D. Itzhak

An anodic behaviour study of an analogical sintered system of austenitic stainless steel in H2SO4 solution

The anodic behaviour of sintered austenitic stainless steel was investigated on an analogical system using wrought plate stainless steel of Type 316. Potentiodynamic polarization measurements were carried out on wrought plate stainless steel of Type 316 in an analogical concentration cell using a special method. The effect of carbides precipitation on the anodic behaviour was investigated on carburized 316 wrought stainless steel containing an average carbon concentration of 0.06 and 0.15 (wt. %). The carburization treatment results in an increase in the critical current density although a clear active passive transition is observed. The anodic behaviour of sintered stainless steel in H2SO4 solution is mainly affected by the existence of interconnected open pores. The local intergranular sensitivity to the corrosive environment is increased due to carbide precipitation.

The effect of Cu addition on the corrosion behaviour of sintered stainless steel in H2SO4 environment

Abstract The influence of Cu addition on the corrosion behaviour of hot pressed sintered stainless steel of Type 316 in H 2 SO 4 solution was investigated. It was found that Cu additions of 0.25-5 wt% enhance the passivation processes of the hot pressed sintered stainless steel. The open-circuit potential of samples containing Cu was + 200 mV(SCE) as compared to −250 mV ( SCE ) in the other case. The potentiodynamic measurements indicate that as a result of the Cu addition the corrosion potential is increased, and the anodic critical current density and the cathodic overpotential for the hydrogen evolution are decreased. It is suggested that the low hydrogen cathodic overpotential caused by Cu addition, combined with the active-passive basic behaviour of the 316 stainless steel matrix, arc the main factors affecting the improved corrosion resistance.

Corrosion behaviour of hot-pressed austenitic stainless steel in H2SO4 solutions at room temperature

Abstract The corrosion behavior of hot-pressed stainless steel (HPSS) of Type 316 was investigated. Samples of HPSS with porosity of 10–30% were prepared. The influence of the porosity factor on corrosion behaviour was investigated by potentiodynamic polarization, open circuit potential, identification of corrosion products by EDAX and SEM observation. It was found that porosity of the HPSS is the main factor affecting corrosion resistance. High porosity results in low corrosion resistance. The open circuit potential of the HPSS is about − 200 mV (VSCE) as compared to 200 mV for the wrought 316 stainless steel. The potentiodynamic measurements indicate that cathodic concentration polarization is achieved at a relatively low current density and secondary active-passive transition is observed at a relatively high current density in HPSS as compared to wrought 316 stainless steel. It is suggested that the main mechanism affecting the low corrosion resistance of HPSS in H 2 SO 4 is the evolution of hydrogen concentration cells due to electrolytic stagnation in the interconnected open pores. As a result the surfaces of the interconnected open pores act as an active anode and the engineering sample surface acts as an active cathode.

The surface composition of sintered stainless steel containing noble alloying elements exposed to A H2SO4 environment

The surface composition of passive hot pressed and sintered stainless steel was investigated by AES analysis. A maximum in the Mo concentration was observed in the depth of about 6 A, in all the sintered stainless steel samples, regardless of the composition, indicating the important role of the Mo in the passivation process of the stainless steel. It was also found that the oxide film thickness of passive samples containing Au, Pd and Pt was about 120–150 A, as compared to 20–30 A for passive sintered 316 stainless steel, or to passive samples containing Ni. The behavior of the passive sintered stainless steel containing Au, Pd and Pt indicates higher cathodic activity and lower electrical isolation of the passive layer, as compared to Ni-containing sampels. Cu is markedly enriched on the surface of passive samples after exposure to H2SO4, as compared to samples containing Au, Pd or Pt.

