Morphological changes of deformed structural steel surface in corrosive environment

Introduction. Internal factors, including phase heterogeneity, crystallographic texture, residual stress amplitude and the concentration of non-metallic inclusions, exert a nonlinear, multi-parametric effect on the corrosion resistance of metallic systems in aggressive environments. This complex interaction significantly complicates the prediction of corrosion degradation kinetics and the assessment of the operational life of metal structures. These parameters modulate the spatial distribution of corrosion defects, their morphology and penetration depth, necessitating a systematic approach to establish quantitative correlations. To gain a more accurate understanding and account for the influence of internal factors on the corrosion process, comprehensive research and analysis are required. The use of mathematical processing methods in the analysis of influence will reveal stronger regularities applicable to the process of corrosion damage. This will enable the development of methods and approaches for optimizing the design, production and operation of metal structures and products, as well as improving their reliability and durability. Purpose of work is to develop a multi-criteria model linking the depth of corrosion damage (an integral indicator of environmental aggressiveness) with microstructural, mechanical, and topographic characteristics of low-carbon steel St3. The objects of the study are samples from rolled sheet metal with varying degrees of residual plastic deformation (ε = 0–7%). Methods of investigation. Grain size, texture, and dislocation density were assessed through microstructural analysis using optical microscopy (Olympus GX53) and scanning electron microscopy (JEOL 6008A). Quantitative morphometry of corrosion damage was performed using digital image analysis (AXALIT software), with median depth determined as a key parameter. X-ray diffraction analysis of residual stresses was implemented to construct tensor stress fields. Results and discussion. Experimental data demonstrates a non-linear increase of the median depth of corrosion damage with the degree of deformation: at ε = 6.6%, a twofold increase in the median depth is observed compared to the undeformed state. Multivariate regression analysis revealed the dominant influence of internal residual stresses on the kinetics of the corrosion damage process (R² = 0.89). The scatter of the determined values for internal stresses is ±5 μm. The observed regularities are associated with the behavior of the material structure during plastic deformation, which occurs most significantly in the {111} <110> directions, leading to the generation of reverse residual stresses. The median depth of corrosion damage reflects the rate of corrosion. The group method of data handling (GMDH) allowed for the synthesis a complex parameter combining various parameters of steel structure. Polynomial approximation of the dependence of the median depth of corrosion damage in 5% HCl on the complex parameter shows high convergence (R² = 0.99) with a determination error of ±1 μm. The developed model confirms that residual stresses are one of the key factors modulating the corrosion activity of deformed St3 steel. The results obtained allow for the optimization of cold treatment of steel to increase the corrosion resistance of metal structures. Further studies are planned to focus on the influence of dynamic loads and temperature gradients on the evolution of dislocation substructures.