Stress Corrosion Cracking on Stainless Steel AISI 304 in MgCl2 Environmet
"This Research of Master degree Thesis was conducted in Research Center for Metallurgy- Indonesian Institute of Sciences from February to May 2008. I would to thank to Prof. DR. Johny Wahyuadi.DEA at University of Indonesia as a supervisor" and also my colleagues in order to succeed my research technically"
ABSTRAK
Baja tahan karat jenis austenitik AISI 304 mempunyai kerentanan terhadap korosi retak tegang di dalam larutan korosif klorida. Baja tipe ini juga rentan terhadap temperatur sensitasi antara 5800C – 8150C. Kerentanan tersebut jelas terjadi pada korosi batas butir. Batas butir mengandung krom karbida. Kombinasi antara internal material logam dan lingkungan memberikan efek korosi retak tegang. Lingkungan MgCl2 merupakan lingkungan korosif yang berperan dalam jenis korosi ini.
Pengujian korosi retak tegang dilakukan dengan metode beban konstan (creep) melalui beban 20 kg/mm2, 25 kg/mm2, 30,5 Kg/mm2, dan 40 Kg/mm2 di larutan 42wt% MgCl2 bersuhu 1060C. Perlakuan material dibagi dua yaitu anil 11000C, tahan 1 jam, kemudian celup cepat air dan tanpa anil. Kedua perlakuan tersebut disensitasi (6000C,7000C,8000C). Pengujian kualitatif karbida, pengujian komposisi bulk, larutan uji (AAS), pengujian kekerasan Vickers, metalografi (foto makro) dan pengujian SEM EDS dilakukan.
Hasil menunjukkan pengujian kekerasan vickers pada suhu sensitasi 7000C mengalami penurunan berkisar 152,06 Hv (anil 11000C) dan 199,1 Hv (non anil 11000C) dibandingkan suhu sensitasi 6000C dan 8000C. Tren sama juga terjadi pada pengujian SCC beban konstan, pada temperatur sensitasi 7000C, waktu patah (tf) lebih pendek dibandingkan suhu sensitasi 6000C & 8000C di dua kondisi material berbeda. Waktu patah tercepat pada beban 25 Kg/mm2 3 detik di kondisi anil 11000C,suhu sensitasi 7000C dan terlama pada beban 30,5 Kg/mm2 86400 detik di kondisi tanpa anil,suhu sensitasi 6000C. Laju pemuluran (iss) tertinggi pada beban 25 kg/mm2 4,80 mm/detik di kondisi anil 11000C,suhu sensitasi 7000C dan terendah pada beban 30,5 Kg/mm2 3.10-8 mm/detik di kondisi tidak anil 11000C.
Bentuk patahan SCC berbentuk intergranular (tidak dianil 11000C). Bentuk patahan transgranular dengan banyak struktur dimple (void-void) nampak banyak di material anil 1100 berbagai suhu sensitasi. Prosentase peningkatan kelarutan Fe kedalam larutan uji antara 484% hingga 2050% , Kation Cr antara 750% hingga 3540%, dan Kation Ni hingga 110%.
ABTRACT
Austenitic Stainless steel (AISI 304) has a susceptibility of stress corrosion cracking inside corrosive chloride solution. This material also is susceptible from sensitizing temperature (5800C-8150C). This susceptibility of material clearly is undergone in intergranular corrosion. Grain boundaries contain chromium carbide. The combination of internal material and environment can contribute a great effect of stress corrosion cracking (SCC). MgCl2 circumstance have main role for SCC as corrosive solution.
SCC test was conducted with constant load method (creep) of 20 Kg/mm2, 25 Kg/mm2, 30,5 Kg/mm2, and 40 Kg/mm2 in 42 wt% MgCl2 solution and constant temperature of 1060C. Material treatment is divided two sides : (1) annealing process (11000C); holding 1 hour then quenching process and (2) without annealing. These two treatments were sensitized at 6000C, 7000C and 8000C. The qualitative test of carbide, the test of bulk chemical composition, solution test (AAS), Vickers hardness test, metallography, and SEM EDS test conducted.
Test results show Vickers hardness value on sensitizing temperature of 7000C that was undergone the decreasing of range 152,06 Hv (annealing of 11000C) and 199,1 Hv (non annealing) by comparing sensitizing temperature of 6000C and 8000C. The same trend also was happen at the test of SCC. On sensitizing temperature of 7000C fracture time (tf) is shorter than sensitizing temperature of 6000C and 8000C in two different material conditions. The shortest fracture time is happened at load of 25 Kg/mm2 that is tf of 3 seconds in annealing condition of 11000C and sensitizing temperature of 7000C. The longest fracture time is also happened at load of 30,5 Kg/mm2 that is tf of 86400 seconds without annealing process and sensitizing 6000C. The Highest Elongation rate (iss) at load of 25 Kg/mm2 is 4,80 mm/s in annealing condition of 11000C for sensitizing temperature of 7000C. The lowest one at load of 30,5 Kg/mm2 is 3.10-8 mm/s without annealing condition of 11000C.
The average shape of fracture of SCC is intergranular form without annealing process of 11000C. The shape of transgranular fracture with surface structure of dimples was undergone at annealing material of 11000C with various sensitizing temperatures. The increasing of dissolution percentage of Fe ions to test solution between 484% to 2050%, from 750% to 3540% (Cr ion), and up to 110% (Ni ion).
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