Self-organized honeycomb-like nanoporous oxide layer for corrosion protection of type 304 stainless steel in an artificial seawater medium

Improvement of Corrosion Resistance of Stainless Steel Welded Joint Using a Nanostructured Oxide Layer

“…EDX measurement ( Figure 5 b) shows that heat treatment causes the F content to decrease to a negligible amount, while steeply increasing the oxygen content. Furthermore, the heat treatment process generated phases [ 42 ] other than austenite or δ-ferrite in the nanoporous oxide layer that was largely amorphous prior to the treatment. XRD measurements ( Figure 3 c) show that the new phases that formed were hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ); of the two, the latter was much more prevalent.…”

“…These features are generally observed in metals coated with a passive film [ 48 , 49 ]. At low frequencies, the two plots show small discrepancies, possibly resulting from impurities at the surface [ 42 ]. The Niquist plots ( Figure 7 a) show a larger radius of depressed capacitive semicircle for the heat-treated anodized welded SS than for the plain SS weld, indicating higher impedance of the former.…”

Electrochemical corrosion resistance of thermal oxide formed on anodized stainless steel

“…The colors of thermal oxide change with the increase of temperature afterwards, which indicates the more iron oxide producing in thermal oxide (Murugan et al , 2001) due to the relative higher diffusion rate of iron through the oxide layer than that of chromium, finally forming an iron rich-oxide layer (Trigwell and Selvaduray, 2005). The uppermost oxide film below 700°C consists primarily of iron-rich oxide, nevertheless of chromium-rich oxide above 700°C (Rastogi et al , 1994; Saha et al , 2019). The iron-rich oxide is considered as most harmful for corrosion resistance of SS in the solution containing chlorides, and chromium-rich oxide is more resistant to corrosion attack (Moltke et al , 1992).…”

“…What extent thermal oxides affect is related closely to the oxide composition, thickness, structure, homogeneity and chromium distribution (Turner and Robinson, 1989; Rastogi et al , 1994). Saha et al found stable and compact thermal oxide with a thickness of ∼235 nm forms beneath the nanoporous passive oxide layer after thermally treated in air at 500°C, and crystalline magnetite phase appeared in thermal oxide film (Saha et al , 2019). Under this condition, thermally treated nanoporous oxide layer improved corrosion resistance compared to the nature passive oxide.…”

“…Unlike electropolishing only to removing the layer formed on the metal surface during mechanical treatment (Babilas et al , 2016), anodization could thicken the natural oxide layer on the metal surface (Wang et al , 2020a, 2020b) and change tremendously the chemical and structural properties of the resulting surface (Babilas et al ,2016; Wang et al , 2020a, 2020b). Moreover, the chromium-rich passive film formed by anodization of SS behaves a stable oxide layer and excellent adhesion between the oxide layer and the substrate, which mitigates effectively the penetration of corrosive ions toward the surface atoms (Saha et al , 2019). Whereas, there is no report on thermal oxides formed on anodized SS, accordingly it is unknown what the special characters of thermal oxide is and whether thermal oxides have influence on corrosion resistance and pitting corrosion of anodized SS.…”