Austenitic stainless steels polarization

Fie. 5.26 Effect of chromium concentration on the polarization of chro-° mium modified type 304 austenitic stainless steel. All alloys con-... 

W.Y.C. Chen and J.R. Stephens, Anodic Polarization Behavior of Austenitic Stainless Steel Alloys with Lower Chromium Content, Corrosion, Vol 35, 1979, p 443-450... 

Fig. 7.19 Effect of temperature on the anodic polarization curves of a modified austenitic stainless steel containing 5.6 wt% Mo in 3.5 wt% NaCl at pH = 3. Redrawn from Ref 31...

S. Ahmad, M.L. Mehta, S.K. Saraf, and I. Saraswat, Anodic Polarization Characteristics of Sensitized 304 Austenitic Stainless Steel in Polythionic Acid Environment, Corrosion, Vol 39,1983, p 330... 

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 . ... 

Electrochemical tmodic polarization tests (ASTM G 5 and G 61 ) are useful corrosion test methods for alloy and process development work on P/M materials. Reference 23 describes the application of potentiostatic anodic polarization to steam-treated P/M carbon steel in neutral salt and acidic environments. References 13 and 19 describe the application of potentiodynamic polarization to sintered austenitic stainless steels. These test methods are very effective in revealing metallurgical weaknesses of sintered stainless steels. Sintered stainless steels, due to their large surface areas, exhibit large corrosion currents, compared to the wrought stainless steels, and frequently the current rises with increcising potential. Furthermore, sintered stainless steels do not always exhibit a pronoimced transition ftom... 

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). 

Figure 1 Anodic polarization curves of the pure metals Fe, Cr, Mo, and Ni and of an austenitic stainless steel, Fel8Crl4.3Ni2.5Mo (at %) (Fel6.7Crl5.0Ni4.28Mo wt%) exposed to 0.1 M HCl + 0.4 M NaCl at 25°C. Sweep rate, 3 mV/s. (From Ref 1.)...

Figure 3 Ion content vs. etch depth for the austenitic stainless steel Fel8Crl4.3Ni2.5Mo polarized at 500 mV (SCE). (From Ref 1.)...

Figure 10 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 Ref. 33.)...

Thus, the relation E =J(I) gives different eorrosion rates for a given metal in a given solution. Figure 1 shows such a relation (polarization curve) in the case of an austenitic stainless steel in an acidic Cl solution. Five domains can be considered for eorrosion and corrosion fatigue damage ... 

Figure 1 General polarization curve for an austenitic stainless steel in acidic Cl solutions.

Current electrochemical noise of the coarse crystalline austenitic stainless steel (a) and the corresponding NC coating (b) under anodic polarization during 600-800 s in 3.5% NaCI solution. ... 

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... 

N Is spectra for austenitic stainless steels (a) As nitrided (b) follow polarization from open circuit to OmV. Sweep rate ImV/s. (From WiUenbruch, R.D., Proceedings of the Sixth International Symposium on Passivity, Part 1, N. Sato and K. Hashimoto, eds., Corros. Sci., 31,179,1990.)... 

The spectra in Figure 6.13 were recorded from a high-alloyed austenitic stainless steel (16.7Cr-15Ni-4.3Mo) after polarization in 0.1 M HCI+0.4 M NaCl at -320 mV (SCE). The potential represents the active dissolution potential slightly above the corrosion potential. The result is from the same study as Figure 6.2. It appears from the spectra of Figure 6.13... 

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... 

Electrolytic etching for stainless steels. Specimen polarized as anode (-t) under a cell voltage of 3 V during 10 s to color ferrite (tan or light blue) and sigma (orange) but does not affect austenite. 

A corrosion study on the austenitic 316 L, 317 L, and 904 L stainless steels was conducted in 98 % phosphoric acid at 170 °C showing passivation regardless of the applied purge gas. When polarized at 0.1 V (hydrogen) and 0.7 V (air) in a phosphoric acid fuel cell environment,... 

Figure 7-30. Comparison of differently measured polarization curves of 5000 h aged duplex stainless steel measured in 0.1 M H2SO4 + 0.01 M KSCN. (a) Integral measurement of the aged material by the potentiostat SVE measurement of (b) austenite grain (c) ferrite grain (d) another ferrite grain (Jiang et al., 1992)...

The electrochemistry of Cl-SCC in duplex (a-y) stainless steels was discussed earlier and displayed in Figure 11.17. The ferrite phase has a higher value, due to its higher Cr content, but dissolves more rapidly (due to its lower Ni content) when both phases are in the active state [124]. The crack approaches (from above, due to IR limifafions) a mixed potential where the austenite is a net cathode and the ferrite a net anode. The ferrite is thus polarized above its normal (isolated) cracking potential and cracks rapidly, while the austenite is below its isolated cracking potential and cracks slowly or not at all. Cracks propagate in the ferrite and tend to be arrested by the austenite. Another situation is possible if toe ferrite can remain passive in the crack while the austenite corrodes now cracking occurs in the austenite and is hindered by the ferrite. 

---