Austenitic stainless steels passivity potentials

The Tafel constant was b = 0.20 V decade-1 for iron electrodes [55] and b = 0.20 V decade-1 for austenitic stainless steels [54] in acid solution. It is noticed that these Tafel constants are greater than those (0.03-0.1 V) usually observed with general dissolution of metals in acid solution. The other mode of localized corrosion is the active mode of corrosion that prevails in the potential range less positive (more cathodic) than the passivation potential, EP, in which potential range the localized corrosion is mainly controlled by the acidity of the occluded pit solution. In the potential range of active metal dissolution, the anodic dissolution current density is also an exponential function of the electrode potential, except for diffusion-controlled dissolution. 

Multiwall carbon nanotube (MWCNT)-reinforced hydroxyapatite composite coatings (80% HAp/20% MWCNT) were deposited on austenitic stainless steel AISI 316L by laser surface alloying (LSA) with a 2.5-kW CW Nd YAG laser (Kwok, 2007). EIS of unprotected AISI 316L and HAp/MWCNT-coated steel obtained at open circuit potential are shown in Figure 7.60 after immersion in 0.9% NaCl solution for 2 h. The Bode plot shows that the total impedance Z has noticeably increased for the steel substrate coated with HAp/MWCNT. While the thin passive oxide film on the stainless steel surface was rendered less protective... 

An interesting aspect of the expression (10) concerns the case of metals and passive alloys because the real polarization potential exhibits a discontinuity around the zone of transition from active to passive state. In fact, if Ip denotes the passivity current density, the value of the discontinuity is of the same order of magnitude as R,IpS because during this transition the current intensity falls very rapidly. The discontinuity may be very pronounced because the values of Ip, which depend on the type of metal, the environment and temperature, may be very high. In the case of the AISI 321H titanium-stabilized, austenitic stainless steel in 1 M HCIO4 -1- 0.3 M NaCl solutions at 25 °C, the value of Ip depends on the thermal history of the specimen [50]. In meiny instances it was found to be about 10 mAcm . ... 

Corrosion of filters occurs in the transpassive state. Their cathodic protection is based on the polarization of steel to a potential characteristic of the passive state. Garner (1998) states that over 120 CP installations have been applied, mainly in North America, for the protection against corrosion of equipment made of austenitic stainless steels operating in bleacheries. More information is given by Webster (1989) and Singbeil and Garner (1987). 

As mentioned before, austenitic stainless steels are susceptible to IGC due to sensitization caused by exposure to high temperatures (450-850 C). The IGC of austenitic stainless steel can also be characterized by normalized classical tests ASTM G28, ASTM A262-86, SEP 1877, AFNOR A05-159 and AFNOR A05-160, currently known as the Strauss, Huey and Streicher tests [54-57]. These methods however are destructive, difficult to perform on site and require sampling that can be harmful to the integrity of materials during service. For this reason, the electrochemical, non-destructive tests commonly known as EPR (electrochemical potentiokinetic reactivation) and DL-EPR (double loop electrochemical potentiokinetic reactivation) were developed to measure the sensitivity of austenitic stainless steels to IGC [58-66]. However, EPR and DL-EPR are based on measurements of characteristic potentials and currents of passive/active zones on potentiody-namic curves in an aqueous solution (linear voltammetry curve from oxygen to hydrogen evolution in the... 

Austenitic stainless steels appear to have significantly greater potential for aqueous corrosion resistance than their ferritic counterparts. This is because the three most commonly used austenite stabilizers, Ni, Mn, andN, all contribute to passivity. As in the case of ferritic stainless steel. Mo, one of the most potent alloying additions for improving corrosion resistance, can also be added to austenitic stainless steels in order to improve the stability of the passive film, especially in the presence of Cl ions. The passive film formed on austenitic stainless steels is often reported to be duplex, consisting of an inner barrier oxide film and outer deposit hydroxide or salt film. 

Passivation is generally believed to take place by the rapid formation of surface-adsorbed hydrated complexes of metals, which are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that, in turn, r idly deprotonates to form an insoluble surface oxide film. Failure in any of these stages would lead to continued active dissolution. The passivation potential is critical to this process, in part because it governs the oxidation state of the metal, which in turn governs its solubility. In addition, the electric field strength has to be sufficient to cause deprotonation of the surface hydroxide phase in order to enable the oxide barrier film to become estabilished. No evidence has been found by surface studies that passivity of austenitic stainless steels is possible by formation of a simple hydroxide film. It is peihaps surprising that the passive film formed on austenitic stainless steels does not always contain each of the alloying elements added to stabilize the austenitic phase, even when such additions appear to improve the chemical stability of the steel. Ni exemplifies this behavior. 

It will also be shown that the thickness of the passive films, while varying with the potential of passivation, is commonly only a few nanometers. Finally, we shall see that in austenitic stainless steels tire active stage of repassivation often results in considerable change in composition at the alloy surface. The possible role of this modified alloy layer in the overall passivation process will be discussed. 

Fe-Cr and Fe-Cr-Ni alloys are of high technical importance, the main benefit for ferritic and austenitic stainless steels resulting from the excellent corrosion resistance of Cr203 layers. Figure 5.31 shows the polarization curve of Fe-15 Cr in 0.5 M H2SO4 [92]. Its characteristic features are determined by the electrochemical properties of the pure alloy components. Hydrogen evolution (with cathodic currents) is observed up to E = -0.2 V followed by the potential range of active dissolution of Cr and Fe + up to OV where passivity starts due to... 

Polarization curves for surface-nitrided and untreated austenitic stainless steels in deaerated 0.1 M HCl where the specimens were permitted to float to the open-circuit potential before polarization. Sweep rate 1 mV/s. (From Willenbruch, R.D., Proceedings of the Sixth International Symposium on Passivity, Part 1, N. Sato and K. Hashimoto, eds., Corros. Sci., 31,179,1990.)... 

A schematic summary of the alloying metals that affect the anodic polarization curve of stainless steel is shown in Fig. 4.16. The addition of 8% nickel to an alloy containing 18% chromium forms austenitic structure SS Type 304. The addition of Mn and N increases the stability of austenitic steel. The chromium content of stainless steel affects the anodic polarization curves as shown in Fig. 4.16. Nickel promotes repassivation in a corrosive environment, but concentrations higher than 30% reduces the passivation current, the critical current density, and increases the critical pitting potential. Nitrogen... 

Pitting corrosion Austenitic staiidess steels owe their corrosion resistance to the formation, from the chrome (Cr) content of the steel, of a passive layer of chromium oxide on the exposed surface. Pitting occurs when the protective oxide film breaks down in small, isolated spots. The rate of attack tends to increase because of the differences in electric potential between the large surrounding passive surface and the active pit. This action is accentuated by the presence of saline solutions. A smooth surface, free of sensitive local minute pits or small depressions, reduces the potential for pitting to commence. The most appropriate quality of stainless for such duties should be selected. 

Figure 7-29. SEM micrographs of the etched surfaces of 5000 h aged duplex stainless steel, (a) At -268 mV(Ag/AgCl) leading to active dissolution in austenite and ferrite (b) -170 mV (Ag/AgCl) (c) -130 mV (Ag/AgCl) leading to active dissolution of ferrite, while austenite is passive at both potentials. Electrolyte 0.1 MH2SO4 + O.OI M KSCN (Jiangetal., 1992).

---