior steel particles lead to small distances between inclusions and favors the initiation of cracks at low stress. The early appearance of material damage was proven by acoustic emissions.
The electrochemical current noise signal of a high-alloy cast steel was investigated in a 0.1 M sulfuric acid solution and in a 5 wt.% sodium chloride solution. In the sulfuric acid solution, the current time signal reveals characteristic spikes of high amplitudes. In the chloride containing solution, spontaneous power drops with a subsequent recovery of comparatively low intensity characterize the noise signal. Both noise records were analyzed by the discrete wavelet transform. For the noise signal in the sulfuric acid solution, the received wavelet coefficients exhibit the highest values in the fine scale, which signal the dominance of short-time corrosion events that were attributed to the observed hydrogen bubble evolution. In the chloride containing medium, the signal decomposition by the wavelet analysis reveals the highest coefficients predominantly in the coarse scale, indicating a preferred initiation of corrosion processes of high duration. The subsequent observations by scanning electron microscopy, reveal an attack by micro pitting, which is associated with the noise events.
The pitting corrosion behavior of a high alloy cast steel is analyzed in a sodium chloride solution by combination of the electrochemical noise technique and the acoustic emission (AE) method. Four corrosion states are identified on the steel surface after the 70 h noise measurement test. The simultaneously monitored AE exhibits two types of acoustic signals. The first type of AE events is attributed to the local passive oxide breakdown and the pit initiation process and the second type of AE events is related to the subsequent associated hydrogen gas bubble evolution.
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