Metal release from stainless steels and the pure metals in different media

Detta är en avhandling från Stockholm : KTH Royal Institute of Technology

Sammanfattning: This study has been triggered by the fact that stainless steel is being increasingly used in new applications, where possible environmental effects may be a matter of concern. When stainless steel is exposed to a given environment, a key issue is the release of small amounts of the main alloying elements iron, chromium, nickel and molybdenum. Published release rate data of these elements turned out to be sparse. Furthermore, only little was known about the role of different parameters that may affect the release rate, such as degree of alloying, exposure time and surface finish. Hence, the aim of this study was to develop methodological means and to provide accurate metal release rates of alloying constituents from different grades of stainless steels- austenitic, ferritic and duplex- when exposed to selected environments: artificial rain and synthetic body fluids. The results and discussion have been summarised in this thesis by formulating and answering ten questions, all believed to be crucial for the understanding of possible environmental effects of stainless steels.Some common conclusions could be drawn, independent of stainless steel grade and exposure condition. Iron was always preferentially released, and the release rates of chromium, nickel and molybdenum (when measured) were significantly lower than of iron, also when considering the bulk proportion of these elements. The release rate of all elements was initially high and decreased with exposure time, mainly because of an observed enrichment of chromium in the passive film formed.The release rates of iron (2 μgcm-2week-1) and nickel (0.08 μgcm-2week-1) from stainless steel from grades 304 and 316 exposed to artificial rain were much lower than corresponding rates for the pure metals (750 μgcm-2week-1 released Fe and 15 μgcm-2week-1 released Ni), whereas chromium exhibited similar release rates from stainless steel and the pure metal (0.1 μgcm-2week-1). This implies that the common procedure to calculate release rates, based on the pure metals and the nominal steel composition, significantly overestimates release rates of iron and nickel from stainless steel, but not of chromium.Total release rates from seven stainless steel grades in synthetic body fluid were found to decrease with increasing alloy content in the following release rate order: grade 409 >> grade 430 > grades 316L ≈ 201 ≈ 2205 ≈ 304 > grade 310. The release rate was highly sensitive to pH of the synthetic body fluid but only slightly sensitive to stainless steel surface finish.

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