Technical note: Effect of Oxide Film on Pitting Susceptibility of 304 Austenitic Stainless Steel

Abstract

Type 304 stainless steel was oxidised at various temperatures in air. The pitting susceptibility oj the oxidised and unoxidised specimens was evaluated inJerrie chloride solution. The increased pitting susceptibility oJthe oxidised specimens at about 500°C is attributed to the change oj chemical composition oj the oxide film on the surface. Table 1 Appearance· of oxide films formed at different temperatures DISCUSSIONThere are several factors which affect the pitting susceptibility of the oxide film, such as the thickness,4 the chemical composition,2 the semiconductor nature of the oxide, 1 and chromium depletion in the alloy beneath the oxide. s Isaacs and Kissel 4 found that below 300°C pitting susceptibility increases with increase in oxide film thickness. The greater pitting susceptibility of the thicker oxides was attributed to the oxide acting as a barrier, restricting the flow of chloride anions from the pit. In the same temperature range, Bianchi et al;1 found the oxygen deficient oxide film (n type) to be more susceptible to pitting corrosion than the oxide film with excess oxygen (p type) with respect to complete stoichiometry. The results of this study (Fig. 2) show that there is no significant difference in pitting susceptibility of (ii) beyond 400°C the pitting density increases with increase in oxidation temperature (iii) the pitting density reaches a maximum value of 40 pits cm -2 at 500°C; beyond 500°C it decreases with increase in temperature (iv) after reaching a minimum value of 5 pits cm -2 at 700°C, the pitting density remains almost constant with further increase in temperature up to 1100°C (v) the pitting density of specimens oxidised above 700°C is much lower than that of specimens oxidised below 400°C. The corrosion rates determined by weight loss measurements are shown in Fig. 2b and show the same trend as observed for pitting density at temperatures of up to 700°C (i.e. both the pitting density and the corrosion rate show a maximum at 500°C). The curve of corrosion rate versus oxidation temperature shows another maximum at 800°C. The corrosion rates of specimens oxidised above 700°C are much higher than those of specimens oxidised below 400°C. The specimens oxidised above 800°C show larger diameter pits than those oxidised at lower temperatures.

Electrochemical corrosion resistance of thermal oxide formed on anodized stainless steel

“…According to Table 1, anodized 304SS with thermal oxide formed at 350°C show the lowest corrosion current densities compared with anodized 304SS with passive oxide and with thermal oxide formed at the other temperatures. Whereas, anodized 304SS after thermally oxidized at 550°C and 750°C and 950°C show higher corrosion current densities, as similar change happen to those as received 304SS with thermal oxide formed by heating in air at 200–1,000 (Rastogi et al , 1994). However, anodized 304SS with thermal oxide formed at 350°C exhibited the best corrosion resistance, when compared to anodized 304SS with thermal oxide formed at the other temperatures.…”

“…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).…”

“…It is also put forward that the iron-rich oxide acts as an ion-selective membrane that absorbs chlorides but prevents these from escaping by diffusion, as resulted in initiating pits. 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).…”

Controlled optimization of Mg and Zn in Al alloys for improved corrosion resistance via uniform corrosion

“…[19][20][21][22][23] This is a direct cause of intergranular corrosion, which leads to stress corrosion cracking. [24][25][26][27][28][29] When Zn is added to this Al-Mg alloy, another secondary phase called the t-(Mg 32 (Al, Zn) 49 ) phase is formed. 23,[30][31][32] In addition, an intermetallic compound called Al 3 Fe causes galvanic corrosion due to the potential difference with the Al matrix.…”

Pitting Corrosion of Some Stainless Steel Alloys Preoxidized at Different Conditions

“…However, Kearns [2] attributed the higher pitting susceptibility of oxides formed at higher temperatures to chromium depletion in the alloy substrate beneath the oxide. Rastogi et al [3] attributed the increased pitting susceptibility of oxidized stainless steel specimens at 500 o C to the change of chemical composition of the oxide film i.e. lowering of Cr/Fe concentration ratio.…”