Differences In The Characteristic Of The Rust Layers On Carbon Steel And Their Corrosion Behavior In An Acidic Medium:Limiting Factors For Cleaner Pickling

With the development of modern industry, the demand of steel products is increasing. Due to the active nature of steel surface, the formation of the rust layer is inevitable under natural state. However, the corrosion of steel caused huge waste of resource and adversely effect on the performance and service lifetime for the steel materials, which has been causing public attention. In order to enhance the performance of steel products, surface treatment and protection should be executed. However, before executing, rust layer should be cleaned out. Acid pickling is one of the common methods in rust removing. However, due to the corrosive characteristic of the pickling liquors, the phenomenon of over-pickling is inevitable, which would cause the waste of metallic matrix and generate pollution. Therefore, analyzing the limiting factor for enhancing the cleaner level of acid pickling and choosing the suitable pickling strategies for various steel samples would be favored for controlling the secondary pollution.In this study, the natural-corroded carbon steel specimens and the carbon steel specimens after high temperature oxidation treatment were prepared under the natural state and the high temperature state. By comparing the differences in the characteristics of rust layers on the prepared specimens. X-ray diffraction (XRD), scanning electron microscopy (SEM) and laser scanning confocal fluorescence microscope were used. Moreover, electrochemical polarization was used to research into the differences for corrosion behaviors for natural-corroded carbon steel specimens, carbon steel specimens after high temperature oxidation and normal carbon steel specimens before and after the addition of inhibitors. After combination of the electrochemical polarization results and the characteristic of different rust layers, the limiting factor for enhancing the cleaner level of acid pickling and the choosing the suitable pickling strategies for different kinds of carbon steel was discussed.Results indicated that the formation process of rust layer heavily influenced their composition, structure, and the interface roughness between the rust layer and the metal matrix, and also influenced the corrosion behavior of samples, which was the key factor limiting the cleaner production level of a pickling process. The rougher the surface between the rust layer and the metal matrix was, the more serious was the "over-pickling". Once the phenomenon of "over-pickling" was effectively controlled, the cleaner level of the pickling process could be enhanced.High concentration of pickling liquors can accelerate the pickling process, while high concentrations of inhibitor can reduce over-pickling.So, for maintaining a suitable pickling speed and at the same time to avoid the over-pickling phenomenon, an optimal pickling formula should be adopted on the basis of the characteristics of rust layers. Therefore, for carbon steel after high temperature oxidation treatment, low-concentration of pickling liquor and high-concentration of inhibitor are favored, and for natural-corroded carbon steel, high-concentration and low-concentration of inhibitor is more suitable.