Abstract
The electrochemical corrosion behavior of laser welded 2205 duplex stainless-steel in artificial seawater environment (3.5% NaCl solutions) with different acidity and alkalinity conditions (different pH values) was investigated using different techniques. Namely, capacitance measurements (Mott–Schottky approach), electrochemical impedance spectroscopy and potentiodynamic polarization measurements. The formation of pitting corrosion on the exposure surfaces of the tested duplex stainless-steel samples was investigated and confirmed by characterizing the surface morphology using field emission scanning electron microscope (FE-SEM). Based on the obtained results, a proportional relation has been found between pH value of the solution medium and the generated film resistance due to the processes of charge transfer, which directly affecting the pitting formation and its specifications. Since the film layer composition created on the duplex stainless-steel surface is changes depending on the pH value, it was found that different bilayer structure type was generated according to the acidity or alkalinity level. The presented bilayer is almost composed from metal oxides, such as iron oxide and chromium oxide, as confirmed by Raman Spectroscopy technique. As the pits size and its quantity increased with decreasing pH value, it can be concluded that the corrosion resistance property of the laser welded 2205 duplex stainless-steel sample is improved on the alkalinity direction of the solution. Vice versa, higher acidic solution has more ability for corrosion.
Highlights
The unique chemical composition and propitious microstructure of duplex stainlesssteel (DSS) made it deservedly nominated to take the lead of the best commercial highstrength materials due to its corrosion resistance and high strength combination
That is because the rich hydrogen ions which accelerating corrosion in low pH medium by attacking and damaging the steel upper surface layer and increasing the total weight losses
Wherever the protective oxide layer got destroyed on some surface portions, the chloride ions slowly penetrate to that unprotective steel surface and corrosion starts
Summary
The unique chemical composition and propitious microstructure of duplex stainlesssteel (DSS) made it deservedly nominated to take the lead of the best commercial highstrength materials due to its corrosion resistance and high strength combination. In some DSS grades after using the laser-welding process, and due to both rapid cooling rate and high heat input, the percentage of ferrite phase in the solidified structures is very close to 100% as reported by some researchers [8–10]. Alkaline materials (e.g., carbonates) can combine with calcium and magnesium from the environment to form scale at high pH, which may help to protect stainless-steel from general corrosion [27]. It is not clear how duplex stainless-steel, with its high chromium content and passive film, will be affected at high pH exactly. To observe the changes in surface morphology scanning electron microscopy (SEM) was used, while the Raman spectroscopy was utilized to study the phases of oxide growth at different intervals on the surface of duplex steel
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