The influence of metallurgic factors on the crevice corrosion resistance of ferritic stainless steels
Aronen, Jenni (2015)
2015
Master's Degree Programme in Materials Science
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2015-09-09
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201508191524
https://urn.fi/URN:NBN:fi:tty-201508191524
Tiivistelmä
Due to their passive oxide layer, stainless steels are very resistant to uniform corrosion. In many environments, however, they tend to suffer from localised corrosion, like pitting and crevice corrosion. These corrosion types occur in aqueous solutions, especially when chloride ions are present. Testing of crevice corrosion is more demanding than testing of pitting due to the challenges in creating reproducible crevices. Yet testing of crevice corrosion is important, because it tends to be more harmful than pitting as the conditions in crevices become easily more aggressive than the conditions on the plain metal surface where pitting takes place.
Austenitic stainless steels are often considered to be more resistant to localised corrosion than ferritic stainless steels. However, the price of ferritic stainless steels is lower and more stable as they contain no nickel alloying. Thus the interest in ferritic stainless steel grades has been increasing during the past years.
The aim of the present study was to examine how metallurgic factors influence the crevice corrosion resistance of ferritic stainless steels. The study includes a literature survey and experimental testing. The test materials contained commercially produced stainless steels and laboratory melts. Two commercial austenitic grades served as reference materials. The laboratory tests consisted of immersion tests and electrochemical measurements.
Vanadium was observed to be the most beneficial alloying element to increase the crevice corrosion resistance of the test materials. A reduction of up to 80 % in the corrosion rate was noticed. Hence vanadium was suggested to be a possible replacement for molybdenum when improved crevice corrosion resistance is desired. Chromium and molybdenum were found to improve the crevice corrosion resistance to some extent. Both elements increased the resistance against crevice corrosion initiation and molybdenum also decreased the corrosion rate. Therefore pitting resistance equivalent number (PREN), which has been established to enable prediction of a stainless steel’s resistance to pitting, can be used to estimate a material’s resistance to crevice corrosion as well. PREN did not, however, correlate with the corrosion rate and should therefore not be applied to predict it. Copper alloying did not significantly alter the crevice corrosion resistance of ferritic stainless steels. Grain boundary etching was observed in the attacked areas of all titanium, niobium or vanadium containing ferritic test materials. No clear reason for this phenomenon could be identified so far.
Austenitic stainless steels are often considered to be more resistant to localised corrosion than ferritic stainless steels. However, the price of ferritic stainless steels is lower and more stable as they contain no nickel alloying. Thus the interest in ferritic stainless steel grades has been increasing during the past years.
The aim of the present study was to examine how metallurgic factors influence the crevice corrosion resistance of ferritic stainless steels. The study includes a literature survey and experimental testing. The test materials contained commercially produced stainless steels and laboratory melts. Two commercial austenitic grades served as reference materials. The laboratory tests consisted of immersion tests and electrochemical measurements.
Vanadium was observed to be the most beneficial alloying element to increase the crevice corrosion resistance of the test materials. A reduction of up to 80 % in the corrosion rate was noticed. Hence vanadium was suggested to be a possible replacement for molybdenum when improved crevice corrosion resistance is desired. Chromium and molybdenum were found to improve the crevice corrosion resistance to some extent. Both elements increased the resistance against crevice corrosion initiation and molybdenum also decreased the corrosion rate. Therefore pitting resistance equivalent number (PREN), which has been established to enable prediction of a stainless steel’s resistance to pitting, can be used to estimate a material’s resistance to crevice corrosion as well. PREN did not, however, correlate with the corrosion rate and should therefore not be applied to predict it. Copper alloying did not significantly alter the crevice corrosion resistance of ferritic stainless steels. Grain boundary etching was observed in the attacked areas of all titanium, niobium or vanadium containing ferritic test materials. No clear reason for this phenomenon could be identified so far.