مقاومت در برابر خوردگی میله های فولادی
Abstract
1-Introduction
2-Material and methods
3-Results and discussion
4-Conclusions
Acknowledgements
References
Abstract
Providing high hardness, low friction coefficient, as well as, relatively good corrosion resistance, chromium-plated coatings are widely used for steel cylinder rods in marine environment. Nevertheless, a uniform network of microcracks in chromium coating is evolving under mechanical loadings during the service-life of cylinder rods. The propagation of these microcracks is in the origin of the premature corrosion of the steel substrate. The aim of the study was to evaluate the relationship between mechanical stresses, the evolution of the microcracks network and the corrosion resistance of chromium coatings. After monotonic pre-loading tests, it was demonstrated by microscopic observations that the microcracks propagated for stress levels higher than the yield stress of the substrate (520 MPa) and have passed instantly through the whole thickness of the coating and reached the steel substrate. The density of microcracks increases with the level of total strain, the inter-crack distance go from 80 µm at 1% of total strain to approximately 65 µm at 5%. Electrochemical measurements have shown that the higher the level of plastic strain applied during the mechanical loading, the more the corrosion potential of the sample decreases until reaching that of the steel substrate of approximately -0.65 V/ECS after 2 hours of immersion. The polarization curves also evidenced an increase in the corrosion current density with the strain level. Moreover, we note the absence of the characteristic passive region of the reference samples that have not undergone any loading. After cyclic loadings, no microcracks propagation was observed after 104 cycles when maximal stress was lower than the yield stress. However, a decreasing of the corrosion potential was observed for samples which were submitted to a cyclic loading. Nevertheless, the current density and the characteristic passive region were not modified.
Introduction
Since 1924 [1], chromium-electroplating process is a well-established practice in industrial needs, such as aerospace, automotive and general engineering [2]. This fact is due to a combination of properties offered to steel by a chromium coating, such as, high hardness, low coefficient of friction and corrosion resistance [3]. These properties depend highly of electroplating parameters: temperature of plating solution, plating current density, concentrations of chemical compounds in plating solution and duration of process [3-4]. Nevertheless, after many decades of practice, some aspects of this process are still not fully understood. One of them is an appearance and evolution of microcracks network during electroplating process [3-5]. The origin of microcracks initiation is related to the residual tensile stresses, when the thickness of chromium coating reaches 0.5 µm [6]. The first explanation of residual tensile stresses is due to the release of trapped atoms of hydrogen during electrolysis, with the following shrinkage of chromium coating [4]. In [3], authors highlight that these trapped atoms of hydrogen could play a role of catalyst to accelerate the chromium phase transformation. Therefore, β-Cr with HCP or FCC crystal arrangement will be transformed in more stable α-Cr (BCC) with shrinkage of 15 % vol. [5]. It has been proved that the presence of these microcracks is favorable for penetration of corrosion agents, such as chlorides [7- 9].