The development in the industrial production of tanks for hot natural water is addressed to the use of stainless steels with high localized corrosion resistance and low costs. The very low nickel content of AISI 444 stainless steel leads to costs lower than those of AISI 304L and 316L; consequently, it seems suitable to substitute these latter steels for industrial production. In the present work, crevice corrosion resistance of AISI 316L and 304L austenitic stainless steels and AISI 444 ferritic stainless steel was studied by means of immersion tests in FeCl3 and cyclic potentiodynamic polarization tests in Cl 1000 mg dm-3 solution. The immersion tests in FeCl 3 1% wt. were carried out at 40 °C using steel sheet samples (25 × 30 mm) with two PTFE crevice formers assembled on the surfaces by means of two O-rings (Fig. 1). Corrosion potential (Ecorr) of the samples was regularly measured during the test. After different immersion times, some samples were pulled out from the solution and weighed, after removing the corrosion products, to measure the weight loss. Polarization tests were performed at 80 °C in Cl 1000 mg dm-3 solution both on samples with crevice former and samples without crevice former (Fig. 2) in order to evaluate crevice and pitting corrosion resistance. The exposed area (2 × 2 cm) was well-defined by shielding the unexposed parts with an epoxy resin resistant to high temperatures. Cyclic potentiodynamic polarization tests were carried out starting from Ecorr - 30 mV until the current density reached 1 mA cm2 (scan rate 0.166 mV s-1); the scan was then reversed and continued until new passivity conditions were achieved. The corrosion potential was measured before the polarization experiments. From the E-log i plots, the values of pitting (Epit), crevice (Ecrev) and protection (Eprot) potential were obtained; from these potentials, the perfect and the imperfect passivity regions were defined. All the tests were performed both on as received stainless steel samples and on samples submitted to a cleaning-passivation treatment by immersion in a solution containing HF and HNO3, to improve their corrosion resistance. The weight loss measurements in FeCl 3 solution (Fig. 3) indicate that AISI 444 is the steel with the lowest corrosion resistance and that the passivation treatment leads to a decrease in corrosion resistance both for AISI 444 and AISI 304L. These results are not confirmed by the corrosion potential values measured during the fist 25 h of immersion In fact, corrosion potentials of the as received AISI 444 and AISI 304L steels (Fig. 4) are lower than those of the passivated samples (Fig. 5), indicating that the passivation treatment increases their corrosion resistance. No remarkable difference was observed for passivated AISI 316L samples with respect to the as received ones. Fig. 6 shows the trend of Epit (as received and passivated steels tested without crevice former), Ecrev (as received and passivated steels tested with crevice former), Eprot and Ecorr obtained from the cyclic polarization curves. Between the as received steels (Fig. 6a), AISI 304L shows the highest pitting corrosion potential whereas AISI 444 and AISI 316L show almost the same pitting resistance. The trend of the Ecrev values of the as received steels, obtained with crevice former (Fig. 6b), is similar to that obtained without crevice former (Fig. 6a), even if all the Ecrev values are lower than the corresponding Epit values. The passivation treatment increases both the pitting and crevice corrosion resistance, in particular those of AISI 444 (Fig. 6c and 6d respectively). Polarization test results indicate that the passivated AISI 444 can be suitable to substitute the austenitic stainless steels in hot natural waters which are not particularly aggressive. The contrast between these results and those obtained with the weight loss measurements in FeCl3 is probably due to the fact that this last solution is much too aggressive to study the effect of the surface finishing on the localized corrosion resistance of the stainless steels.

Studio della corrosione interstiziale degli acciai inossidabili AISI 316L, AISI 304L e AISI 444 (A study on crevice corrosion of AISI 316L, AISI 304L and AISI 444 stainless steels)

BELLEZZE, Tiziano
Writing – Review & Editing
;
ROVENTI, Gabriella
Writing – Review & Editing
;
FRATESI, Romeo
Supervision
2008

Abstract

The development in the industrial production of tanks for hot natural water is addressed to the use of stainless steels with high localized corrosion resistance and low costs. The very low nickel content of AISI 444 stainless steel leads to costs lower than those of AISI 304L and 316L; consequently, it seems suitable to substitute these latter steels for industrial production. In the present work, crevice corrosion resistance of AISI 316L and 304L austenitic stainless steels and AISI 444 ferritic stainless steel was studied by means of immersion tests in FeCl3 and cyclic potentiodynamic polarization tests in Cl 1000 mg dm-3 solution. The immersion tests in FeCl 3 1% wt. were carried out at 40 °C using steel sheet samples (25 × 30 mm) with two PTFE crevice formers assembled on the surfaces by means of two O-rings (Fig. 1). Corrosion potential (Ecorr) of the samples was regularly measured during the test. After different immersion times, some samples were pulled out from the solution and weighed, after removing the corrosion products, to measure the weight loss. Polarization tests were performed at 80 °C in Cl 1000 mg dm-3 solution both on samples with crevice former and samples without crevice former (Fig. 2) in order to evaluate crevice and pitting corrosion resistance. The exposed area (2 × 2 cm) was well-defined by shielding the unexposed parts with an epoxy resin resistant to high temperatures. Cyclic potentiodynamic polarization tests were carried out starting from Ecorr - 30 mV until the current density reached 1 mA cm2 (scan rate 0.166 mV s-1); the scan was then reversed and continued until new passivity conditions were achieved. The corrosion potential was measured before the polarization experiments. From the E-log i plots, the values of pitting (Epit), crevice (Ecrev) and protection (Eprot) potential were obtained; from these potentials, the perfect and the imperfect passivity regions were defined. All the tests were performed both on as received stainless steel samples and on samples submitted to a cleaning-passivation treatment by immersion in a solution containing HF and HNO3, to improve their corrosion resistance. The weight loss measurements in FeCl 3 solution (Fig. 3) indicate that AISI 444 is the steel with the lowest corrosion resistance and that the passivation treatment leads to a decrease in corrosion resistance both for AISI 444 and AISI 304L. These results are not confirmed by the corrosion potential values measured during the fist 25 h of immersion In fact, corrosion potentials of the as received AISI 444 and AISI 304L steels (Fig. 4) are lower than those of the passivated samples (Fig. 5), indicating that the passivation treatment increases their corrosion resistance. No remarkable difference was observed for passivated AISI 316L samples with respect to the as received ones. Fig. 6 shows the trend of Epit (as received and passivated steels tested without crevice former), Ecrev (as received and passivated steels tested with crevice former), Eprot and Ecorr obtained from the cyclic polarization curves. Between the as received steels (Fig. 6a), AISI 304L shows the highest pitting corrosion potential whereas AISI 444 and AISI 316L show almost the same pitting resistance. The trend of the Ecrev values of the as received steels, obtained with crevice former (Fig. 6b), is similar to that obtained without crevice former (Fig. 6a), even if all the Ecrev values are lower than the corresponding Epit values. The passivation treatment increases both the pitting and crevice corrosion resistance, in particular those of AISI 444 (Fig. 6c and 6d respectively). Polarization test results indicate that the passivated AISI 444 can be suitable to substitute the austenitic stainless steels in hot natural waters which are not particularly aggressive. The contrast between these results and those obtained with the weight loss measurements in FeCl3 is probably due to the fact that this last solution is much too aggressive to study the effect of the surface finishing on the localized corrosion resistance of the stainless steels.
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11566/50153
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus 2
  • ???jsp.display-item.citation.isi??? 1
social impact