The effect of Sn addition on the corrosion behaviour of sintered stainless steel in H2SO4

Abstract The influence of Sn addition on the corrosion behaviour of hot-pressed sintered stainless steel of type 316 in H2SO4 solutions was investigated. The samples were investigated by SEM observation, open circuit potential during an immersion test and potentiodynamic polarization. It was found that Sn additions of 0.5–3 wt% enhance the densification processes and as a result the amount of the interconnected open pores was decreased. The corrosion resistance of hot-pressed sintered stainless steel was substantially improved by the Sn addition. The weight loss of the samples containing Sn is negligible after 200 h of immersion compared to corrosion. The potentiodynamic measurements indicate that as a result of the Sn addition, the critical current density and the passivation current density were decreased, as compared to the hot-pressed stainless steel. The cathodic polarization curves indicate surface poisoning, causing higher over-potential for the hydrogen evolution.

Pitting corrosion evaluation by computer image processing

Abstract Pitting corrosion of AISI 304L stainless steel in a solution of FeCl3 is evaluated by computer image processing methods. Pitting probability and histograms of pit-areas are computed. Pitting probability is defined as the ratio of pitted area over the total area. The pitting probability in this work was found to be 9.73%. Computer image processing is shown to be a promising tool for statistical evaluation of pitting corrosion.

The corrosion behavior of double pressed, double sintered stainless steel containing noble alloying elements

Abstract Hot pressed and sintered stainless steel samples are usually active in a H 2 SO 4 environment, at ambient temperature and corrode up to total loss. The double pressing, double sintering (DPDS) process results in a closed, round pore morphology. Samples obtained by this process exhibit a clear passive behavior in a 1 N H 2 SO 4 environment. The pore morphology of the sintered stainless steel has a crucial role in its environmental behavior. When a closed pore morphology is achieved, its behavior is similar to wrought stainless steel. The noble alloying elements enhance the self passivation processes, and enable passivation in samples with some degree of open porosity.

The effect of noble alloying elements Ag, Pt and Au on the corrosion behavior of sintered stainless steel in an H2SO4 environment

The effect of cathodically active noble alloying elements, Ag, Pt and Au on the environmental behavior of hot pressed and sintered stainless steel in H2SO4 environment was investigated. It was found that Ag additions slightly improve the corrosion resistance and remain as inclusions in the sintered stainless steel. Pt additions of about 2 wt% were found to be sufficient to preserve the passive state. High Pt content results in high tendency for breakdown of the passive layer, due to very high cathodic activity. Au additions strongly affect pore morphology by decreasing the density of the open pores, most probably due to liquid phase sintering. The samples containing Au additions of various concentrations exhibit clear anodic active-passive transition with a wide range of passivation. Samples containing 5 wt% Au remain passive.

The influence of alkali-halide additions on the stress corrosion cracking of an austenitic stainless steel in MgCl2 solution

Abstract The influence of NaCl, NaBr, NaI additions on the stress corrosion cracking (s.c.c.) of Type 304 stainless steel in 38 wt% aqueous MgCl 2 solution at 135°C has been investigated. Slow strainrate and U-Bend tests, potentiodynamic measurements and fractographic observation were employed. The results indicate clearly that the addition of 1N NaI or NaBr to the boiling MgCl 2 solution prevents stress corrosion cracking of the steel while NaCl additions accelerates cracking. It is suggested that the addition of NaI inhibits mainly the cathodic process. The I − ion is oxidized by the dissolved oxygen through the following reaction: 41 − + 4H + + O 2 → 2I 2 + 2H 2 O. Due to this reaction O 2 and H + are not reacted cathodically with the metal. This process enhanced the cathodic inhibition effect and results in preventing s.c.c. of the AISI 304 stainless steel in 38 wt% MgCl 2 boiling solution.

The stress corrosion cracking of welded austenitic stainless steels in MgCl2 solutions in the presence of NaI additions

Abstract The influence of Nal additions on the stress corrosion cracking (SCC) of Type 304L and 316L welded stainless steels in 42 wt% aqueous MgCl 2 solution at 154°C has been investigated. The results indicate clearly that addition of 1 N NaI to the boiling MgCl 2 solution prevents SCC of the welded steels. The I − can act as an effective cathodic inhibitor. The correlation between mechanical properties, fracture morphology and the mechanism of the inhibition behaviour of I − on SCC is discussed.