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We use the dynamic speckle pattern method for the investigation of pitting corrosion. Pitting corrosion is a major failure mechanism in metals caused by the depassivation of a small area. The sample is Custom 450 stainless steel in 3.5 wt% NaCl solution and a potential of 350 mV SCE is applied to onset the corrosion. The analysis of the speckle patterns resulting from the laser scattering from the sample results in the quantitate evaluation of pitting during the experiment. The analysis shows that the growth rate of pitting in longer times decreases. This method for corrosion evaluation is low-cost, in situ, and easy to implement, and it is especially useful when the samples are difficult to reach for microscopic studies.

Corrosive factors along with mechanical loads on the gas turbine compressor blades, cause phenomena such as pitting corrosion, stress corrosion cracking and corrosion fatigue. Due to erosion of particles in the presence of a corrosive environment, pitting happens on the blade surfaces, which is a source of subsequent cracks. Therefore, it is necessary to get knowledge of its mechanism in order to prevent the phenomena as much as possible. The main purpose of this paper is to investigate the growth of pitting corrosion in CUSTOM 450 stainless steel and to obtain strain values in the growing pits at the maximum bending region. In this regard, a two-point bending specimen was made and subjected to a potentio-static test under the potential of 350 mVSCE in the 3.5 wt% sodium chloride solution. Then the propagated pits were numerically examined. By the digital image correlation method, the local strain was calculated in the pits and a relation was presented to obtain the maximum strain time. Therefore, growth direction of pitting corrosion could be estimated by having maximum strain region. Finally, by simulating the pitting corrosion process of a stress-free sample under the potential of 350 mVSCE in 3.5 wt% sodium chloride solution in COMSOL Multiphysics software, variations in the concentration of ions, electric potential, and corrosion current density were shown in the existing pit. The potential was decreased by moving in-depth and the maximum current density was found at the depth of 18 μm. Thus, without the need of advanced laboratory facilities for surface scanning and analysis, useful information from surface corrosion conditions could be obtained

A key application of CUSTOM 450 alloy is in the construction of gas turbine compressor blades. The study of pitting corrosion can prevent the failure of many gas turbine compressor blades. In this study, a reflective digital holography microscopy method was employed to investigate the growth of pitting corrosion in depth. To this end, a constant potential of 350 mVSCE in a 3.5 wt.% NaCl solution was applied to the specimen. The generated pits were simulated in three dimensions, and it was indicated that pitting corrosion rate was decreased as time passed. Comparing the obtained experimental data with the data gathered from the real industrial environment surrounding a compressor installation, an accelerated test was proposed. By the proposed accelerated test, it is possible to produce a pit similar to the one that will be initiated and propagated at any time in the future in real conditions.

In this study, the electrochemical polarization tests were performed on tensioned and non-tensioned CUSTOM 450 specimens in a 3.5 wt% NaCl solution to investigate pitting potential and stable pit initiation time. A potentiodynamic test was conducted to determine the exact amount of pitting potentials. According to the potentiostatic tests, a relation between applied potential and the stable pit initiation time was obtained. Concerning this relation, stable pitting time can be predicted without experimental works. Optical microscopy was used to evaluate the shape of the pits. Tensile stress led the pit to experience the “pit to crack” step. The corrosion rate of samples was studied by the determination of mass loss. Mass loss measurements and current density–time curve in potentiostatic tests demonstrated the rate of pitting corrosion decreased as time passed. Finally, the depth of the pits was measured by the eddy current technique. The results showed that tensile stress facilitated deeper pit development.

In this study, pitting corrosion was investigated numerically and experimentally under actual operational conditions of a gas turbine compressor. First, the samples cut from a blade made of Custom 450 stainless steel were exposed to different corrosion times in a 3.5 wt% NaCl solution. After the creation of pits in the samples, the dimensions of the corrosion pits were measured using a 3D profilometer device. The stoichiometry equations governing the corrosion of the Custom 450 alloy in a chloride solution were extracted and applied to the models. The results of this simulation were validated by comparing the depth of the simulated pits and the experimental samples. Subsequently, simulation was conducted for 48 months on a compressor having pits. There is a good agreement between the simulation results and the results obtained from the Turbo machine laboratory of the Texas A&M University. In addition, pits grew under the influence of corrosion, joined together, and made an equivalent pit.

Since compressors have components rotating at high speed, the most common cause of failure in compressors is their high cycle fatigue. In the present study, one of the rotating blades on the first row of an axial flow compressor was prematurely fractured. The crack position and direction were determined, using nondestructive tests. The results indicated that fatigue cracking initiated at two corrosion pits on the surface of the pressure side of the blade in response to corrosive compounds in the site atmosphere. The total life of the blade, from corrosion pit initiation to unstable fatigue crack growth, was calculated. The blade life had three stages: pit initiation and pit growth, pit transition to initial crack and stable crack growth reaching a critical value where the final fracture occurred. Pit growth and its transition to initial cracking were described. The time an equivalent pit requires to reach initial fatigue cracking was subsequently calculated. The number of cycles was calculated via two methods until initial cracking reached unstable growth. The first calculation was performed, using ZENCRACK fracture mechanics code. These results demonstrated that the rate of fatigue crack growth was at first slow and stable. After a period of time, the stress intensity factor reached a critical value where the crack began to propagate unstably. A MATLAB code was written based on Paris model as the second method for estimating fatigue life. The results obtained from drawing a comparison between these methods revealed that ZENCRACK crack shape simulation was in agreement with the fatigue beach marks.